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Resource limitation and population ecology of white-eared kob Fryxell, John M. 1985

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RESOURCE LIMITATION AND POPULATION ECOLOGY OF WHITE-EARED KOB by JOHN M. FRYXELL B.Sc.  (hon), U n i v e r s i t y of B r i t i s h Columbia, Vancouver, 1978  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES (Department of Zoology) We accept t h i s t h e s i s as conforming to the r e q u i r e d standard  THE UNIVERSITY OF BRITISH COLUMBIA 1985 @ @  John M. F r y x e l l ,  In  presenting  degree  this  at the  thesis in  University of  partial  fulfilment  of  of  department  this or  thesis for by  his  or  scholarly purposes may be her  representatives.  permission.  Department of The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3  for  an advanced  Library shall make  it  agree that permission for extensive  It  publication of this thesis for financial gain shall not  DE-6(3/81)  requirements  British Columbia, I agree that the  freely available for reference and study. I further copying  the  is  granted  by the  understood  that  be allowed without  head of copying  my or  my written  ii  ABSTRACT In t h i s study I • examine the e f f e c t of seasonal resource l i m i t a t i o n on the behavior and population dynamics of whiteeared kob, Kobus kob leucotis, i n the Boma region of the southern Sudan. This p o p u l a t i o n , numbering over 800,000, migrates s e a s o n a l l y between savannah g r a s s l a n d s i n areas with low r a i n f a l l and ephemerally swamped g r a s s l a n d s in areas with high rainfall. The aims of the study were: (1) to examine whether kob m i g r a t i o n t r a c k s ephemeral d i s t r i b u t i o n s of food or water resources, (2) to t e s t the hypothesis that the Boma kob p o p u l a t i o n i s l i m i t e d by food a v a i l a b i l i t y , (3) to determine i f c a l f production i s cued to seasonal peaks i n food abundance, and (4) to evaluate the effect of breeding synchrony on l e k k i n g behavior and male c o m p e t i t i o n . Seasonal c l i m a t i c changes produced pronounced changes in the distribution and abundance of both green forage and water supplies. Dry season m i g r a t i o n p r i m a r i l y tracked limited s u p p l i e s of water. Within the dry season range, kob aggregated at high d e n s i t i e s (over 1,000 per km ) i n l o w - l y i n g meadows that supported grass re-growth when l i t t l e green grass was a v a i l a b l e elsewhere in the ecosystem. However, s o u t h e r l y movements i n the' wet season were not e x p l a i n a b l e by the resource h y p o t h e s i s , s i n c e both food and water were widely d i s t r i b u t e d d u r i n g the wet season. I suggest that kob may move southward i n order to a v o i d surface f l o o d i n g d u r i n g the wet season. 2  Kob m o r t a l i t y during the dry seasons of 1982 and 1983 was c o n s i d e r a b l y higher than estimated m o r t a l i t y during the wet season. Unusual rainfall during the dry season of 1982 provided a " n a t u r a l experiment" to t e s t the food limitation hypothesis. Adult m o r t a l i t y was s i g n i f i c a n t l y lower d u r i n g the dry season of 1982 than d u r i n g the more t y p i c a l dry season of 1983. C a l f m o r t a l i t y d i d not vary s i g n i f i c a n t l y between y e a r s . Adult m o r t a l i t y r a t e s were r e l a t e d to dry season d u r a t i o n . Dry season m o r t a l i t y was r e l a t e d to sub-maintenance food intake and declining fat reserves. The age s t r u c t u r e of the kob p o p u l a t i o n in 1983 suggests that l a r g e - s c a l e m o r t a l i t y (ca. 40%) occurred i n the 1980 drought that immediately preceded t h i s study. These f i n d i n g s support the food l i m i t a t i o n h y p o t h e s i s . Kob e x h i b i t e d a 4 month p e r i o d of c a l f p r o d u c t i o n d u r i n g the l a t e wet season, when food a v a i l a b i l i t y was h i g h e s t . As a consequence, females continued l a c t a t i o n through the dry season p e r i o d of food scarcity. I suggest that kob reproductive phenology may result from an o b l i g a t o r y delay d u r i n g which females r e s t o r e t h e i r f a t r e s e r v e s p r i o r to c a l v i n g or s e l e c t i o n p r e s s u r e s imposed by p r e d a t i o n d u r i n g the v u l n e r a b l e post-partum period. Synchronous breeding i n the Boma kob was related to increased r a t e s of aggression between males and i n c r e a s e d c o l o r dimorphism, i n comparison to the asynchronous breeding Uganda  kob, Kobus kob thomasi. Male aggression served not only to e s t a b l i s h dominance r e l a t i o n s between males on leks, but also disrupted the mating a c t i v i t i e s of neighboring males. Young a d u l t males s u f f e r e d higher a g e - s p e c i f i c m o r t a l i t y than females, possibly resulting from injuries i n c u r r e d during strenuous f i g h t i n g on l e k s . In order to analyze the age s t r u c t u r e of the kob p o p u l a t i o n , I d e v i s e d a new method f o r e s t i m a t i n g a g e - s p e c i f i c m o r t a l i t y r a t e s that i s f r e e of the r e s t r i c t i v e assumptions that u n d e r l i e most c o n v e n t i o n a l techniques. The proposed method has somewhat g r e a t e r sampling v a r i a t i o n , but i s c o n s i d e r a b l y more robust, than two c u r r e n t methods. Moreover, the proposed method p e r m i t s c a l c u l a t i o n of a g e - s p e c i f i c m o r t a l i t y at frequent intervals during p e r i o d s of p o p u l a t i o n f l u c t u a t i o n and, under some c i r c u m s t a n c e s , p o p u l a t i o n numerical trends may be directly determined from age s t r u c t u r e .  TABLE OF CONTENTS ABSTRACT LIST OF TABLES LIST OF FIGURES ACKNOWLEDGEMENTS CHAPTER 1 . GENERAL INTRODUCTION CHAPTER 2. SEASONAL MIGRATION IN RELATION TO RESOURCES Introduction Methods A e r i a l surveys Ground Observations The study area P h y s i c a l f e a t u r e s and v e g e t a t i o n types Climatic seasonality Results A e r i a l survey r e s u l t s Seasonal changes i n green biomass Seasonal changes i n water a v a i l a b i l i t y White-eared kob p o p u l a t i o n numbers Seasonal m i g r a t i o n of white-eared kob Dry season ground o b s e r v a t i o n s Kob d i s t r i b u t i o n p a t t e r n s F a c t o r s a f f e c t i n g forage abundance Feeding s e l e c t i o n f o r p l a n t p a r t s Food a v a i l a b i l i t y r e l a t i v e t o kob requirements Discussion Seasonal changes i n resource d i s t r i b u t i o n Seasonal kob m i g r a t i o n B e n e f i t s of m i g r a t i o n CHAPTER 3. FOOD LIMITATION AND KOB MORTALITY PATTERNS Introduction Methods Population density Carcass d e n s i t y Sex and age d i s t r i b u t i o n Tooth cementum l i n e s Post-mortem examination Results Age d i s t r i b u t i o n i n the l i v e p o p u l a t i o n Age d i s t r i b u t i o n at death Age-specific mortality Sex r a t i o Seasonal changes i n body c o n d i t i o n Seasonal changes i n m o r t a l i t y r a t e s Dry season m o r t a l i t y i n r e l a t i o n to food i n t a k e Cumulative dry season m o r t a l i t y Discussion Causes of m o r t a l i t y d u r i n g the dry season Predation Disease Age s t r u c t u r e of the kob p o p u l a t i o n Evidence f o r food l i m i t a t i o n  i i vi vii ix 1 8 8 10 10 13 17 17 20 24 24 24 .... 27 29 29 35 35 45 49 53 55 55 56 59 61 61 62 62 63 65 66 67 68 68 68 72 72 76 79 82 85 89 89 89 92 93 96  V  CHAPTER 4. KOB REPRODUCTIVE PHENOLOGY 100 Introduction 100 Methods 101 Results 103 Discussion 111 CHAPTER 5. BREEDING SYNCHRONY AND MALE AGGRESSION 114 Introduction 114 Methods 119 Sexual dimorphism 119 Lek o b s e r v a t i o n s 119 Results 121 Sexual dimorphism 121 Temporal change i n mating and a g o n i s t i c behavior ....121 Spatial distribution of females and a g o n i s t i c encounters 124 Functions of f i g h t i n g behavior 128 Consequences of f i g h t i n g 131 Discussion 133 CHAPTER 6. AGE-SPECIFIC MORTALITY: AN ALTERNATIVE APPROACH 137 Introduction 137 The model 138 Methods 140 C a l c u l a t i o n of a g e - s p e c i f i c m o r t a l i t y 141 Sampling d i s t r i b u t i o n s 144 E f f e c t s of an unstable age d i s t r i b u t i o n 146 Results 147 Discussion 151 Advantages of the proposed method 151 Comparisons between methods 154 CHAPTER 7. GENERAL DISCUSSION 158 Food l i m i t a t i o n 158 Seasonal m i g r a t i o n 160 Breeding phenology 162 Breeding synchrony and male a g g r e s s i o n 163 Age-specific m o r t a l i t y patterns 164 General c o n c l u s i o n s 165 LITERATURE CITED 168 APPENDIX 1. PLANT SPECIES COLLECTED 183  LIST OF TABLES Table 2.1 Akobo monthly c l i m a t i c normals Table 2.2 Kob p o p u l a t i o n estimates 1980-1983 Table 2.3 Dry season grass biomass estimates Table 2.4 N u t r i t i o n a l analyses of meadow vs. t a l l grasses. Table 2.5 Feeding s e l e c t i o n f o r l e a f vs. stem t i s s u e Table 3.1 Dry season weekly m o r t a l i t y estimates Table 3.2 M o r t a l i t y i n r e l a t i o n to food intake Table 5.1 Changes i n lek behavior over the dry season Table 5.2 Female movements f o l l o w i n g d i s r u p t i o n Table 6.1 Data used to c a l c u l a t e q using method C Table 6.2 Skewness and k u r t o s i s of sampling distributions obtained using 3 a l t e r n a t i v e methods x  21 30 39 41 52 80 84 123 132 143 149  vii  LIST OF FIGURES F i g u r e 1.1 Map of the southeast Sudan 4 F i g u r e 1.2 L i n e s of i n v e s t i g a t i o n 7 F i g u r e 2.1 C a l i b r a t i o n of photometer readings 14 F i g u r e 2.2 Map of the study area 15 F i g u r e 2.3 Map of woody v e g e t a t i o n cover 19 F i g u r e 2.4 Mean monthly r a i n f a l l t o t a l s 22 Figure 2.5 North-south g r a d i e n t i n seasonal green biomass abundance 25 F i g u r e 2.6 Seasonal d i s t r i b u t i o n of green biomass 26 F i g u r e 2.7 Seasonal d i s t r i b u t i o n of water s u p p l i e s . 28 F i g u r e 2.8 Seasonal d i s t r i b u t i o n of kob 31 F i g u r e 2.9 Kob d i s t r i b u t i o n i n r e l a t i o n to nearest water. . 33 F i g u r e 2.10 Kob dry season c o n c e n t r a t i o n s 36 F i g u r e 2.11 P o p u l a t i o n densities i n woodland vs. meadow habitats 37 F i g u r e 2.12 Kob d i u r n a l movements onto meadows 38 F i g u r e 2.13 Dry season meadow grass growth 42 F i g u r e 2.14 Dry season l e a f and stem t i s s u e growth 43 F i g u r e 2.15 Kob aggregation around meadows 46 F i g u r e 2.16 Kob abandonment of meadows 47 Figure 2.17 Dry season grass growth in r e l a t i o n to s o i l moisture 48 F i g u r e 2.18 S o i l moisture d e c l i n e over the dry season 50 F i g u r e 2.19 Feeding c o n c e n t r a t i o n near water 51 F i g u r e 3.1 Age s t r u c t u r e of the l i v e p o p u l a t i o n 69 F i g u r e 3.2 Age s t r u c t u r e of found c a r c a s s sample 70 F i g u r e 3.3 Age f r e q u e n c i e s of c a r c a s s e s by sex 71 F i g u r e 3.4 A g e - s p e c i f i c m o r t a l i t y curve 73 F i g u r e 3.5 Carcass f r e q e n c i e s i n r e l a t i o n to d i s t a n c e from observers 75 F i g u r e 3.6 Dry season d e c l i n e i n f a t r e s e r v e s 77 F i g u r e 3.7 Dry season a d u l t m o r t a l i t y r a t e s (Ajwara) 81 F i g u r e 3.8 Dry season c a l f m o r t a l i t y r a t e s (Ajwara) 83 Figure 3.9 Adult m o r t a l i t y as a f u n c t i o n of dry season duration 86 Figure 3.10 Graphical explanation of the body c o n d i t i o n / m o r t a l i t y hypothesis 88 Figure 3.11 Cumulative m o r t a l i t y during a drought in N a i r o b i N a t i o n a l Park 90 F i g u r e 3.12 Kob p o p u l a t i o n estimates 1979-1983 95 F i g u r e 4.1 Frequencies of b i r t h s by month 1.04 F i g u r e 4.2 Female r e p r o d u c t i v e c o n d i t i o n January to May. ..105 F i g u r e 4.3 Dry season body c o n d i t i o n of l a c t a t i n g vs. nonl a c t a t i n g females 107 F i g u r e 4.4 A c t i v i t y p a t t e r n s of males vs. females 108 F i g u r e 4.5 Seasonal changes i n c a l f / f e m a l e r a t i o 109 Figure 5.1 Sexual selection in relation to breeding synchrony 116 F i g u r e 5.2 Hypothetical cost-benefit relations of male fighting intensity 118 F i g u r e 5.3 Kob l i v e weights at age 122  viii  Figure 5.4 Changes i n a g o n i s t i c behavior r a t e s over the breeding season 125 F i g u r e 5.5 Female s p a t i a l d i s t r i b u t i o n on l e k s 126 F i g u r e 5.6 Female group s i z e s with s i n g l e males 127 F i g u r e 5.7 S p a t i a l d i s t r i b u t i o n of f i g h t s between males. ..129 F i g u r e 6.1 A f r i c a n b u f f a l o a g e - s p e c i f i c m o r t a l i t y c u r v e s ..142 F i g u r e 6.2 Sampling v a r i a t i o n r e s u l t i n g from the use of 3 a l t e r n a t i v e methods 148 Figure 6.3 Age-specific mortality estimated from an u n s t a b l e age d i s t r i b u t i o n 150 F i g u r e 6.4 Sampling v a r i a t i o n as a f u n c t i o n of denominator value 156  ix  ACKNOWLEDGEMENTS A number of people a s s i s t e d i n the f i e l d work r e p o r t e d i n t h i s study, f r e q u e n t l y under t r y i n g circumstances: Alpayo Dani, Michael Earle, Dana F r y x e l l , Beko Ladu, C h a r l e s L o r i n g , Ipote Luke, Y u s i f Mohammed, and Baba Terkumpte. Steve Cobb, Conrad E v e l i n g , Tim Fyson, Jens H e s s e l , C h r i s H i l l m a n , Karen Ross, P h i l Snyder, and Tim Tear assisted ably i n a e r i a l surveys. In a d d i t i o n , a number of i n d i v i d u a l s and o r g a n i s a t i o n s p r o v i d e d invaluable logistic support: Barbie A l l e n , the Arensons, L i z D a v i s , Peter Garment, GTZ, the Haspels, I n t e r f r e i g h t L t d . , Mike Norton-Griffiths, the Juba Boatyard, Phillip Winter, John Olander and the mujanim. In t h e i r own ways, a l l c o n t r i b u t e d t o many memorable adventures. I thank my t h e s i s committee, Ray H i l b o r n , Charley Krebs, Peter Murtha, Mike P i t t , Tony S i n c l a i r , and Jamie Smith f o r their u n h e s i t a t i n g a t t e n t i o n and h i g h - q u a l i t y c r i t i c i s m . In a d d i t i o n t o my t h e s i s committee, Lee Gass, John Eadie, Don Ludwig, John Greever, Dave T a i t , B i l l N e i l l , and Dolph S c h l u t e r made v a l u a b l e comments on e a r l i e r d r a f t s of v a r i o u s p a r t s of the thesis. Fellow graduate students, s t a f f , and f a c u l t y of the Institute of Animal Resource Ecology provided a s i m u l t a n e o u s l y c r i t i c a l and c o o p e r a t i v e atmosphere that c o n t r i b u t e d g r e a t l y t o the development of these i d e a s . This p r o j e c t was funded i n l a r g e p a r t by the New York Z o o l o g i c a l S o c i e t y . A d d i t i o n a l f i n a n c i a l support was p r o v i d e d by the N a t u r a l Sciences and E n g i n e e r i n g Research C o u n c i l (Canada), the N a t i o n a l Geographic Society, the Frankfurt Zoological S o c i e t y , and the Charles A. Lindbergh Fund. The M i n i s t r y of W i l d l i f e Conservation of The Democratic Republic of the Sudan k i n d l y provided permission t o work i n Boma N a t i o n a l Park and a s s i s t e d i n o b t a i n i n g other necessary documents as well. I am g r a t e f u l t o a l l f o r t h e i r a s s i s t a n c e . Finally, I would l i k e t o give s p e c i a l thanks t o 4 people: Tony S i n c l a i r , f o r numerous deeds beyond the normal c a l l of duty; C h r i s Hillman, f o r h i s companionship, sage a d v i c e , and innumerable f a v o r s ; S h i r l e y F r y x e l l , f o r her s t e a d f a s t support over the y e a r s ; and Sue Pennant, f o r enduring even my w i l d e s t follies.  1  CHAPTER 1.  Assumptions fundamental resources  of  food  resource  hypotheses are  adaptations, individuals Resources  GENERAL INTRODUCTION  in  commonly habitat  (Geist may  limitation  large-mammal  presumed suitability,  1974;  Jarman  determine  underlie  ecology.  to  constrain  and  social  1974;  Limited foraging  behavior  Owen-Smith  Sinclair  resource  p o p u l a t i o n d e n s i t i e s through d e n s i t y -  1977; McCullough 1979;  scarcity  p a r t i t i o n i n g or 1963;  Bell  may  structure  facilitation  evidence  Fowler  1981).  communities  (Vesey-Fitzgerald  1971; Jarman and S i n c l a i r  Direct  1985),  populations. evidence  that  availability.  and  wildebeest there  large-mammal  (Sinclair  which  populations  (1968)  K l e i n concluded  available vegetation. similar  ungulate  1960;  niche Lamprey  1977), kangaroo  are  al.  1985)  circumstantial  limited  by  food  documented the r a p i d increase and onto  St.  Matthew  was caused by the overgrazing of  Caughley  eruptions,  (1970) which  l a t e r e x p l a i n e d using  a  interactions  herbivores  between  through  et  i s considerable  c a t a s t r o p h i c d i e - o f f of r e i n d e e r i n t r o d u c e d Island,  Finally,  of food l i m i t a t i o n has been documented f o r  Also,  Klein  (Bobek  1979).  roe deer (Bobek 1977), A f r i c a n b u f f a l o ( S i n c l a i r (Bayliss  of  1982).  dependent e f f e c t s on recruitment, m o r t a l i t y , or d i s p e r s a l 1977;  many  reviewed  evidence  for  Caughley and Lawton (1981)  theoretical  model  and  based  vegetation.  on  trophic  Coe et a l .  (1976) demonstrated that l a r g e - h e r b i v o r e biomass i n A f r i c a n game reserves was p o s i t i v e l y c o r r e l a t e d  with  annual  rainfall  and,  2  presumably,  vegetation  abundance.  Sinclair  (1977)  s i m i l a r p o s i t i v e c o r r e l a t i o n between A f r i c a n b u f f a l o density  and  (1980)  and  evidence  annual Clarke  suggesting  In  rainfall. and  Leader-Williams  Henderson  food l i m i t a t i o n  (1981)  found  and  age  1969;  G e i s t 1971;  al.  1981;  1982),  McCullough  1985).  Smith 1973; Fowler  (Grubb  Clutton-Brock  (Grubb  1974;  1981;  1974;  et a l .  1975;  Lett 1982;  et  populations  Gasaway et a l . Crete  (1985)  are  1977;  (1983), Mech and suggested  w h i t e - t a i l e d deer was  that  Sinclair  which  food  predation carrying  was  not  limited  limiting. wildebeest  wolf  emu  survival  populations  Berry  (1981)  and  i n moose and density  numbers  "control"  large-  or d i s e a s e .  and  l e v e l s for  Smuts (1978) suggested  c a p a c i t y set by v e g e t a t i o n abundance.  al.  response.  prey p o p u l a t i o n s at low  (1980) argued that dingo p r e d a t i o n may  et  Messier  population  population  1981;  availability  predators  juvenile  et a l .  that some  Karns (1977), and  r e l a t e d to  that p r e d a t o r s c o u l d maintain  by  et  Houston 1982), and  s p e c u l a t e d that food  limited  Gross  al.  u l t i m a t e l y r e s p o n s i b l e f o r the density-dependent  herbivore  in  Lett  Clutton-Brock  However, there i s a l s o e m p i r i c a l evidence  and  responses  (Woodgerd 1964;  Gambell  Sinclair  Many of these authors  chamois.  s t u d i e s i n d i c a t e that many  reproduction  1979;  survival  1979;  adult survival  was  Fowler and  juvenile  McCullough  at f i r s t  Ricketts  demographic  large-mammal p o p u l a t i o n s e x h i b i t density-dependent fecundity  a  population  and  i n r e i n d e e r and  a d d i t i o n , numerous long-term  found  that  below  lion the  Caughley et a l . some  kangaroo  in A u s t r a l i a . argued  that combined e f f e c t s of disease  and  3  p r e d a t i o n l i m i t e d wildebeest game  reserve.  p o p u l a t i o n numbers  C h r i s t i a n et a l .  in a  (1960) suggested  Botswana  that d e n s i t y -  dependent s t r e s s d i s e a s e caused the c a t a s t r o p h i c m o r t a l i t y of a herd  of  suggest  i n t r o d u c e d Sika deer.  Finally,  r e s u l t s from Serengeti  that an introduced d i s e a s e , r i n d e r p e s t , l i m i t e d  buffalo  and wildbeest  p o p u l a t i o n s at low numbers p r i o r to e r a d i c a t i o n of  the  i n the e a r l y 1960's ( S i n c l a i r  disease  Norton-Griffiths  1979).  Thus, while there i s evidence populations  of  large  availability,  this  i s by no means  study,  1977; S i n c l a i r and  that a t  herbivores  least  are  by  food  true.  In  this  universally  the white-eared  L i c h t e n s t e i n and P e t e r s ,  kob  natural  limited  I i n v e s t i g a t e whether a l a r g e migratory  A f r i c a n antelope,  some  p o p u l a t i o n of an  (Kobus  kob  leucotis,  1854), i s l i m i t e d by food abundance and  examine the e f f e c t s of food l i m i t a t i o n on kob movement p a t t e r n s , breeding study  phenology, and mating system.  The c e n t r a l theme of my  i s t h a t food resource l i m i t a t i o n determines  most important The  many  of the  l i f e h i s t o r y c h a r a c t e r i s t i c s of h e r b i v o r e s .  white-eared  kob p o p u l a t i o n under study numbers about  830,000 i n the Boma N a t i o n a l Park region of the southern (Fig.  1.1).  of t r o p i c a l woody  The Boma ecosystem i s composed of broad  savannah grasslands punctuated  vegetation.  Like  most  African  g r a s s l a n d s a r e subject t o seasonal extremes monthly r a i n f a l l extreme  ranges from l e s s than  variation  i n monthly  expanses  by a patchy  cover  savannahs,  the Boma  in rainfall;  20mm t o over  rainfall  Sudan  causes  150mm. reduced  abundance d u r i n g the dry season, from January t o A p r i l ,  of  mean This food  and at  4  F i g u r e 1.1 Map  o f the S.E. Sudan. Watercourses and i n t e r n a t i o n a l  b o u n d a r i e s i n d i c a t e d by s o l i d l i n e s , and townships i n d i c a t e d by boxes.  roads i n d i c a t e d by broken  lines,  5  t h i s time food may be l i m i t i n g In  Chapter  f o r the kob.  2, I d e s c r i b e seasonal c l i m a t i c changes i n the  Boma ecosystem and consequent e f f e c t s on abundance  the  distribution  of green grass and water s u p p l i e s .  Having shown that  resource d i s t r i b u t i o n changes d r a m a t i c a l l y throughout the I t e s t whether kob migratory  or  green  forage  (Pennycuick  p a t t e r n s at three d i f f e r e n t entire  kob  year,  movements are responses to temporal  changes i n the d i s t r i b u t i o n of scarce s u p p l i e s of water 1975)  and  1975).  (Western  I examine movement  s c a l e s : 1) annual m i g r a t i o n  of the  p o p u l a t i o n between wet and dry season ranges 200 km  a p a r t , 2) dry season movement p a t t e r n s between d i f f e r e n t h a b i t a t types, and 3) f o r a g i n g movements of i n d i v i d u a l s w i t h i n  specific  dry season h a b i t a t s . In  Chapter  3,, I i n v e s t i g a t e the e f f e c t  of dry season  food  s c a r c i t y on kob demographic p a t t e r n s , p a r t i c u l a r l y m o r t a l i t y . t e s t the hypothesis kob  that dry season  abundance  determines  p o p u l a t i o n numbers through n u t r i t i o n - r e l a t e d m o r t a l i t y .  a d d i t i o n , I examine the impact occurred age  food  at  the  structure  of  large-scale  I  mortality  In that  outset of the study by e v a l u a t i n g the c u r r e n t  of  the p o p u l a t i o n .  Finally,  I  suggest  an  e m p i r i c a l model of kob p o p u l a t i o n m o r t a l i t y as a f u n c t i o n of dry season d u r a t i o n . I examine the phenology of c a l f p r o d u c t i o n use  these  1983a)  that  synchronized  data  to t e s t the hypothesis  production to  periods  of of  young  in  peak  food  abundance i s l i m i t i n g d u r i n g the dry  i n Chapter 4.  (Sadleir  1969; S i n c l a i r  mammals abundance.  season,  I  this  should  be  If  food  hypothesis  6  p r e d i c t s that kob In  Chapter  synchrony changes eared  should produce young d u r i n g the wet 5,  I  (resulting in  food  kob.  The  investigate  presumably  the  from  polygyny.  Oring's  test  breeding  polygyny and  of  breeding  adaptation  to  seasonal  Boma p o p u l a t i o n has a lek system (Bradbury a  moderate  effect  abundance) on the mating system of the white-  Bradbury and Gibson 1983), with I  Emlen  and  synchrony  competition  presumably  should  to o r i g i n a l data order  increase  population,  evaluate  I d e r i v e d a new This  conventional  methods in Chapter  study  shown i n F i g . derive  from  1.2, a  is  age  described  by  (Kobus kob  structure estimating and  of  comparing thomasi)  of  the  kob  age-specific  compared  to  two  6.  assumption  limitation  f o r the white-eared kob p o p u l a t i o n .  summarize  my  c o n c l u s i o n s , and c o n s i d e r  of t h i s approach.  that  degree  the l i n e s of i n v e s t i g a t i o n  unifying  of  kob.  method for  survival.  As  method  the  1981;  degree  the  between males f o r females,  for the white-eared to  high  (1977) h y p o t h e s i s  p u b l i s h e d data on the c o n s p e c i f i c Uganda kob  In  season.  of  in  this  periodic  food  In Chapter 7,  the m e r i t s and  I  drawbacks  7  climatic seasonality  movement patterns  resource "limitation"  foraging  population dynamics  F i g u r e 1.2 L i n e s o f i n v e s t i g a t i o n employed i n t h i s  breeding phenology  * male competition  study.  8  CHAPTER 2.  SEASONAL MIGRATION IN RELATION TO RESOURCES  Introduction Long d i s t a n c e movements are c h a r a c t e r i s t i c herbivores  in  African  savannah  ecosystems.  p o p u l a t i o n s e x h i b i t nomadic movements season  (Delaney  and  Happold  of  large  Although  apparently  1979),  many  most  some  unrelated  large  herbivore  migrations  f o l l o w seasonal changes i n resource d i s t r i b u t i o n  abundance  (Sinclair  1983b),  as do m i g r a t i o n s  from a wide v a r i e t y of other taxa A f r i c a n savannah changes  in  rainfall  (Phillipson  1978;  Sinclair  1979).  As  n u t r i t i o n a l q u a l i t y as they is  exhibit  g r a s s l a n d p r o d u c t i v i t y due  McNaughton  usually  a  response  Laredo and Minson 1973;  well,  grasses  Reid et a l .  cease  Strugnell  many  rainfall  and high d i g e s t i b i l i t y .  perennial  pronounced  and  grasses  grasslands  are  decline in  this  1973;  process  Egan 1977).  1957;  Early in  leaves with high p r o t e i n  At l a t e r  growth  stages,  many  v e g e t a t i v e p r o d u c t i o n and t r a n s l o c a t e roots  leaving highly l i g n i f i e d , poorly d i g e s t i b l e ,  t i s s u e s above ground.  Pigott  s e a s o n a l i t y (Plowes  most of the s o l u b l e c o n s t i t u e n t s back i n t o the bases,  seasonal  to p e r i o d i c v a r i a t i o n i n  1975;  the growing season, grasses produce new content  of s p e c i e s drawn  flower and mature, and  to  and  (Dingle 1980).  ecosystems  1975;  to  As a r e s u l t , g r a z i n g animals  and  stem  low p r o t e i n in  savannah  faced with s e a s o n a l l y v a r i a b l e food q u a l i t y  a b s o l u t e abundance: in the wet  season food i s  distributed,  in the dry season food i s s c a r c e ,  and  nutritious;  abundant,  and  widely  9  unevenly  d i s t r i b u t e d , and of r e l a t i v e l y poor q u a l i t y .  The most d e t a i l e d s t u d i e s of l a r g e - h e r b i v o r e Africa 1975;  are  on  Maddock  tracks  per  the S e r e n g e t i wildebeest p o p u l a t i o n 1979).  seasonal  high q u a l i t y year)  changes  studies  in  suggest  that  migration  the d i s t r i b u t i o n and abundance of  Wildebeest  substantial dry season,  move i n t o  production  low-rainfall  of  wildebeest migrate  (1000mm per year) t a l l  (400mm  when these  nutritious  grasses.  northwards i n t o high  grass areas that produce  q u a n t i t i e s of n u t r i t i o u s regrowth as the dry season  in  (Pennycuick  .short grass p l a i n s during the wet season,  the  rainfall  These  forage.  areas support During  migration  result  of  limited  infrequent  rainstorms.  However,  Western  (1975)  p o i n t e d out that i n many A f r i c a n  savannah ecosystems, water s u p p l i e s become r e s t r i c t e d d u r i n g the dry season, As  a  due to the evaporation of w a t e r - f i l l e d  consequence,  metabolic  water requirements  grazers) are supplies obtain  species  obliged  to  that solely  requirements  water  and,  as  movements t o watering  from  unable  to  meet  from t h e i r forage  concentrate  d u r i n g the dry season. sufficient  are  depressions.  around  their  ( i . e . most  permanent  water  In c o n t r a s t , many browsers can their  forage  to  meet  body  a consequence, do not e x h i b i t dry season  points.  Thus, the a v a i l a b i l i t y  of  water  c o n s t r a i n s the dry season f o r a g i n g options of g r a z e r s . In  this  of white-eared or both.  chapter, I examine whether the seasonal m i g r a t i o n kob t r a c k s s h i f t i n g d i s t r i b u t i o n s of food, water,  I f kob  migration  seasonally r e s t r i c t e d  i s an  adaptation  for  exploiting  resources, I p r e d i c t the f o l l o w i n g :  10  1.  Resources  should  be  unevenly  d i s t r i b u t e d throughout the  ecosystem. 2.  Resource  abundance  should  follow  distribution  should  be  predictable  seasonal  trends. 3.  - Kob  correlated  with  resource  distribution. 4.  Kob should  locations  have  remain  in  locations  only  as  long  as  greater abundance of resources than  those  surrounding  areas. In  this  chapter,  I  first  describe  c h a r a c t e r i s t i c s of the Boma ecosystem. changes  in  the d i s t r i b u t i o n  p o p u l a t i o n , as determined the  proposed  of  seasonal  Then, I examine seasonal  food,  water,  from a e r i a l surveys.  predictions  against  climatic  findings  and  the kob  Finally, from  both  surveys and ground o b s e r v a t i o n s i n the dry seasons of  I  test aerial  1982 and  1 983.  Methods  Aerial  surveys  The d i s t r i b u t i o n and abundance of l a r g e mammal p o p u l a t i o n s , forage,  water  resources,  and  habitat  c h a r a c t e r i s t i c s of the  e n t i r e study area were measured by systematic a e r i a l f u l l d e s c r i p t i o n of the a e r i a l  survey  procedures  surveys. employed  A is  given i n N o r t o n - G r i f f i t h s (1978). Aerial  surveys  of  the whole study area were conducted i n  11  early A p r i l  1980, October  survey  a s m a l l e r p o r t i o n of the study area was performed i n  May  of  1980, and J u l y  1982.  1982. A e r i a l survey teams c o n s i s t e d of  seat  observer,  and  two  responsible  for correct  maintenance  of  rear  seat  seat  observer  recorded  readings, o b s e r v a t i o n s of water vegetation  cover  of  addition,  pilot,  a  a  front  The p i l o t was  transect  lines,  f l y i n g h e i g h t , and f o r s i g n a l l i n g the  s t a r t and f i n i s h of t r a n s e c t l i n e s . front  a  observers.  orientation  constant  In  At 1 minute  photometer  intervals,  the  and radar a l t i m e t e r  sources, and estimates of  and woody s p e c i e s composition.  woody  The rear seat  observers counted a l l animals observed w i t h i n a s t r i p  of  width  width was  on  both  sides  of  the  aircraft.  Transect  d e l i n e a t e d by 2 f i b r e g l a s s rods a t t a c h e d to e i t h e r When  rear  seat  observers  encountered  i n d i v i d u a l s , they took c o l o r photographs  wing  fixed  strut.  groups of more than 25 of  the  entire  group  and, time p e r m i t t i n g , a l s o made a v i s u a l count of group numbers. All  rear  seat o b s e r v a t i o n s were recorded immediately on a tape  recorder as w e l l as the time i n t e r v a l of the o b s e r v a t i o n . Transect width was c a l i b r a t e d at the end of each survey flying  at  known height p e r p e n d i c u l a r to an a i r s t r i p marked o f f  in 20m i n t e r v a l s by white sand bags. the  markers,  resultant regression  As the plane  passed  over  the rear seat o b s e r v e r s counted the number of bags  seen i n s i d e the rods. heights,  by  were strip was  taken. width used  Several  passes,  at  a  wide  range  of  A r e g r e s s i o n l i n e was drawn through the estimates to  calculate  at  given  transect  a c t u a l survey from height above ground data.  heights;  this  width d u r i n g the  12  T r a n s e c t width v a r i e d between 150 and 300m surveys.  Transects  were spaced at r e g u l a r  the study a r e a , with an surveys  above ground.  and 200km per hour.  final  determined  a  2 surveys, f l i g h t  such as prominent White-eared  Flight  During the f i r s t  using  ratio  l i m i t s for Jolly's  of  i n t e r v a l s over The  above ground,  speed  three  varied  first  the f i n a l 2 between  2 surveys, f l i g h t  calculated  150  position  During the  l i n e s were o r i e n t e d to known  landmarks,  kob  p o p u l a t i o n estimates f o r each survey were  the  sample  for a l l  transects  divided  area to the t o t a l area.  surveys  2 f o r unequal  were  calculated  sampling u n i t s  from  the  weighted  according  (Jolly  population  v a r i a n c e s of the four surveys ( N o r t o n - G r i f f i t h s  by  Confidence  1969).  white-eared kob p o p u l a t i o n estimate f o r the e n t i r e study was  2  rocky o u t c r o p s .  individual  method  the  G l o b a l N a v i g a t i o n System.  c a l c u l a t e d as the observed t o t a l the  10km  orientation.  were flown at a h e i g h t of 91m  surveys at 76m  was  east-west  for  The  period  estimates 1978).  to  and  Seasonal  d i s t r i b u t i o n p a t t e r n s were o b t a i n e d by summing animal counts f o r 10km  segments along each t r a n s e c t  l i n e , then d i v i d i n g t h i s  by the area observed to a r r i v e at the d e n s i t y per Green readings.  biomass The  a l l o w i n g passage wavelengths  was  estimated  photometer  from  respectively.  km . 2  digital  photometer  c o n s i s t s of two probes with  of l i g h t at 800nm ( i n f r a red) and Red  light  675nm  vegetation.  light  is  reflected  by  filters (red)  i s absorbed by c h l o r o p h y l l  in green v e g e t a t i o n , while i t i s r e f l e c t e d by non-green Infrared  count  both  green  and  matter. non-green  Thus, the r a t i o between these r e f l e c t a n c e values i s  13  c o r r e l a t e d with al.  1973; McNaughton 1979).  against  of  green  biomass  (Tucker  c l i p plots  2  used to  (Fig.  estimate  et  Photometer readings were c a l i b r a t e d  measurements of the dry weight of green matter  in 0.25m was  the abundance  2.1). The r e s u l t i n g  green  biomass  readings obtained during a e r i a l  from  obtained  regression line  digital  photometer  surveys.  Ground Observations At and  five  study s i t e s  (Fig.  2.2), I measured grass biomass  kob p o p u l a t i o n d e n s i t y along 5km  dry  season  within  (January  0.25m  clip  2  transects.  1 to A p r i l plots  Subsequent  i n t o green  l e a f , green  at  throughout  the  I measured grass biomass 0.5km  intervals  along  c l i p p i n g , grass samples were s o r t e d  stem, and brown f r a c t i o n s , weighed  triple-beam balance, d r i e d Preliminary  30).  taken  to  transects  on  a  i n the sun f o r 8 h r s , and re-weighed.  observations  confirmed  little  weight  change  f o l l o w i n g 8 h r s of sun d r y i n g . Kob p o p u l a t i o n d e n s i t y was estimated by encountered Strip of  width  each  transect  within a s t r i p  was measured using a r a n g e - f i n d e r a t the beginning  transect; strip  thereafter  width.  transects  in  observers  In order to reduce  the  100m  summed  the e f f e c t  of  d i s t r i b u t i o n p a t t e r n s , I performed  the e a r l y morning.  i n t e r v a l s d u r i n g the dry season, population  estimated  Animal t o t a l s were subsequently  d i u r n a l movements on observed  kob  a l l kob  100m e i t h e r s i d e of a moving v e h i c l e .  for 0.5km t r a n s e c t i n t e r v a l s .  all  counting  At approximately  monthly  I made s i m i l a r measurements  of  d e n s i t y along a 25km t r a n s e c t that b i s e c t e d the  14  dry green biomass (gm/m ) 2  F i g u r e 2.1 C a l i b r a t i o n o f photometer  r e a d i n g s . The r a t i o o f r e f l e c t a n c e  v a l u e s r e c o r d e d by the 800nm and 675nm probes was p o s i t i v e l y r e l a t e d t o 2 green g r a s s d r y weight (r =0.812; p < 0 . 0 5 ) .  15  Figure 2.2 Map of the study area. Study s i t e s are indicated by triangles (l=Gom swamps; 2=Wangchira; 3=Ajwara; 4=Neubari; 5=Ungwala).  16  kob dry season  range.  D i u r n a l changes i n kob d i s t r i b u t i o n p a t t e r n using two methods. counts  of  kob  F i r s t , a t the Ajwara  present  throughout  meadow on s e l e c t e d dates over the  were  recorded  study s i t e I made t o t a l  the day on a 0.5km  grassy  2  course  of  the dry  season.  Second, at the Gom Swamp study s i t e I repeated a 4km t r a n s e c t at i n t e r v a l s throughout Grass  growth  determined  from  constructed  in  the day. over  the  course  0.25m  clip  plots  2  Soil  the dry  taken  season  inside  exclosures  from  regressions  of  moisture  content  throughout  the dry  season  study s i t e .  Soil  samples  re-weighed.  The  was  a t the  (300-500gm) were o b t a i n e d a t a  depth of 10cm below the s u r f a c e , weighed, sun-dried f o r 2 and  green  time.  recorded f o r 7 meadow and 1 wooded g r a s s l a n d e x c l o s u r e s Ajwara  was  both swampy meadows and wooded g r a s s l a n d areas.  Grass growth r a t e s were c a l c u l a t e d biomass over  of  difference  between  wet  days,  and dry weight  i n d i c a t e d r e l a t i v e water c o n t e n t . Kob  feeding  between  green  exclosures.  rates  were  biomass  calculated  measurements  from  the  inside  difference  and  outside  Since a t a l l study s i t e s no other g r a z i n g ungulates  were observed, I assume that a l l g r a s s biomass removed was taken by kob. In order t o particular  plant  determine  parts,  I  whether  measured  kob  were  feeding  on  the l e n g t h of l e a v e s and  stems i n s i d e and o u t s i d e e x c l o s u r e s a f t e r  kob had  been  feeding  for s e v e r a l weeks. The  nutritional  quality  of  stoloniferous  grasses  was  17  estimated  by a n a l y s i s o f : (1) crude  protein  v i t r o d i g e s t i b i l i t y , and (3) detergent al.  1964; Van  chemical  Soest  analyses  Laboratory,  content,  f i b e r content  (2) i n  (Johnson et  1963a, b; Van Soest and Marcus 1964). A l l  were  performed  by  the W i l d l i f e  Habitat  Washington S t a t e U n i v e r s i t y , Pullman, WA.  Plant s p e c i e s i d e n t i f i c a t i o n s were made by the East A f r i c a n Herbarium, N a i r o b i , Kenya.  The  study  P h y s i c a l f e a t u r e s and v e g e t a t i o n Physical 2.2. the  features  area  types  of the Boma region are d e p i c t e d i n F i g .  The study area, c o v e r i n g some 28,000 km , i s bordered 2  Boma  escarpment  i n the south-east,  by  the Kangen/Pibor R i v e r  system i n the west, and the Akobo River to the n o r t h - e a s t .  The  e n t i r e region i s i n a watershed that u l t i m a t e l y empties i n t o the Nile  River  v i a the Sobat R i v e r .  the area, except around  the  nutrient  escarpment.  by  chernozems,  rich,  have  a  high  Most  cracked  i n the dry season.  being wetted, and, as a r e s u l t , the  surface.  of  the  plains  are  or b l a c k - c o t t o n s o i l s , which are clay  c o n s i s t e n c y from being h i g h l y adhesive and  over most of  i n the v i c i n i t y of the c e n t r a l Maruwa H i l l s and  Boma  characterized  Terrain i s f l a t  content,  and  range  in  i n the r a i n s to rock-hard  These s o i l s s e a l q u i c k l y a f t e r subsequent r a i n f a l l  puddles  on  The s o i l s a t the base of the Boma escarpment are  r e d - c o l o r e d , v o l c a n i c loam, and  are considered  nutrient  rich  18  (Willimott basalt  1956).  monoliths  Red-colored  sandy s o i l s are a l s o present  and  hills  small  that  southern  and eastern p a r t s of the study  present  over  near  are s c a t t e r e d i n the  area.  Termitaria  are  most of the r e g i o n , and are p a r t i c u l a r l y abundant  in the n o r t h . Much of the study area has some degree of woody cover,  being  p a r t i c u l a r l y dense (>25%  and around the Boma escarpment cover  (Fig.  cover)  2.3).  i n the  vegetation north-east  Intermediate  woody  (10-25%) occurs along the margins of the Kong-kong R i v e r ,  in the f a r south, and L i g h t cover except  (< 10%)  for  i n patches  of the n o r t h - e a s t e r n woodlands.  c h a r a c t e r i z e s much of the r e s t of the  expanses  of  open g r a s s l a n d i n the north and  c e n t r a l p a r t s of the study  of deciduous,  fraqrans.  The  growing  species,  Balanites aegyptiaca. north  are  composed  s e y a l , and  Acac i a  grasslands  are  Zanzibarica.  The  characterized  by  m e l l i f e r a , Acac i a  The of  composed  of  a  broadleaf t r e e s , dominated by Combretum  Kong-kong River area  thicket  south  area.  Most of the areas of dense woody cover are community  area,  is  characterized  by  low-  dominated by Z i z i p h u s m a u r i t i a n a  and  lightly  the  wooded  grasslands  in  scattered Balanites aegyptiaca, Acacia  sieberiana.  The  dominated  by  far  southern  scattered polycantha,  southern  extensive  lightly  stands  bushed  grasslands  patches  of  and  unidentified  an  of  low-growing  wooded Acac i a are Acac i a  Combretum  species. V i r t u a l l y the e n t i r e Boma region supports cover,  excluding  areas  of  gallery  substantial  f o r e s t and  grass  t h i n s t r i p s of  19  Figure 2.3 Map of woody vegetation cover, as estimated during a e r i a l surveys  (shaded=25% cover; hatched=10-25%; stippled=l-10%; open  Each g r i d square i s 10 x 10 km.  1%) .  20  r i p a r i a n woodland along the margins of the the  extensive  growth. is  The  Combretum  common:  grass  ioclados,  pyramidalis,  visit  contortus,  and  Panicum  tentatively  characterizes  watercourses. study  the  Due  region  each year.  Setaria  ioclados.  identified  the  flood  during  the  wet  incrassata,  A stoloniferous as  plain  to s u r f a c e f l o o d i n g , i t was  sites  grass  i n sandier areas, a wide v a r i e t y of grasses  Heteropoqon  species,  Even  substantial  r u f a , which grows to h e i g h t s of 2-3m  In the f a r south and  Sporobolus  support  River.  predominant grass s p e c i e s over much of  Hypparhenia  are  woodlands  Akobo  season,  Echinochloa  margins of major not and  feasible  to  the absence of  r e p r o d u c t i v e s t r u c t u r e s during the dry season f i e l d  study p e r i o d  permitted only t e n t a t i v e i d e n t i f i c a t i o n s of  grass  species.  full  samples i s given i n  list  Appendix  of  all  identified  vegetation  A  1.  Climatic seasonality Mean monthly records wind  direction,  township  are  temperatures  and  given  of  temperature,  evaporation in  Table  19.9°C i n December.  rainy  season,  throughout  2.1.  humidity,  from the nearby Akobo Mean  daily  maximum  range between 38.9°C i n March to 30.8°C i n August.  Mean d a i l y minimum temperatures and  piche  relative  the  from year.  range between  R e l a t i v e humidity May  to  November,  Seasonal  changes  23.2°C  in  April  i s h i g h e s t during the but in  is  considerable  evaporation  rate  f o l l o w seasonal temperature v a r i a t i o n . Seasonal  changes i n r a i n f a l l  are i l l u s t r a t e d  in F i g .  2.4.  Table- 2.1 Monthly climatic normals f o r Akobo township (see F i g . 1.1). Data are mean values for 27 years of records between 1941 and 1970 (from the Meteorological Department, Government of the Sudan)..  month  max temp  min temp  r e l a t i v e humidity 6:00 12:00  wind direction  evaporation piche  Jan.  36.6  20.0  47%  27%  NE  9.7  Feb.  37.9  21.6  41  24  NE  11.2  Mar.  38.9  23.1  49  27  SE  10.4  Apr.  37.5  23.2  62  35  SE  8.2  May  34.9  22.2  74  47  SE  5.7  Jun.  32.6  21.5  82  55  SE  4.1  Jul.  30.9  21.5  87  62  SE  2.9  Aug.  30.8  21.1  88  65  SE  2.6  Sep.  32.0  22.7  86  59  SE  2.9  Oct.  33.5  21.8  82  54  SE  3.5  Nov.  34.6  21.2  75  46  SE  4.7  Dec.  35.4  19.9  59  34  NE  7.1  *  Pachalla (1232mm)  150  • 1.5  100  1.0  60  0.5  Boma (1430mm) 200  2.0  E E  150  1.5  O ^  «  100"  1.0  —  CO 50-  C C © o  wE  CD CO O > °  0.5  150i  Loelli  T 1.5  (603mm)  100-  1.0  50  0.5  N  D  J  F  M  A  M  J  J  A  .5  •*- Z!  S  O  month  Figure 2.4 Mean monthly r a i n f a l l t o t a l s f o r Pachalla, Boma Plateau, and L o e l l i  (see F i g . 2.2) indicated by histograms. Triangles  indicate  c o e f f i c i e n t of v a r i a t i o n (S/mean) for monthly r a i n f a l l t o t a l s .  23  At a l l s i t e s , inclusive.  rainfall  i s h i g h e s t between  Between year v a r i a t i o n  coefficient  of v a r i a t i o n ) i s  indicating  that  rainfall  April  in r a i n f a l l  highest  during  and  November,  (measured as the  dry  the  season,  i s most u n p r e d i c t a b l e at t h i s time of  year. Seasonal closely  rainfall  related  s o u t h - e a s t e r l y and moves  north  the  1972).  during  air  from  rainfall  the  (Fig.  north  (1232  isohyet  2.4) mm  ITCZ i s the  northern  the  winds  Atlantic  tropics.  from  hemisphere  winter.  Sudan each March or a  northerly  These winds b r i n g in coast.  This  hemisphere summer, and  northern  shift  i n t e r f a c e between  the  "During  However, d u r i n g most  rainfall  totals  to  a  moisture-  the dry  season,  of  the  dry  season  f o r the P a c h a l l a and  suggest a g r a d i e n t per year) to low  T h i s trend i s  maps  are  typical.  annual  sites  Sudan  r e s u l t s from thunderstorms that a r i s e from the  hot, dry winds are  per y e a r ) .  the  (Table 2.1).  E t h i o p i a n Highlands.  Mean  southern  ITCZ passes through the southern  southerly d i r e c t i o n  sporadic  The  during  early A p r i l , prevailing  laden  the  n o r t h - e a s t e r l y winds in  south of the equator After  in  to movements of the I n t e r - T r o p i c a l Convergence  Zone (ITCZ) ( G r i f f i t h s  zone  patterns  for  the  from high r a i n f a l l  rainfall  consistent  with  e n t i r e southern  (603  heavy  the mm  rainfall  Sudan ( G r i f f i t h s  R a i n f a l l on the Boma escarpment i s unusually i t s high e l e v a t i o n .  in  i n the south published  Loelli  1972).  because  of  24  Results  A e r i a l survey  results  Seasonal changes i n green biomass There  was c o n s i d e r a b l e  seasonal and s p a t i a l v a r i a t i o n i n  green biomass over the study area ( F i g . season  (July),  biomasses less  the northern  half  2.5). In the e a r l y wet  of the r e g i o n  supported  i n excess of 300 gm/m , while southern areas supported 2  than  300 gm/m .  region supported supported  In the l a t e wet season, the northern  2  less  more  than  200 gm/m  2  than 200 gm/m . 2  while  southern  In both the e a r l y and  areas  l a t e wet  seasons, green biomass i n c r e a s e d from south t o north ( e a r l y ANOVA,F=42.8, p<0.05; l a t e wet:  ANOVA, F=60.5, p<0.05).  In the  dry season, green biomass abundance was low i n a l l areas F=2.0,  n.s.),  but was g r e a t e s t along the margins  2.6). Less  green biomasses escarpment.  5%  escarpment  of the study area had dry  i n excess of 50 gm/m , mostly 2  around  season  the Boma  In a l l seasons, the s p a t i a l d i s t r i b u t i o n of green  biomass  was patchy,  unevenly  distributed.  These  than  (ANOVA,  of the Kong-  kong, Kangen, and Oboth watercourses and on the Boma (Fig.  wet:  results  possibly  suggest  indicating  that  rainfall  that annual grass p r o d u c t i o n rates  were h i g h e s t i n the north and on the Boma escarpment, in the southern a r e a s .  was  and lowest  T h i s t r e n d i s c o n s i s t e n t with the north-  25  500-,  dry 300-  100-  E  T  co 500-,  CO CO  1  r  1  1  1  1  late wet  co  E o  — B  300-  c  CD CD  i_  CO  100-  c to  1  ~i  CD  1  1  1  1  1  1  1  1  1  1  r  E  early wet 500  300  100  i  1  1  1 5  south  1  9  13  17  transect  1 21  i  i  25  north  Figure 2.5 North-south gradient i n seasonal green biomass abundance, estimated from d i g i t a l photometer readings during a e r i a l surveys. Shaded bars indicate transects i n which kob were observed during s p e c i f i c a e r i a l surveys  (dry: A p r i l 1980;  late wet: October 1980;  early wet: July 1982).  late wet  o OOO OO o OOO  0 0 0 °  early wet  o o O O O  0-25 gin/m 26-50 61-100 101-200 201-300 301-400  1  O >*oo  dry: A p r i l 1980 survey late wet: October 1980 early wet: July 1982 OOOOOO^^ O O o OOO  OO  OO o  o  oXt  Figure 2.6 Seasonal d i s t r i b u t i o n of green biomass, estimated from d i g i t a l photometer readings during a e r i a l surveys.  27  south r a i n f a l l g r a d i e n t d e s c r i b e d e a r l i e r . green but  biomass were  values  somewhat  watercourses  In the  dry  season,  were g e n e r a l l y low a c r o s s the ecosystem,  greater  along  the  margins  of  northern  and at higher e l e v a t i o n s .  Seasonal changes i n water a v a i l a b i l i t y The Fig.  seasonal  2.7.  distribution  During  distributed  the  throughout  rains,  the  water s u p p l i e s i s shown i n  water  region.  were inundated; standing water Swamping  of  supplies  covered  several  d i d not occur south of the Maruwa H i l l s ,  but  watercourses  during  widely  The n o r t h - c e n t r a l p l a i n s  escarpment, or i n the Combretum woodlands t o plains,  were  the  thousand  km . 2  near the Boma east  of  the  the r a i n s there were l a r g e numbers of f u l l  and waterholes s c a t t e r e d  throughout  these  areas.  By the l a t e wet season, most of the ephemeral swamped g r a s s l a n d s in  the  north  watercourses than dry  dried  out  and waterholes.  and  water was c h i e f l y r e s t r i c t e d to  The south was l e s s  the n o r t h , p o s s i b l y as a r e s u l t of lower season,  water  was  mainly  restricted  well  rainfall. to  the  watercourses: the Oboth, Neubari, and Akobo R i v e r s . remaining  swamps,  drainage system.  supplied In the northern  The l a r g e s t  c a l l e d the Gom Swamps, a r e part of the Oboth Some  woodlands and around  waterholes  remained  the Boma escarpment.  in  the  Combretum  Figure 2.7 Seasonal d i s t r i b u t i o n of water supplies, estimated during a e r i a l surveys. S o l i d dots indicate standing water observed at l e a s t once per i n a given transect. Boundaries  10km  of the study area as indicated i n F i g . 2.6.  29  White-eared  kob p o p u l a t i o n  During  1980-82,  numbers  the  Boma  kob  approximately 830,000, making i t the ungulate  population  in  kob  second  largest  numbered migratory  i n A f r i c a , a f t e r the S e r e n g e t i w i l d e b e e s t  ( S i n c l a i r and N o r t o n - G r i f f i t h s change  population  1982).  There was no  significant  p o p u l a t i o n numbers over the course of the study  (Table 2.2; t - t e s t s f o r a l l p a i r s of census t o t a l s ,  n.s.).  Seasonal m i g r a t i o n of white-eared kob The seasonal d i s t r i b u t i o n p a t t e r n shown  in Fig.  l a r g e herds  2.8.  south  grasslands  white-eared  kob i s  During the r a i n s , kob were c o n c e n t r a t e d i n  of  where  of  the Kangen  rainfall  River,  i s low.  in  By  lightly  October,  wooded  kob  moved  northwards along the Kangen R i v e r , c r o s s i n g the r i v e r a t s e v e r a l points. Post  Large numbers of kob proceeded as f a r n o r t h  before  crossing  the Kangen R i v e r .  as  A e r i a l reconnaissance  f l i g h t s and f i e l d o b s e r v a t i o n s i n d i c a t e d that kob moved dry season range adjoining  by  January.  often  in  escarpment.  dry  The southward m i g r a t i o n was  remained  season  range,  the v i c i n i t y  sporadic;  the  s t a t i o n a r y f o r a number of days and even  r e v e r s e d d i r e c t i o n on o c c a s i o n . were  The  i n t o the  the Neubari/Oboth River system, was maintained u n t i l  the onset of r a i n s . herds  early  Pibor  of  By  May,  the Maruwa  The wet season range was  migratory Hills  reached  kob  and by  herds  the Boma  early  July.  The dry season and wet season ranges were 150 to 200km a p a r t . The Oboth River system was the only watercourse that  showed  30  Table 2.2 Boma white-eared kob population estimates from a e r i a l censuses (1980-1982). The mean population estimate f o r the entire period was calculated from the weighted estimates of i n d i v i d u a l censuses  (Norton-  G r i f f i t h s 1978) .  survey date  population estimate  95% confidence i n t e r v a l  A p r i l 1980  888,880  + 70 %  October 1980  651,020  + 93 %  May 1982  869,553  + 47 %  July 1982  964,932  + 104 %  mean  831,081  + 31%  31  F i g u r e 2.8  Seasonal d i s t r i b u t i o n o f w h i t e - e a r e d kob. 2 dots i n d i c a t e s kob d e n s i t y per km .  S i z e o f the  solid  32  continued  flow during the dry season,  of water were a v a i l a b l e the  dry  season.  range was Fig.  i n woodland waterholes  Aerial  b e t t e r endowed  2.9  although  surveys with  suggest  water  closer.  distribution p<0.05).  was  This  to  water  random  surrounding  supplies.  supplies  than  (Kolomogorov-Smirnov  histogram  indicates  part  of  areas.  d i s t r i b u t i o n of kob i n  r e l a t i o n to d i s t a n c e to the nearest water concentrated  during  that the dry season  than  d e p i c t s the r e l a t i v e frequency  limited supplies  that  Kob  expected  test,  few  were  kob  if  D=0.49, (<5%)  were  l o c a t e d f a r t h e r than 20km from the nearest known water s u p p l i e s , and most kob locations  (75%) were w i t h i n 10km.  in  the  study  area  During  the wet  were f u r t h e r than  season,  no  20km from the  nearest water s u p p l i e s . If kob m i g r a t i o n t r a c k s the seasonal a v a i l a b i l i t y of then  one  would  expect  a  p o p u l a t i o n d e n s i t y and green kob  positive  correlation  biomass.  As shown  between  kob  Fig.  2.5,  in  vacated h i g h biomass northern areas and c o n c e n t r a t e d  south during the e a r l y wet season  season  (July).  By  food,  the  i n the  late  wet  (October), kob began to move north i n t o the h i g h biomass  swamped g r a s s l a n d s .  By the l a t e dry  areas  the  occupied  southern areas  by  results  (April),  d i d not d i f f e r  i n terms of green  A e r i a l survey between  kob  season  northern  significantly  from  biomass.  showed  no  significant  kob p o p u l a t i o n d e n s i t y and green  biomass d u r i n g the dry  season ( r = 0 . 0 l , n . s . ) .  Since  choice  the a c c e s s i b i l i t y of water s u p p l i e s , I  2  examined  of  habitats the  same  by data,  kob  correlation  excluding  are  all  restricted  ecosystem  in  their  locations  33  100  80  c o tj 60 00 3  a o a  40  n o  20  20  ca  a  40  CO  >. "D 3  60  CO  80  100  J  <10  20-29 10-19  40-49 30-39  60-69 50-59  distance to water (km)  F i g u r e 2.9 Kob d i s t r i b u t i o n i n r e l a t i o n t o the n e a r e s t water s u p p l i e s d u r i n g the d r y season  ( A p r i l ) a e r i a l survey.  34  further  than  20km from water.  no c o r r e l a t i o n patterns  (r =0.0l, n.s.).  migrated  the  grass biomass to  end  the  down.  After  available  dry  low  levels.  the  distribution  not r e f l e c t  time  kob  forage  actually  be  to  of green biomass in a l l regions at  the  season, before doing  at  this,  A  clearer  test  might  kob had an o p p o r t u n i t y to graze i t I  found  a  substantially  higher  c o r r e l a t i o n between kob p o p u l a t i o n d e n s i t y d u r i n g the dry and wet  the  amount of green biomass that was  season,  (r =0.l87; 2  prior  to  migration  into  season  a v a i l a b l e in the  the  dry  season  late range  p<0.05).  Thus,  while  there  was  weak  evidence  that kob  t r a c k e d forage abundance d u r i n g the t r a n s i t i o n seasons, t h e r e was  no  distributions  green  There  still  season range, since kob q u i c k l y reduce  abundance  of the wet  forage  in the l a t e dry season may  patterns  into  consider  However,  2  observed  distribution  R e s u l t s i n d i c a t e there was  was  distribution  of  more of  i n d i c a t i o n that migration biomass  substantial kob  nearby water s u p p l i e s .  from wet  was  at  evidence  correlated  other that  tracked  migration to  shifting  times of the the  dry  dry  year. season  with the a v a i l a b i l i t y  of  35  Dry  season ground  Kob  distribution Habitats  three  main  cover  of  observations  patterns  in the kob dry season range types:  Acac i a  aeqyptiaca,  and  1)  may  be  lumped  wooded g r a s s l a n d with a s c a t t e r e d woody  sieberiana, Ziziphus  Combretum  mauritiana  fraqrans,  ,  and  Balanites  an understory  Hyparrhenia r u f a grass; 2) t a l l H.rufa open g r a s s l a n d s ; swampy  meadows  Echinochloa  with  dense  pyramidalis,  low  and  growing  of  and  3)  s t o l o n i f e r o u s grass,  i n t e r s p e r s e d with o c c a s i o n a l patches of  t a l l e r clump-forming grasses. that  into  Data  from  25km  transects  show  kob were unevenly d i s t r i b u t e d across the dry season range, were concentrated  around the swampy  meadows  (Fig.  2.10).  Much lower p o p u l a t i o n d e n s i t i e s were found i n areas of t a l l grassland  or  i n the wooded g r a s s l a n d s  meadows ( F i g .  the  day,  most  kob  remained moved  in  off  woodlands d u r i n g the h o t t e s t part of the day d i u r n a l movement p a t t e r n became season  progressed.  Large  swampy meadows throughout the  more  as  surrounding  much  green  wooded  into  meadows  surrounding  (Fig.  2.12). as  numbers of kob continued  This  the  dry  to feed  on  night.  biomass  grasslands  though t o t a l biomass was  swampy  pronounced,  C l i p p l o t measurements (Table 2.3) times  the swampy  2.11).  Although s u b s t a n t i a l numbers throughout  that surround  open  similar  in  show that there was  swampy  meadows  than  nine in  (t-test,  t=3.66,  p<0.05) even  (t-test,  t=0.10,  n.s.).  The  w ater  water  7  9  1 1  13  15  d i s t a n c e along t r a n s e c t  17  19  21  23  25  (km)  Figure 2.10 Kob dry season aggregation near swampy meadows, as indicated by mean numbers seen per km during 25km transects from January to A p r i l , 1983 (n=3).  37  Jan.  Feb.  Mar.  date  F i g u r e 2.11 Kob p o p u l a t i o n d e n s i t i e s  i n wooded g r a s s l a n d  swampy meadow ( s o l i d l i n e ) h a b i t a t s a t the Ajwara study dry season  ( v e r t i c a l bars=95% c o n f i d e n c e  limits).  (broken l i n e ) v s . s i t e d u r i n g the 1983  38  8:00  12:00  16:00  time of day  F i g u r e 2.12 Kob d i u r n a l movements onto meadows. Maximum kob numbers were p r e s e n t i n the e a r l y morning and l a t e a f t e r n o o n . Note the i n c r e a s i n g tendency t o abandon meadows a t mid-day as the d r y season p r o g r e s s e d from January t o March.  Table 2.3 Dry season grass standing crops i n meadows vs. wooded grasslands at the Ajwara study s i t e i n 1983. Meadows had more green biomass than wooded grasslands (t-test; t=3.66; p<0.05), but no other s i g n i f i c a n t differences were observed.  MEADOWS 2) mean dry wt. (gm/0.25m location  date  brown  green  total  Neubari  sample size  2-3-83  7.7  9.1  16.0  7  Neubari  23-3-83  1.0  2.3  3.3  2  Gom  14-3-83  5.9  7.9  13.8  5  Gom  1-4-83  1.7  4.9  6.6  5  Wangchira  13-2-83  1.8  3.7  5.5  11  Wangchira  21-3-83  9.5  17.3  26.8  8  Ungwala  24-3-83  42.1  7.5  49.6  • 6  Ajwara  16-1-83 to 14-4-83  7.5  15.8  23.3  18  mean  9.7  8.5  18.2  8 (  SE  4.5  1.8  5.0  WOODED GRASSLANDS Neubari  2-3-83  29.7  0.4  30.1  7  Neubari  23-3-83  24.3  0.1  24.4  6  Gom  14-3-83  14.1  0.8  14.9  6  Gom  1-4-83  0.3  1.1  1.4  6  Wangchira  13-2-83  1.1  0.9  2.0  11  Ungwala  24-3-83  6.2  1.1  7.3  5  Ajwara  16-1-83  38.4  2.1  40.5  17  16.3  0.9  17.2  7 (  5.2  0.2  5.2  mean SE  40  stoloniferous  grasses  remained green throughout  long a f t e r t a l l e r g r a s s e s green  grasses  grasses  are  (Wilson  obtained  had  flowered  generally  1976), kob that f e d i n the meadows  forage  higher  higher  in protein  protein  than  Small  1985).  gut  (Owen-Smith  senescent  that  for  for in  grass samples c o l l e c t e d  I  (Table 2.4).  estimate content.  from meadows c o n t a i n e d on  17% crude p r o t e i n , w e l l above the minimum  kob  their  1982; Demment and Van Soest  that kob r e q u i r e forage with a minimum 9% crude p r o t e i n  average  food  s i n c e they  compensate  F o l l o w i n g Owen-Smith's (1982) c a l c u l a t i o n s ,  Stoloniferous  fed in  require  content than l a r g e r ruminants,  capacity  Since  s h o u l d ' have  those  ruminants  need to process food a t a f a s t e r r a t e to smaller  senesced.  more p r o t e i n r i c h than  surrounding wooded g r a s s l a n d s . with  and  the dry season  requirements  In c o n t r a s t , senescent Hyparrhenia  grass  surrounding wooded areas was probably too low i n p r o t e i n  be  utilized  by  kob,  s i n c e other s t u d i e s i n nearby g r a s s l a n d s  r e p o r t crude p r o t e i n v a l u e s of 2% i n mature ( J o n g l e i I n v e s t i g a t i o n Team, u n p u b l i s h e d ) . Hyparrhenia  to  grass  Since kob r a r e l y f e d  on  tall  that dry season  grazing  was  l a r g e l y c o n f i n e d t o meadows because only these areas  offered  forage of s u f f i c i e n t Moreover, stoloniferous throughout when  tall  manifested stems:  g r a s s , I conclude  Hyparrhenia  quality.  results grasses  from  continued  the dry season grasses  exclosure  (Fig.  to  produce  indicate  that  new green  growth  2.13; ANOVA, F=178.7,  were no longer p r o d u c t i v e .  i n e l o n g a t i o n of stem and l e a f  ANOVA,  plots  tissue  p<0.0l),  T h i s growth was ( F i g . 2.14;  F=50.8, p<0.05; l e a v e s : ANOVA, F=106.8, p<0.05).  41  Table 2.4 N u t r i t i o n a l analyses of above-ground tissues from stoloniferous vs. Hyparrhenia grasses. No differences were observed between stoloniferous grass fractions except i n crude protein content (ANOVA; p<0.05) . S t a t i s t i c a l comparisons were not possible between Hyparrhenia and stoloniferous grasses because variances were not stated i n Mefit-Babtie  report  (1983). CP=crude  protein; IVDDM= iri v i t r o d i g e s t i b l e dry matter; NDF=neutral-detergent soluble f r a c t i o n ; ADF=acid-detergent soluble f r a c t i o n ; LIG=lignin content; AIA= acid insoluble ash.  grass type  part  n  CP  IVDDM  NDF  ADF  stoloniferous  green leaf  19  21.7  39.3  66.8  30.1  8.8  4.8  stoloniferous  green stem  17  16.1  45.7  62.8  30.9  12.1  4.7  stoloniferous  brown  10  18.2  43.3  64.8  32.5  9.2  4.4  Hyparrhenia'*' * rufa  total plant  n.a.  n.a.  n.a.  n.a.  n.a.  n.a.  1.7  LIG  AIA  1. From: Mefit-Babtie, 1983 ( A p r i l ) . Development Studies i n the Jonglei Canal Area. Range Ecology Survey, Livestock Investigations, and Water Supply. A report to the Jonglei Executive Organ, Government of the Sudan, Vol. 1-10.  42  Figure 2.13 Dry season meadow grass growth, Ajwara study s i t e . Values 2 determined by 0.25m (r =0.78; p<0.05) . 2  c l i p - p l o t s from exclosures erected i n mid-January, 1983  43  Figure 2.14a Dry season leaf growth, Ajwara study s i t e  (r =0.82;  p<0.05).  44  y = 1.32x + 21.7 200n  time (days)  F i g u r e 2.14b Dry season stem growth, Ajwara study s i t e  (r =0.68; p<0.05).  45  These  results  during  the  continue little  suggest  d r y season  t o produce  green  In  concentrate  in feeding  high protein  fell  this  range. in late  rainfall  substantial  By c o n t r a s t , February,  was i n s u f f i c i e n t  amounts  of  green  kob d i s p e r s e d  the  widespread  to  the  unusual  70mm  new  waterholes,  appeared  i n the t a l l  grasslands.  distributions  away from t h e swampy meadows i n  o c c u r r e n c e of t h i s  i nthe  of the d r y season.  wooded  1982 and 1983 t r a n s e c t  rainfall  approximately  form  regrowth  between  that  i n 1982  i n the middle  comparison  flush  A  suggest  response  of g r e e n  to  growth  2.15). Circumstantial  season  refugia  supplies. regrowth, dried  nearby  Factors affecting  Totally  Thus,  the importance  high  of  of nearby  produce as nearby  quality  these  dry water  substantial waterholes  f o o d a l o n e was  requirements;  they  s u p p l i e s of water.  f o r a g e abundance  moisture  Hyparrhenia  to  kob d r y "season h a b i t a t  s e a s o n g r a s s growth  saturated  several  t h e s e a r e a s a s soon  2.16).  t o meet  soil  from  t h e meadows c o n t i n u e d  (Fig.  needed  residual  illustrate  kob a b a n d o n e d  up  Dry  observations  also  While  insufficient  Tall  densities  (swampy meadows) t h a t  no d r y s e a s o n  open g r a s s l a n d s a n d i n  also  high  f o o d when o t h e r g r a s s l a n d s have  Hyparrhenia  (Fig.  at  refugia  1983 t h e r e was e s s e n t i a l l y  rain  While  kob  growth.  kob d r y s e a s o n of  that  soils  rates  (Fig.  i n swampy meadows d e p e n d e d on 2.17;  ANOVA, F=16.0, p<0.05).  were a p p r o x i m a t e l y 40% water  wooded  by  weight.  g r a s s l a n d s s u r r o u n d i n g swampy meadows  50-, 1982 b e f o r e rain 30-  10  50  1982  -i  after rain  E  n  30-  o  6 c  10  70  -i  1983 50-  30  10-  1  2  3  4  5  6  7  8  distance from water (km)  F i g u r e 2.15 Kob a g g r e g a t i o n near meadows, as a f f e c t e d by d r y season rainfall.  Data from 25km t r a n s e c t s show t h a t kob d i s p e r s e d away from  meadows f o l l o w i n g 70mm o f r a i n f a l l  i n the 1982 d r y season. In 1983, kob  remained c o n c e n t r a t e d near meadows throughout the d r y season.  Figure 2.16 Kob abandonment of meadows following disappearance of water supplies. The mean number of kob present on meadow declined  following  evaporation of the central water hole (solid l i n e ) , despite the fact that food intake over the same period was increasing  (broken l i n e ) .  48  Figure 2.17 Dry season stoloniferous grass growth rate as a function of 2 s o i l moisture content, Ajwara study s i t e (r =0.86; p<0.05) .  49  had  s o i l moisture values of l e s s than  18% d u r i n g the dry  season  and none of the e x c l o s u r e s i n the wooded g r a s s l a n d s produced dry season  grass  re-growth.  Soil  moisture  d e c l i n e d even i n the  meadows over the course of the dry season  ( F i g . 2.18;  ANOVA,  F=17.2, p<0.05). Kob  maintained  a  u n i f o r m l y short sward (stems < 25mm)  the swampy meadow areas: new regrowth as  forage.  This  was  immediately  e x p l a i n s why the dry season  meadows was l i t t l e  different  from the  in  utilized  grass biomass i n  unproductive  surrounding  wooded g r a s s l a n d s . While  new  regrowth  concentrated t h e i r waterholes  feeding  close  ( F i g . 2.19; males:  X =788.2, p<0.05).  Since  moisture  kob  2  content,  order to e x p l o i t to  occurred  to  the margins  "XL2 = 212.7, growth  was  of  drying  p<0.0l;  females:  related  to  soil  probably concentrated i n those areas i n  f a s t e r growing grasses, although I have no data  test t h i s d i r e c t l y .  concentrate  grass  over the e n t i r e meadow, kob  Females showed  a  greater  tendency  to  c l o s e to the water margin than d i d males ( % =39.2, 2  p<0.05).  Feeding s e l e c t i o n f o r p l a n t p a r t s Comparisons between exclosures of  suggest  grass  (l-2m),  stems.  However,  stoloniferous  inside  and  outside  that a t times kob s e l e c t e d f o r c e r t a i n p a r t s  the g r a s s p l a n t s (Table 2.5).  grasses  structure  kob kob  grasses.  selected f e d on  When green both  feeding  on  young  tall  leaves but f e d l i t t l e on stems  and  leaves  of  There was a small d i f f e r e n c e i n p r o t e i n  164  20  40  60  80  days since Jan. 19  Figure 2.18 S o i l moisture decline over the dry season i n meadows at the 2 Ajwara study s i t e (r =0.44; p<0.05) .  51  females  30-  n=1193  O  10  o o males  30  n=802  10  0  15  30  45  60  75  distance from water (meters)  Figure 2.19 Kob feeding concentration near drying waterholes, Ajwara study s i t e . Although both sexes concentrated i n the region 15-30m from the water's 9  edge, this tendency was most pronounced i n females (X =39.2; p<C0.05).  Table 2.5 Kob dry season feeding selection f o r leaf vs. stem tissue. There was no s i g n i f i c a n t difference i n t a l l grass stem lengths inside and outside exclosures, while there were s i g n i f i c a n t differences i n th lengths of t a l l grass leaves, short grass stems, and short grass leave (t-tests; p<0.05) .  mean stem or leaf length plant part  inside  outside  t  t a l l grasses stems  961 + 235  874 + 180  t a l l grasses leaves  175 + 20  31 + 4  30.4 , *  short grasses stems  137 + 14  22 + 4  20.3 , *  short grasses leaves  290 + 23  36 + 6  27.5 , *  1.3 , n.s.  53  content between s t o l o n i f e r o u s 2.4).  However,  Owen-Smith  substantial difference between  in  grass (1982)  leaves  lignified  when  grasses  feeding  while  content  grasses. on  they  and  leaves  (Table  has documented elsewhere a  protein  leaves and stems of t a l l  selected  stems  and  Thus, kob a p p a r e n t l y  tall,  showed  digestibility  presumably  no  highly  such s e l e c t i o n when  feeding on s t o l o n i f e r o u s g r a s s e s .  Food a v a i l a b i l i t y There of  r e l a t i v e to kob  is l i t t l e  free-ranging  requirements  i n f o r m a t i o n a v a i l a b l e on food  African  ungulates,  requirements from data on domestic kob  in  size  and  feeding  so  I  estimated kob food  sheep, which are  habits.  The  During  kj/W " . 0  dry  75  l a c t a t i o n , energy  season  was  16.53  energy a v a i l a b l e ratio  536  of  requirements i n c r e a s e to  (n=14; SE=2.1).  i n forage was  6.94  Thus,  Soest  1985).  non-lactating  season  meet t h e i r energy Kob  daily  the  kJ/gm (=16.53 kJ/gm x 0.42). available  (Demment and  On t h i s b a s i s , males should r e q u i r e 1.23  kg  of  forage  per  1.56  day.  females c o n t i n u e d l a c t a t i n g throughout  (Chapter 4 ) .  1335  digestible  r e q u i r e d over d i g e s t i b l e energy  females  p r o p o r t i o n of kob  during  the  p r o v i d e s an estimate of minimum forage requirements Van  (NRC  kJ/gm (n=21; SE=0.042) and the dry matter  42.3%  energy  to  energy  kJ/W  The energy content of s t o l o n i f e r o u s grass  d i g e s t i b i l i t y was  The  similar  maintenance  requirements of rams and n o n - l a c t a t i n g ewes i s 1975).  requirements  These females would r e q u i r e 3.08  and  A large the dry  kg/day  to  needs. food i n t a k e d u r i n g the 1983  dry season  averaged  54  0.93  k g / i n d i v i d u a l (n=5;  significantly and  l e s s than  lactating  lactating selected  SE=0.20). requirements  females  females  T h i s rate of food intake  (t=-l0.80;  (t=-1.49;  of males (t=-3.15;  p<0.05)  p<0.05),  of  non-  though  kob  n.s.).  Thus,  the best feeding l o c a t i o n s a v a i l a b l e  range, they  were  still  unable  was  to  meet  but  not  even  i n the dry season  their  daily  forage  requirements. A  series  availability requirements. season  range  distributed  of  simple  during  the  The  green  was over  wet  season  biomass  approximately an  the food a v a i l a b l e per almost  calculations  indicates  0.l3kg/m . 2  2  requirement.  exceeded  kob  i n the e a r l y  wet  Since  kob  at l e a s t 2.7kg, or  This i s a  conservative  e s t i m a t e , s i n c e i t ignores g r a s s l a n d p r o d u c t i v i t y d u r i n g the season  and  the p o s s i b i l i t y of kob  S i m i l a r c a l c u l a t i o n s i n the l a t e wet availability  was  at  least  in  theory,  the wet  there was  wet  foraging in a d j o i n i n g areas. season i n d i c a t e  4.2kg/individual/day.  f a c t o r s , such as sward s t r u c t u r e ( B e l l a c t u a l forage a v a i l a b i l i t y ,  were  f o r no more than 90 days,  i n d i v i d u a l per day was  twice the d a i l y kob  food  probably  available  area of 1500km  that  1971),  might  that  food  While other constrain  these c a l c u l a t i o n s demonstrate t h a t ,  s u f f i c i e n t green  biomass a v a i l a b l e d u r i n g  season to meet kob e n e r g e t i c needs.  55  Discussion  Seasonal  changes i n resource  It has seasonal untrue,  been  than  often  temperature  assumed  temperate  particularly  tropical  is  is  linked  Inter-Tropical  the  tropics  are  to  1950).  This is  savannahs.  While  annual  in r a i n f a l l .  Zone  (ITCZ).  wind p a t t e r n s produce i n c r e a s e d r a i n f a l l  savannahs  In A f r i c a ,  trans-equatorial  Convergence  less  (Dobzhansky  o f t e n minor, most t r o p i c a l  have marked seasonal v a r i a t i o n seasonality  that  regions  in  variation  distribution  rainfall  movements  of  the  Resultant s h i f t s in in  the  wake  of  the  ITCZ. As  a  result  of  highly  e s s e n t i a l to the white-eared  seasonal  rainfall,  kob p o p u l a t i o n v a r i e d  i n d i s t r i b u t i o n and abundance.  resources  considerably  In t h i s study, both green  forage  and water s u p p l i e s became i n c r e a s i n g l y r e s t r i c t e d d u r i n g the dry season.  During  the wet  season,  abundant s u p p l i e s of both  and water were a v a i l a b l e throughout There peak green season, south.  was  a  north-south  biomass  green  in  the  biomass was  During  the  dry  continued grass re-growth watercourses,  the Boma r e g i o n .  g r a d i e n t i n annual  Boma  highest  ecosystem.  i n the north and  season, occurred  rainfall  During  the  lowest  and wet  i n the  swampy meadows that produced along  the  major  as w e l l as permanent water s u p p l i e s .  grass or water was  forage  Little  a v a i l a b l e elsewhere d u r i n g the dry  These r e s u l t s are c o n s i s t e n t with the f i r s t  northern green  season.  two p r e d i c t i o n s  56  outlined  at the beginning  were u n e v e n l y of  an  distributed  increasing  of t h i s chapter. throughout  rainfall  Both  t h e ecosystem, as a  gradient  from  south  Resource abundance e x h i b i t e d pronounced v a r i a t i o n year  because of seasonal c l i m a t i c  Seasonal The southern  Boma w h i t e - e a r e d wooded g r a s s l a n d s  early  River  i n t h e wet  system  European v i s i t o r s  migratory  in  all  As  well,  season dry  Finally,  suggests  from year  season.  since  Comparisons observations  the  that the  1940's  (Cave  1947; Weeks 1947; A n d e r s o n 1949; a l l observers  commented  f o r' at  on  the  suggesting least  forty  areas,  widely  that  vary  northern  r e c o r d s o f kob m i g r a t i o n were  t o year.  to  the  from  these h i s t o r i c a l but  the  separated  locations.  wet s e a s o n r a n g e may v a r y  in location  S i n c e kob  a l o n g one s p e c i f i c range  to  numbers o f kob i n t h e m i g r a t o r y h e r d s ,  from s o u t h e r n  This  each year  study and a n e c d o t a l  kob h a v e been a b u n d a n t i n t h e r e g i o n  years.  north.  throughout the  t o t h e Boma r e g i o n s u g g e s t  1940; L y e t h  1949).  extraordinary that  the  phenology has changed l i t t l e  Cruikshank  Zaphiro  to  changes.  kob p o p u l a t i o n m i g r a t e d  between o b s e r v a t i o n s from t h i s  and  result  kob m i g r a t i o n  Neubari/Oboth  by  food and water  river  in  concentrate  in  the  s y s t e m , one w o u l d e x p e c t  comparatively  little  dry  season  t h e d r y season  in location  from year t o  year. During chiefly  the  dry  restricted  concentrated  in  season,  to  the  locations  permanent Neubari/Oboth adjoining  water River this  supplies system.  were Kob  watercourse,  57  particularly  in  the  Gom  Swamps.  Dry season movement towards  permanent water s u p p l i e s was c o n s i s t e n t  with  Western's  (1975)  hypothesis that most g r a z e r s must migrate s e a s o n a l l y i n order t o obtain  sufficient  water  supplies.  Laboratory t r i a l s  indicate  that the c o n s p e c i f i c Uganda kob r e q u i r e s water on a d a i l y (Schoen  1971).  However, w i t h i n t h e i r dry season high d e n s i t i e s nutritious  meadows  range,  kob c o n c e n t r a t e d at  (over 1,000/km ) i n feeding r e f u g i a that  regrowth  were  throughout  the dry season  green biomass.  Many  when surrounding of  depressions  remained  flooded  Hyparrhenia g r a s s l a n d s . retained  residual  soil  As  a  longer  These  than  consequence,  moisture  ensured  ecosystem.  low-lying  the  soil  tall  i n the  moisture longer than surrounding s o i l s . supported  Thus, m i g r a t i o n to ephemerally  when  swampy  surrounding  substantial  the  these These  p r e d i c t i o n , that kob  the dry season.  of both n u t r i t i o u s forage and  were  findings  of  swamped g r a s s l a n d s d u r i n g the dry  availablity  resources  This  regrowth  s t o l o n i f e r o u s grasses high i n p r o t e i n throughout  season  these  o l d meander channels of the Oboth R i v e r that have  been subsequently cut o f f from the main flow.  meadows  produced  2  g r a s s l a n d s showed l i t t l e  water  basis  scarce  are  distribution  elsewhere  consistent should  be  in  with  my  correlated  the third with  resource abundance. During the wet season, however, kob migrated rainfall  areas  that  than i n the n o r t h . that  migration  south i n t o low  supported lower biomasses of green  This i s  inconsistent  facilitates  the  with  the  exploitation  forage  hypothesis  of  shifting  58  d i s t r i b u t i o n s of important  resources.  don't  kob  i n the northern  suggest  that kob can not do so because much of the north becomes during  restlessness season,  as  rainfall,  the wet  following was  seen  season.  extensive i n February  kob aggregated  on a year-round  simply  remain  flooded  grasslands  Why  Kob  basis?  exhibited  rains,  even  1982.  signs  I  of  during the dry  Following  70mm of  i n t o l a r g e r groups and began moving i n  columns southwards, v a c a t i n g areas that s e v e r a l days  previously  had  When  supported  upwards of 1000 i n d i v i d u a l s per km . 2  temporary r a i n s ceased,  kob returned i n l a r g e  swampy meadows i n the n o r t h . of  1983, northern  fell  around  regrowth.  areas  the Boma Kob  By c o n t r a s t , i n February  may  plateau,  producing  the onset  areas  consistent  with  the h y p o t h e s i s  i n order t o a v o i d waterlogged The  chapter  restlessness until  this  areas.  prediction,  These  northern  the beginning  of  this  resources  a r e more  abundant  than  in  Dry season o b s e r v a t i o n s were c o n s i s t e n t with since  produced  p r o t e i n content)  observations  t h a t kob vacate  kob d i s p e r s e d away from swampy meadows  e i t h e r when nearby water s u p p l i e s d r i e d up or rainfall  Thus,  kob should remain i n a given l o c a t i o n only as  long as food and water surrounding  of A p r i l .  s o i l s and s u r f a c e f l o o d i n g .  fourth prediction o u t l i n e d at i s that  grass  of r a i n s i n t h e i r l o c a t i o n as the  proximate cue t o begin the m i g r a t i o n south. are  and March  luxurient  showed no s i g n s of migratory  utilise  t o the  r e c e i v e d no r a i n , while 50mm of r a i n  the north r e c e i v e d heavy r a i n s i n the middle kob  numbers  these  new  green  re-growth  i n surrounding Hyparrhenia  when  d r y season  (with presumably high grasslands.  59  In c o n c l u s i o n , kob movements from the wet  season  t o dry  season ranges a p p a r e n t l y t r a c k e d i n c r e a s i n g l y scarce s u p p l i e s of both  food  and  water.  However,  i n c o n s i s t e n t with the resource kob  moved  into  grasslands  wet  season  exploitation with  lower  movements  hypothesis,  green  that  another  factor,  such  as  since  biomass and l e s s  abundant water s u p p l i e s than the areas they had l e f t . likely  were  avoidance  It of  seems  surface  f l o o d i n g , e x p l a i n s t h i s southward m i g r a t i o n .  B e n e f i t s of m i g r a t i o n Numerous nutritional  studies intake  stock suggest protein  than al.  selective  of  rumen  (Laredo  tropical  temperate 1973).  can  feeding.  to  retention  time,  and  Minson  grasses grasses  effects  on  possibly  forage  appetite  1973; Reid et a l . 1973; Egan  important  in  savannah  g e n e r a l l y have lower (Minson  and  digestibilities 1970; Reid  et  below which they begin t o l o s e weight, because they  are f o r c e d  nutritional  their  own  insufficiency  minimum  McLeod  grasslands  requirements  metabolize  have  t o the  forage p r o t e i n  to  Grazers  their  i s related  due  and  improve  Studies on domestic  forage,  This i s p a r t i c u l a r l y  because  grazers  that the rate of food intake  digestibility,  1977).  by  content  suppression  suggest  body  reserves  (Chalmers  to  compensate f o r  1961; Bredon  and  Wilson  1963). Pastoralists  living  i n the Boma region a l s o migrate  t h e i r c a t t l e herds between wet season ranges west of River  to  the Kong-kong  River  and  the  with Pibor  Gom Swamps d u r i n g the dry  60  season. of  Similar pastoralist  the Sahel r e g i o n  Thus,  traditional  designed  to  evolved  behaviors  for  envi ronments.  Africa  methods  maximize  f i n d i n g s suggest strategy  of  in  m i g r a t i o n s were once common i n much  of  secondary natural  that m i g r a t i o n  coping  (Sinclair animal  and  husbandry,  production, herbivore is  a  Fryxell  presumably  apparently  populations.  particularly  with resource s c a r c i t y  1985).  mimic These  successful  i n h i g h l y seasonal  61  CHAPTER 3.  FOOD LIMITATION AND KOB MORTALITY PATTERNS  Introduction Numerous s t u d i e s suggest ungulate  populations  Sinclair 1980;  1977;  Fowler  (Klein  that food a v a i l a b i l i t y  1968;  McCullough  Caughley  1979;  1981; Clutton-Brock  Leader-Williams  et a l .  1982).  s t u d i e s i l l u s t r a t e changes i n recruitment function  of p o p u l a t i o n s i z e  Clutton-Brock ungulate  et  population  resources a r e populations et a l .  al.  (Bobek  1985).  density  1982).  or  while  dependent responses,  limitation  and R i c k e t t s  Many  of  these  have  comparatively  actually  measured  the  few  documented  effect  on  that  consumer  1977; B a y l i s s 1985; S i n c l a i r  ungulate  populations  there i s l i t t l e  do  evidence  exhibit that  food  i s responsible.  An a l t e r n a t i v e view i s t h a t h e r b i v o r e p o p u l a t i o n s be  1977;  or s u r v i v a l r a t e s as a  However,  1977; S i n c l a i r  Thus,  Bobek  (e.g. McCullough 1979; Fowler 1981;  studies  limiting  1970;  l i m i t s many  limited  by  can not  food abundance because only a small f r a c t i o n of  p o t e n t i a l l y e d i b l e food i s ever consumed ( H a i r s t o n et a l .  1960;  Slobodkin  least  some  et a l .  1967; Van Valen  large-mammal  vegetation Karns  limited carrying  1977;  Smuts  1983), or d i s e a s e Giffiths If  populations  1973). may  capacity  white-eared  at  maintained  below  the  predation  (Mech  and  by  1978; Caughley et a l .  ( C h r i s t i a n et a l .  1979; Berry  be  Moreover,  1980; Gasaway et a l .  1960; S i n c l a i r and  Norton-  1981). kob  are  limited  by food a v a i l a b i l i t y , I  62  p r e d i c t the f o l l o w i n g : 1.  S u r v i v a l and/or  years  of l i m i t i n g  food  scarcity.  2.  There  condition 3.  recruitment  rates  should  decline  during  food abundance and d u r i n g seasonal p e r i o d s of  should  be  a  direct  correspondence  between  body  i n d i c e s and s u r v i v a l r a t e s .  Survival  in  c o r r e l a t e d with food In t h i s chapter consider  trends  population  specific  locations  should  positively  availability. I  examine  these  predictions.  First,  I  i n p o p u l a t i o n numbers and age s t r u c t u r e of the  i n r e l a t i o n to a l a r g e - s c a l e drought  1980, immediately  be  preceding t h i s study.  that o c c u r r e d i n  Second,  I  report  on  f i e l d o b s e r v a t i o n s of m o r t a l i t y and recruitment of y e a r l i n g s f o r 1982  and  1983.  T h i r d , I r e l a t e dry season m o r t a l i t y r a t e s t o  r a t e s of food intake and i n d i c e s of body c o n d i t i o n . compare  my  limitation  findings  against  the  predictions  Finally,  of  I  the  food  at  four  hypothesis.  Methods  Population density I recorded p o p u l a t i o n d e n s i t y and c a r c a s s numbers study  sites,  from  s i t e s from January numbers counted  were  February to  estimated  April,  to  April,  1983  1982 and at f i v e  (Fig.  2.2).  Population  i n two ways: at the Ajwara study  t o t a l numbers of kob present at v a r i o u s times  study  site I  throughout  63  the day on a 0.5km  grassy meadow; a t a l l other  2  estimated  population  each study  site.  vehicle,  Two observers  and  sites  I  d e n s i t y from 5 km t r a n s e c t s that b i s e c t e d  v e h i c l e , counted animals the  study  stood  i n the back  of  a  moving  seen w i t h i n a 100m s t r i p e i t h e r s i d e of  recorded a l l o b s e r v a t i o n s at 0.1km i n t e r v a l s  into a tape-recorder.  The d r i v e r maintained  a  straight  course  and c a l l e d out 0.1km i n t e r v a l s t o the observers. Strip  transects,  following  c a l i b r a t i o n with a range-finder a t the outset of each  transect.  Observers  width  were  was  estimated  consistently  width t o w i t h i n 10m accuracy. the use of a range-finder introduced  by  able  to  High  population  bias  estimate  the 100m s t r i p densities  to  introduced  from outside the l e g i t i m a t e  be  inconsequential.  In  by  diurnal  p a t t e r n s (see  movement  order  Chapter 2 ) , a l l t r a n s e c t s were performed i n the e a r l y (07:00-09:00).  On  one  occasion,  I  performed  estimates.  Since  totals  varied  4  replicate  replicates  ( c o e f f i c i e n t of v a r i a t i o n  assume  transects  were a r e l i a b l e  that  of the  relatively  between these  =  to  morning  t r a n s e c t s throughout the day, t o t e s t the r e p e a t a b i l i t y density  made  i m p r a c t i c a l during t r a n s e c t s , but bias  i n c l u s i o n of animals  s t r i p width was l i k e l y minimize  during  little  0.28),  I  index of kob p o p u l a t i o n  density.  Carcass  density  Carcass  d e n s i t y was determined i n two  ways.  t o t a l counts of c a r c a s s e s were made over a 0.5km  2  of  four observers  on f o o t , separated  At  Ajwara,  area by a team  by d i s t a n c e s of 40 meters.  64  Thus, each observer  was r e s p o n s i b l e f o r  meters  side.  to  either  searching  a  strip  A l l c a r c a s s e s encountered were marked  with a numbered s t r i p of red tape around a horn or limb. marked a d u l t c a r c a s s e s , 61% were subsequently following  initial  were recovered, from survey  recovered  Of the 6  weeks  o b s e r v a t i o n , while 39% of c a r c a s s e s of c a l v e s  suggesting  carcasses  remained  to survey, m o r t a l i t y estimates were biased  downwards  by c a r c a s s disappearance  by  a  two  that while  many  or m u t i l a t i o n by  At a l l other study transects,  20  sites,  scavengers.  c a r c a s s e s were counted d u r i n g  observer  team from the back of a moving  vehicle.  In a l l cases, c a r c a s s e s were counted  transect  lines  used  5km  along  the  same  for l i v e population density estimates.  I  measured d i s t a n c e to each c a r c a s s using a tape measure, and a l l carcasses  located  subsequent  outside  50  meters  were  disregarded  analyses.  Weekly m o r t a l i t y r a t e s (m) were estimated  by:  m = c / n t where, m=finite  weekly m o r t a l i t y r a t e  c=number of carcasses/km  2  s i n c e previous  n=mean kob p o p u l a t i o n d e n s i t y t=time i n weeks  survey  in  65  Sex and age  distribution  At r e g u l a r i n t e r v a l s seasons,  I  throughout  recorded  sex  p r o p o r t i o n s of y e a r l i n g and of  kob.  dark  the  ratio,  and  calf/female  2-year o l d males from  1983  dry  ratio,  and  large  groups  Sexes were e a s i l y d i s t i n g u i s h e d s i n c e mature males have  brown to ebony pelage and  are h o r n l e s s and tawny brown  lyre-shaped horns, while  in  color.  Yearling  short s t r a i g h t horns as long as the ears slightly  smaller  than females three  1982  than  (Fig.  6.3)  females.  and 2 year o l d males are s i m i l a r I estimated age  males  (Buechner 1974)  (Buechner 1974). i n c o l o r to  have  and  Two-year o l d males are  and have s l i g h t l y curved horns  times as long as the ears  females  are  larger two  to  Both y e a r l i n g  females.  s t r u c t u r e of the p o p u l a t i o n  older  than  2  years from a random shot sample of 38 i n d i v i d u a l s , s i n c e I c o u l d detect  no  v i s i b l e d i f f e r e n c e s between a d u l t s of d i f f e r e n t  The usual hunting p r o t o c o l i n v o l v e d s e l e c t i n g from  which to sample, slowly approaching  shooting  the  population unusually  nearest densities  simple.  animal and  Under  of ease  the  the of  of  kob  i n a v e h i c l e , and  then  sex  circumstances,  counting  molars.  kob  (from both the shot sample and  the  wanted.  cementum a n n u l i from f i r s t  made I  High sampling  consider  behavior by  or age c l a s s e s g r e a t l y b i a s e d the r e s u l t s .  ages of 155 by  I  group  approach  u n l i k e l y that hunter preference or avoidance sex  a  age.  certain  I estimated from  it  the  carcasses)  i n c i s o r s and  first  66  Tooth cementum  lines  Whole t e e t h used f o r counting fixed the  in  10%  field.  they cut  formalin  cementum  a  rubbery c o n s i s t e n c y  first  i n 5% n i t r i c  acid  until  (3-5 days) and immediately  i n t o 10-15 micr t h i c k l o n g i t u d i n a l s e c t i o n s through the pulp  c a v i t y , using a freeze hours,  microtome.  t e e t h were s t a i n e d  After  a i r drying  minutes, and f i n a l l y  After a i r drying  r i n s e d i n tap water again  f o r 24  for  24  i n H a r r i s haematoxylin f o r 4 minutes,  r i n s e d i n tap water f o r 4 minutes, bathed i n Scott 3  were  f o r f i v e minutes p r i o r to shipment from  They were then d e c a l c i f i e d  reached  annuli  hours,  slides  were  solution  for  f o r 4 minutes.  then  mounted  in  Permount. I  examined  tooth  s e c t i o n s under a compound microscope at  both 10x and 40x m a g n i f i c a t i o n .  The three c l e a r e s t s e c t i o n s out  of at l e a s t 9 s e c t i o n s per t o o t h were counted and mean the  annuli  whole  that  number.  the f i r s t  Ring  cementum  counts rest l i n e  from  yearling  per  year.  Cementum  l i n e counts r e q u i r e d  judgement, s i n c e l i n e s were f r e q u e n t l y d i f f i c u l t In order  t o a s s e s s the r e l i a b i l i t y  counts  that  (r =0.85, 2  remained c o n s i s t e n t  were aged. p<0.05)  by a  single  some degree of to d i s t i n g u i s h .  of the technique, I recounted  cementum l i n e s from a random 20% sub-sample of the individuals  kob  i s l a i d down d u r i n g  the dry season f o l l o w i n g b i r t h , t h e r e a f t e r followed line  A  count was determined f o r each tooth and rounded to  nearest  indicate  recorded.  Close  indicated  initial  155  c o r r e l a t i o n between the two that  throughout the study.  counting  criteria  67  Post-mortem All  examination  shot  animals  and  fresh  c a r c a s s e s encountered along  t r a n s e c t s were subjected to post-mortem examination. the  total  carcass,  remaining  alimentary  head-tail  length,  length.  rumen (with and tract heart  without  I  c o n t e n t s ) , and  using a s p r i n g balance. girth,  shoulder  I a l s o measured the horn l e n g t h  weighed  I  height,  and  the  measured and  distance  tibia between  horn t i p s for a l l males. Body  condition  in  ungulates  measurement of the f a t content 1972;  content  of femur bone marrow using a  of  bone  1981;  (Hanks 1981; marrow  Riney  Riney  over hot c o a l s .  marrow  1982).  1982).  assessed (Sinclair  I estimated  modification First,  from the center of the  weighed in an aluminum t r a y p r i o r  commonly  bone  Duncan  procedures  Hanks  of  is  and  the f a t standard  I e x t r a c t e d 5-10  femur.  to being heated  T h i s procedure evaporated  of  by  T h i s sample  gm was  f o r a set time  a l l the water  present  in the bone marrow sample, l e a v i n g a r e s i d u e of f a t and p r o t e i n . Since m o b i l i z e d marrow f a t i s r e p l a c e d by water, post-heating marrow  fat  subsequent  weight  over  content. weight  ratio  p r e - h e a t i n g weight r e f l e c t s  Preliminary  change  the  following  h e a t i n g ; t h e r e a f t e r 5 minutes was  trials the  maintained  indicated first as a  5  of  the bone little  minutes of  standard.  68  Results  Age d i s t r i b u t i o n  i n the 1 i v e p o p u l a t i o n  The d i s t r i b u t i o n of ages i n the shown  in  Fig.  3.1.  the small sample s i z e . largest  age  1983  live  population  Data were p o o l e d from both sexes, due to Individuals 5  classes.  to  7  years  No i n d i v i d u a l s o l d e r than  found i n the shot sample, although i n d i v i d u a l s up old  were observed among c a r c a s s e s .  class  was  somewhat  Recruitment  is  smaller  than  Frequency  formed  the  10 years were to  13  years  of the 3 year age  adjacent  of young to age 1 was approximately  age  classes.  10% of the t o t a l  p o p u l a t i o n i n both 1982 and 1983.  Age d i s t r i b u t i o n at death The similar  age d i s t r i b u t i o n of c a r c a s s e s ( F i g . i n shape to the s t a n d i n g  age  distribution.  frequent age c l a s s e s were from 5 t o 7 y e a r s . and  young  adults  3.2) was  somewhat The  most  However, subadults  (1-4 years) were l e s s common among c a r c a s s e s  than i n the l i v e p o p u l a t i o n , while o l d e r age c l a s s e s  were  more  common. Age  distribution  3.3, -X. = 15.0, 2  males  died  numbers of differences  p<0.0l). than  old  at  suggest  differed  between sexes ( F i g .  A g r e a t e r p r o p o r t i o n of 4 to 7 year  similar males  death  were that  aged  females.  found  than  old  Conversely, smaller old  females.  These  males s u f f e r e d s i g n i f i c a n t l y g r e a t e r  69  24  live population  20  C O  16 +  D  12 +  jjo  8 -  a o a  4•  6  7  8  9  10  1112  13  age  Figure 3.1 Age structure of the 1983 l i v e population (1 year and o l d e r ) . Frequencies of individuals aged 1-3 estimated d i r e c t l y from observations of herd composition. Frequencies of age classes 3 years and older estimated from a random shot sample. Data f rom h>oth sexes pooled (n—38) •  24  carcasses 20-  C O  16  CO  a o a  8 -  4-  1  ••I  o  1  2  3  4  5  6  1  i"*  7  8  9  10  111213  age  Figure 3.2 Age structure of the found carcass sample (1 year and o l d e r ) . Data from both sexes pooled (n=10 3).  20  12 •  4 -  12-  20  4-5  6-7  8-9  10-11  age classes  Figure 3.3 Age frequencies of male vs. female carcasses  (males=50  females=45). Female t o t a l was adjusted to equal male t o t a l f o r 2  s t a t i s t i c a l comparison ( X =15.0; p<0.05) .  72  r i s k of m o r t a l i t y as young to middle-age a d u l t s than females.  Age-specific The of  mortality  a g e - s p e c i f i c m o r t a l i t y , or q ,  illustrating  differences  m o r t a l i t y agents according q  x  curve i n d i c a t e s  little  f o r o l d e r age  consistent  with  (Caughley 1966). age  appears  domestic  Sex  more  vulnerability  to  (Caughley 1966).  mortality  of  groups ( F i g .  African 1963;  3.4).  The  in  increase  form  to  buffalo,  1977;  (Caughley  Boma  kob  relatively increasing  general  form  mammalian  q^  in q  increasing  with  published  waterbuck,  Sinclair  seen i n thar  This  means  potential  youngsters,  published  curvilinear  similar  Hickey  increase  curves  curves  and  dall  Spinage 1982)  for sheep  than  the  (excluding  calves)  in  in  of  1966).  ratio The  both  sex  1982  r a t i o i n the and  1983  (males=2550,females=3434; equal sex sexes.  r a t i o at b i r t h ,  l i v e population was  X  died  biased  s i n c e there was likely  c a l f and  than females.  to be  skewed  2=130  '' 8  If we  females an  then m o r t a l i t y must d i f f e r between  the  the dry  P<0.001).  season i n d i c a t e d that more  However, these r e s u l t s may  some evidence that  seen d u r i n g  female carcasses  favor  assume  Carcass t o t a l s d u r i n g  males  more  to age  previous  The  sheep,  (Deevey 1947; linear  high  in  m o r t a l i t y of young to middle-aged a d u l t s , and  mortality is  curve i s a u s e f u l  x  transects.  declined  male The  have been  carcasses  were  frequency of both  significantly  at  increasing  73  age  Figure 3 . 4 Kob age-specific mortality curve, from pooled data for both sexes. Age-specific mortality rates were calculated using equation 1, as explained  i n Chapter 6 .  74  distance  from the v e h i c l e  c a l v e s : "Y_2 = 96.8,  3.5)(females: % =]0.9,  at  ( "X2 = 5.9,  n.s.).  p<0.05;  2  p<0.05), while the  observed  than  (Fig.  number  of  male  carcasses  d i f f e r e n t d i s t a n c e s from the v e h i c l e v a r i e d  females  T h i s suggests that  and  calves  observed c a r c a s s t o t a l s  during  for  males  were  carcass  females  little  more  visible  t r a n s e c t s , and  and  calves  were  that  biased  downwards. In  order  multiplied  carcass  correction  totals  factors all  equivalent  in  of  individuals totals  frequency  applying  the  for for  2.99  were  seen, consequently  After  account  factors  correction transects  to  this  sightability  youngsters  and  1.77  derived  by  less  than  from  and  females  respectively.  assuming 10m  greater  by  during  from observers were  distances  factor,  I  These  that  to o b s e r v a t i o n s from,the  correction  bias,  there  should  be  0-10m  range.  were  still  s i g n i f i c a n t l y more male and l e s s female c a r c a s s e s than one would expect  from  (males=85, addition,  the  p r o p o r t i o n of each sex i n the l i v e p o p u l a t i o n  females(adjusted)=94; circumstantial  of  kob  males  2  p<0.05).  by  also result  l o c a l tribesman.  from  preferential  During 2 communal  hunts that I observed, s i g n i f i c a n t l y more males and l e s s were  taken  p<0.001). the  live  than These  expected results  population  In  evidence suggests that the skewed sex  r a t i o i n the l i v e p o p u l a t i o n may hunting  -X = 4.18,  (males=l34,  females=72,  females X 54.8, 2=  suggest that the skewed sex r a t i o i n  results  hunting m o r t a l i t y between sexes.  from  differential  natural  and  60  males  40 J  n= 6 9 20  CO  C  60  CO  >  females  40  © CO  O  n  s  39  20-|  "ca  o  c£  60-1  calves  40  n = 99 20  10  20  30  distance  40  50  (m)  F i g u r e 3.5 Numbers o f c a r c a s s e s observed a t i n c r e a s i n g d i s t a n c e observers.  from  76  Seasonal changes in body c o n d i t i o n Several  previous  unable to maintain decline fat  (Hanks  reserves,  adequate  1981).  3.6b;  food  intake  Animals f i r s t bone  ( S i n c l a i r and  condition,  declined dramatically (Fig.  i n d i c a t e that when ungulates are  before m o b i l i z i n g  stages of emaciation Body  studies  as  Kruskal-Wallis,  fat  reserves  up t h e i r subcutaneous  marrow  fat  Duncan 1972;  measured  during  use  their  at  advanced  Hanks 1981).  by bone marrow f a t content,  the course of the  1983  dry  season  H=16.4, p<0.05).  Males remained in  comparatively b e t t e r c o n d i t i o n than females (Mann-Whitney, U=90, p<0.05),  but  condition. shot  both sexes showed s i m i l a r r a t e s of d e c l i n e  Bone marrow f a t content  individuals  condition  probably  reflects  food; thus, s t r e s s e d  body f a t reserves Indices  of  between y e a r s . not  that  decrease  In to  1982  (Fig.  during  the  poorest body  Decline  in  supplies  the  dry  season  3.6a), body c o n d i t i o n i n 1983  (over  of  range  (Fig.  70mm) f e l l  middle of the dry season.  (Chapter  2).  from t h i s unusual dry  increased  kob  This  in  differed  indices did  3.6b).  In  1982,  late  February,  rainfall  produced a  season r a i n f a l l  green forage intake,  i n f a t reserves  high  dry  Increased abundance of green grass  resulting  decline  body  mobilize  temporary f l u s h of green growth over l a r g e areas i n the kob season  to  requirements.  in  seen  unusual amounts of r a i n f a l l  die.  in  compared  i n d i v i d u a l s were f o r c e d to  condition  levels  to  submaintenance  to meet metabolic body  carcasses  individuals  were a l s o the most l i k e l y  condition protein  suggests  in  i n body  seen in  preventing  1983.  i n 1982 the  may  have  precipitous  Figure 3.6a Dry season measurements of bone marrow f a t content (1982). Data combined for both sexes. S o l i d l i n e connects monthly mean values.  78  Figure 3.6b Dry season measurements of bone marrow f a t content (1983). Data combined for both sexes. Solid l i n e connects monthly mean values.  79  Seasonal changes i n m o r t a l i t y Weekly m o r t a l i t y during  the dry  During the a  proportion  0.0030 and  dry of  the  week.  Adult years.  Calf mortality  rates  3.1;  production which  conditions  The  led  mortality In  that  lower than the m o r t a l i t y  rate  improved  the  body  p<0.05). dry  reflected  in  1983  This  is  season  produces  in elevated  mortality  1982  increased  the  condition  season  caused  dry and  season  range,  decreased  adult  rates. mortality  Data  r a t e s of a d u l t s  increased  season (Table  3.1;  significantly  Mann-Whitney,  from the most i n t e n s i v e l y monitored study  Ajwara, i l l u s t r a t e  the dry  U=7,  season r a i n f a l l of  over the course of the dry  rates  season d i f f e r e d between dry  are  to  difference  1982  that  as  adults.  in the  of green grass over much of to  3.1.  v a r i e d between  a four f o l d  in the dry  Mann-Whitney,  unusual dry  1983,  p<0.05).  was  r a t e s between c a l v e s and  consistent, with the hypothesis  rates.  i n Table  r a t e s v a r i e d from 0.0070  Thus, there  significantly  (0.0076)(Table  are given  periods  values ranged from 0.0027  mean weekly m o r t a l i t y r a t e  (0.0039) was  stressful  1983  week, while 1983  mortality  The  and  i n i t i a l adult population,  0.0779 per week in 1983. i n weekly m o r t a l i t y  1982  l o c a l i t i e s and  season, a d u l t m o r t a l i t y r a t e s , expressed  0.0044 per  to 0.0195 per  rates for d i f f e r e n t  seasons of  1982  rates  this  trend  at Ajwara i n c r e a s e d season, at an  (Fig.  3.7).  Adult  by a f a c t o r of 4 during  apparently  constant  rate  U=6, site,  mortality  the course of of  increase.  E s t i m a t e s of c a l f m o r t a l i t y suggest a s i m i l a r trend although statistically  significant  (Table  3.1,  Mann-Whitney, U=3,  not  n.s.).  80  Table 3.1 Dry season weekly mortality estimates (1982 and 1983). Mean adult mortality rates d i f f e r e d between years (Mann-Whitney; U=7; p<0.05), but there was no s i g n i f i c a n t difference between c a l f mortality rates i n the 2 years.  adult weekly mortality rate  c a l f weekly mortality rate  location  date  Gom  Feb.-March 1983  0.0032  0.0155  Gom  March-April 1983  0.0068  0.0627  Wangchira  Jan.-Feb. 1983  0.0061  0.0650  Wangchira  Feb.-March 1983  0.0195  0.0741  Neubari  Jan.-Feb. 1983  0.0027  n.a.  Neubari  Feb.-March 1983  0.0054  n.a.  Ungwala  Jan.-Feb. 1983  0.0061  n.a.  Ungwala  Feb.-March 1983  0.0060  n.a.  Ajwara  Jan. 1983  0.0054  0.0070  Ajwara  Feb. 1983  0.0081  0.0779  Ajwara  March-April 1983  0.0143  0.0634  x  0.0076  x  0.0522  SE  0.0032  SE  0.0245  Ajwara  Jan.-Feb. 1982  0.0030  0.0206  Ajwara  March-April 1982  0.0039  n.a.  Gom  Jan.-Feb. 1982  0.0043  0.0508  Ungwala  Jan.-March 1982  0.0044  n.a.  x  0.0039  x  0.0357  SE  0.0009  SE  0.0107  y=0.00106x+ 0.0026 0.016  dry season residence time (weeks)  Figure 3.7 Dry season adult mortality rates at Ajwara (1983). Means indicated by s o l i d dots, 95% confidence 2 bars (r =0.98; p<0.05) .  i n t e r v a l s indicated by v e r t i c a l  82  C a l f m o r t a l i t y at the Ajwara study the dry season ( F i g .  site  i n c r e a s e d r a p i d l y during  3.8).  What p r o p o r t i o n of annual m o r t a l i t y occurs season? the  I was  wet  unable to measure m o r t a l i t y r a t e s d i r e c t l y  season.  However,  m o r t a l i t y during the wet that  during  the  dry  a d u l t s during the  1982  0.0076  per week.  e n t i r e year, been  18%,  33%.  season. and  If one  Since  at the  1983  calculations probably  dry  during  suggest that  much  lower  than  The  average m o r t a l i t y r a t e of  dry  seasons  was  0.0039  and  a p p l i e s these m o r t a l i t y r a t e s over 1982  1983  r a t e , t o t a l m o r t a l i t y would  the have  r a t e , t o t a l m o r t a l i t y would have been  recruitment  approximately  simple  season was  then, at the  and  o u t s i d e the  of  young  into  the  population  was  10% per year, annual a d u l t m o r t a l i t y g r e a t e r than  10% would have produced a d e c l i n e i n p o p u l a t i o n  numbers.  Since  censuses show no evidence  of p o p u l a t i o n d e c l i n e over t h i s p e r i o d  (Table  that  2.2),  I  argue  s u b s t a n t i a l l y higher d u r i n g remainder of the  Dry  the  dry  intake  than Ajwara c a l v e s was although  were  than  during  the  d u r i n g the  1983  Estimated  intake  dry season was  g r e a t e r at  Gom  m o r t a l i t y of both a d u l t s  and  higher at the l o c a t i o n with lower food  t h i s i n f e r e n c e c o u l d not be s t a t i s t i c a l l y  inconclusive,  probably  year.  (Table 3.2).  limited  evidence  rates  season  season m o r t a l i t y i n r e l a t i o n to food Food  the  mortality  number of r e p l i c a t e s (Ajwara n=3; these  results  provide  availability, t e s t e d due  Gom  additional  n=2).  to  While  supporting  that m o r t a l i t y r a t e s during the dry season were r e l a t e d  0.10-,  Figure 3.8 Dry season c a l f mortality rates at Ajwara  (1983). Means  indicated by s o l i d dots, 95% confidence intervals indicated by v e r t i c a l  84  Table 3.2 Dry season mortality i n r e l a t i o n to green grass consumption rates at Gom and Ajwara study s i t e s .  grass consumed (kg/ind/day)  location  Gom  Ajwara  adult mortality  calf mortality  x  1.27  0.0050  0.0391  SE  0.07  0.0013  0.0167  x  0.71  0.0093  0.0494  SE  0.26  0.0022  0.0177  n  85  to food  availabilty.  Cumulative dry A  season m o r t a l i t y  linear  increase  implies a c u r v i l i n e a r season  progresses.  in  weekly  increase The  mortality  r a t e s of  adults  in t o t a l a d u l t deaths as the  regression  dry  l i n e drawn through estimates  of weekly m o r t a l i t y rates from the Ajwara study s i t e means the  number  of a d u l t deaths r e l a t i v e to i n i t i a l  r e l a t e d to the dry  dy dt so  season residence  numbers (y)  was  by:  = 0.00l06t + 0.0026  that y = 0.00053t  where  The of  time (t)  that  dry  2  + 0.0026t + c  y= a d u l t d e a t h s / i n i t i a l adult p o p u l a t i o n t= time i n weeks c=0.0, by d e f i n i t i o n  r e s u l t i n g p l o t of t o t a l adult m o r t a l i t y as season d u r a t i o n  i s shown i n F i g .  t o t a l a d u l t m o r t a l i t y changed r e l a t i v e l y weeks of the dry  3.9.  by  year  to year d i f f e r e n c e s i n both kob  habitat conditions.  However, as long as  in weekly m o r t a l i t y increase  function  Note, that  season, t h e r e a f t e r m o r t a l i t y r a p i d l y  exact parameters of t h i s model should be  a curvilinear  a  l i t t l e over the  The  increase  size  first  strongly  affected  density  there  a  is  5  escalated.  population  r a t e s as the dry  i n t o t a l population  while  and  constant  season progresses,  mortality  necessarily  y = . 0 0 0 5 3 x 2 + .0026x  dry season duration (weeks)  Figure 3.9 Cumulative adult mortality over the 1983 dry season, us Ajwara data.  87  follows. T h i s c u r v i l i n e a r increase have  i n cumulative a d u l t m o r t a l i t y  may  r e s u l t e d from i n c r e a s i n g v u l n e r a b i l i t y to m o r t a l i t y agents  as the dry season progressed, due to d e c l i n e i n body Let  us  assume  that  individuals within a distribution;  that  at  a  given  population i s , some  time  has  a  condition.  the body c o n d i t i o n of bell-shaped  individuals  are  frequency  i n much b e t t e r  c o n d i t i o n and some i n d i v i d u a l s are i n much poorer c o n d i t i o n  than  the bulk of the p o p u l a t i o n .  mean  body  condition  periods  of food  population  of  the  population  the  Fig.  would  become v u l n e r a b l e  population  3.9.  Fig.  declines  3.6b,  steadily  s c a r c i t y , then an i n c r e a s i n g p r o p o r t i o n  t h e i r weakened c o n d i t i o n of  I f , as shown i n  mortality  should  increase  population  has d i e d  The  vulnerable  predicts  that  curvilinearly  until  1/2  a v a i l a b l e food  i s less  Unfortunately,  1968;  than  the  show  a  Note that the h y p o t h e s i s does not  the  maintenance  requirements  that of  population.  there  Catastrophic  been documented  of  s u p p l i e s become i n c r e a s i n g l y scarce, only  i n d i v i d u a l s w i t h i n the  adequately.  adult  (assuming a symmetrical d i s t r i b u t i o n of body  r a t e of i n c r e a s e . food  portion  cumulative  c o n d i t i o n ) , t h e r e a f t e r cumulative a d u l t m o r t a l i t y would  that  the  would increase at an i n c r e a s i n g r a t e , as i n hypothesis  demand  of  to m o r t a l i t y agents, due t o  ( F i g . 3.10).  This  diminishing  during  are  no  data to t e s t the h y p o t h e s i s  d i e o f f s of ungulate  populations  a number of times ( C h r i s t i a n et a l .  1960; K l e i n  C h i l d 1972; Keep 1973) but none of these s t u d i e s  m o r t a l i t y at i n t e r v a l s over the p e r i o d of demise.  have  recorded  Hillman  and  88  O  c  CD 3  rj  body condition index  Figure 3.10 Graphical representation of the body condition/mortality hypothesis. As the body condition of individuals within a population declines during periods of food s c a r c i t y  (T=l to 3), an increasing  proportion of the population becomes vulnerable to mortality agents (shaded portion under the curve).  89  Hillman  (1977)  estimated  an extended drought Their  results  cumulative possibly  in Nairobi  show  adult  m o r t a l i t y a t monthly i n t e r v a l s National  the p r e d i c t e d  mortality  Park  ( F i g . 3.11).  curvilinear  f o r kongoni  during  increase  and  wildebeest,  obvious  sources  in and  f o r zebra.  Discussion  Causes of m o r t a l i t y d u r i n g the dry season I  now  mortality  consider  during  the  the  three  dry  most  season:  predation,  of kob  disease,  and  malnutrition.  Predation Predation  by  w i l d c a r n i v o r e s was probably  over most of the Boma kob dry season range. carnivores  occur  and cheetah,  in  their  inconsequential  While a v a r i e t y  of  the area, i n c l u d i n g l i o n , hyena, l e o p a r d ,  numbers  were  low.  Leopard  and  cheetah  s i g h t i n g s were c o n f i n e d t o f o r e s t e d areas near the Boma P l a t e a u . Kob for  used  these  areas d u r i n g the southward m i g r a t i o n , but only  a few days each year.  areas  frequented  locations,  lion  L i o n and hyena o c c u r r e d  by kob d u r i n g the d r y season. and  hyena  of  the need  savannah  However, i n most  were r a r e l y seen or heard.  systems of l i o n and hyena preclude because  in  migration  with  the  Social herds,  t o feed non-mobile young ( S c h a l l e r 1972;  (from Hillman and Hillman 1 0 7 7 )  '  0  T  1 2  1  1 4  1  r-  6  dry season length (months)  Figure 3.11 Cumulative adult mortality of kongoni, wildebeest, and zebra during a drought i n Nairobi National Park, Kenya (from data i n Hillman and Hillman 1977).  91  Hanby and Bygott  1979).  Resident prey numbers were low  i n the  northern g r a s s l a n d s , p o s s i b l y because these areas become f l o o d e d each  wet  season.  T h i s suggests that Boma p r e d a t o r s were rare  because of a low biomass basis.  An  themselves  alternative actively  mortality  is  carnivores  of  available  hypothesis  persecuted  they  by  actually  p e r s . com.).  local  kill  however,  5000  the  kob  were  6  tribesmen.  tribesmen  are This  tolerate  or  people  in  migration  1982  does  substantial.  During  i n communal hunts near Pibor and  not  1983.  come  In  many  years,  c l o s e enough to Murle  hunts.  During the course of the dry season,  were  carnivores  livestock  more  killed  s e t t l e m e n t s f o r s u c c e s s f u l communal  both Murle  year-round  insignificant.  P r e d a t i o n of kob by man may be  Post by Murle  that  a  The low c a r n i v o r e d e n s i t y means that  p r e d a t i o n on kob was probably  migration,  is  on  probably s m a l l , s i n c e l o c a l p a s t o r a l i s t s  unless  (J.Arenson,  prey  s u b s i s t e n c e hunting by  and Anuak t r i b e s o c c u r r e d on a r e g u l a r b a s i s .  There  major v i l l a g e s with approximately 500 persons each  that  were i n c l o s e contact with kob herds.  Assuming  that  an  adult  kob would p r o v i d e 20kg of meat, that each person a t e 1kg of meat per  day,  and  that  kob  hunters  r e p l e n i s h e d t h e i r l a r d e r s as  needed, over a 3 month d r y season perhaps been  slaughtered.  from s u b s i s t e n c e  In the wet season,  hunters,  so  offtake  15,000 kob might  kob were l a r g e l y was  probably  have  isolated minimal.  Thus, I estimate that no more than 20,000 kob were harvested per year  by  human p r e d a t o r s .  By comparison,  at l e a s t 44,000 and 84,000 a d u l t  kob  died  results of  i n d i c a t e that  natural  causes  92  during  the  respective  estimate t h a t 2-4  dry  seasons of 1982  and  1983.  Thus, I  times as many kob d i e d from n a t u r a l causes  as  died from hunting. Two First,  points  there  was  relatively  no evidence to  and  counted  of i n j u r y or m u t i l a t i o n by  hunters.  that  subsistence  hunters  r e l a t i v e l y easy  trade  hunt k i l l s are f o r s u b s i s t e n c e purposes. to k i l l  behavior.  take  There i s no means  p r e s e r v i n g meat, there i s no commercial  all  kob,  because of  Thus, i t i s l i k e l y  dependent p r e d a t i o n by  in  were only  assume  Carcasses  constant numbers of kob per year.  adequately  meat,  and  be s t r e s s e d about hunting m o r t a l i t y .  a f f e c t e d by hunting.  Second, i t i s f a i r  of  to  i t i s u n l i k e l y that the s u b s t a n t i a l m o r t a l i t y observed  the f i e l d was when  need  their  sheer  in  It i s  abundance  that there i s l i t t l e d e n s i t y  man.  Di sease There was resulted  from  examinations parasite  no evidence unusual  of  loads  shot or  that m o r t a l i t y d u r i n g the dry disease  individuals  ubiquitous  occurrence. showed  disease  no  season  Post-mortem unusually  symptoms.  There  high are  comparatively  few d i s e a s e s of s u f f i c i e n t p a t h o g e n i c i t y and  transmission  r a t e s to have l a r g e - s c a l e e f f e c t s , the few that do  include bovine (L.  t u b e r c u l o s i s , pneumonia, r i n d e r p e s t , and  K a r s t a d , p e r s . com.).  by d i s e a s e  anthrax  Since a l l study s i t e s showed s i m i l a r  i n c r e a s e s i n m o r t a l i t y r a t e s over the dry season, that widely  high  i t is unlikely  separated kob herds would have been e q u a l l y a f f e c t e d outbreak.  93  Many  pathogens  are  relatively  harmless  n u t r i t i o n a l c o n d i t i o n of an a f f e c t e d i n d i v i d u a l critical as  level  the  (Scrimshaw et a l . , 1968).  proximate  insufficiencies individuals  Age  cause  are  of  declines  to  a  when  nutritional  responsible  for  making  vulnerable.  s t r u c t u r e of the kob p o p u l a t i o n The  age d i s t r i b u t i o n of the kob p o p u l a t i o n  pronounced  bulge  population  has  frequency smaller  in  the  reached  of any given than  5-7  a  o l d age  stationary  age  preceding  year  class  age  i n 1983 showed a  age  must  be  classes.  groups.  years  equivalent  Since  the  to or  5 to 7 year o l d 1  or  2  o l d , t h i s i m p l i e s t h a t the p o p u l a t i o n must have undergone  s i z e a b l e change distribution  previous  could  to  have  Alternatively, to  1983.  arisen  young i n 1976 to 1978, with  due  If a  distribution,  i n d i v i d u a l s were markedly more common than i n d i v i d u a l s  a  This  relative  young  adult  further  age  information  of  the age  decrease  in  mortality  subsequent  we  can  not  years.  between 1978 and 1983  of  both c a l v e s and o l d proportion  c l a s s e s i n the s u r v i v i n g p o p u l a t i o n .  hypotheses (Caughley Much  in  from i n c r e a s e d recruitment of  i f the p o p u l a t i o n decreased  increased  bulge  a d u l t s , t h i s would produce an i n c r e a s e d r e l a t i v e  in  the  Thus, d i s e a s e may a c t  death,  ultimately  unless  discriminate  between  of  Without these  1977).  the southern  Sudan experienced  1979-80, immediately p r e c e d i n g  t h i s study.  drought c o n d i t i o n s As a r e s u l t , many  94  u s u a l l y r e l i a b l e watering the  Pibor  township  c a t t l e losses (J.Arenson,  due  p o i n t s d r i e d up  west to  inadequate  p e r s . com.).  (G.  Schaller  census  in  March  D.  forage  1980,  80,000  1980,  kob  late  (Table 2.2;  a sizeable  reduction  c o n d i t i o n s i n 1979 The  for  carcasses  kob  standing  age  curve  x  very  young  very  d i s t r i b u t i o n observed i n 1983  than  rather  s u r v i v a l or b i r t h r a t e s from 1976 7  year  recruitment  in  (range=128,000 yearlings  drought  may,  was  from  of  then  about  1.5  therefore,  a  aerial  since that  drought  kob.  be the most h e a v i l y  due  to  Current  suggest  145,000  10%  was  million  increased the  1980  a  frequencies mean  yearlings  minimum  per  year  assumes that p r i o r to of  the  total  t h i s i m p l i e s that the kob  have  Under  temporary i n c r e a s e in c a l f  to 1978.  I f one  composed  old  c l a s s e s during  classes  179,000).  calves),  to 1980  age  1976-1978  similarly  (excluding prior  old  to  an  I suggest that the bulge in the  population  to  estimated  numbers  resulted  and  o l d e r age  5  I  i n d i c a t e s that m o r t a l i t y r i s k  m o r t a l i t y of both c a l v e s and  of  February  Thus, i t i s l i k e l y  numbers  Therefore,  decline  in  1980.  both  groups.  water  A e r i a l censuses  population  s t r e s s f u l environmental c o n d i t i o n s these may a f f e c t e d age  of  during  in the drought.  kob  white-eared kob q  i s highest  lack  observers  t - t e s t , n.s.). in  and  and  Western p e r s . com.).  suggest no subsequent change in March  in  Large numbers of dead kob near d r y i n g  and  that there were over  pastoralists  of Boma N a t i o n a l Park reported heavy  waterholes were r e p o r t e d by independent 1980  and  (Fig.  caused  a  3.12). 40%  1980  population population The  reduction  1980 in kob  95  Figure 3.12 1979  Kob population estimates 1979-1983. Population estimates a f t e r  from a e r i a l censuses.  1979 population estimate as explained i n the text.  96  population  numbers.  Evidence f o r food  limitation  My r e s u l t s support limits  the  chapter  I  Boma  the hypothesis  kob  outlined  population.  a  series  that At  of  food  the  availability  beginning  predictions  to  of t h i s  test  the  hypothesis. The  first  prediction  of  the  food  s u r v i v a l r a t e s should d e c l i n e during years abundance and d u r i n g seasonal was evidence year  to  hypothesis  p e r i o d s of food  scarcity.  depending  on  the  severity  of  The short dry season of 1982 produced  less  mortality  adult  than  the  long  dry  observed no s i g n i f i c a n t d i f f e r e n c e s between m o r t a l i t y or recruitment the  1979-80  to  consequently 1975).  rates.  During  concentrate  Thus,  i n many  season  considerably  season of 1983. I years  i s consistent  in juvenile  with  drought c o n d i t i o n s ,  near  can not o b t a i n  dry  from  Moreover, l a r g e - s c a l e m o r t a l i t y  drought  l i m i t a t i o n hypothesis. forced  There  that adult m o r t a l i t y rates v a r i e d considerably year,  that  of below-average food  conditions.  during  is  the  few remaining  adequate  food  the  food  grazers  are  waterholes and  supplies  (Corfield  droughts, h e r b i v o r e s d i e of inadequate  food rather than lack of water. M o r t a l i t y r a t e s of both a d u l t s and c a l v e s i n c r e a s e d the  dry  season.  As shown i n Chapter 2, food intake d u r i n g the  dry season was i n s u f f i c i e n t Many period  previous of  during  to meet kob e n e r g e t i c  requirements.  s t u d i e s have suggested that the dry season i s the  greatest  nutritional  stress  in  African  savannah  97  grasslands few  (e.g.  Western 1975;  1977).  s t u d i e s that have a c t u a l l y measured both  and  mortality  season  rates,  mortality  The should body  Sinclair  in  c o r r e l a t e d with food  the  inverse c o r r e l a t i o n  condition  of  Boma  poorest  population,  i n c r e a s i n g the  condition  (1985) found that  (Duncan 1975; The  were  dry  season  Sinclair  was  rates  to  also  is  1975;  declining  among  Sinclair  1977).  be  food  correlated  d i f f e r e d between the  l i m i t s kob  population  m o r t a l i t y r a t e s increased m o r t a l i t y was  insufficient  fat  reserves,  during  Similar  t o p i suggest African  with  this  ungulates  food  intake.  2 study s i t e s for which I  i n t a k e , but due  suggest  rates  l i m i t a t i o n hypothesis i s  sample s i z e s , a s t a t i s t i c a l comparison was  availability  and  I n d i v i d u a l s in the  and  common  p r e d i c t i o n of the  observations  there  rates  the most l i k e l y to d i e .  able to estimate r a t e s of food  These  closely  mortality  which suggests that  led  that m o r t a l i t y r a t e s should Mortality  not  that  to the  possible.  dry  season  numbers.  Both a d u l t and  the  season,  dry  small  but  food calf adult  more c l e a r l y r e l a t e d to food a v a i l a b i l i t y than  calf  mortality.  1968;  Grubb 1974;  A  dry  I found d i r e c t correspondence  r i s k of m o r t a l i t y .  relationship  third  was  mortality  f i n d i n g s from A f r i c a n b u f f a l o , wildebeest, cause-effect  the  availability  wildebeest  between  elevated  s u p p l i e s of high p r o t e i n food ultimately  food  of  food hypothesis i s that  herbivores.  kob  al.  In one  availability.  second p r e d i c t i o n of the  be an  the  et  Serengeti  between body c o n d i t i o n and in  Sinclair  limited  Sinclair  number of previous  1977;  Sinclair  et  was  studies (Klein  al.  1985)  have  98  also  found  adults. and/or 1964;  that food a v a i l a b i l i t y determines  There calf  that  natality  s u r v i v a l are r e l a t e d to food a v a i l a b i l i t y  (Woodgerd  considerably  Gross 1969;  Clutton-Brock c a l v e s may in  is  Geist  et  al.  more  1971,-Grubb 1982;  evidence  1974;  Houston  abundance  milk throughout the s t u d i e s survival  1982).  the dry season  showing come  (Chapter  5).  density-dependent  from  to  measure  temperate  In c o n t r a s t , a l l of  responses  populations  in  that  juvenile terminate  scarcity.  not  only  mortality  affected  competing f o r scarce r e s o u r c e s , but individuals  are  is  T h i s would tend to confound any  density-dependent is  kob  variation  i n c r e a s e in m o r t a l i t y r a t e s of a d u l t s as  season p r o g r e s s e s .  Mortality  dependent  nutritional deficit,  the  attempt  directly.  by the number of  individuals  also  the  length  of  upon sub-maintenance s u p p l i e s .  the lower  the  effects  longer i n d i v i d u a l s must r e l y on t h e i r own a  White-eared  of the most i n t r i g u i n g r e s u l t s from t h i s study  a p p a r e n t l y constant  1979;  because t h e i r mothers continue to provide  l a c t a t i o n p r i o r to the winter p e r i o d of food  dry  McCullough  be p a r t i a l l y b u f f e r e d a g a i n s t year to year  forage  One  m o r t a l i t y r a t e s of  body r e s e r v e s to  these r e s e r v e s become, and  time The meet the  more v u l n e r a b l e they are to v a r i o u s p o t e n t i a l m o r t a l i t y agents. One the  of the consequences of a  direct  relationship  between  d u r a t i o n of p e r i o d s of food s c a r c i t y and m o r t a l i t y r a t e s i s  that many large-mammal p o p u l a t i o n s may  be prone  population  Caughley and Lawton  fluctuations  (Klein  1968;  S i n c l a i r and N o r t o n - G r i f f i t h s 1979; similar  to  the  Boma kob may  Bayliss  to  1985).  large-scale 1981;  Populations  be a f f e c t e d s u b s t a n t i a l l y by even  99  moderate droughts. play  at frequent  Thus, resource l i m i t a t i o n intervals  (Schoener 1982)  to see l a r g e f l u c t u a t i o n s over living  time  in  i n h i g h l y v a r i a b l e environments.  should  and we  herbivore  come  should  into expect  populations  100  CHAPTER 4.  KOB REPRODUCTIVE PHENOLOGY  Introduction Many  ungulate  populations  p r o d u c t i o n corresponding (reviews  in Sadleir Since  high  (Caughley  1966; S i n c l a i r  Clutton-Brock  et a l .  as  of  means  seasonal  peaks  calf  1977; Bunnell  mortality  1977;  improving  i s common  McCullough  1982), breeding the  1982;  1979;  Sinclair  i n ungulates Fowler  survival  lactation  and  as a r e s u l t , l a c t a t i n g  diminished  f a t reserves during the c a l v i n g  of v u l n e r a b l e young.  periods  et a l .  1982).  period  relative  to  (Sinclair  By timing c a l f production to  of the year when food i s i n g r e a t e s t abundance, females  should be b e t t e r a b l e t o meet the demands of young,  through  females f r e q u e n t l y s u f f e r  both n o n - l a c t a t i n g females and males i n the p o p u l a t i o n Clutton-Brock  1981;  synchrony may have evolved  Females supply a l l the food f o r t h e i r newborn o f f s p r i n g  1977;  in calf  to p e r i o d s of pronounced food abundance  1969; S c h a l l e r  1983a).  a  show  thereby  increasing  feeding  offspring survival.  dependent  Thus, there i s  presumably strong s e l e c t i v e pressure  for populations  seasonal  c a l v e s d u r i n g the p e r i o d of  environments  to  produce  living  in  g r e a t e s t food abundance. In t h i s chapter, the  white-eared  I compare the  kob,  Kobus  Uganda kob, Kobus kob thomasi. lives  kob The  reproductive  r a i n f a l l patterns  (Chapter  of  l e u c o t i s , t o the c o n s p e c i f i c migratory  i n savannah grasslands of the southern  seasonal  phenology  white-eared  kob  Sudan with markedly  2 ) , while the Uganda kob i s  101  non-migratory and l i v e s i n savannah seasonality  (Buechner  et a l .  In response to c l i m a t i c available  as  throughout  food  to  white-eared  kob  of  pronounced  abundant  As  both  a  and  adult  during  On  to  consequence and  widely  from  April  distributed.  of  swamps of  resource  scarcity,  white-eared  (Chapter 3).  i s highest  the b a s i s of t h i s  ephemeral  juvenile  the dry season  showed that food a v a i l a b i l i t y season.  predictably  the dry season, from November t o March, green  extent.  mortality  varies  During the r a i n s ,  grass becomes i n c r e a s i n g l y r e s t r i c t e d limited  pronounced  1966; Modha and E l t r i n g h a m 1976).  the year (Chapter 2 ) .  during  less  s e a s o n a l i t y , the amount of green biomass  to October, forage i s both However,  areas with  during  kob  is  In Chapter 2 I the  late  wet  i n f o r m a t i o n , kob s h o u l d time the  p r o d u c t i o n of young t o c o i n c i d e with the l a t e wet season.  Methods  Seasonal  changes  determined  from  in a  female  random  reproductive  shot sample.  examined f o r evidence of l a c t a t i o n measurement  of  mammary  d i s s e c t i o n of u t e r i ) .  gland  wet  were  A l l shot females were  (by mammary  palpation  and  weight), and pregnancy (by  I measured f e t a l t o t a l weight  and d e s c r i b e d the f e t a l developmental were  condition  Conception  dates  subsequently estimated from the r e g r e s s i o n of f e t a l  weight  on age r e p o r t e d by Buechner et a l . estimated  by  stage.  and l e n g t h ,  (1966).  Birth  dates  were  e x t r a p o l a t i n g the 240 day g e s t a t i o n l e n g t h of the  Uganda kob (Buechner  et a l .  1966)  from  estimated  conception  102  dates.  I a l s o noted the c o n d i t i o n of the o v a r i e s ,  the presence of corpora Morrison  lutea,  based  on  the  description  months  old) in  calf/female ratios the  ground  migrating May). 867 339  the  i n samples  (December herds  kob  to  taken  population  (individuals <  were determined from  of the p o p u l a t i o n observed both on  April),  from  and  from  photographs  low-flying a i r c r a f t  Sample s i z e s f o r ground o b s e r v a t i o n s ranged individuals,  individuals.  females  because  s-shaped horns readily  while  from  96  of pronounced  from  distinguishable  c o l o r dimorphism  (Chapter 6 ) . Males  coloration  distinguish confined  1 to 3 y e a r s  calculations  third  after  3  years.  and presence of o l d were  size  also  dark  I t was d i f f i c u l t to  on  photographs,  so  I  to those counts from w e l l d e f i n e d , c l o s e  (taken at f l y i n g h e i g h t s of l e s s than 30m above  Calves l e s s than 6 months o l d a r e  the  from  females d u r i n g ground o b s e r v a t i o n s  immature males from females  up a e r i a l photos ground).  until  to  a e r i a l sample s i z e s ranged from 243 to  Adult males were e a s i l y  distinguished  of  (November and  due t o presence of horns, even though males do not develop adult  in  (1971 ).  Seasonal changes i n the p r o p o r t i o n of c a l v e s 12  particularly  of  adult  approximately  one  females (Chapter 6) and were e a s i l y  d i s t i n g u i s h e d both on the ground and from a e r i a l  photographs.  D i u r n a l a c t i v i t y budgets of a d u l t kob d u r i n g the dry season were e s t i m a t e d from l a r g e aggregations on lOmin  intervals,  each  individual  f e e d i n g , ruminating, r e c l i n i n g ,  within  swampy  meadows.  At  40m was recorded as  standing, or o t h e r .  1 03  Results S i x t e e n out of 17 f e t u s e s examined were January  and  April  (Fig.  September and December. the wet  4.1),  implying a b i r t h peak between  The b i r t h peak o c c u r r e d at the  reproductive  condition  observed seasonal trend i n February,  approximately  calf 80%  suggests  that  80%  (Fig.  end  of  females  (January to A p r i l ) ,  r e f l e c t e d the  In  January  of females were l a c t a t i n g . days  (Buechner  (Fig.  declined  over  The  proportion  females  Fig.  of  the course of the dry season  r e a c h i n g approximately 25% by  4.2).  Since  1974),  May.  In January and February, none of the sampled pregnant  and  of the females observed i n January had  produced c a l v e s i n the preceeding 6 months. lactating  4.2)  production.  kob l a c t a t e f o r approximately 180  the  between  season, as p r e d i c t e d .  Female  4.2  conceived  females  were  By the end of A p r i l , approximately 80% of  were  pregnant.  These  findings  are  i n c o n s i s t e n t with the c o n c e p t i o n p e r i o d  (Fig.  the r e g r e s s i o n of f e t a l weight on age.  I t i s p o s s i b l e that some  of the females sampled  i n January and February  small f e t u s e s that were undetected. procedure  may  were more l i k e l y case,  both  have  4.1  might  females.  had  females In  any  i n d i c a t e that most conceptions  May.  Approximately 80% of the female p o p u l a t i o n and  have  A l t e r n a t i v e l y , the sampling  than pregnant  and 4.2  occurred between January and  year,  estimated by  been b i a s e d such that non-pregnant  to be sampled  Figs.  4.1)  somewhat  only one o f f s p r i n g was  conceived  produced per female.  were capable of c o n c e p t i o n as y e a r l i n g s  each  Females  (2 females of age 2 were  104  co 6-  o c  CO cr co  4 -  2-  0 = 120000E E  CO  c  CO N  0  1  h  month  F i g u r e 4.1 Upper h i s t o g r a m - f r e q u e n c i e s o f b i r t h s by month, e s t i m a t e d from f e t a l w e i g h t (n=17). Lower h i s t o g r a m - mean monthly r a i n f a l l f o r P a c h a l l a township  (1953-1962).  totals  80 -  40 •  0  40 -  80  J  n=5  F  M  A  n=13  n=9  n=ll  month  M n=4  Figure 4.2 Female reproductive condition (January-May) from pooled data (1982 and 1983). Monthly sample sizes as indicated below histograms.  1 06  sampled as  and both were l a c t a t i n g ,  yearlings).  i n d i c a t i n g that they  I observed no i n s t a n c e s of m u l t i p l e f e t u s e s , i n  accordance with records from Uganda kob L i k e the Uganda kob were i n v a r i a b l y  (Buechner  1961b)  females  were  it  et a l .  white-eared  kob  exposed  n u t r i t i o n a l s t r e s s during the l a t t e r as  (Buechner  1966). embryos  implanted in the r i g h t horn of the u t e r u s .  Reproductive  period,  conceived  extended  stages  of  considerable the  w e l l i n t o the dry season  u=14,  p<0.0l).  body c o n d i t i o n than adult despite  the  than  body  Lactating males  indicate  particularly  from  (Mann-Whitney,  that  males  lactation at  a  time  vulnerable  to  nutritional  Calf mortality  per  food  p<0.05)  of  4.4).  imposed  year  when  stress to  i n the dry season may  These  increased kob  (Chapter  high  were 3).  mortality:  week from l a t e January to A p r i l have r e s u l t e d  (Chapter in  part  l i m i t e d c a p a c i t y of females to feed young adequately.  Although c a l v e s began to forage on this  u=90,  (Fig.  itself  reserves  1.5-7.5%  the  (Mann-  females were a l s o i n poorer  During the dry season, c a l v e s were s u b j e c t  3).  females  and  f a c t that females devoted a g r e a t e r p o r t i o n of the  demands on f a t  between  in  non-lactating  d a y l i g h t hours to feeding than d i d findings  were  poorer  (as measured by bone marrow f a t content; S i n c l a i r 1981),  season  4.3).  condition  Whitney,  dry  (Fig.  females  Hanks  the  lactation  Lactating  Duncan 1972;  in  to  source  was  their  insufficient  own  before  weaning,  to make up f o r the l o s s of  mother's m i l k . Seasonal changes i n c a l f / f e m a l e r a t i o 4.5  (curve A ) .  are  shown  in  Fig.  There was a steady i n c r e a s e i n the p r o p o r t i o n of  Figure 4.3 Dry season body condition of l a c t a t i n g non-lactating mean values.  (solid dots) vs.  (open dots) females (1983 data). Lines connect monthly  108  100  CD  c o cd > CD 0}  o 3 100 o  other stand  o  \j iimlnale  60  recline  -I  feed  20-I  males 8:00  1 2:00  16:00  time of day  F i g u r e 4.4 A c t i v i t y budgets o f male v s . female kob on meadows observations=1025;  (male  female observations=1379). Note t h a t males spent  s i g n i f i c a n t l y l e s s time f e e d i n g than d i d females  2  ("X =269.6; p < 0.001)  109  Figure 4.5 Seasonal changes i n calf/female r a t i o , estimated  i n two ways.  Curve A indicates calf/female r a t i o observed from f i e l d observations herd composition  of  (open triangles= 1981-1982; open c i r c l e s = 1982-1983).  Curve B indicates calf/female r a t i o predicted from frequencies of b i r t h s by month (Fig. 4.1)  and f i e l d estimates of c a l f mortality rates (Fig. 3.8).  Lower histogram - mean monthly r a i n f a l l t o t a l s for Pachalla (1953-1962).  11 0  females  with  calves  from  decline in calf/female virtually (Fig.  October  ratios  a l l calves  from  only  until  in c o n t r a s t t o f i e l d arisen  ratio  to  May.  calf/female  ratio  the end of the c a l v i n g p e r i o d  observations.  some time, a common behavior 1966).  mid-March  the  because new-born kob hide  (Leuthold  mid-March, and a sharp Since  were born between September and December  4.1), one would expect  increased  to  This  to  (December),  discrepancy  may  i n t h i c k e t s and t a l l  field  observations  have  grass f o r  i n young of many A f r i c a n  As a r e s u l t ,  have  ungulates  of c a l f / f e m a l e  may have been biased downwards p r i o r to the date when a l l  c a l v e s j o i n e d the a d u l t herds f o l l o w i n g the h i d i n g out p e r i o d . I c a l c u l a t e d an ratios  (Fig.  month ( F i g .  4.5,  of c a l v e s d u r i n g at  the  d e c l i n e s over calf/female  curve  seasonal  B)  rate  4.2), and f i e l d  the dry season conclusion  the  dry  ratios  by  curve  of  among  estimates  reproductive  the  age  of m o r t a l i t y r a t e s  (Chapter 3 ) . The expected curve  of the c a l v i n g p e r i o d  season  calf/female  from the frequency of b i r t h s by  4.1), the f e c u n d i t y  females (80%, F i g .  peaks  expected  to  end  levels  of  the  (December) and  close dry  to  observed  season  (April).  S i m i l a r i t y between curves A and B i n the l a t e dry season "suggest that by A p r i l a l l young mortality  was  have  apparently  joined  highest  the in  the  p e r i o d , l e v e l l i n g o f f with the onset of the between  field  observations  natality  rates  between the two y e a r s .  and  (open  reproductive  herds.  latter rains.  of c a l f / f e m a l e r a t i o  1981-82 (open t r i a n g l e s ) and 1982-83 that  adult  dry season Similarity  (Fig.  circles),  timing  Calf  varied  4.5) i n suggests little  111  Discussion White-eared southern  kob  Sudan  living  showed  seasonal  breeding r e s u l t i n g  wet  (September  i n the  There  a post-parturition  season  As a r e s u l t ,  gave b i r t h to a s i n g l e o f f s p r i n g per year. produce  w i t h i n 2 months f o l l o w i n g p a r t u r i t i o n  (Buechner  average while  1.35  Consequently,  young per  white-eared  females  O b s e r v a t i o n s of l a c t a t i o n confirmed  Buechner  female  individual  kob  per  ungulates Sinclair the  dry  are  4 months females Uganda  conceive  al.  kob produce  (Buechner  0.80  1966;  1974),  young per year. in  this  et a l . ' s (1966) view that Kobus kob  study females  Thus, seasonal breeding  potential  of  white-eared  kob  kob  calf  was  p r o d u c t i o n occurred i n the l a t e  both  abundant  and  widespread.  wet  These  c o n s i s t e n t with a l a r g e number of s t u d i e s on other  living 1983a).  in  seasonal  environments  (Sadleir  1969;  However, females continued l a c t a t i o n w e l l  season,  when  food  supplies  were  scarce.  into As  consequence, l a c t a t i n g females experienced g r e a t e r d e p l e t i o n fat  on  kob.  season, when food results  year  i n 2 year o l d females  reduced the annual r e p r o d u c t i v e  White-eared  Uganda  produced  are capable of conception as y e a r l i n g s .  r e l a t i v e t o Uganda  December).  females et  the  in c a l f  By c o n t r a s t ,  year and  1974).  throughout  of  individual  the  Buechner  calves  to  i n t e r v a l of approximately  before females conceived a g a i n .  kob  grasslands  synchronous  production was  late  in  reserves  than  n o n - l a c t a t i n g females.  s u p p l i e s were a v a i l a b l e e a r l y  i n the wet  a of  Since abundant food  season,  i t i s therefore  p u z z l i n g t h a t c a l f p r o d u c t i o n d i d not occur e a r l i e r  in  the  wet  1 12  season. One  possible  explanation  s u b s t a n t i a l p e r i o d to recover before g i v i n g b i r t h and  is  that  emerged  inadequate  from  feeding new  young ( S i n c l a i r  females  lactation. reserves  have  dry  season,  of  adequate  females  this  timing may  young d u r i n g the wet  T h i s hypothesis  females recover rains,  lactation  1983a).  If  season, females would have  Thus, the observed  between producing  that  stressful  a  p o s s i b l y with  f a t reserves to s u s t a i n l a c t a t i o n even under abundant  food c o n d i t i o n s . off  the  require  f a t reserves l o s t during  kob c a l v e s were born i n the e a r l y wet just  females  recovery  c o u l d be  throughout  fat reserves q u i c k l y  would  be  season  time  and  by  wet  next  measuring  season  after  ensuring  p r i o r to the  tested  the  r e f l e c t a trade-  the  fat  period.  onset  of  If the  i n c o n s i s t e n t with the c o n d i t i o n recovery  hypothesis. Alternatively, selective eared  forces  it  is  possible  ephemeral  in the south swamped  Chapter 2). northward  migration  adaptation.  wet  grasslands  in  from  production There  each year  each  Calf production  t i m i n g of c a l f  October may  migration  season  are  3).  to  to  other  The  white-  rainfall, high  place to  largely  December. as  an  lack  Thus, c a l f  i n t o the dry season range may  season  during The  the  observed  anti-predator  few p r e d a t o r s present the  short  rainfall,  north in the dry  function  are a p p a r e n t l y  s p e c i e s (Chapter  from low  the  takes  northern g r a s s l a n d s , p o s s i b l y due r e s i d e n t prey  there  a c t i n g on r e p r o d u c t i v e phenology.  kob p o p u l a t i o n migrates  grass areas  that  of  i n the  year  round  production  during  reduce the l i k e l i h o o d of  11 3  p r e d a t i o n on v u l n e r a b l e young and There might be species  is  clear  tested. have  white-eared  kob  the  Boma  ecosystem,  migratory  hypothesis  several  movements  ungulate  s i m i l a r to the  (e.g. t i a n g , zebra, and e l a n d ) , while others  the  then  northward  calving  migration  as  If kob  an  I p r e d i c t that other migratory  adopt a s i m i l a r s t r a t e g y , exhibit  i n which the p r e d a t i o n  (e.g. b u f f a l o , l e l w e l , and o r i b i ) .  during  adaptation,  In  way  north-south  non-migratory calves  a  females.  peaks  while in  the  non-migratory e a r l y wet  should be a c l e a r d i f f e r e n c e in the timing between non-migratory and migratory  populations.  produce  anti-predator species species  season. of  are  calf  should should  Thus, there production  1 14  CHAPTER 5.  BREEDING SYNCHRONY AND MALE AGGRESSION  Introduction Four  features  characterize  lek  mating systems (Bradbury  1981): (1) males congregate at communal d i s p l a y sole  purpose  young;  of  attracting  (3) d i s p l a y  interest  to  sites  from the d i s p l a y s i t e . lek  systems  no  defensible  resources  of  these  an extreme form of polygyny and, since  there i s presumably intense sexual s e l e c t i o n on males 1977;  lekking  species  hypotheses choice  Borgia  1979;  are  about  the  (Emlen and Oring  Wrangham  1980;  of  conditions,  female c h o i c e depends mainly on the d i s p l a y q u a l i t i e s of  Oring  the  (4) females choose males to mate with  As a consequence  constitute  for  females; (2) males do not care f o r  contain  females; and  sites  Bradbury  particularly evolution  and well  males,  (Emlen and  Gibson 1983). suited  Thus,  to  testing  and adaptive f u n c t i o n of mate  1977; B o r g i a  1979;  Wittenberger  1978;  Bradbury 1981; Bradbury and Gibson 1983;  Foster  1983). A critical  element i n lek systems  aggressively  for  courts.  most  restricted  In  to  occupation lekking  males  of  is  achieved  by  populations,  compete  matings  are  largely  that are l e k members, and s a t e l l i t e  holders  of  Moreover,  most  a small number of c e n t r a l  w i t h i n the lek (Buechner 1961a; Wiley Robel and B a l l a r d  males  p r e f e r r e d lek t e r r i t o r i e s , or  are excluded from breeding a c t i v i t i e s . are  that  1973;  1974; but see L e u t h o l d  Lill  1974,  males  matings courts 1976;  1966; Floody and Arnold  115  1975).  As  a  r e s u l t , there i s a h i g h l y skewed d i s t r i b u t i o n of  mating success among males i n the p o p u l a t i o n 1977;  Borgia Among  1979; Bradbury and Gibson lekking  ungulates,  c e n t r a l c o u r t s f o r only a peripheral  courts  few  days,  can  control  while  they  more  energy  (Buechner  and S c h l o e t h  a few males have access males  on  defense  1965).  Oring  preferred  can  control  weeks (Buechner et a l .  T h i s d i f f e r e n c e i n s i t e occupancy c o u l d a r i s e expending  and  1983).  males  for several  (Emlen  and  from c e n t r a l males  courting  of  There i s some evidence  to c e n t r a l  1966).  positions  females  that only  because  dominant  r e t u r n r e p e a t e d l y f o r b r i e f p e r i o d s of tenure w i t h i n l e k s  f o l l o w i n g temporary bachelor  herds  (Buechner  Bradbury and Gibson Little  periods  has  breeding  (1977) suggested  et  association  al.,  with  unpublished  surrounding MS,  cited in  1983).  attention  synchronized  of  on  been  paid  to  the  l e k mating systems.  that extreme breeding  effect  of  Emlen and Oring  synchrony should d i m i n i s h  the degree of polygyny, s i n c e i n d i v i d u a l males can only o b t a i n a few matings when simultaneously at  many  (Fig.  females  come  males would be a b l e to o b t a i n of  populations difficult  tenure should  condition  in be  a  many  individual  matings  the  during  p r e f e r r e d lek p o s i t i o n . less  expected  synchrony, s i n c e  polygynous,  since  short  Asynchronous i t would  be  f o r males t o r e t a i n a dominant p o s i t i o n f o r extended  p e r i o d s of time, p r o v i d i n g new o p p o r t u n i t i e s Therefore,  breeding  5.1). Pronounced polygyny may be  intermediate degrees of breeding  period  into  one  would  f o r other  p r e d i c t more polygyny and intense  males. sexual  116  F i g u r e 5.1 H y p o t h e t i c a l r e l a t i o n s h i p between degree o f b r e e d i n g synchrony and degree o f polygyny  (from Emlen and O r i n g  1977).  11 7  selection Two  i n p o p u l a t i o n s with intermediate breeding predictions arise  sexual  selection  is  from  this  greater  hypothesis.  in  dominant  males  should  c o n d i t i o n s f a v o r i n g polygyny populations). are able to competing  Natural obtain  obtain  greater  synchronous  more  moderately  synchronous  should favor i n d i v i d u a l s that  benefits  relative  to  (Fig.  a  migratory  leucotis, eared  costs,  population  i n the southern  kob  kob thomasi  on  so  moderately  5.2).  In order to t e s t t h i s h y p o t h e s i s , I examined in  pronounced.  males on l e k s should be more a g g r e s s i v e i n  synchronous p o p u l a t i o n s  if  g r e a t e r b e n e f i t s under  ( i . e . for  selection  First,  moderately  p o p u l a t i o n s , sexual dimorphism should a l s o be Second,  synchrony.  of  Sudan.  the white-eared The  behavior  lek  behavior  kob, of  Kobus  the  white-  leks i s s i m i l a r to that of the Uganda kob,  (Buechner and S c h l o e t h 1965), so I s h a l l  on q u a n t i t a t i v e d i f f e r e n c e s between the two  races.  kob  Kobus  concentrate The  chief  difference  is  that mating i s r e s t r i c t e d to a 4 month p e r i o d i n  white-eared  kob  (Chapter  (Buechner  1974).  5) while Uganda  Thus,  a  This  synchronous than  hypothesis  breed  year-round  comparison of the p o p u l a t i o n s  t e s t the hypothesis that sexual s e l e c t i o n moderately  kob  is  more  intense  i n asynchronous l e k k i n g p o p u l a t i o n s .  kob and more intense competition among males.  I a l s o i n v e s t i g a t e d the consequences of a g g r e s s i o n on l e k s .  Previous s t u d i e s on the Uganda kob  that lek aggression determines (Buechner 1961a).  in  p r e d i c t s a g r e a t e r degree of sexual dimorphism  in the white-eared  males  will  between  suggest  only  dominance r e l a t i o n s between males  However, recent work suggests  that d i s r u p t i v e  118  F i g u r e 5.2 P o s t u l a t e d male c o s t / b e n e f i t curves intensity  (b^ = b e n e f i t s t o males i n h i g h l y s y n c h r o n i z e d o r asynchronous  populations;  b^ = b e n e f i t s t o males i n moderately s y n c h r o n i z e d  c = c o s t s t o males; f  i n r e l a t i o n to f i g h t i n g  populations;  = f i g h t i n g i n t e n s i t y a t which b^-c i s g r e a t e s t ; and  = f i g h t i n g i n t e n s i t y a t which b -c i s g r e a t e s t ) .  119  behavior  i s common  Foster  1983;  hypothesis,  i n some  Trail  lekking  1985).  This  species  (Wrangham  suggests  an  1980;  alternative  that male aggression may serve t o d i s r u p t the mating  a c t i v i t i e s of neighboring  males.  Methods  Sexual  dimorphism  I measured s i z e dimorphism i n a d u l t kob from a random sample  of 18 males and 24 females.  Animals were weighed, and a  v a r i e t y of other body measurements were recorded. (with  shot  A  non-random  respect t o age) sample of 10 immature males and 11 c a l v e s  were a l s o taken. horn  shape,  horn  These age groups were e a s i l y l e n g t h , and coat c o l o r .  recognizable  by  Ages of a d u l t s were  determined by counts of t o o t h cementum a n n u l i (Chapter 3 ) .  Lek  observations Observations  observations  of lek behavior  were compiled  per month f o r January t o A p r i l ,  different I tape  of  1983. Because kob  p o p u l a t i o n s were t r a n s i e n t , no l e k s were a c t i v e e n t i r e 4 month p e r i o d .  from 8 hours  throughout  I t h e r e f o r e combined o b s e r v a t i o n s  the  from 5  l e k s , each numbering between 20 and 65 males.  made  recorded  r e c o r d e r , separated  transcribed  observations  f o r 10 minute periods on a  by 10 minute i n t e r v a l s d u r i n g  the records onto data sheets.  which  I  For each o b s e r v a t i o n  1 20  p e r i o d I counted the t o t a l number of males and at  the  beginning  of  females  present  the i n t e r v a l , and noted the l o c a t i o n s of  females on the l e k .  During the o b s e r v a t i o n p e r i o d s I noted a l l  incidences  f o l l o w i n g b e h a v i o r s : (a) mount attempts - a  of  the  mount i n which the penis i s e r e c t and the physical occur; such  contact  male  "prancing", " l i p - c u r l i n g " ,  and S c h l o e t h 1965); displays  with  complete  makes  with the female, although p e n e t r a t i o n may  (b) sexual s o l i c i t s - male e x h i b i t s as  actually  (c) t h r e a t s  -  precoital  behaviors  or " l e g - s t r i k i n g " males  engage  not  (Buechner  in  agonistic  heads lowered and ears extended, and prepare f o r  sparring a c t i v i t y ; when  a  (d) chases  -  male  chases  another  male,  usually  second male moves onto the court of the  first,  or as a r e s u l t of t h r e a t or f i g h t behavior; (e) f i g h t s  males make p h y s i c a l c o n t a c t with the horns, u s u a l l y a f t e r behavior.  Only  recorded  for  terminated  the  each  most  escalated  incident.  Since  virtually  attempts  lek: central,  location  i n t e r m e d i a t e , and  circular  calculate  of  all  peripheral.  females  predict  fights  Since  and  expected  were  leks  were  i n shape, the r a t i o s of the r e l a t i v e areas of  values  I used  distributed  these  ratios  f o r the s p a t i a l d i s t r i b u t i o n of  females and of a g g r e s s i v e encounters on the l e k . if  fights  w i t h i n 3 c o n c e n t r i c zones of equal width on the  the three zones were approximately 1:3:5. to  all  was  fight.  In a d d i t i o n , I recorded the  roughly  behavior  i n a r e t r e a t by the l o s e r and chase by the v i c t o r , I  simply scored such events as a  mount  agonistic  threat  at  For  example,  random on the l e k , one would  f i v e times as many females i n the perimeter zone  as  in  121  the  central  zone.  Results  Sexual  dimorphism  Adult  white-eared  kob  degree of c o l o r dimorphism. colored, ears,  with  and  contrast, of  highly  throat,  d i f f e r markedly Adult  contrasting  and  are  white  males  are  in  white  ventrally.  when males are approaching f u l l (Buechner  et  al.  1966).  There  weight  (Buechner and G o l l e y than  Uganda  =  1.37;  1967) kob  ).  The  By  Females  pronounced  appears at 3 years of age,  body s i z e  dimorphism between the two races male/female  in color.  leucotis f i r s t  ebony  patches on the face,  thomasi males are an even tawny brown c o l o r .  dimorphism  smaller  leucotis  entirely  both r a c e s resemble thomasi males  color  from Uganda kob i n  is  (Fig.  and  sexual  maturity  no d i f f e r e n c e i n weight 5.3)(  leucotis  thomasi male/female weight = White-eared  kob  are,  adult 1.41  however,  (mean l i v e weight of l e u c o t i s males =  55kg, mean l i v e weight of thomasi males = 90kg).  Temporal change i n mating and a g o n i s t i c behavior The number of females observed on between' January and A p r i l per  (Table 5.1).  leks  declined  slightly  However, mount attempts  hour d i d not change s i g n i f i c a n t l y over the 4 month  breeding  122  70-, (4)  (6)  (3) ( 2 )  males (6)  Oi  5  0  C)  Ui  (4)  (5)  CD CD >  females  (6)  30 (4)  (11)  1 0 - «'  birth  ~i  1  r  i  i  5  3  i  7  age  F i g u r e 5.3 Kob l i v e weights a t age (males=solid d o t s ; females=open d o t s ) . Mean v a l u e s i n d i c a t e d by d o t s , range by v e r t i c a l b a r , and sample s i z e s i n brackets.  123  Table 5.1 Changes i n mating behavior over the breeding season  (January-  A p r i l , 1983)(mean values per hour, SE of mean shown i n parentheses). Differences between means evaluated by Kruskal-Wallis Test (df=3;*:p< 0.05; **:p<0.001) .  January  February  March  April  differences between means?  28.7(1.7)  26.4(0.9)  51.5(4.2)  18.4(0.7)  H=42.5,**  no. females  7.6(1.7)  7.0(0.7)  4.8(0.3)  5.3(0.4)  no. mounts  21.1(4.0)  20.6(3.9)  17.1(3.5)  19.6(5.1)  no. males  H=11.0,* H=3.5,n.s.  no. fights per male  2.06(0.33)  0.98(0.17)  0.39(0.07)  0.42(0.03) H=30.7,**  no. chases per male  0.64(0.09)  0.26(0.05)  0.12(0.03)  0.13(0.03) H=26.6,**  no. threats per male  0.25(0.08)  0.09(0.03)  0.13(0.03)  0.08(0.03) H=4.7,n.s.  1 24  season.  These  r e s u l t s suggest that matings were evenly spread  over the e n t i r e breeding p e r i o d , as i n the Uganda kob  (Buechner  1974). However,  the  mean  number of f i g h t s per male per hour d i d  decrease s i g n i f i c a n t l y over the breeding season, ranging from peak  of 2.06 f i g h t s per hour  in March ( F i g .  i n January to 0.39 f i g h t s per hour  5.4). The mean value f o r the 4 month p e r i o d was  1.07 f i g h t s per hour.  When f i g h t s a r e combined  with  a  threat  r a t e of 0.13 per hour, there was a mean a g o n i s t i c encounter of 1.20 per hour over the breeding season, which than  the  a  value of 0.38 per hour  rate  i s much g r e a t e r  f o r the Uganda kob, c a l c u l a t e d  in the same way from Floody and A r n o l d ' s (1975) d a t a .  Agonistic  encounters were t h e r e f o r e three times as frequent i n the  white-  eared kob p o p u l a t i o n as the Uganda kob p o p u l a t i o n .  However, the  agonistic  value by the  encounter  r a t e d e c l i n e d to a comparable  end of the white-eared kob breeding season.  Spat i a l d i s t r i b u t i o n of females and a g o n i s t i c  encounters  More than 3 times as many females as expected in  the  c e n t r a l area of the d i s p l a y grounds  df=2, p<0.00l). Uganda  5.5, ^ =296, 2  were  female  1961b; Floody and A r n o l d 1975), as w e l l as  that of most other l e k k i n g s p e c i e s areas  (Fig.  T h i s i s c o n s i s t e n t with the behavior of  kob (Buechner  central  concentrated  preferred,  (Bradbury  and  as  1981).  many  as 12 d i f f e r e n t  females were on o c c a s i o n l o c a t e d with  a  were  ( F i g . 5.6). Clumping of  generally  not  highly  clumped  single  Although  females was even l e s s common away from c e n t r a l  male,  or  females  intermediate  125  3.2-,  O  JC Q)  2.44  CO  E -»-• CO  i_  c o  1.64  CO CO  CD  i_  O)  D) CO  c  0.8  CO CD  E  0.0J an  Feb  Mar  Apr  date  Figure 5.4 Changes i n aggressive behavior over the breeding season (dots=means, v e r t i c a l bars=95% confidence i n t e r v a l s ) .  126  "D  CD 1 6 0 > CD CO  central  intermediate  perimeter  o O  c  8 0 -  CD 3  rj CD  0  "D  CD O CD CL X CD  80-  >» O  c  CD  160-  CT  CD  240-  location of females  Figure 5.5 Female s p a t i a l d i s t r i b u t i o n on leks. Upper histogram shows observed values, lower histogram  shows expected frequencies under the  n u l l hypothesis that females choose p o s i t i o n at random.  127  1204  80 4  o c  CD 3  cr 40  4  no. females with single male  F i g u r e 5 . 6 Female group s i z e s w i t h i n d i v i d u a l c o u r t i n g  males.  1 28  areas.  A  mean of 5.7  observation  period,  and  d i s t r i b u t e d among 3.2 Agonistic central  checks  these  different  encounters  positions  p<0.00l),  females were l o c a t e d on the lek f o r each  on  females  lek  (Fig.  ? C = 116.1,  5.7,  2  rate  in  the  Spot  i n d i c a t e d that males were spread  Thus, the  increased  on  df=2,  in p e r i p h e r a l areas.  r e l a t i v e l y evenly over the lek (based on expected 3 c o n c e n t r i c zones).  average  more frequent than expected  although males a l s o fought  of male d i s t r i b u t i o n  on  males.  were  the  were  out  numbers i n the  agonistic  center of the lek d i d not simply r e s u l t  encounter from more  males being present. These r e s u l t s  are  consistent  with  Floody  and  Arnold's  (1975) o b s e r v a t i o n s of a c l o s e c o r r e l a t i o n between the number of mating  attempts  and  the  number  i n d i v i d u a l Uganda kob males. in  central  agonistic  Both behaviors  encounters  were  by  concentrated  areas i n Uganda kob, as i s the usual case f o r other  l e k k i n g s p e c i e s (Bradbury areas tended those  of  1981).  In a d d i t i o n , f i g h t s i n c e n t r a l  to be both more prolonged and more  energetic  than  i n more p e r i p h e r a l a r e a s .  F u n c t i o n s of f i g h t i n g Buechner  behavior  (1961a) suggested  that a g g r e s s i v e behavior  kob c o n t r i b u t e s to the establishment of dominance the d i s p l a y ground.  females  fights.  were present  positions  on  Males compete s t r e n u o u s l y f o r p o s s e s s i o n of  c e n t r a l c o u r t s and a g o n i s t i c encounters full-scale  i n the  In with  204  of  either  305  frequently escalate into f i g h t s observed  combatant,  (67%), no  suggesting  that  129  central  intermediate  perimeter  0 > 120i_ 0 CO .Q O >» O c 0 60cr CD  0  T5  0 O CD  60  a x 0  O c 120 0  CJ 0  180-  location of fights  Figure 5.7 Spatial d i s t r i b u t i o n of male f i g h t s . Upper histogram shows observed values, lower histogram shows the expected frequencies under the n u l l hypothesis that the number of fights i s related to the number of males in zones of d i f f e r e n t s i z e s .  130  these  fights  were  concerned  solely  with  deciding  the  p o s i t i o n s of the males i n v o l v e d .  However, i n the other  fights,  present  at  l e a s t one  the combatants.  female was  This suggests  that  lek  33%  of  i n the c o u r t of one  many  fights  may  of  disrupt  mating a c t i v i t i e s of neighboring males. To  examine  this  hypothesis,  I  compared  the  p r o b a b i l i t i e s of a male being engaged i n a f i g h t female  was  or  was  not p r e s e n t .  cases.  I  calculated  rule  that  a  not o r i e n t e d in  both  the c o n d i t i o n a l p r o b a b i l i t i e s of a  fight  o c c u r r i n g , given that a female was to Bayes'  given  If f i g h t i n g was  towards d i s r u p t i o n , f i g h t i n g should be e q u a l l y  relative  (Scheaffer  and  or was  likely  not present,  Mendenhall  1975),  according using  the  f o l l o w i n g p r o b a b i l i t i e s c a l c u l a t e d from 91 o b s e r v a t i o n s p e r i o d s :  A = p ( f i g h t ) = 0.18 A' = p(no f i g h t ) = 0.82 B = p(female present f i g h t ) = 0.33 B' = p(female present no f i g h t ) = 0.07 C = p(no female present f i g h t ) = 0.67 C = p(no female'present no f i g h t ) = 0.93 (A)(B)  p(fight  female present) =  p(fight  no female present) =  Thus,  the  probability  four times g r e a t e r i f a suggests  of  (A)(B) +  (A')(B')  = 0.509  (A)(C) (A)(C) +  a f i g h t o c c u r r i n g was  female  =  (A')(C)  approximately  was  present  than  that neighboring males may  initiate  fights  e i t h e r d i s r u p t mating or to s t e a l  females.  0.137  not.  This  i n order to  131  During changed  14 out  of  location  observation  periods  subsequent to f i g h t s .  the male that was  c h a l l e n g e d was  before the f i g h t and of  91  0.9  On  (15%)  these  females  14 o c c a s i o n s ,  c o u r t i n g on average 4.1  females a f t e r the f i g h t .  As  females  a  result  t h i s d i s r u p t i v e behavior, a t o t a l of 57 females moved to  l o c a t i o n s , while 22 observation behavior. Table  females  periods  (85%)  Nearly  39%  lek,  left  males.  while  females  the lek e n t i r e l y .  36%  females  resulted  no  are  the  disruptive  summarized  in  moved to a  original  partner  The m a j o r i t y  resulted in  77  (64%)  females  of  leaving  i n females moving to neighboring  These f i n d i n g s i n d i c a t e that d i s r u p t i v e behavior g r e a t l y  i n c r e a s e d the frequency although  a  challenging fighting.  of movements  by  females  s u b s t a n t i a l p r o p o r t i o n of females  original partners.  forced  by  than returned to t h e i r  movements of non-disrupted the  during  twice as many d i s r u p t e d females  neighboring male (40%) and  position  characterized  Subsequent movements of  5.2.  (21%),  changed  new  male At  females  The data are inadequate subsequently  worst, back  the  to t e s t whether  disrupting  lek,  returned to t h e i r  mated with females  however,  on  males  the  s c a t t e r e d by temporarily  i n t o a " p o o l " from which they might  later  gain.  Consequences of  fighting  Vigorous competition to s u b s t a n t i a l r i s k s . I  recorded  4  f o r access to females may  During  expose males  16 hours of o b s e r v a t i o n s at  i n s t a n c e s of s e r i o u s wounding during 305  leks, fights;  Table 5.2 Movements of disrupted vs. non-disrupted  females (proportion  t o t a l observations i n parentheses).  disrupted not disrupted  returned to o r i g i n  moved to neighbour  l e f t lek  t o t a l no. females  12 (0.21)  23 (0.40)  22 (0.39)  57  0 (0.00)  8 (0.36)  14 (0.64)  22  1 33  all  of these were  combatants.  horn  Agonistic  f r e q u e n t l y j o i n e d by initial  to  the  encounters  a  third  male,  sides  or  between  2  who  often  abdomen males  of were  directed  his  blows to the unprotected f l a n k s and abdomen of the other  combatants. showed  wounds  Also,  symptoms  distribution  3  of  of  out recent  ages  season) d i f f e r e d  at  the  (obtained d u r i n g the breeding the  on  the  display  sexes  ground  c o m p e t i t i o n between males may  r a t i o s observed i n both white-eared Uganda kob  Moreover,  leks  (Fig.  3.8, female  young to mid-aged a d u l t s , and these are the age  c l a s s e s most a c t i v e Aggressive  death  wounds.  A l a r g e r p r o p o r t i o n of male than  2  were  horn  s i g n i f i c a n t l y between  X = 15.0, df = 3, p<0.0l)'. carcasses  of 5 male c a r c a s s e s found near  (Leuthold  1966).  e x p l a i n the skewed sex  kob  (58-62%  females)  and  (59% females; Buechner 1974) p o p u l a t i o n s .  P i scussion Emlen  and  Oring's  moderate b r e e d i n g  synchrony  e i t h e r asynchronous sexual  One  hypothesis  should  should  be  of  increased  than  breeders p r e d i c t s that for  the  selection  coloration.  moderately Uganda  kob.  i s increased  in  differed  from the c o n s p e c i f i c Uganda kob i n the degree  of c o l o r dimorphism,  and  polygynous  asynchronous  sexual  s p e c i e s with  sexual dimorphism markedly  size  that  more  greater  white-eared kob than the  consequence  be  or h i g h l y synchronous  selection  synchronous  (1977)  but not i n s i z e  White-eared  kob  dimorphism.  Dark c o l o r a t i o n has no known adaptive value other than male advertisement.  Few  savannah-dwelling  antelope  species  are  1 34  characterized  by  extreme  melanism,  presumably  disadvantages of being h i g h l y v i s i b l e to  because  predators.  of the  Moreover,  dark pelage absorbs c o n s i d e r a b l y more s o l a r r a d i a t i o n than  light  pelage, making i n d i v i d u a l s more prone to heat s t r e s s i n t r o p i c a l climatic  conditions  temperatures the  for  s t u d i e s i n hot dark  Western  1977).  stress  coat  color  in  I  argue  that  of males may  of  males  a r b i t r a r y mating  may cues  dark c o l o r a t i o n  from  30-39°  if  so  Previous  c a t t l e herds  ( F i n c h and  black  and  white  f o r sexual advertisement.  that  otherwise  non-adaptive  they are used by females as  1980,  1981;  example of "runaway" sexual s e l e c t i o n from  C,  selection  Kirkpatrick  i n white-eared kob males may  A second p r e d i c t i o n  maximum  strong  contrasting  serve l a r g e l y  (Lande  daily  suggest  African  evolve,  Mean  i s considerable.  grasslands  t h e o r e t i c a l work has shown  traits  Thus,  thermal  savannah  against  Recent  1972).  i n the Sudan study area range  potential  coloration  (Finch  (Fisher  1982).  c o n s i t u t e an  1958).  i n c r e a s e d sexual s e l e c t i o n  i s that  white-eared kob males should compete more v i g o r o u s l y than Uganda kob f o r p o s s e s s i o n of p r e f e r r e d l e k c o u r t s . study suggest that the o v e r a l l is  significantly  higher  f o r the moderately  Uganda kob,  not c l e a r .  for  Uganda  I t may  kob.  The  simply r e f l e c t  white-  supporting  However, f i g h t i n g frequency d e c l i n e d  p e r i o d the white-eared kob a g g r e s s i o n r a t e was  this  encounters  synchronous  over the course of the breeding season, and by the  reported  from  frequency of a g o n i s t i c  eared kob than f o r the asynchronous hypothesis.  Results  the  substantially end  of  the  s i m i l a r to v a l u e s  s i g n i f i c a n c e of t h i s d e c l i n e " i s that  dominance  relations  on  1 35  the d i s p l a y grounds are g r a d u a l l y e s t a b l i s h e d over the course of the  breeding p e r i o d .  to gain by mating the  supplanting  males i n p r e f e r r e d  i n j u r y remain the same.  mating  strategy  may  simply  courts  season when most females have a l r e a d y  r i s k s of  male  A l t e r n a t i v e l y , males may  have l e s s  late  in  been served,  Thus, the most  the while  successful  i n v o l v e f i g h t i n g v i g o r o u s l y e a r l y in  the mating season when p o t e n t i a l f i t n e s s gains are h i g h e s t ,  and  less  are  vigorously  later  on.  The  data  from  this  study  inadequate to d i s c r i m i n a t e between these hypotheses. Lekking  males  incur  a  high  risk  consequence of i n t r a s e x u a l competition observed  several  instances  males f r e q u e n t l y a t t a c k e d engaged  in t e r r i t o r i a l  of  of  mortality  wounding, and  the undefended f l a n k s of  specific  mortality  Brock et a l . mortality sexual  (1985)  i s higher  dimorphism.  reproductive-aged conditions adult  sex  of  have  D i s t r i b u t i o n of ages at death  and  a skewed sex  recently  I argue, males may  strong  in  be higher  sexual  a c t i v i t i e s of  display  that  juvenile  that  mortality  selection.  of  T h i s may  explain  the  observed  in  1982).  i n f l u e n c e s dominance r e l a t i o n s  grounds, but males.  high  than that of females under  Clutton-Brock et a l .  neighboring  age-  Clutton-  r a t i o s skewed towards females f r e q u e n t l y  on  strong  ratio.  suggested  addition,  A g g r e s s i v e behavior not only males  that  f o r males than females i n species with  ungulates (Cowan 1950;  of  males  r e s u l t i n p r e d i c t a b l e changes i n  patterns  I  lekking  other  d i f f e r e d s i g n i f i c a n t l y between the sexes, suggesting i n t r a s e x u a l s e l e c t i o n may  a  f o r access to females.  serious  disputes.  as  i t a l s o d i s r u p t s the mating Males  that  were  courting  136  females  were  much  unattended males. directed  more  likely  to  Thus, much of  towards  be engaged in f i g h t s than  the  successfully  observed  mating  original  male  was  Females  were  males.  s c a t t e r e d immediately f o l l o w i n g d i s r u p t i v e substantial proportion  aggression  fights,  although  of females subsequently returned  partners.  Disruptive  r e d i s t r i b u t e s females onto new  to  a  their  fighting periodically  c o u r t s , which i s advantageous  to  c h a l l e n g i n g males. Such  d i s r u p t i v e behavior has  not  been p r e v i o u s l y  described  for mammalian l e k s , although i t occurs in some i n s e c t species may  (Foster  1983;  Trail  be a s u c e s s f u l s t r a t e g y  white-eared  kob  with  1985).  in  lekking  populations  r e s t r i c t e d breeding seasons.  f o r the owners of p r e f e r r e d c e n t r a l  rather than incur the  disruptive  that  behavior  in Foster  disruptive  selection  in white-eared  kob.  species  do b e t t e r  courts  to  by  weaken,  importance  the e v o l u t i o n of lek mating 1983).  behavior  Results may  from  choice  this  be more prevalent  p r e v i o u s l y assumed, p a r t i c u l a r l y under c o n d i t i o n s sexual  the  s u b s t a n t i a l r i s k s of f i g h t i n g .  systems (Wrangham 1980; suggest  like In  Recent reviews of lek behavior have examined the of  bird  I suggest that d i s r u p t i o n  that breed year-round, p e r i p h e r a l males on l e k s may waiting  and  of  study than  increased  a s s o c i a t e d with a r e s t r i c t e d mating season as  137  CHAPTER 6.  AGE-SPECIFIC MORTALITY: AN ALTERNATIVE APPROACH  I n t r o d u c t ion Age-specific models  survivorship  used by e c o l o g i s t s ( L e s l i e  G a d g i l and Bossert 1977).  Since  cohorts  1970; S c h a f f e r  over  their entire l i f e many  from samples of the p o p u l a t i o n Caughley  "static" l i f e (r)  1977).  age  Two  x  distribution.  Stearns  1976;  estimate  of  the  number  life  most  of  Despite  r a t e of increase producing  x  the  used  meet  adequately  individuals  is  , to discount  age  distribution  in a population  at a  (s  x  ,  the  relative  of age x that d i e over a s p e c i f i e d  1 year).  converted  1966).  of new-born young), or (B)  of  time i n t e r v a l , u s u a l l y  the  methods estimate age-  standing  frequencies  individuals  a  the widespread use of these  table studies  frequently  age  1970;  (Seber 1973; Ricker  the p o p u l a t i o n  d i s t r i b u t i o n of ages a t death  by e  Gulland  i n d i r e c t methods of c a l c u l a t i n g  the standing  distribution  1957;  survivorship  assumptions on which they are based (Caughley  the  For method A, the standing  t o an l  x  age  schedule by m u l t i p l y i n g s  the e f f e c t s of the p o p u l a t i o n  increase on the observed age d i s t r i b u t i o n . standing  population  spans (Lowe 1969; C o n n e l l  s p e c i f i e d time r e l a t i v e to the number  values  many  f o r s e v e r a l generations,  s p e c i f i c s u r v i v o r s h i p from (A) (s ,  1974;  age s t r u c t u r e  t a b l e s assume t h a t  methods, few p u b l i s h e d underlying  to  1945; Beverton and H o l t  studies  These  has remained constant  stable  basic  i t i s r a r e l y p o s s i b l e t o monitor the f a t e s of true  Sherman and Morton 1984),  1975;  is  For  method  x  r a t e of B,  the  d i s t r i b u t i o n of ages a t death i s l i k e w i s e converted to  138  a 6^ schedule by m u l t i p l y i n g schedule  is  then  used  to  schedule, and the r a t i o d / l x  specific  mortality  the s^  calculate is  x  rate,  .  assumption that the p o p u l a t i o n and  require  precise  values  used  by  the to  e  .  This  d  corresponding  1  estimate  the  x  age-  Both methods A and" B r e l y on the has a  stable  estimates of p o p u l a t i o n  age  distribution,  numbers over  several  generations i n order to c a l c u l a t e r . I  propose an a l t e r n a t i v e method of e s t i m a t i n g  mortality of  rates based on P i e l o u ' s  these  restrictive  currently  age-specific  (1977) approach, that  assumptions,  and  is  compare i t to methods  i n use.  The Simply put, q  x  model  v a l u e s i n d i c a t e the p r o b a b i l i t y of dying f o r  i n d i v i d u a l s i n each age c l a s s over the time i n t e r v a l from t+1.  free  Thus,  it  i s the p r o p o r t i o n  t  to  of i n d i v i d u a l s of age x that  d i e over a given time i n t e r v a l r e l a t i v e to the number  alive  the beginning of the i n t e r v a l .  (1977),  % = —  As i n d i c a t e d by P i e l o u  at  '  a.  where  c n  In  = number of age x i n d i v i d u a l s that d i e during the year, and = i n i t i a l number of age x i n d i v i d u a l s .  field  studies  it  is  rarely  possible  to  count a l l  1 39  individuals those  of known age i n a p o p u l a t i o n or even  that  estimated class  die  i n a given year.  count a l l  However, these values may be  by m u l t i p l y i n g the sampled  frequencies  f o r both l i v e and dead p o p u l a t i o n s  estimates.  to  of  by o v e r a l l  each  age  population  Note that  c = C f and ry. = N g. x  where  C = N = fx 9x  x  £c  ,  x  =• sampled p r o p o r t i o n at age x among c a r c a s s e s , and = sampled p r o p o r t i o n s at age x among l i v e i n i t i a l population.  k f.  Thus  where In  equation 1  =  Qx  k = C / N . some  distribution  field may  situations,  be  unavailable  the  initial  f o r one  However, i t i s p o s s i b l e to d e r i v e these values age  d i s t r i b u t i o n at the end of the i n t e r v a l .  h  standing  reason  age  or another.  from the standing Suppose  = sampled p r o p o r t i o n s at age x+1 among l i v e p o p u l a t i o n subsequent to annual m o r t a l i t y and N' = t o t a l p o p u l a t i o n s i z e at end of i n t e r v a l .  Then n  x  x + 1  = N'h  x + 1  + Cf  x  and equation  1 can be r e w r i t t e n as  1 40  C f'x N'  h  x+l  x  In order to c a l c u l a t e a q one  requires  an  equation 2  + C f  estimate  schedule using  x  of  annual  d i s t r i b u t i o n of ages at death, and either  at  the  m o r t a l i t y was of  and  monitored.  population  There are no  beginning  numbers  or  the  the  equations 1 or  mortality,  standing  end  the  end  of the p e r i o d an  of the o b s e r v a t i o n  f u r t h e r assumptions of a  constant r a t e of p o p u l a t i o n  standing  age d i s t r i b u t i o n  Equation 2 a l s o r e q u i r e s at  stable  2,  age  i n which estimate period.  distribution  increase.  Methods To  i n v e s t i g a t e the u t i l i t y of the proposed technique,  approaches  were  employed.  specific  mortality  published  values  Syncerus c a f f e r . simulations,  First,  derived (Sinclair  Second,  using  I  by  I compared estimates of  the  1977)  proposed for  of age  the  age-specific mortality.  three  is  following  African buffalo  violated,  African  series  of  by  to  computer  methods  T h i r d , I t e s t e d the  age-  buffalo,  to estimate  alternative  the of  accuracy  assumption of a s t a b l e  calculating  a temporary p e r t u r b a t i o n  population.  technique  data,  methods when the u n d e r l y i n g  distribution  mortality  a  buffalo  sampling v a r i a t i o n r e s u l t i n g from three estimating  the  performed  Sinclair's  three  of a  age-specific hypothetical  141  C a l c u l a t i o n of a g e - s p e c i f i c Sinclair the  mortality  (1977) d e r i v e d a l i f e  Serengeti  ecosystem  t a b l e f o r female b u f f a l o  in  from a found sample of 246 c a r c a s s e s 2  years and o l d e r .  S u r v i v a l of c a l v e s  (age  estimated independently by counts of these age  1 to 2) was  c l a s s e s per 100  (age 0 to 1) and  females from a e r i a l photographs  At the time of the study the  population  constant  ,  rate  (r = 0.077)  so  was  Sinclair  yearlings  (Sinclair  1977).  increasing calculated  at cL^  m u l t i p l y i n g the standing d i s t r i b u t i o n of ages at death by e i n c o r p o r a t e the e f f e c t of p o p u l a t i o n i n c r e a s e . schedule i s shown i n F i g .  older  over the same three year p e r i o d  s k u l l sample was the  shot  collected  sample  and  the  by to  r x  resultant  q  x  6.1.  S i n c l a i r a l s o shot a random sample of 80 and  The  a  females  2  years  (1967-1969) that the  ( S i n c l a i r unpublished).  The ages  c a r c a s s sample were determined  of from  counts of t o o t h cementum a n n u l i . The s t a n d i n g age d i s t r i b u t i o n converted to a g age  x  from the l i v e p o p u l a t i o n  d i s t r i b u t i o n by d i v i d i n g the frequency of each  group i n the shot sample by the t o t a l  sample s i z e , as shown  i n Table 6.1.  The  standing d i s t r i b u t i o n of ages  converted  an  f  to  distribution  x  by  (246).  For  subsequent  m o r t a l i t y , I smoothed both the  I  estimated q  x  death  was  the number of  total  number  of  c a l c u l a t i o n s of a g e - s p e c i f i c  and the  t a k i n g a three p o i n t running average  at  dividing  c a r c a s s e s of age x i n the found sample by the carcasses  was  g^  distributions  f o r s e q u e n t i a l age  from the r a t i o of f  x  by  classes.  over g , m u l t i p l i e d by x  the annual m o r t a l i t y r a t e of a d u l t s (0.0596) averaged  over  the  142  Figure 6.1 A f r i c a n buffalo age-specific mortality curve estimated using methods B and C. S o l i d dots indicate q  estimates obtained using method B  x  (data from S i n c l a i r 1977). Open dots indicate q^ estimated using the new method, C.  143  Table 6.1 Age d i s t r i b u t i o n data f o r female African b u f f a l o , from a found carcass sample ( S i n c l a i r 1977) and a l i v e sample ( S i n c l a i r personal communication). Age-specific mortality rate, q^, calculated according to the new method, C (*: estimated from polynomial regression of q values 2 2 for age classes 2-13; q = 0.0564 + 0.0214x + 0.00290x ; r =0.950). X  live sample  smoothed f  smoothed  age  carcass sample  2  9  5  0.037  0.075  0.029  3  9  7  0.045  0.109  0.025  4  15  14  0.046  0.154  0.018  5  10  16  0.057  0.163  0.021  6  17  9  0.052  0.141  0.022  7  11  9  0.062  0.096  0.038  8  18  5  0.072  0.084  0.051  9  24  6  0.083  0.054  0.092  10  19  2  0.099  0.038  0.155  11  29  1  0.100  0.029  0.206  12  25  4  0.100  0.025  0.238  13  19  1  0.081  0.021  0.230  14  16  0  0.065  n.a.  0.325*  15  13  0  0.049  n.a.  0.388*  16  7  0  0.032  n.a.  0.456*  17  4  1  0.016  n.a.  0.531*  18  1  0  0.004  n.a.  0.611*  X  g  x  q  x  1 44  three  year  period  of  study  standing age d i s t r i b u t i o n was sample,  equation  1  is  (Sinclair  1977).  not c o l l e c t e d p r i o r to  still  prior  to  the  rate  for  Due  to the r e l a t i v e l y  the  one  i n d i v i d u a l o l d e r than  sample,  found  several  c o l l e c t i o n p e r i o d , presumably producing a  s t a b l e age d i s t r i b u t i o n . shot  the  the  a p p r o p r i a t e because the b u f f a l o  p o p u l a t i o n had been i n c r e a s i n g at a constant years  Although  only  taken.  I t h e r e f o r e estimated q  using  a  polynomial  curve  values for fitted  to  small  age  size  of  13 years  was  classes  data  for  14-18  ages  2-13  (r =0.950). 2  Sampling The procedure  distributions methodology for  employed  simulating  survival  vectors.  executed  to  A  assess  sampling  series the  follows  of  computer  sampling  age  survivorship  mortality  ages at death; and equation  1  variation  (C)  above.  equations  rates  different  age  age-specific Demographic  were  (Caughley  of  were  age-specific methods:  (A)  distribution;  (B)  age-  estimated from the standing d i s t r i b u t i o n of  A f r i c a n b u f f a l o study were used mortality  simulations  estimated from the r a t i o of adjacent  f r e q u e n c i e s from the standing  specific  (1985)  d i s t r i b u t i o n s for specified  m o r t a l i t y estimates d e r i v e d from three age-specific  Polacheck's  data  f o r each  calculated  1977):  mortality from  Sinclair's  method.  according  estimated  to  from (1977)  Age-specific the f o l l o w i n g  1 45  method A:  q  = 1 - (s /s _  x  x  x  )  1  X-l  method B:  q  = d /l-)_d.  x  method C:  q  Sinclair's  x  i  = k f /g  x  x  x  (1977) schedules f o r 1  and  d ,  x  the  population  expected  rate  standing  distribution  of  of  increase,  age  ages  (s )  and  x  death (s' ).  distributions  numbers  were  the  random  f r e q u e n c i e s of s , s x  number  between  assigned  to  distribution, individual  0  age  x  number  , f,  created  by  be age  individuals.  to a s p e c i f i c age c l a s s  to .  the  For  cumulative  example,  0  in  the  simulated  for  each  assigned  to age c l a s s  distributions For  Age-specific  simulated  mortality  if a  random  age  age  between 0.138 and 0.230 was drawn, one  method  were C,  mortality  distribution,  repeated 400 times to generate sampling specific  by  relative  standing  1, and so on.  simulated  each  of  d i s t r i b u t i o n s were simulated f o r 500 i n d i v i d u a l s , sample of 1000.  generating  and 0.138 was drawn, one i n d i v i d u a l would be class  would  or  x  i f a number  methods A and B, sampled  "X  from a uniform d i s t r i b u t i o n between 0 and  1.0 and a s s i g n i n g each random number comparing  and a  X  were taken as the expected v a l u e s f o r method C.  Simulated age pseudorandom  standing  The smoothed f  X  distributions  for  were used to c a l c u l a t e the  distribution  at  corrected  x'  rates  for  the f giving  were  For 1000  and g  x  a total  calculated  and t h i s procedure was distributions  of  age-  r a t e s using each of the three methods. A l l  146  s i m u l a t i o n s were programmed in C Language and D i g i t a l VAX operating  11/750 computer, running  first  result  rate  (1977) l  produced  followed by a t h i r d year to normal.  produced  by  4.2  and  d  x  schedules,  x  assuming  a  Then, f o r a h y p o t h e t i c a l the  standing  2 years of i n c r e a s e d m o r t a l i t y ,  at twice (1)  numbers from 50,000  increasing  in the t h i r d year This  t h a t would  mortality  rates  the 2 years of i n c r e a s e d m o r t a l i t y ,  population  distribution.  x  distribution  i n which a g e - s p e c i f i c  During  I a r b i t r a r i l y set each q scenario  stable  of 50,000 b u f f a l o , I c a l c u l a t e d  distribution  returned  the  of increase (r=0.077).  female p o p u l a t i o n  This  UNIX  distribution  calculated  from S i n c l a i r ' s  constant  age  under a Berkeley  system.  E f f e c t s of an unstable age I  were executed on a  scenario  a  the values temporary  to  as  low  to 49,000', and would  mimic  shown i n Table  1.  decrease i n b u f f a l o as  45,000,  before  (2) an u n s t a b l e the  effects  age of  temporarily  harsh weather c o n d i t i o n s , f o r example a drought, on  an otherwise  increasing population.  methods  B, and  A,  of the t h i r d  year.  I then c a l c u l a t e d q  x  using  C for the h y p o t h e t i c a l p o p u l a t i o n at the  end  147  Results A g e - s p e c i f i c m o r t a l i t y estimates d e r i v e d (from  Sinclair  6.1.  1977)  There i s  indicating  close  that  the  and  the  methods  between method  the (C)  two  q  yields  curves,  x  comparable  r e s u l t s to the t r a d i t i o n a l method (B). The f i t between the curves  is  thereafter  close  for  age  B  new method C are shown i n F i g .  agreement proposed  using  classes  up to age 13, and  f o r o l d e r age c l a s s e s whose q  x  values  were  two  diverges estimated  using a f i t t e d polynomial curve. Fig.  6.2  the three  shows the standard  alternative  variation,  methods.  particularly  deviations Method  the lowest v a r i a t i o n at a l l ages.  to  B  age  classes  o l d e r age c l a s s e s . variability Both  Method C  methods  greatest  was  of  comparable  increased for  magnitude  greater  A and C produced sampling d i s t r i b u t i o n s that skewed and k u r t o t i c (Table 6.2). for  methods  A  and  A l l of  method  B  kurtotic.  were  In c o n t r a s t , d i s t r i b u t i o n s d e r i v e d  neither  skewed  or  kurtotic.  d i s t r i b u t i o n s d e r i v e d using method B are methods  A  comparative  demonstrated  in  were from  Thus, sampling  approximately  normal,  and C produce d i s t r i b u t i o n s t h a t , w h i l e ' b e l l -  shaped, show some departure from The  the  C were p o s i t i v e l y  71% of method C and 44% of method A d i s t r i b u t i o n s  significantly  while  the  2 through 9, but s h a r p l y  Method A had an order  distributions  skewed.  d e r i v e d by  than method B, even f o r younger age c l a s s e s .  were s i g n i f i c a n t l y sampling  had  x  f o r younger age c l a s s e s , w h i l e method B  had  for  A  for q  robustness  Fig.  6.3.  normality. of  the  three  Age-specific  methods  mortality  is  curves  148  149  Table 6.2 Skewness and kurtosis of simulated sampling d i s t r i b u t i o n s , as indicated by g^ and g^ s t a t i s t i c s (Snedecor and Cochrane 1967). Values s i g n i f i c a n t l y d i f f e r e n t from 0.0 (p'< 0.05) indicated; by.*.  Method A g  Age  <J  0  0.60*  0.52*  1  0.33*  2  1  2  Method B g^^ g  Method C 2  g  g  ±  2  0.03  -0.19  n.a.  n.a.  0.14  -0.01  -0.25  n.a.  n.a.  0.37*  0.25  -0.03  -0.06  0.31*  -0.03  3  0.41*  0.43  0.23  0.07  0.39*  -0.02  4  0.72*  0.87*  0.02  -0.38  0.31*  -0.06  5  0.71*  1.64*  0.32*  0.02  0.57*  0.15  6  0.46*  0.20  0.15  -0.05  0.60*  0.62*  7  0.22  -0.30  0.23  -0.15  0.70*  0.52*  8  0.44*  0.26  0.16  -0.33  0.48*  9  0.52*  0.39  0.10  -0.22  0.65*  0.67*  10  0.50*  0.56*  0.02  -0.06  1.42*  3.64*  11  0.56*  0.37  0.05  -0.04  1.29*  2.82*  12  0.61*  0.31  0.03  0.09  1.74*  6.08*  13  1.14*  2.05*  -0.05  0.11  2.04*  7.66*  14  0.82*  0.81*  0.02  0.01  1.50*  1.51*  15  2.81*  14.57*  -0.06  -0.17  1.26*  0.87*  -0.13  150  F i g u r e 6.3 A g e - s p e c i f i c m o r t a l i t y curves  estimated  from an u n s t a b l e age  d i s t r i b u t i o n , as d e s c r i b e d i n the t e x t , u s i n g 3 a l t e r n a t i v e methods, (assuming t h a t the f a t e s o f a l l members o f the p o p u l a t i o n a r e known, i . e . , t h e r e i s no sampling  variation).  151  estimated  u s i n g methods A or B were biased by a s l i g h t  from the u n d e r l y i n g assumption of while  method  C  was not a f f e c t e d .  6.3 i n v o l v e d a short much  like  a  the  (2 year)  presumed  stable  frequent  period  of  increased  To the degree  should  expect  distribution,  few  mortality,  e f f e c t of a temporary drought, with a Such p e r t u r b a t i o n s  i n many n a t u r a l p o p u l a t i o n s  1982).  distribution,  The s c e n a r i o producing F i g .  subsequent r e t u r n to normal c o n d i t i o n s . be  age  departure  that  such  may  (Wiens 1977, Schoener  perturbations  do  occur,  we  n a t u r a l p o p u l a t i o n s to e x h i b i t a s t a b l e age  s e r i o u s l y l i m i t i n g the u s e f u l n e s s of methods A and  B.  Di s c u s s i o n  Advantages of the proposed method Under circumstances table c a l c u l a t i o n s justified  f a m i l i a r to most f i e l d b i o l o g i s t s ,  using  conventional  because of r e s t r i c t i v e assumptions.  t h i s problem i s t o use independently standing at death. directly  are  rarely  One way to a v o i d  obtained estimates  of the  age d i s t r i b u t i o n and the standing d i s t r i b u t i o n of ages Because a g e - s p e c i f i c m o r t a l i t y r a t e s from  population  v u l n e r a b l e , there population  techniques  life  i s no  are c a l c u l a t e d  deaths r e l a t i v e t o those need  to  know  long-term  s i z e , or whether the age d i s t r i b u t i o n  potentially trends  in  i s stable.  As  152  in other an  methods of e s t i m a t i n g  underlying  assumption  representative promote  a g e - s p e c i f i c m o r t a l i t y , there  that  sampled  of the p o p u l a t i o n  the uneven s p a t i a l d i s t r i b u t i o n of c e r t a i n age  classes,  population numbers.  sample or a c a r c a s s  1  and  2  mortality, This  However,  it  is  do  expressed  often  requires  a  less  population is  numbers no  over  underlying  reached a s t a b l e age One  difficult  If  inaccurately,  the  downwards, but  of  view,  techniques  of  total  proportion  of  initial  to  obtain.  to o b t a i n a s i n g l e p r e c i s e than  to  extended  assumption  measure  precisely  time p e r i o d .  Moreover,  that  the  population  k  (total o^.  annual  curve  will  r a t i o of the  mortality) be  f  is  displaced  and  x  estimated upwards  remain the same.  or As  (1966) suggested, the shape of the q^ curve i s perhaps b a s i s f o r comparisons of a g e - s p e c i f i c  experienced by d i f f e r e n t p o p u l a t i o n s . the  proposed  that  rely  method on  fundamentally change the  is  From t h i s  preferable  underlying  shape of the  selection  to  calculation  i n t e r v a l s during  of  age-specific  periods  point  traditional  assumptions  that  can  curve.  Another b e n e f i t of the proposed technique i s that the  has  of the proposed technique i s that the shape  the shape of the curve w i l l  the most robust pressures  biased.  estimate  statistic  curve i s determined only by the  distributions.  Caughley  a  be  distribution.  advantage x  an  as  effort  an  sample may  require  estimate of t o t a l annual m o r t a l i t y  of the q  are  factors  Equations  as a whole.  frequencies If s o c i a l  then e i t h e r a shot  there  age  is  mortality  of r a p i d p o p u l a t i o n  r a t e s at  i t allows frequent  fluctuation,  which  153  is  not p o s s i b l e u s i n g c u r r e n t methods.  insights  into  changes  in  c o i n c i d e n t with p o p u l a t i o n Using determine simply  the  proposed  whether  from  a  the  annual m o r t a l i t y . age  age-specific  selection  pressures  fluctuations. method C i t should a l s o be p o s s i b l e to  population  age  Such data may y i e l d new  is  distributions  increasing and  or  decreasing  a s i n g l e estimate of  L e t us assume that data on both the  standing  d i s t r i b u t i o n and s t a n d i n g d i s t r i b u t i o n of ages at death are  collected  f o r a given p o p u l a t i o n , and a q^  u s i n g method C. calculate  q^  Then, suppose that the s^ values  curve  u s i n g method B, under the n u l l  method r=0.0  u s i n g method B l i e s below the  estimated  data i s then used to  that p o p u l a t i o n numbers have remained c o n s t a n t . generated  is  hypothesis  I f the q  curve  x  generated  curve using  C ( i . e . a g e - s p e c i f i c m o r t a l i t y under the assumption is  less  than  m o r t a l i t y ) , then t h i s Conversely,  an  unbiased  estimate  of  that  age-specific  i m p l i e s that the p o p u l a t i o n i s d e c r e a s i n g .  i f q^ v a l u e s c a l c u l a t e d under the n u l l hypothesis of  r=0.0 are g r e a t e r than the unbiased e s t i m a t e s , then t h i s i m p l i e s that the p o p u l a t i o n i s a c t u a l l y This  reasoning  remains unchanged. that  i s valid  only  i f age-specific  natality  I f n a t a l i t y r a t e s i n c r e a s e at the same  time  a g e - s p e c i f i c m o r t a l i t y r a t e s a r e i n c r e a s i n g , then by using  the above procedure  one might f a l s e l y conclude  was d e c r e a s i n g when, i n f a c t , increasing. If  increasing.  Such  that a population  i t was remaining constant or  situations,  however, may be r a r e i n nature.  increased age-specific mortality rates result  shortage of resources  even  (e.g. d u r i n g a drought  from a r e l a t i v e  or as a  result  of  1 54  large-scale should  habitat  similarly  changes)  cause  a  then  t h i s process, i f anything,  decrease  in  natality.  The  only  r e a l i s t i c circumstances under which such a s c e n a r i o seems l i k e l y is  when  m o r t a l i t y changes are due  the outbreak  of  an  epidemic  to an unusual event, such as  or  introduction  c o i n c i d e n t a l with an i n c r e a s e i n n a t a l i t y  of  predators,  rates.  Comparisons between methods The  s i m u l a t e d sampling d i s t r i b u t i o n s allow us to judge the  r e l a t i v e p r e c i s i o n of the three procedures f o r specific  mortality.  Method B, based on the d  d i r e c t l y from the standing d i s t r i b u t i o n of less  subject  unrealistic  or l e s s than 0). method  Strictly  schedule d e r i v e d  x  ages  at  death,  is  Moreover, using method B i t i s i m p o s s i b l e to  generate b i o l o g i c a l l y  0 and  age-  to sampling v a r i a t i o n than e i t h e r method A or the  proposed method C.  1.0  estimating  C  values  ( i . e . greater  Method A can generate q  can  generate  q^  values  x  values less  greater  than  than than 1.0.  on the b a s i s of sampling v a r i a t i o n , method B should be  p r e f e r r e d to method C, which i s i n turn p r e f e r a b l e to method A. Method C i s s i m i l a r age are  classes,  i n p r e c i s i o n to Method  imprecise  estimation  o l d e r age c l a s s e s u s i n g method C may in  most  o l d e r age groups groups,  so  for  younger  but i s l e s s p r e c i s e f o r o l d e r age c l a s s e s .  s e v e r a l reasons why  First,  B  populations is  much  less  the  not cause  of  survival  serious  There for  problems.  number of i n d i v i d u a l s i n the  than  numbers  in  younger  age  that sampling e r r o r s would be small r e l a t i v e t o the  1 55  whole p o p u l a t i o n . no longer their  r e p r o d u c t i v e l y a c t i v e , so i n a c c u r a c i e s  frequencies  models. with  Second, in many s p e c i e s o l d e r i n d i v i d u a l s are  Third, i f q  precision,  discount classes  will  have  values  x  little  (e.g. Chapman 1964,  Sampling  variation  becomes  c l a s s e s using a l l three methods. mortality  is  derived  may  of  the  used  This is  because  frequency  variation  (Fig.  is,  the  6.4).  As  older  age  The  f o r methods A,  greater  age  age-specific  f r e q u e n c i e s , that are  i n c r e a s i n g l y smaller as a f u n c t i o n of age. expected  for  more pronounced f o r o l d e r  from r a t i o s of two  equations  known  1977).  themselves subject to sampling v a r i a t i o n . each  are  be employed to  variation  Caughley 1966,  population  classes  procedures  the e f f e c t of high sampling  estimating  bias e f f e c t on  f o r younger age  curve-fitting  in  denominator  B, and C, becomes  The  the  in  smaller that  effect  of  the  random  a consequence, sampling v a r i a t i o n  i n c r e a s e s f o r o l d e r age c l a s s e s . Methods A and distribution  has  i s not a f f e c t e d .  B produce i n a c c u r a t e q  x  curves  when a  stable  not been reached, while the proposed method C T h i s problem w i l l  be most  serious  for  long-  l i v e d s p e c i e s , s i n c e i t would take many years at a constant of  population  increase  to  produce a s t a b l e age  Thus, method C i s more robust  q^  p r e c i s e procedure a v a i l a b l e .  assumptions of a s t a b l e age population  increase are not met,  B),  However, when the  d i s t r i b u t i o n and  B.  estimation  the standing d i s t r i b u t i o n of ages at death (method most  distribution.  than e i t h e r methods A or  When the u n d e r l y i n g assumptions are met,  rate  constant  is  from the  underlying rate  of  method C o f f e r s an a l t e r n a t i v e  156  0.47-  0.33H  0.19^  cr*  A  0.05-  •-  0.15  0.09  0.03  c o  A- A A A AAA  B 0.08-1  CO  >  CO "D  I 0.04-  CO "D C CO  0.00-  CO  0.60  0.20  1.00  0.42H  6.28-  0.1  4-  0.00-  A A a A_0 . 1 2  0.04  0.20  denominator  Figure 6.4 Sampling v a r i a t i o n of q value i n the equations used.  x  estimates as a function of denominator  approach with a t o l e r a b l e  loss in p r e c i s i o n .  1 58  CHAPTER 7.  GENERAL DISCUSSION  Food  limitation  S e v e r a l f a c t o r s are known to (1)  food  1979; (2)  availability  Fowler  1981;  predation  Sinclair  1977;  limited  at  and  Berry  different  by  rinderpest  early  1960's  1979).  A  (Sinclair  temporary  and  by  1982), 1980;  ( C h r i s t i a n et a l .  different  1960;  f a c t o r s (May  be  1977;  studies indicate  buffalo populations  1977;  period  (Sinclair  S i n c l a i r et a l . that  Houston  Caughley et a l .  Sinclair of  and  increase  these p o p u l a t i o n s are now  food a v a i l a b i l i t y  reported  McCullough  were  limited  p r i o r to a s u c c e s s f u l e r a d i c a t i o n program in the  e r a d i c a t i o n , and  1982;  1977;  1982;  For example, long-term  that Serengeti wildebeest  populations:  Moreover, a given p o p u l a t i o n may  times  1979).  Sinclair  1977;  (3) d i s e a s e  1981).  ungulate  et a l .  Karns  1983), and  Peterman et a l .  1977;  Clutton-Brock  (Mech  Gasaway et a l .  (Bobek  limit  some  1977;  1985). Alaskan  Norton-Griffiths  followed rinderpest apparently  S i n c l a i r and  limited  by  Norton-Griffiths  S i m i l a r l y , Gasaway et a l .  (1983)  moose p o p u l a t i o n s are r e g u l a t e d at  low p o p u l a t i o n d e n s i t i e s by wolf  predation, following  of  prey p o p u l a t i o n numbers.  harsh winters that depressed  r e s u l t s i n d i c a t e that m u l t i p l e e q u i l i b r i a may  a  be more  series These  prevalent  than p r e v i o u s l y assumed. At  the  outset  v a r i a t i o n suggested  of  this  study, extreme seasonal  that the white-eared  climatic  kob p o p u l a t i o n might be  l i m i t e d by food a v a i l a b i l i t y d u r i n g the dry  season  period.  I  159  tested  t h i s hypothesis  by attempting  to f a l s i f y a number of i t s  predictions.  R e s u l t s i n d i c a t e d that kob  dry  when  season,  food  intake  c o n s i d e r a b l y g r e a t e r than at reserves  (a  was  mortality  below  other  times  during  the  requirements, of  the  was  year.  Fat  measure  of  body c o n d i t i o n ) d e c l i n e d d r a m a t i c a l l y  d u r i n g the dry season  at  the  increasing.  Finally,  same  unusual  time  that  rainfall  mortality  during  the  season, which produced i n c r e a s e d food a v a i l a b i l i t y , decreased  adult mortality.  i s no reason  to r e j e c t the  On  Food a v a i l a b i l i t y d u r i n g environmental  conditions  and  of  specific  time,  increased  competition  p o p u l a t i o n numbers. considerable this  study  sensitive conditions. should  year suggest to  the  for  food  most  of  serve  savannah  adult  sub-maintenance  from year  herbivore to  year.  from  intra-  ecosystems  mortality  If  extended  to r e g u l a t e  in r a i n f a l l .  T h i s p r e d i c t s that savannah  fluctuate considerably  for an  resulting  should  total  duration  in  there  herbivore population density.  to year v a r i a t i o n that  resulted  the dry season i s a f f e c t e d by both  mortality  However,  dry  hypothesis.  environmental c o n d i t i o n s were to remain constant period  1982  the b a s i s of t h i s evidence,  food  was  kob show  Results  from  is  highly  dry  season  populations  1 60  Seasonal m i g r a t i o n Two  hypotheses  have been advanced  to e x p l a i n the a d a p t i v e  f u n c t i o n of m i g r a t i o n by l a r g e h e r b i v o r e s .  The  first  suggests that h e r b i v o r e s migrate i n order to take ephemeral  distributions  Maddock 1979). that  food  supplies  The second h y p o t h e s i s  herbivores  unable  requirements  from forage  periods  permanent  to  of  to are  water  1975)  their  obliged  to  supplies.  advantage  (Pennycuick  (Western  meet  hypothesis  1975;  suggests  metabolic  migrate During  of  water  during wet  dry  periods,  h e r b i v o r e s d i s p e r s e i n t o surrounding areas to forage at  will.  M i g r a t i o n p a t t e r n s of white-eared kob were c o n s i s t e n t both  hypotheses,  since  during  the  dry  season  with  water  was  r e s t r i c t e d to the same areas that o f f e r e d the g r e a t e s t abundance of  green f o r a g e .  Like many problems in  ecology  the examination of d i s t r i b u t i o n p a t t e r n s was the  scale  of  observation.  Data  (Krebs  1985),  greatly affected  from a e r i a l surveys  by  indicated  p o o r l y that kob dry season movements t r a c k e d the a v a i l a b i l i t y of food and water s u p p l i e s , while ground relationship  more  clearly.  Choice  o b s e r v a t i o n c o u l d e a s i l y lead one the resource a c q u i s i t i o n In to  of  the  to f a l s e l y  wrong reject  resources  animals i n v o l v e d . herbivores,  due  Furthermore,  (McNaughton  in  Not to  1984),  the as  this  scale  of  either  of  hypotheses.  a d d i t i o n to the problems of s c a l e , i t i s o f t e n  measure  1971).  surveys demonstrated  terms  that  a l l vegetation  are may  difficult  meaningful be  f o r the  available  e f f e c t s of v e g e t a t i o n s t r u c t u r e shown  continual  in  this  cropping  study  and  to  (Bell  elsewhere  i s a common f e a t u r e of  161  many grazing ecosystems. abundance  are  not  Thus,  simple  necessarily  measures  of  resource  indices  of  resource  valid  availability. Many s t u d i e s have concluded shifting  resource  that  distributions  animal  migration  (Dingle 1980; S i n c l a i r  However, most s t u d i e s ( i n c l u d i n g t h i s one) have indirect,  correlative  evidence.  e n t i r e l y d i f f e r e n t primary or  reducing  parasite  population  remain t r a n s i e n t . secondarily  under  periodic  track  movement  resources.  strong  selective  I t should then be of s e l e c t i v e  "choose"  a  1983).  based  on  Animal m i g r a t i o n may have an  by  h a b i t a t s , and only s e c o n d a r i l y hypothetical  been  f u n c t i o n , such as a v o i d i n g  loads  tracks  migratidn  route  that  study  and  predators into  new  Consider  an  pressure  to  advantage  to  also  tracks  resources. For example, r e s u l t s Africa  (Pennycuick  t r a c k s resources. season, no  from  1975;  this  Western 1975) suggest  This p a t t e r n i s most  when resources a r e s c a r c e .  convincing  elsewhere  explanation  during  the  wet  why  apparent  evidence  can not be r e g u l a t e d unable  that by  these  same  that migration in  1979).  move  season, s i n c e the d r y season ranges  migratory predators,  ungulates since  There i s some i n the Serengeti  the  predators  to f o l l o w the herds because of the need t o feed  Bygott  the d r y  populations  young during a long p e r i o d of dependency ( S c h a l l e r and  in  However, t h e r e i s c u r r e n t l y  a l s o have abundant forage d u r i n g the wet season. supporting  elsewhere  are  immobile  1972;  Hanby  Thus, m i g r a t i o n may have e v o l v e d p r i m a r i l y as  an a n t i - p r e d a t o r a d a p t a t i o n , with a secondary s e l e c t i o n  pressure  1 62  to  cue movements to resource d i s t r i b u t i o n .  ecosystems other s p e c i e s (and sometimes persist  without  migration,  of scarce resources  production abundant study  i n these same  population  sub-groups)  i t i s c l e a r that m i g r a t i o n  i n search  i s not o b l i g a t o r y f o r s u r v i v a l .  Breeding Mammals  Since  living  in  phenology  seasonal  environments  should  of young to the p e r i o d of the year when food i s most  (Sadleir  1969;  Sinclair  1983a).  Results  are c o n s i s t e n t with t h i s h y p o t h e s i s ,  from  However, white-eared when  kob continued  food  limitation  and  females or abundant  had  males  early  lower in in  was  in  most  to  pronounced.  considerable  the  the  population.  wet  season,  Since  early  replenishing (Sinclair  prior  birth  to  (Sinclair selection  also  i t i s p u z z l i n g that c a l f  produce  young  wet season because of an o b l i g a t o r y time l a g f o r  1983a). has  was  earlier.  f a t reserves  phenology  a  nutritional  food  i s p o s s i b l e that females are not able to  the  As  f a t reserves than e i t h e r n o n - l a c t a t i n g  p r o d u c t i o n d i d not occur It  highest.  t o l a c t a t e throughout the dry  consequence, females were exposed stress,  this  s i n c e kob gave b i r t h  d u r i n g the l a t e wet season, when food a v a i l a b i l i t y was  season,  time  T h i s hypothesis  evolved  1983a).  following  rather  the  onset  of  the  i m p l i e s that kob r e p r o d u c t i v e  to ensure adequate female body c o n d i t i o n than  food  intake  during  lactation  A l t e r n a t i v e l y , b i r t h t i m i n g may r e f l e c t  pressures.  rains  Kob  calves  northward m i g r a t i o n to the dry season  other  are born l a r g e l y during the range.  I  propose  that  1 63  calving  at  this  time  may  serve as an a n t i - p r e d a t o r s t r a t e g y ,  s i n c e there are few predators present d e l a y i n g c a l f p r o d u c t i o n u n t i l the  i n the northern a r e a s .  late  a v o i d p r e d a t i o n on v u l n e r a b l e young and  wet  season,  females  i n the  By  kob  may  immediate  post-calving period.  Breeding The  white-eared  1981), i n which specialized mates.  synchrony and male aggression kob  males  display  higher  that  grounds,  male  consistent  Uganda with  more a g g r e s s i v e season)  compete  from  for  kob.  on  choose t h e i r  synchrony  hypothesis  sexual dimorphism should  Results  from  kob  this  p r e d i c t i o n s : white-eared  (particularly  position  which females  (1977) breeding  (Bradbury  be  than  i n the  study  were  kob males were  in the e a r l y p a r t of the  breeding  and e x h i b i t pronounced c o l o r dimorphism' r e l a t i v e to the kob.  addition,  aggression.  dominance  I  Previous  that lek a g g r e s s i o n  considered studies  i n ungulates  relations  among  lek  other s p e c i e s (Foster 1983) also  mating system  synchronous white-eared  these  c o n s p e c i f i c Uganda In  lek  aggression and  i n the moderately  asynchronous  a  aggressively  Emlen and Oring's  predicts  has  disrupt  R e s u l t s from  the this  conflicts  occurred  in  presumably  concerned  with  consequences  serves p r i m a r i l y  establish  to  males, but recent evidence  showed  male  suggested  that  activities  the  of  (Buechner 1961a) have  suggests  breeding study  the  that  absence  establishing  male of the of  aggression  for may  neighboring males. majority females,  dominance  of  male  and  were  relations.  1 64  However, males fought more f r e q u e n t l y when females were present, causing  a r e d i s t r i b u t i o n of females on the l e k , i n d i c a t i n g that  disruption related  i s an  important  leks.  Fighting  to i n c r e a s e d m o r t a l i t y of young a d u l t males,  that competition the kob  f e a t u r e in kob  was  suggesting  f o r mates causes the skewed a d u l t sex r a t i o  in  population.  Age-specific m o r t a l i t y patterns Most  conventional  methods  of  estimating  m o r t a l i t y r a t e s r e l y on r e s t r i c t i v e assumptions rate  of  population  i n c r e a s e and  p r a c t i c e , these assumptions  are  l i m i t i n g the u s e f u l n e s s of these  age-specific  of  a  constant  a s t a b l e age  distribution.  seldom  (Caughley  met  In  1966),  approaches.  I d e v i s e d an a l t e r n a t i v e method for e s t i m a t i n g a g e - s p e c i f i c mortality and  age  naturally  rates  distributions occurring  assumptions. for  the  proposed simulation The  based  The  on the annual p o p u l a t i o n m o r t a l i t y r a t e from  both  deaths  method was  that  live  i s f r e e of these  buffalo  (Sinclair  to  conventional  1977).  methods  I  and  restrictive  compared  using  Monte  data the Carlo  techniques. advantages  of the proposed method a r e :  robust than c o n v e n t i o n a l methods because i t has u n d e r l y i n g assumptions, intervals  population  i l l u s t r a t e d using demographic  African two  the  to  circumstances  (2)  fluctuating i t may  it  can  be  populations,  be used to estimate  i n c r e a s i n g or d e c r e a s i n g .  The  (1) i t i s more  less  applied and  (3)  restrictive at  frequent  under  some  whether a p o p u l a t i o n i s  proposed method i s , however, l e s s  165  precise  than  calculating  distribution  of  values  carcasses,  directly  provided  from  that  the  the age underlying  assumptions a r e adequately met.  General Evidence  from t h i s study  conclusions suggests  that the Boma  kob p o p u l a t i o n i s l i m i t e d by the a v a i l a b i l i t y during  the  adaptations migration  dry  season.  i n response allows  White-eared  to t h i s  pressure.  these  refuge  scarce  late  areas  limitation  wet  Seasonal green  elsewhere  in  Year-round r e s i d e n c y i s precluded by f l o o d i n g of during  the wet  season.  p r o d u c t i o n of young to c o i n c i d e with peak the  forage  refuge areas that supply  forage and water when these resources are the ecosystem.  green  kob e x h i b i t a number of  selection  kob to u t i l i z e  of  white-eared  season.  explains  Thus,  several  Kob  forage  time  abundance  adaptation t o seasonal important  aspects  the  of  in  resource kob  life  history. These  findings  suggest  that  other  savannah-dwelling  h e r b i v o r e s may be l i m i t e d s i m i l a r l y by food abundance.  However,  there a r e 2 f e a t u r e s of the Boma  probably  uncommon  ecosystem  that  i n most other savannah ecosystems.  dry season adequate forage i s r e s t r i c t e d to a proportion  of  the Boma ecosystem.  are  F i r s t , d u r i n g the relatively  small  Kob p o p u l a t i o n d e n s i t i e s i n  the dry season range commonly exceed 1000 i n d i v i d u a l s / k m . 2  few other ungulate  p o p u l a t i o n s occur  even  reserves.  in  natural  at  such  high  Very  densities,  Second, predators are rare i n the  Boma ecosystem, u n l i k e most other savannah ecosystems i n A f r i c a .  166  Thus, in many other  savannah ecosystems p r e d a t i o n  may  exert  a  much g r e a t e r impact on h e r b i v o r e numbers. In the course work  became  of t h i s study,  apparent.  s e v e r a l u s e f u l l i n e s of f u t u r e  First,  although  food  inadequacy  was  u l t i m a t e l y r e s p o n s i b l e for much of the dry season m o r t a l i t y , the proximate cause of death in many cases was related  disease.  There  has  and domesticated  work  has  been  animals  done  on  nutrition-  been a good d e a l of r e s e a r c h  the r e l a t i o n s h i p between n u t r i t i o n and man  probably  r e s i s t a n c e to d i s e a s e  (Scrimshaw et a l .  natural  ungulate  1968)  but  Second, populations However,  we  Caughley have  similar  know l i t t l e  has  very  young  Chapter impact  6, of  information  it  about how  Using  population  be  of  new  technique  possible on  curves.  age-specific mortality rates  rapid  the  be  perturbations would  mortality  change.  very o l d kob were probably  should  given  that many v e r t e b r a t e  I s p e c u l a t e d that d u r i n g the  and  than young a d u l t s .  shown  age-specific  are a f f e c t e d d u r i n g periods of  both  a  Such  respond to changes in food abundance.  (1966)  example, in t h i s study  in  little  populations.  r e s e a r c h might provide a u s e f u l means of p r e d i c t i n g how herbivore population w i l l  into  I  1980  particular  age use  drought  more a f f e c t e d outlined  to determine the  specific  in  relative  groups. in  For  This  age-structured  p o p u l a t i o n models. T h i r d , although  n a t u r a l predators  r a r e , hunting pressure may i n f e r r e d that hunting related  mortality  i n the Boma ecosystem are  be of much g r e a t e r  i s probably  significance.  l e s s important  in the Boma kob,  than  I  nutrition-  but t h i s a s s e r t i o n needs to  167  be t e s t e d . have  There have been few s t u d i e s  to  my  knowledge  examined man as a p r e d a t o r , but t h i s information  particular  importance i n the many ecosystems, l i k e  harvesting  by man i s e s s e n t i a l l y  unrestricted.  that  may be of  Boma,  where  1 68  LITERATURE CITED  Anderson, A.B. 1949. Seasonal Sport. J_: 1 6- 1 8 .  game m i g r a t i o n . Sudan W i l d l i f e and  B a y l i s s , P. 1985. The p o p u l a t i o n dynamics of red and western grey kangaroos i n a r i d New South Wales, A u s t r a l i a . I . P o p u l a t i o n trends and r a i n f a l l . J . Anim. E c o l . 54:111-125. B e l l , R.H.V. 1971. A g r a z i n g ecosystem i n the S e r e n g e t i . S c i . Am. 224:86-93. Berry, H.H. 1981. Abnormal l e v e l s of disease and p r e d a t i o n as l i m i t i n g f a c t o r s f o r wildebeest i n the Etosha N a t i o n a l Park. Madoqua 12:242-253. Beverton, R.J.H. and S.J. H o l t . 1957. On the dynamics of exploited f i s h populations. F i s h e r i e s Investigations, London, S e r . 2. Bobek, B. 1977. Summer food as the f a c t o r l i m i t i n g roe deer p o p u l a t i o n s i z e . Nature 268:47-49. B o r g i a , G. 1979. Sexual s e l e c t i o n and the e v o l u t i o n of mating systems. I n : Sexual s e l e c t i o n and competition i n i n s e c t s (Blum, M.S. and N.A. Blum, e d s . ) . p. 19-80. Academic Press, N.Y. Bradbury, J.W. 1981. The e v o l u t i o n of l e k s . I n : N a t u r a l s e l e c t i o n and s o c i a l behavior: recent r e s e a r c h and new theory (Alexander, R.D. and D.W. T i n k l e , e d s . ) . p. 138169. C h i r o n Press, N.Y. Bradbury, J.W. and R.M. Gibson. 1983. Leks and mate c h o i c e . I n : Mate c h o i c e (Bateson, P., ed.). p. 109-138. Cambridge Univ. P r e s s , Cambridge. Bredon, R.M. and J . Wilson. 1963. The chemical composition and n u t r i t i v e value of grasses from semi-arid areas of Karamoja, as r e l a t e d to ecology and types of s o i l . E. A f r . a g r i c . F o r . J . 29:134-142.  1 69  Buechner, H.K. 1961a. T e r r i t o r i a l Science 133:698-699. Buechner, H.K. 1961b. U n i l a t e r a l Nature 190:738-739.  behavior i n Uganda kob.  implantation  i n the Uganda kob.  Buechner, H.K. and R. S c h l o e t h . 1965. Ceremonial mating behavior in Uganda kob (Adenota kob thomasi Neumann). Z. T i e r p s y c h o l . 22:209-225. Buechner, H.K., J.A. Morrison and W. L e u t h o l d . 1966. Reproduction i n Uganda kob w i t h s p e c i a l r e f e r e n c e to b e h a v i o r . I n : Comparative b i o l o g y of r e p r o d u c t i o n i n mammals (I.W.Rowlands, e d ) . p. 69-88. Sym. Z o o l . Soc. Lon. Academic Press, London. Buechner, H.K., L e u t h o l d , W. and Roth, H.D. (Unpublished MS) Lek t e r r i t o r y occupancy i n male Uganda kob a n t e l o p e . In: Bradbury and Gibson (1983). Buechner, H.K. and F.B. G o l l e y . 1967. P r e l i m i n a r y e s t i m a t i o n of energy flow i n Uganda kob. In:. Secondary p r o d u c t i v i t y of t e r r e s t r i a l ecosystems (K. P e t r u s e w i c z , e d . ) . p. 243-254. P o l . Acad. S c i . I n s t . E c o l . Buechner, H.K. 1974. I m p l i c t i o n s of s o c i a l behavior i n the management of Uganda kob. In: The behaviour of ungulates and i t s r e l a t i o n to management ( G e i s t , V. and F. Walther, e d s . ) . p. 853-870. V o l . 2. I.U.C.N. Morges, S w i t z . Buechner, H.K. and Roth, H.D. 1974. The l e k system i n Uganda kob a n t e l o p e . Amer. Z o o l . 14:145-162. B u n n e l l , F.L. 1982. The lambing p e r i o d of mountain sheep: s y n t h e s i s , hypotheses, and t e s t s . Can. J . Z o o l . 60:1-14. Caughley, G. 1966. M o r t a l i t y p a t t e r n s i n mammals. Ecology. 47: 906-918. Caughley, G. 1970. E r u p t i o n of ungulate p o p u l a t i o n s with emphasis on Himalayan thar i n New Zealand. Ecology 51:5372.  1 70  Caughley, G, 1977. A n a l y s i s of v e r t e b r a t e p o p u l a t i o n s . John Wiley and Sons. N.Y. Caughley, G., G.C. G r i g g , J . Caughley, and G.J.E. H i l l . 1980. Does dingo p r e d a t i o n c o n t r o l the d e n s i t i e s of kangaroos and emus? Aust. W i l d l . Res. 7:1-12. Caughley, G. and J.H. Lawton. 1981. P l a n t - h e r b i v o r e systems. I n : T h e o r e t i c a l ecology: p r i n c i p l e s and a p p l i c a t i o n s (May, R.M., e d . ) . p. 132-166. B l a c k w e l l , Oxford. Cave, F.O. and A. Cruickshank. 1940. A note on game m i g r a t i o n i n the s o u t h e a s t e r n Sudan. Sudan Notes and Records 23:341344. Chalmers, M.I. 1961. P r o t e i n s y n t h e s i s i n the rumen. I n : D i g e s t i v e p h y s i o l o g y and n u t r i t i o n i n the ruminant (Lewis, D., e d . ) . p. 205-222. Butterworth, London. Chapman, D.G. 1964. A c r i t i c a l study of the P r i b i l o f f u r s e a l p o p u l a t i o n e s t i m a t e s . F i s h e r i e s B u l l e t i n 63:657-669. C h i l d , G. 1972. O b s e r v a t i o n s on a wildebeest d i e o f f A r n o l d i a 5:1-13.  i n Botswana.  C h r i s t i a n , J . J . , V. F l y g e r , and D.E. D a v i s . 1960. F a c t o r s i n the mass m o r t a l i t y of a herd of S i k a deer, Cervus nippon . Chesapeake S c i . U79-95. C l a r k e , C.M.H. and R.J. Henderson. 1981. N a t u r a l r e g u l a t i o n of a non-hunted chamois p o p u l a t i o n . N.Z. J . E c o l . 4:126-127. C l u t t o n - B r o c k , T.H., S.D. Albon and F.E. Guiness. 1985. P a r e n t a l investment and sex d i f f e r e n c e s i n j u v e n i l e m o r t a l i t y i n b i r d s and mammals. Nature 313:131-133. C l u t t o n - B r o c k , T.H., F.E. Guiness, and S.D. Albon. 1982. Red deer: behavior and ecology of two sexes. Edinburgh Univ. Press, Edinburgh.  171  Coe,  M.J., D.-H. Cummings, and J . P h i l l i p s o n . 1976. Biomass and production of l a r g e A f r i c a n h e r b i v o r e s i n r e l a t i o n t o r a i n f a l l and primary p r o d u c t i o n . Oecologia 22:341-354.  C o n n e l l , J.H. 1970. A predator-prey system i n the marine i n t e r t i d a l r e g i o n . I . Balanus qlandula and s e v e r a l predatory species of T h a i s . E c o l o g i c a l Monographs 40:4978. C o r f i e l d , T.F. 1975. Elephant m o r t a l i t y i n Tsavo N a t i o n a l Park, Kenya. E. A f r . W i l d l . J . jM:339-368. Cowan, I.McT. 1950. Some v i t a l s t a t i s t i c s of b i g game on overstocked mountain range. Trans. N. Am. W i l d l . Conf. 15: 581-588. Deevey, E.S. 1947. L i f e t a b l e s f o r n a t u r a l p o p u l a t i o n s of animals. Quart. Rev. B i o l . 22:283-314. Delaney, M.J. and D.C.D. Happold. 1979. Ecology mammals. Longman, London.  of A f r i c a n  Demment, M.W. and P.J. Van Soest. 1985. A n u t r i t i o n a l e x p l a n a t i o n f o r body-size p a t t e r n s of ruminant and nonruminant h e r b i v o r e s . Am. Nat. 125:641-672. D i n g l e , H. 1980. Ecology and e v o l u t i o n of m i g r a t i o n . I n : Animal m i g r a t i o n , o r i e n t a t i o n , and n a v i g a t i o n (Gauthreaux, S.A., ed.). p. 1-101. Academic Press, London. Dobzhansky, T. 1950. E v o l u t i o n i n the t r o p i c s . Amer. S c i . 38: 209-221. Duncan, P. 1975. Topi and t h e i r of N a i r o b i .  food supply. PhD. t h e s i s . Univ.  Egan, H.R. 1977. N u t r i t i o n a l s t a t u s and intake r e g u l a t i o n i n sheep. V I I I . R e l a t i o n s h i p s between the v o l u n t a r y intake of herbage by sheep and the protein/energy r a t i o i n the d i g e s t i o n products. A u s t r . J . A g r i c . Res. 28:907-915.  1 72  Emlen, S.T. and L.W. O r i n g . 1977. E c o l o g y , sexual s e l e c t i o n , and the e v o l u t i o n of mating systems. Science 197:215-223. F i n c h , V.A. 1972. Energy exchanges w i t h the environment of two East A f r i c a n a n t e l o p e s , the e l a n d and the h a r t e b e e s t . I n : Comparitive physiology of d e s e r t animals (Maloiy, G.M.O., ed.). Sym. Z o o l . Soc. Lon. 31:315-326. F i n c h , V.A. and D. Western. 1977. C a t t l e c o l o u r s i n p a s t o r a l herds: n a t u r a l s e l e c t i o n or s o c i a l p r e f e r e n c e . Ecology 58: 1384-1392. F i s h e r , R.A. 1958. The g e n e t i c a l theory of n a t u r a l s e l e c t i o n (2nd e d i t i o n ) . Dover P u b l i c a t i o n s , N.Y. Floody, O.R. and A.P. A r n o l d . 1975. Uganda kob ( Adenota kob thomasi ) : t e r r i t o r i a l i t y and the s p a t i a l d i s t r i b u t i o n s of sexual and a g o n i s t i c b e h a v i o r s a t a t e r r i t o r i a l ground. Z. T i e r p s y c h o l . 37:192-212. F o s t e r , M.S. 1983. D i s r u p t i o n , d i s p e r s i o n , and dominance i n l e k breeding b i r d s . Am. Nat. 122:53-72. Fowler, C.W. 1981. Density dependence as r e l a t e d to l i f e s t r a t e g y . Ecology 62:602-610.  history  Fowler, C.W. and T. Smith. 1973. C h a r a c t e r i z i n g s t a b l e p o p u l a t i o n s : an a p p l i c a t i o n t o the A f r i c a n elephant p o p u l a t i o n . J . W i l d l . Mgmt. 37:513-523. G a d g i l , M. and W.H. Bossert. 1970. L i f e h i s t o r i c a l of n a t u r a l s e l e c t i o n . Am. Nat. 104:1-24.  consequences  Gambell, R. 1975. V a r i a t i o n s i n r e p r o d u c t i o n parameters a s s o c i a t e d with whale stock s i z e s . I n t . Whaling Comm., Rep. 25:182-189 Gasaway, W.C, R.O. Stephenson, J . L . D a v i s , P.E.K. Shepherd, and O.E. B u r r i s . 1983. I n t e r r e l a t i o n s h i p s between wolves, prey, and man i n i n t e r i o r A l a s k a . W i l d l . Monogr. 84:1-50.  1 73  G e i s t , V. 1971. Mountain sheep: a study i n behavior e v o l u t i o n . Univ. of Chicago Press. Chicago.  and  G e i s t , V. 1974. On the r e l a t i o n s h i p of s o c i a l e v o l u t i o n and ecology in ungulates. Amer. Z o o l . 44:205-220. G r i f f i t h s , J.F. 1972. Wet and dry t r o p i c s . In: Climates of A f r i c a ( G r i f f i t h s , J.F., e d . ) . p. 221-258. E l s e v i e r Pub. Co., Amsterdam. Gross,  J.E. 1969. Optimal y i e l d i n deer and elk p o p u l a t i o n s . Trans. N. Am. W i l d l . Conf. 34:372-386.  Grubb, P. 1974. P o p u l a t i o n dynamics of the Soay sheep. In: I s l a n d s u r v i v o r s : the ecology of the soay sheep of St. K i l d a ( J e w e l l , P.A., C. M i l n e r , and J.M. Boyd, e d s . ) . p. 242-272. Athlone P r e s s . A t l a n t i c Highlands, N.J. G u l l a n d , J.A. 1977. The a n a l y s i s of data and development of models. In: F i s h p o p u l a t i o n dynamics (Gulland, J.A., ed.). John Wiley and Sons, London, UK. H a i r s t o n , N.G., F.E. Smith, and L.B. Slobodkin. 1960. Community s t r u c t u r e , p o p u l a t i o n c o n t r o l , and c o m p e t i t i o n . Am. Nat. 94:421-425. Hanby, J.P. and J.D. Bygott. 1979. P o p u l a t i o n changes i n l i o n s and other p r e d a t o r s . In: S e r e n g e t i : dynamics of an ecosystem ( S i n c l a i r , A.R.E. and M. N o r t o n - G r i f f i t h s , e d s . ) . Univ, of Chicago P r e s s , Chicago. Hanks, J . 1981. C h a r a c t e r i z a t i o n of p o p u l a t i o n c o n d i t i o n . In: Dynamics of l a r g e mammal p o p u l a t i o n s (Fowler, C.W. and T.D. Smith, e d s . ) . p. 47-74. John Wiley and Sons. N.Y. H i l l m a n , J.C. and A.K.K. H i l l m a n . 1977. M o r t a l i t y of w i l d l i f e i n N a i r o b i N a t i o n a l Park d u r i n g the drought of 1973-1974. E. A f r . W i l d l . J . 15:1-18. Hickey, F. 1963. Sheep m o r t a l i t y i n New A g r i c u l t u r a l i s t . 15:1-3.  Zealand.  New  Zealand  1 74  Houston, D.B. 1982. The northern Yellowstone e l k : ecology and management. Macmillan. N.Y. Jarman, P.J. 1974 The s o c i a l o r g a n i s a t i o n of antelopes i n r e l a t i o n t o t h e i r ecology. Behaviour 48:215-267. Jarman, P.J. and A.R.E. S i n c l a i r . 1979. Feeding s t r a t e g y and the p a t t e r n of resource p a r t i t i o n i n g i n ungulates. I n : S e r e n g e t i : dynamics of an ecosystem ( S i n c l a i r , A.R.E. and M. N o r t o n - G r i f f i t h s , e d s . ) . p. 130-163. Univ. of Chicago Press, Chicago. Johnson, R.R., B.A. Dehority, K.E. McClure, and J . L . Parsons. 1964. A comparison of i n v i t r o fermentation and chemical s o l u b i l i t y methods i n e s t i m a t i n g forage n u t r i t i v e v a l u e . J . Anim. S c i . 23:1124-1128. J o l l y , G.M. 1969. Sampling methods f o r a e r i a l census of w i l d l i f e p o p u l a t i o n s . E. A f r . a g r i c . F o r . J . 34:46-49. Keep, M.E. 1973. F a c t o r s c o n t r i b u t i n g t o a p o p u l a t i o n c r a s h i n Nduma Game Reserve. The Lammergeyer 19:16-23. K i r k p a t r i c k , M. 1982. Sexual s e l e c t i o n and the e v o l u t i o n of female c h o i c e . E v o l u t i o n 36:1-12. K l e i n , D.R. 1968. The i n t r o d u c t i o n , i n c r e a s e , and c r a s h of r e i n d e e r on S t . Mathews I s l a n d . J . W i l d l . Manage. 32:350367. Krebs, C.J. 1985. Ecology: the experimental a n a l y s i s of d i s t r i b u t i o n and abundance ( T h i r d e d i t i o n ) . Harper and Row. N.Y. Lamprey, H. 1963, E c o l o g i c a l s e p a r a t i o n of the large-mammal s p e c i e s i n the T a r a n g i r e Game Reserve, Tanganyika. E. A f r . a g r i c . F o r . J . J_:63-92. Lande, R. 1980. Sexual dimorphism, sexual s e l e c t i o n , and a d a p t a t i o n s i n p o l y g e n i c c h a r a c t e r s . E v o l u t i o n 34:292-305.  1 75  Lande, R. 1981. Models of s p e c i a t i o n by sexual s e l e c t i o n on polygenic t r a i t s . Pro. Nat. Acad. S c i . USA 78:3721-3725. Laredo, M.A. and D.J. Minson. 1973. The v o l u n t a r y i n t a k e , d i g e s t i b i l i t y , and r e t e n t i o n time by sheep of l e a f and stem f r a c t i o n s of f i v e grasses. Aust. J . a g r i c . Res. 24: 875-888. Leader-Williams, N. and C. R i c k e t t s . 1980. P o p u l a t i o n dynamics and m o r t a l i t y of r e i n d e e r i n t r o d u c e d onto South Georgia I s l a n d . J . W i l d l . Manage. 44:640-657. L e s l i e , P.H. 1945. On the use of matrices i n c e r t a i n mathematics. Biometrika 33:183-212.  population  L e t t , P.F., R.K. Mohn, and D.F. Gray. 1981. The impact of the determination of density-dependent processes on the management s t r a t e g y f o r the north-western A t l a n t i c harp s e a l . I n : Dynamics of l a r g e mammals (Fowler, C.W. and T.D. Smith, e d s . ) . p. 135-158. John Wiley and Sons, N.Y. Leuthold, W. 1966. V a r i a t i o n s i n t e r r i t o r i a l behavior of Uganda kob, Adenota kob l e u c o t i s (Neumann,1896). Behaviour 27: 214-257. Leuthold, W. 1977. A f r i c a n ungulates.  Springer-Verlag. B e r l i n .  Lill,  A. 1974. S o c i a l o r g a n i z a t i o n and space u t i l i z a t i o n i n the lek-forming white-bearded manakin, M. manacus t r i n i t a t i s . Z. T i e r p s y c h o l . 36:513-530.  Lill,  A. 1976. Lek behavior i n the golden-headed manakin, P i p r a e r y t h r o c e p h a l a , i n T r i n i d a d (West I n d i e s ) . Adv. i n E t h o l . 18:1-83.  Lowe, V.P.W. 1969. P o p u l a t i o n dynamics of the red deer elaphus L.) on Rhum. J . Anim. E c o l . 38:425-457.  (Cervus  Lyeth, R.E. 1947. The m i g r a t i o n of game i n the Boma a r e a . Sudan Notes and Records. 30:191-192.  1 76  Maddock, L. 1979. The " m i g r a t i o n " and g r a z i n g s u c c e s s i o n . In: S e r e n g e t i : dynamics of an ecosystem ( S i n c l a i r , A.R.E. and M. N o r t o n - G r i f f i t h s , e d s . ) . p. 104-128. Univ. of Chicago P r e s s . Chicago. May, R.M. 1977. Thresholds and breakpoints i n ecosystems with a m u l t i p l i c i t y of s t a b l e s t a t e s . Nature 269:471-477. McCullough, D.R. 1979. The George reserve deer herd: p o p u l a t i o n ecology of a K - s e l e c t e d s p e c i e s . Univ. of Michigan P r e s s . Ann Arbor, Michigan. McNaughton, S.J. 1979. G r a s s l a n d - h e r b i v o r e dynamics. In: S e r e n g e t i : dynamics of an ecosystem ( S i n c l a i r , A.R.E. and M. N o r t o n - G r i f f i t h s , e d s . ) . p. 46-81. Univ. of Chicago Press, Chicago. McNaughton, S.J. 1984. Grazing lawns, animals i n herds, p l a n t form, and c o e v o l u t i o n . Am. Nat. 124:863-886. Mech, L.D. and P.D. Karns. 1977. Role of the wolf i n a deer d e c l i n e i n the S u p e r i o r N a t i o n a l F o r e s t . U.S. Dept. A g r i c . Serv. Res. Pap. NC-148, North Cent. F o r . Exp. S t n . , S t . Paul, Minn. 23 p. M e s s i e r , F. and M. C r e t e . 1985. Moose-wolf dynamics and the n a t u r a l r e g u l a t i o n of moose p o p u l a t i o n s . O e c o l o g i a 65:503512. Minson, D.J. and M.N. McLeod. 1970. The d i g e s t i b i l i t y of temperate and t r o p i c a l g r a s s e s . Pro. 11th I n t . G r a s s l . Cong. p. 719. Modha, K.L. and S.K. E l t r i n g h a m . 1976. P o p u l a t i o n ecology of the Uganda kob ( Adenota kob thomasi Neumann) i n r e l a t i o n t o the t e r r i t o r i a l system i n the Rwenzori N a t i o n a l Park, Uganda. J . Appl. E c o l . 13:453-473. M o r r i s o n , J.A. 1971. Morphology of corpora l u t e a i n the Uganda kob antelope, Adenota kob thomasi (Neumann). J . Reprod. F e r t . 26:297-305.  177  N a t i o n a l Research C o u n c i l . 1975. N u t r i e n t requirements of sheep. N a t i o n a l Research C o u n c i l , N a t i o n a l Academy of S c i e n c e s , Washington, DC. N o r t o n - G r i f f i t h s , M. 1978. Counting animals. (Second e d i t i o n ) . Handbook no. 1, A f r i c a n W i l d l i f e L e a d e r s h i p Foundation, N a i r o b i , Kenya. 139 p. Owen-Smith, N. 1982. F a c t o r s i n f l u e n c i n g the consumption of p l a n t products by l a r g e h e r b i v o r e s . I n : Ecology of t r o p i c a l savannas (Huntley, B.J. and B.H. Walker, e d s . ) . p. 359-404. S p r i n g e r - V e r l a g , B e r l i n . Pennycuick, L. 1975. Movements of migratory w i l d e b e e s t p o p u l a t i o n between 1960 and 1973. E. A f r . W i l d l . J . 13:6587. Peterman, R.M., W.C. C l a r k , and C.S. H o l l i n g . 1979. The dynamics of r e s i l i e n c e : s h i f t i n g s t a b i l i t y domains i n f i s h and i n s e c t systems. I n : P o p u l a t i o n dynamics (Anderson, R.M., B.D. Turner, and L.R. T a y l o r , e d s . ) . p. 321-341. B l a c k w e l l , Oxford. P h i l l i p s o n , S. 1975. R a i n f a l l , primary p r o d u c t i o n , and " c a r r y i n g c a p a c i t y " of Tsavo N a t i o n a l Park ( E a s t ) , Kenya. E. A f r . W i l d l . J . 13:171-201. P i e l o u , E.C. 1977. Mathematical New York.  e c o l o g y . John Wiley and Sons.  Plowes, D.C.H. 1957. The seasonal v a r i a t i o n of crude p r o t e i n i n twenty common v e l d grasses a t Matopos, Southern Rhodesia, and r e l a t e d o b s e r v a t i o n s . Rhod. A g r i c . J . 54:33-55. Polacheck, T. 1985. The sampling d i s t r i b u t i o n of a g e - s p e c i f i c s u r v i v a l estimates from an age d i s t r i b u t i o n . J . W i l d l . Manage. 49:180-184. Reid, R.L., A . J . Post, F . J . Olsen, and J.S. Mugerwa. 1973. S t u d i e s on the n u t r i t i o n a l q u a l i t y of g r a s s e s and legumes in Uganda. I . A p p l i c a t i o n of i n v i t r o d i g e s t i b i l i t y techniques to s p e c i e s and stage of growth e f f e c t s . Trop. A g r i c , T r i n . 50:1-16.  178  R i c k e r , W.E. 1975. Computation and i n t e r p r e t a t i o n of b i o l o g i c a l s t a t i s t i c s of f i s h p o p u l a t i o n s . B u l l e t i n of the F i s h e r i e s Research Board of Canada 191. Riney, T. 1982. Study and management of large mammals. John Wiley and Sons. N.Y. Robel, R.J. and W.B. B a l l a r d . 1974. Lek s o c i a l o r g a n i z a t i o n and r e p r o d u c t i v e success i n the g r e a t e r p r a i r i e c h i c k e n . Amer, Z o o l . 14:121-128. S a d l e i r , R. 1969. Ecology of r e p r o d u c t i o n i n w i l d and domestic mammals. Methuen and Co. London. S c h a f f e r , W.M. 1974. S e l e c t i o n f o r optimal l i f e h i s t o r i e s : the e f f e c t s of age s t r u c t u r e . Ecology 55:291-303. S c h a l l e r , G.B. 1972. The S e r e n g e t i l i o n : a study of p r e d a t o r prey r e l a t i o n s . Univ. Chicago Press, Chicago. S c h a l l e r , G.B. 1977. Mountain monarchs. Wild sheep and goats of the Himalaya. Univ. Chicago Press, Chicago. S c h e a f f e r , R.L. and W. Mendenhall. 1975. I n t r o d u c t i o n to p r o b a b i l i t y : theory and a p p l i c a t i o n s . Duxbury Press, North S c i t u a t e , Mass. Schoen, A. 1971. The e f f e c t of heat s t r e s s and water d e p r i v a t i o n on the environmental p h y s i o l o g y of the bushbuck, the reedbuck, and the Uganda kob. E. A f r . a g r i c . For. J . 37:17. Schoener, T.W. 1982. The c o n t r o v e r s y over c o m p e t i t i o n . Am. S c i . 70 : 586-595.  interspecific  Scrimshaw, N.S., C E . T a y l o r , and J.E. Gordon. 1968. I n t e r a c t i o n s of n u t r i t i o n and i n f e c t i o n . Geneva: WHO no. 57.  1 79  Seber, G.A.F. 1973. The e s t i m a t i o n of animal abundance and r e l a t e d parameters. Hafner Press, New York, New York, USA Sherman, P.W. and M.L. Morton. 1984. Demography of B e l d i n g ' s ground s q u i r r e l s . Ecology 65;1617-1628 . S i n c l a i r , A.R.E. 1975. The resource l i m i t a t i o n of t r o p h i c l e v e l s in t r o p i c a l g r a s s l a n d ecosystems. J . Anim. E c o l . 44:497520. S i n c l a i r , A.R.E. 1977. The A f r i c a n b u f f a l o . Univ. of Chicago P r e s s . Chicago. S i n c l a i r , A.R.E. 1983a. The adaptations of A f r i c a n ungulates and t h e i r e f f e c t s on community f u n c t i o n . I n : T r o p i c a l savannas ( B o u r l i e r e , F. e d ) . Ecosystems of the world 13:401-426. S i n c l a i r , A.R.E. 1983b. The f u n c t i o n of d i s t a n c e movements i n v e r t e b r a t e s . In: The ecology of animal movements (Greenwood, P.J. and I.R. Swingland, e d s . ) . Clarendon Press, Oxford. S i n c l a i r , A.R.E., H. D u b l i n , and M. Borner. 1985. P o p u l a t i o n r e g u l a t i o n of Serengeti wildebeest: a t e s t of the food h y p o t h e s i s . Oecologia 65:266-268. S i n c l a i r , A.R.E. and P. Duncan. 1972. Indices of c o n d i t i o n i n t r o p i c a l ruminants. E. A f r . W i l d l . J . 10:143-149. S i n c l a i r , A.R.E. and J.M. F r y x e l l . 1985. The S a h e l : ecology of a d i s a s t e r . Can. J . Z o o l . 63:987-994. S i n c l a i r , A.R.E. and M. N o r t o n - G r i f f i t h s . 1979. S e r e n g e t i : dynamics of an ecosystem. Univ. of Chicago P r e s s . Chicago. S i n c l a i r , A.R.E. and M. N o r t o n - G r i f f i t h s . 1982. Does c o m p e t i t i o n or f a c i l i t a t i o n r e g u l a t e ungulate p o p u l a t i o n s i n the S e r e n g e t i ? A t e s t of hypotheses. Oecologia 53:364-369.  180  Slobodkin, L.B., F.E. Smith, and N.G. H a i r s t o n . 1967. R e g u l a t i o n in t e r r e s t r i a l ecosystems, and the i m p l i e d balance of nature. Am. Nat. 101:109-124. Smuts, G.L. 1978. I n t e r e l a t i o n s between p r e d a t o r s , prey, and t h e i r environment. B i o s c i e n c e 28:316-320. Snedecor, G.W. and W.G. Cochrane. 1967. S t a t i s t i c a l methods (6th e d i t i o n ) . Iowa State Univ. P r e s s , Ames. Spinage, C A . 1982. A t e r r i t o r i a l a n t e l o p e : the Uganda waterbuck. Academic Press, London. Stearns, S . C 1976. L i f e - h i s t o r y t a c t i c s : a review Quart. Rev. of B i o l . 51:3-47.  of the i d e a s .  S t r u g n e l l , R.G. and C D . P i g o t t . 1978. Biomass, shoot p r o d u c t i o n , and g r a z i n g of two g r a s s l a n d s i n the Rwenzori N a t i o n a l Park, Uganda. J . E c o l . 66:73-76. Trail,  P. 1985. C o u r t s h i p d i s r u p t i o n m o d i f i e s mate c h o i c e i n l e k - b r e e d i n g b i r d . Science 227 : 778-780.  Tucker, C.J., L.D. M i l l e r , and R.L. Pearson. 1973. Measurement of the combined e f f e c t of green biomass, c h l o r o p h y l l , and l e a f water on canopy s p e c t r o r e f l e c t a n c e of the s h o r t - g r a s s p r a i r i e . Proc. of the Second Annual Remote Sensing of E a r t h Resources Conf., Univ. of Tenn., Tullahoma, Tenn. Van  Soest, P.J. 1963a. Use of d e t e r g e n t s i n the a n a l y s i s of f i b r o u s feeds. I . P r e p a r a t i o n of f i b e r r e s i d u e s of low n i t r o g e n c o n t e n t . J . Assn. O f f i c i a l Agr. Chem. 46:825-829.  Van  Soest, P.J. 1963b. Use of d e t e r g e n t s i n the a n a l y s i s of f i b r o u s feeds. I I . A r a p i d method f o r the d e t e r m i n a t i o n of f i b e r and l i g n i n . J . Assn. O f f i c i a l Agr. Chem. 46:829-835.  Van  Soest, P.J. and W.G. Marcus. 1964. Method f o r the d e t e r m i n a t i o n of c e l l - w a l l c o n s t i t u e n t s i n forages u s i n g detergents and the r e l a t i o n s h i p between t h i s f r a c t i o n and v o l u n t a r y intake and d i g e s t i b i l i t y . J . Dairy S c i . 47:704705.  Van V a l e n , L. 1973. P a t t e r n s and the balance of nature. E v o l u t i o n a r y Theory J_:31-49. V e s e y - F i t z g e r a l d , D.F. 1960. Grazing s u c c e s s i o n amongst East A f r i c a n game animals. J . Mammal. 41:161-17 0. Weeks, J.T. 1947. L e t t e r to e d i t o r . Sudan Notes and Records 30: 218-219. Western, D. 1975. Water a v a i l a b i l i t y and i t s i n f l u e c e on the s t r u c t u r e and dynamics of a savannah large-mammal community. E. A f r . W i l d l . J . 13:265-286. Wiens, J.A. 1977. On c o m p e t i t i o n and v a r i a b l e Amer. S c i . 65:590-597.  environments.  Wiley, R.H. 1973. T e r r i t o r i a l i t y and non-random mating i n sage grouse, Centrocercus urophasianus . Animal Behavior Monogr. 6:87-169. . W i l l i m o t , S.G. 1956. S o i l s and v e g e t a t i o n of the Boma P l a t e a u and e a s t e r n d i s t r i c t E q u a t o r i a . Sudan Notes and Records. 39:9-20. Wilson, J.R. 1976. V a r i a t i o n i n l e a f c h a r a c t e r i s t i c s with l e v e l of i n s e r t i o n on a g r a s s t i l l e r . I . Development r a t e , chemical composition and dry matter d i g e s t i b i l i t y . Aust. J . a g r i c . Res. 27:343-354. Wittenberger, J . F . 1978. The e v o l u t i o n of mating grouse. Condor 80:126-137.  systems i n  Woodgerd, W. 1964. P o p u l a t i o n dynamics of bighorn sheep on Wildhorse I s l a n d . J . W i l d l . Manage. 28:381-391. Wrangham, R.W. 1980. Female c h o i c e of l e a s t c o s t l y males: a p o s s i b l e f a c t o r i n the e v o l u t i o n of l e k s . Z. T i e r p s y c h o l . 54:357-367.  Zaphiro, D. 1949. Notes on L o e l l i Sport. 1:6-17.  game. Sudan W i l d l i f e and  1 83  APPENDIX 1.  PLANT SPECIES COLLECTED  A c a c i a nubica Benth.  Mimosaceae  A c a c i a polyacantha  Mimosaceae  Acac i a Senegal  Willd.  (L.) W i l l d .  Mimosaceae  Acac i a s e y a l DC.  Mimosaceae  Acac i a s i e b e r iana DC. v a r . s i e b e r i a n a  Mimosaceae  Acacia Z a n z i b a r i c a  (S. Moore) Taub.  Mimosaceae  Achyranthes aspera  L.  Amaranthaceae  A l y s i c a r p u s glumeceus (Vahl)  DC.  Leguminosae  Asparagus a f r i c a n u s Lam.  Liliaceae  Balanites aegyptiaca  Balanitaceae  (L.) D e l .  Balanites r o t u n d i f o l i a Butyrospermum  paradoxum  (Van Tiegh.) B l a t t e r (Gaertn.)  Hepper  ssp. n i l o t i c u m (Kotschy)  Hepper  Balanitaceae Sapotaceae  Cadaba f a r i n o s a Forssk. s s p . f a r i n o s a  Capparaceae  Capparis  Capparaceae  tomentosa  C e l t i s toka  Lam.  (Forssk.) Hepper & Wood  Ulmaceae  Combretum f r a g r a n s F. H o f f .  Combretaceae  Cordia s i n e n s i s Lam.  Boraginaceae  Crateva adansoni i Dichrostachys  forma v e l sp. a f f .  DC.  Capparaceae  c i n e r e a (L.) Wight & Arn.  Mimosaceae  ssp. c i n e r e a Diospyros  m e s p i l i f o r m i s A.  DC.  Ebenaceae  Dobera g l a b r a (Forssk.) R. B r .  Salvadoraceae  Echinochloa  pyramidalis  Gramineae  Echinochloa  staqnina  (Lam.) H i t c h c . & Chase  (Retz.) P. Beauv.  Gramineae  184  Eragrostis ci1ianensis  ( A l l . ) F.T. Hubb.  Gramineae  Ficus lutea Vahl  Moraceae  F i c u s sur F o r s s k . forma v e l sp. a f f .  Moraceae  F u e r s t i a a f r i c a n a T.C.E. F r .  Labiatae  Gardenia t e r n i f o l i a  Rubiaceae  Schumach. & Thonn.  Gardenia v o l k e n s i i K. Sch.  Rubiaceae  Grewia b i c o l o r  Ti1iaceae  Grewia m o l l i s Grewia tenax  Juss.  Ti 1iaceae  Juss. (Forssk.) F i o r i  Tiliaceae  Harrisonia abyssinica  Oliv.  Simaroubaceae  Heteropogon c o n t o r t u s  (L.) Roem. & S c h u l t .  Gramineae  Hyparrhenia f i l i p e n d u l a Hyparrhenia r u f a  (Nees) S t a p f .  Hyperthelia dissoluta W.D.  (Hochst.) Stapf  Gramineae Gramineae  (Nees ex Steud.)  Gramineae  Clayton  Lonchocarpus l a x i f l o r u s G u i l l .  & Perr.  Papilionaceae  Loudetia arundinacea (A. Rich.) Steud.  Gramineae  Maerua a n g o l e n s i s  Capparaceae  DC.  Maerua o b l o n g i f o l i a A. R i c h . Maerua pseudopetulosa ( G i l g & Bened.) De Wolf Maytenus s e n e g a l e n s i s  (Lam.) E x e l l  -Capparaceae Celestraceae Mimosaceae  Mimosa p i g r a L. Nauclea l a t i f o l i a  Capparaceae  Rubiaceae  Smith  Ozoroa i n s i q n i s D e l . ssp. i n s i g n i s  Anacardiaceae  Panicum coloratum L.  Gramineae  Piliostigma  thonningii  Pseudocedrela k o t s c h y i  (Schumach.) Milne-Redh.  Caesalpiniaceae  (Schweinf.) Harms  Meliaceae  185  Rhus n a t a l e n s i s  Krauss  Anacardiaceae  Securineqa v i r o s a ( W i l l d . )  Baill.  Euphorbiaceae  Sehima nervosum ( W i l l d . ) Stapf  Gramineae  Setaria  Gramineae  i n c r a s s a t a (Hochst.) Hack.  Sorghum purpureo  sericeum  (Hochst. ex A. Rich.)  Gramineae  Aschers & Schweinf. Sporobolus  helvolus  ( T r i n . ) Dur. & S c h i n z .  Sporobolus  i o c l a d o s ( T r i n . ) Nees  Steganotaenia a r a l i a c e a Hochst.  Gramineae Gramineae  forma v e l . a f f . Araliaceae  Stereospermum kunthianum Cham.  Bignoniaceae  Tamarindus i n d i c a L.  Caesalpinaceae  Terminalia  b r e v i p e s Pampan  Combretaceae  Triumfetta  rhomboidea Jacq.  Tiliaceae  Ximenia  caffra  Sond.  Z i z i p h u s m a u r i t i a n a Lam.  Olacaceae Rhamnaceae  

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