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Aggression and self-regulation of population size in deermice Healey, Michael Charles 1966

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AGGRESSION AND SELF-REGULATION OF POPULATION SIZE IN DEERMICE by MICHAEL CHARLES HEALEY B.Sc, University of B r i t i s h Columbia, 1964 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in the Department of Zoology We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA March, 1966 I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r a n a d v a n c e d d e g r e e a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l m a k e i t f r e e l y a v a i l a b l e f o r r e f e r e n c e a n d s t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e H e a d o f my D e p a r t m e n t o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t b e a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n D e p a r t m e n t The U n i v e r s i t y o f B r i t i s h C o l u m b i a V a n c o u v e r 8, C a n a d a ABSTRACT Sadleir (1965) proposes that the s u r v i v a l of juvenile deermice i s determined by the aggressiveness of the adult population. During the summer, when adult aggres-sion i s high, juvenile s u r v i v a l i s poor, but i n the f a l l , when adult aggression i s low, juveniles survive well. The purpose of this study i s to examine some of the consequences of Sadleir's hypothesis experimentally. Sadleir bases his hypothesis on the observation that the aggressiveness of males changes seasonally. This premise has been reexamined and confirmed. How adult aggression affects juveniles was studied f i r s t i n the lab-oratory. Juveniles grow poorly when competing with adults in t h e i r home cage. Males appear to be more active aggres-sors than females, but only aggressive males are capable of i n h i b i t i n g juvenile growth. Even though juveniles grew slowly when competing with aggressive adults, they seldom died from encounters with adults. In order to avoid the crowded conditions and con-finement i m p l i c i t i n the laboratory experiments, the rel a t i o n s h i p between adult aggressiveness and juvenile growth and s u r v i v a l was reexamined i n f i e l d experiments. Two p a r t l y i s o l a t e d plots of habitat were used, and on these plots a r t i f i c i a l populations of aggressive or d o c i l e male i i i deermice were established. Juveniles were then released onto the plots, and t h e i r growth and s u r v i v a l followed. In the f i e l d , as i n the laboratory, juveniles grew poorly when competing with aggressive adults. Since emigration was not r e s t r i c t e d i n the f i e l d , however, juveniles disappeared i n s i g n i f i c a n t l y greater numbers when the adult population was aggressive than when the adult population was d o c i l e . In addition to these experiments, the success of immigrants onto trapped out plots and plots with a resident population was examined. Immigrants were more successful i n e s t a b l i s h -ing themselves on trapped out p l o t s . A l l the data collected support Sadleir's hypothe-s i s , and i t seems reasonable to conclude that the c o r r e l a -t i o n he drew between adult aggressiveness and juvenile s u r v i v a l i s r e a l . However, the data collected also provide some i n t e r e s t i n g clues as to the organization of deermouse populations. An organization i s proposed i n which the s o c i a l unit i s an animal and i t s immediate neighbours. Within the s o c i a l unit mutual antagonism i s reduced. But the members of the unit maintain a high l e v e l of aggressive-ness, and are intolerant of any stranger that wanders into t h e i r home ranges. The system proposed would prevent immigrants from s e t t l i n g , while conserving energy by reducing antagonism between f a m i l i a r animals. The system would also e f f e c t i v e l y regulate population s i z e . i v Table of Contents Page INTRODUCTION 1 LABORATORY EXPERIMENTS 3 The Cycle i n Aggressiveness 3 Methods. 3 Results 6 Defence of a. Home Cage 8 Methods 9 Results 10 Sex of Aggressor 12 Methods 12 Results 13 Aggressiveness of Males 13 Methods 13 Results 14 Spontaneous A c t i v i t y i n Males 14 Methods 15 Results 16 FIELD EXPERIMENTS 16 Isolated Plot Experiments 16 Methods. . 17 Results 19 V Page A r t i f i c i a l Immigration onto Trapped Out a n d Natural PLoTs 21 Methods 21 R e s u l t s 22 Homing i n Peromyscus 23 Neighbour and Stranger Responses 24 Methods. . 24 R e s u l t s . 25 DISCUSSION 25 The S o c i a l System i n Peromyscus 27 T h e , S e l e c t i v e Advantage o f the S e l f -R e gulatory System 33 SUMMARY 35 ACKNOWLEDGMENTS 37 REFERENCES 38 Appendix I 42 Appendix II 43 v i L i s t of Tables Table Facing Page I Contingency tables f o r spring and f a l l changes i n aggressiveness (from Figs. 1 and 2) 7 II Changes i n testes sizes of males i n the experiments on aggression cycle. Testes sizes range from: none palpa-ble (1), small testes(2), medium testes (3), large testes (4). Brac-keted figures i n 1963 are males moved back into the laboratory i n October (see t e x t ) . . . 9 III Growth and s u r v i v a l of juveniles after 14 days i n the maze: A. com-peting with adults on home ground, B. alone, C. competing with adults on,neutral ground 11 IV Survival and growth of juveniles i n the maze with t e r r i t o r i a l adult males or adult females 14 v i i Table Facing Page V Survival of juveniles on plots A and B when adult aggression was constant 19 VI Growth rates of juveniles on plots A and B when adult aggression was constant 19 VII Survival of juveniles on plots A and B when adult aggression changed 19 VIII Growth rates of juveniles on plots A and B when adult aggression changed 19 IX Survival of animals released onto trapped out plots and onto plots with resident populations 23 X Nine instances of homing i n P. m. austerus 25 XI Mean number of aggressive responses of neighbours and strangers dur-ing 10 min. bouts i n a neutral arena 25 v i i i L i s t of Figures Figure Facing Page 1 Changes i n the aggressiveness of experimental and control males during the f a l l of 1963 and 1964. . . . 7 2 Changes i n the aggressiveness of experimental and control males during the spring of 1965 7 3 Weight changes of juveniles i n the maze with aggressive males ( s o l i d c i r c l e s ) and juveniles i n the maze with d o c i l e males (open c i r c l e s ) , 15 4 Diagram of plots A and B showing approximate locations of trap s i t e s 19 5 Relationship between four aggressive acts and t o t a l aggres-sion 44 1 INTRODUCTION Animal populations fluctuate i n numbers, but the fluctuations occur within cert a i n d e f i n i t e l i m i t s . That i s to say, whole populations do not go on increasing i n d e f i -n i t e l y , and they seldom become extinct. The way these fluctuations are regulated remains a mystery. It seems unlikely, i n the l i g h t of present knowledge, that popula-t i o n s i z e i s governed e n t i r e l y by factors operating from out-side the population. Therefore the number of animals i n a population must be determined p a r t l y by the animals them-selves. This paper deals with such a self-regulatory mechanism i n the deermouse (Peromyscus maniculatus austerus). The working hypothesis used i n t h i s study was advanced by Sadleir (1965). The data which led to i t s development are reviewed by him. B r i e f l y , during most of the breeding season (June through August)- mice are scarce. In s p i t e of the low population density few juveniles are recruited. In September and October, however, recruitment i s very rapid, and the population density increases, so that i n early winter there are two or three times as many mice as i n summer. This f a l l r i s e i n the s u r v i v a l rate of i juveniles i s correlated with a decline i n the aggressiveness of the adult males. Sadleir hypothesized that juveniles survive poorly i n the early part of the breeding season because they are competing with aggressive adults for 2 habitat, and that s u r v i v a l i s better i n the f a l l because the adults are less aggressive. I have attempted to test Sadleir's hypothesis experimentally both i n the laboratory and i n the f i e l d . For convenience the experiments are presented under the headings "Laboratory" and " F i e l d " , even though doing so p a r t l y d i s -rupts the l o g i c a l sequence i n which the experiments were performed. The methods and r e s u l t s of each experiment are presented as a unit. The sort of s o c i a l system these r e s u l t s indicate f o r Peromyscus maniculatus i s taken up i n the d i s -cussion. Animals used i n the experiments were either caught i n the f i e l d or were f i r s t generation laboratory stock. No d i s t i n c t i o n w i l l be made between adults of these two types since the process of s e l e c t i n g experimental animals counter-acted any q u a l i t a t i v e differences that may have resulted from rearing animals i n the laboratory. The term juvenile i n t h i s paper ref e r s to animals three to four weeks o l d . These animals generally weighed between 11 and 13 grams. They had not moulted the grey juvenile pelage and were undeveloped sexually. A l l the juveniles were f i r s t generation laboratory stock. Except where noted a l l the laboratory experiments were performed i n an enclosed ventilated room kept constantly 3 on 13L-11D reversed daylight. This l i g h t i n g schedule was s u f f i c i e n t to keep the adults continuously i n breeding con-d i t i o n . Plots used for f i e l d experiments w i l l be described l a t e r . LABORATORY EXPERIMENTS The Cycle i n Aggressiveness Methods Sadleir (1965) presents data which show a spring increase followed by a f a l l decrease i n the aggressiveness of male deermice. Since these observations constitute an important premise i n Sadleir's argument, I decided to retest them. The f a l l decrease was retested i n 1963 and 1964, and the spring increase i n 1965. In the experimental s i t u -ation s i x randomly chosen male deermice were housed on the Zoology building roof i n 1963, and i n an open shed i n 1964 and 1965. These animals were thus subject to natural f l u c t u -ations i n daylength and temperature. In the control s i t u -ation s i x males were kept i n the constant environment of the laboratory. The animals l i v e d separately i n 15 i n . diameter s t e e l washbasins provided with sawdust l i t t e r and excess food and water. The basins were stored,on a "Dexion" s t e e l 4 rack and food and water were checked at least every two days. The aggressiveness of experimental and control mice was measured i n encounters with members of a graded seri e s whose aggressiveness r e l a t i v e to one another was known. The organization of the graded seri e s i s discussed i n Appendix I (see also Sadleir 1965). The procedure for these encounters was as follows: Three subjects from the experimental group and three from the control group were tested each week. In 1963 subjects were selected randomly each week except f o r two r e s t r i c t i o n s : No animal was fought more than two weeks i n succession, and each animal met each member of the graded serie s only once during the experiment. In 1964 and 1965 experimental and control groups were each randomly divided into two sets of three animals, and the sets were alternated each week. Again encounters were organized so that each experimental or control male met a member of the graded series only once. Twenty-four hours p r i o r to the encounters subjects were weighed and placed i n 2 f t . by 1 f t . by 1 f t . glass sided arena cages and provided with food and water. Before an encounter the cage containing the subject to be tested was placed on an observation platform and the water dish removed. The observation platform was lighted from above by a single 40 watt red bulb. The subject was given 5 min. to s e t t l e down, then a member of the graded series was introduced and the ensuing a c t i v i t y observed from behind 5 a s c r e e n f o r 5 min. The animals from the graded s e r i e s had patches of f u r c l i p p e d o f f so that they could be d i s t i n g u i s h e d i n the arena. D e s c r i p t i o n s of what o c c u r r e d were spoken i n t o a tape r e c o r d e r and t r a n s c r i b e d l a t e r . In October 1963, three members of the experimental group were brought back i n t o the l a b o r a t o r y . T h i s was done to see i f longer daylength and hi g h e r temperatures would cause an i n c r e a s e i n the aggres-s i v e n e s s of animals which had n a t u r a l l y become d o c i l e i n the f a l l . A l l encounters were recorded except d u r i n g the i n i t i a l weeks of 1963 when c o n t r o l encounters were run every week but were recorded o n l y every three weeks ( F i g . 1). In 1963 the temporal p a t t e r n of events was examined by r e c o r d i n g the d a t a i n 10 s e c . time i n t e r v a l s . Every act which o c c u r r e d w i t h i n each 10 sec. i n t e r v a l was recorded, but i f a p a r t i c u -l a r act o c c u r r e d more than once i t was s t i l l scored o n l y once (cf S a d l e i r 1965). In 1964 and 1965 simple frequency counts were made. The f o l l o w i n g c a t e g o r i e s of behaviour were recorded i n 1963 (from E i s e n b e r g 1962 except where noted): Threat (see S a d l e i r 1965), chase, f i g h t , a g g r e s s i v e grooming (Grant and Mackintosh 1963), grooming, washing, naso-nasal, naso-anal, e x p l o r i n g , and mutual u p r i g h t . E x p l o r i n g and washing were not recorded i n 1964 and 1965. From these a c t s the t o t a l number 6 of threats and chases i n a 5 min. encounter was selected as the best index of aggressiveness. The reasons for t h i s choice are given i n Appendix I I . Results The r e s u l t s of the two f a l l experiments are pres-ented i n F i g . 1 and the re s u l t s of the spring experiment i n Fig . 2. Occasionally animals escaped when they were being taken from the basins f o r testing. If t h i s happened on the roof or out i n the shed i t usually meant the animal was l o s t . Sometimes animals escaped from the testing arenas. Conse-quently not a l l weeks show three encounters. The t o t a l number of points on each graph was divided i n half f i r s t by a v e r t i c a l l i n e , then by a horizontal l i n e (dashed l i n e s Figs. 1 and 2 ) . If the mice are becoming less aggressive the points should be clustered i n the upper l e f t and lower right quadrats formed by the dashed l i n e s . And t h i s i s what happened i n the two f a l l experiments. If the mice are becoming more aggressive the points should c l u s t e r i n the lower l e f t and upper right quadrats. This was the case i n the spring experiment. The s i g n i f i c a n c e of t h i s c l u s t e r i n g of points can be tested by means of a analysis i n a 2 X 2 contingency table. These tables are presented i n Table I. The cl u s t e r i n g O l -IO w « . o « at I 0 1-- I 0 9 0 8 Ofr OS 0 2 01 x a p u i U 0 I 8 8 3 J 6 6 V — I — 0 2 09 T— 0 8 I 1 1 1 i 0 * O C 0 1 01 x a p u i U O ) S S O J 6 & V n at B X tn 0 9 I 0 8 0 * O S 0 3 01 x a p u | U O ; S S 9 J 6 6 V — I — 0 6 io at 0 9 I 0 6 i Ofr 0 6 0 2 01 x a p u i uois8»jB6v I-•i <-11 • •! I I I • I 1 ol 4}-1 I Figure 1. Changes i n the aggressiveness of experimental and control males during the f a l l of 1963 and 1964. • = Aggression., score f o r one animal. x = Aggression scores f o r experimental males moved back into the laboratory i n October 1963. o <0 o X c o » IO (/> <U O »- CM o» < o Experimental* 1965 i T — i — r May March April o o 10 at * " O c IO (0 a> o» < CM Control* 1965 T -March April May Figure 2. Changes i n the aggressiveness of experimental and control males during the spring of 1965. Table I Contingency t a b l e s f o r s p r i n g and f a l l changes i n aggr e s s i v e n e s s (from F i g s . 1 and 2 ) . F a l l D e c l i n e 1963 EXPERIMENTALS CONTROLS f i r at Second H a l f H a l f h i g h 12.5 4 16.5 9 6 15 Agg r e s s i o n low 4 N 12.5 16.5 6 9 15 16.5 16.5. 33 15 15 30 X 2= 8.75 P<0.005 X 2= 1.20 P>0.25 F a l l D e c l i n e 1964 - 11 a 19 13 10 23 8 11 19 10 13 23 19 19 38 23 23 46 X 2= 0.95 P>0.25 X 2= 0.72 P>0.25 S p r i n g Increase 1965 3.3 7.7 11 7. 5 6 13.5 7.7 3.3 11 6 7.5 13.5 11 11 22 13. 5 13.5 37 X 2- 3.52 P<0.10 X 2= 0.33 P>0.50 7 of points for the experimental animals i s s i g n i f i c a n t only in the f a l l of 1963, however, the data from 1963 and 1964 may be lumped to give a X 2 of 9.7 with two degrees of freedom, which i s s i g n i f i c a n t (P<0.01). None of the graphs for con-t r o l animals show s i g n i f i c a n t c l u s t e r i n g . Lumping the data from control animals f o r the two f a l l experiments does not give a s i g n i f i c a n t r e s u l t (X 2 = 1.92, df 2, P>0.25). The c l u s t e r i n g of points for experimental animals i n spring 1965 approaches s i g n i f i c a n c e and aggrees with what Sadleir observed. S i m i l a r l y the animals which were moved back into the laboratory i n October 1963 increased markedly i n aggressiveness (Fig. 1). Aggressiveness, then, appears to be controlled by changing conditions of l i g h t and temperature. The main point to be made, however, i s that Sadleir's observations are confirmed; seasonal changes do occur i n the aggressive-ness of male deermice. A l l the animals were weighed each week, and no change i n weight was associated with the change i n aggres-siveness. Nor was there any rela t i o n s h i p between siz e and aggressiveness i n control animals. The s i z e of each animal's testes was also estimated every week (except i n 1963) by gently squeezing the testes into the scrotum between finger and thumb. Four categories 8 of t e s t i s s i z e were recognized: large, medium, small, and none discernable. As the experimental animals became less aggres-sive t h e i r testes got smaller, and as they became more aggres-sive t h e i r testes got larger (Table I I ) . H i s t o l o g i c a l sections were made of the four cate-gories of testes mentioned above. These showed that large and medium sized testes were a c t i v e l y producing sperm, while testes smaller than medium were not. Presumably testes which are a c t i v e l y producing sperm are also a c t i v e l y producing testosterone. Beeman (1947) showed that testosterone l e v e l s affect aggressiveness i n white mice and Whitaker (1940) showed that day-length affected the breeding of Peromyscus  leucopus. In deermice, therefore, the whole cycle i n aggressiveness i s probably related to sexual maturity and testosterone l e v e l s . Defence of a Home Cage Burt (1940) and S t i c k e l (1960) have presented e v i -dence that Peromyscus occupy i n d i v i d u a l home ranges during the breeding season. Unpublished data of my own support t h i s view. Some authors (McCabe and Blanchard 1950, Howard 1949) have suggested that i n the winter Peromyscus band together i n small groups. Whether or not the summer home ranges are defended remains an open question. However, i t i s important to know whether adult aggression i n summer i s associated with Table II Changes i n testes sizes of males i n the experiments on aggression cycle. Testes sizes range from: none palpable (1), small testes (2), medium testes (3), large testes (4). Bracketed figures i n 1963 i ^ . are males moved back into the laboratory i n October (see t e x t ) . A. F a l l Decline 1963 Experimentals Controls B. F a l l Decline 1964 Experimentals Controls C. Spring Rise 1965 Experimentals Controls Date Aug. Sep. Oct. Nov. mean 3.7 1.7 1.3(2.2) 1.4(3.4) range 3-4 1-3 1-2(1-3) 1-3(3-4) mean 3.2 3.2 2.9 range 1-4 1-4 1-4 Date J u l . Aug. Sep. Oct. Nov. mean 4.0 3.7 2.6 2.2 1.8 range 4 1-4 1-4 1-4 1-3 mean 3.9 3.9 3.6 3.7 3.7 range 3-4 3-4 1-4 1-4 2-4 Date Mar. Apr. May mean 2.9 3.5 4.0 range 1-4 1-4 4 mean 3.6 3.8 3.9 range 2-4 3-4 3-4 9 t e r r i t o r i a l i t y ( i . e . defence of an area) or whether adult aggression per se i s a l l that i s necessary to reduce juvenile s u r v i v a l . Methods The e f f e c t s of t e r r i t o r i a l behaviour were tested by releasing juveniles into the home cage of a pair of adult deermice. Adult aggression divorced from t e r r i t o r i a l behavi-our was examined by releasing juveniles and adults together into a s i m i l a r , but unfamiliar cage. The colony maze described by Sadleir (1965), divided into three regions (A, B, C Table I I I ) , was used f o r the cages. In region A an adult pair was released. At the same time two randomly chosen juveniles were isol a t e d i n small subregions of A, B, and C. A second adult pair was isola t e d i n a separate subregion of C. After two days the iso l a t e d sections were connected to t h e i r respective maze regions. This produced the following s i t u a t i o n s : (1) In region A juveniles were dispersing into an area occupied by adults i n breeding condition. (2) In region B juveniles were dispersing into an unoccupied area. (3) In region C juveniles and adults were dispersing together into an unoc-cupied area. Regions A and C were made twice as large as B so 10 that each mouse had the same amount of p o t e n t i a l l i v i n g space. Food and water were s u p p l i e d i n excess. Experiments were run f o r 14 days a f t e r the i s o l a t e d s e c t i o n s had been connected to the main body of the maze. For the f i r s t hour a f t e r the i s o l a t e d s e c t i o n s were connected to the maze a r e c o r d was made of the a n t a g o n i s t i c a c t s i n r e g i o n s A and C. For the remainder of the experiment spot checks were made of the p o s i t i o n s of the a d u l t s and j u v e n i l e s i n the maze. T h i s was done to see i f the j u v e n i l e s had f r e e use of the maze and to see i f they a s s o c i a t e d with the a d u l t s . At the end of each experiment the j u v e n i l e s were removed and weighed. Experiments were performed i n b l o c k s of two. For the second experiment i n each block the same a d u l t s were used but t h e i r r o l e s were r e v e r s e d . That i s , the p a i r that had been on home ground became the p a i r on n e u t r a l ground and v i c e v e r s a . R e s u l t s J u v e n i l e s grew much more s l o w l y i n r e g i o n A than i n e i t h e r of the o t h e r two r e g i o n s (Table I I I ) . The proba-b i l i t y t h a t the d i f f e r e n c e s i n growth are due.to chance i s low (P<0.10). The r e s u l t i s not s t a t i s t i c a l l y s i g n i f i c a n t but agrees w e l l with the f i n d i n g s of o t h e r authors (cf Barnet t 1958). Only three j u v e n i l e s d i e d , two i n r e g i o n A Table I I I Growth and s u r v i v a l of j u v e n i l e s a f t e r 14 days i n the maze: A. competing with a d u l t s on home ground, B. alone, C. competing w i t h a d u l t s on n e u t r a l ground. Maze Region A B C No. Released 8 8 8 No. S u r v i v i n g 6 8 7 X Wt. Increase 0.92 g. 2.81 g. 2.64 g. S.E. 0.73 0.70 0.66 11 and one in region C. On the average there was no difference in the amount of aggression between adults and juveniles in regions A and C, however, the way the aggression occurred was rather different. In region A the adults generally moved into the isolate section almost as soon as i t was opened and attacked the juveniles i t contained. In region C, on the other hand, altercations occurred when the adults and juveniles met while exploring the maze. The two juvenile deaths in region A were a result of direct adult attacks as described above. The juvenile that died in region C invaded the adult isolate section before the adults had l e f t i t . He was severely attacked, and never recovered from the beating. The adults ignored him after he l e f t their home ground in their isolate section, however. In spite of the aggressive interaction in region C the juveniles in this region grew as well as the juveniles in region B where no adults were present. There i s some evidence that strange surroundings may produce fear in rats (Montgomery 1955) and i t may be argued that the adults in region C were inhibited by the strangeness of the maze. It is unlikely that they would have explored the maze as quickly or shown as much aggression as they did i f they were inhibi-ted, by the strangeness' of the maze. A l l the evidence supports 12 the conclusion that, i n the laboratory at least, adults must have f a m i l i a r surroundings before they can affect juvenile growth. Sex of Aggressor Methods It i s not possible to decide from the previous experiment which sex, i f either, has the greater e f f e c t upon the juveniles. In order to answer t h i s question juveniles were released into maze regions controlled by either males or females. The colony maze used i n the previous experiment was divided into eight v e r t i c a l columns. Pairs of adult males were released into columns 1, 3, 6, and pairs of adult females into columns 2, 5, and 7. Columns 4 and 8 were used fo r control areas. Two days aft e r the adults were released, pairs of juveniles were introduced into each column. Aggres-sion between adults and juveniles was recorded for the f i r s t hour aft e r the juveniles were introduced. The experiment was terminated after 14 days and the surviving juveniles were weighed. A small additional experiment was run using only one column of males and one of females, after the f i r s t experiment was completed. 13 Results Survival was 100% i n the columns containing female adults, but much less i n the columns containing male adults (X 2 = 7.27 P<0.01) (Table IV). Also males were much more aggressive toward juveniles than females were. Too few juveniles were used i n thi s experiment to permit any meaning-f u l analysis of growth rates. It should be pointed out, however, that females do not seem to have affected the growth rates of the juveniles. Aggressiveness of Males The r e s u l t s of the previous experiment suggested that i t would be best to concentrate on the behaviour of the males, and how t h e i r behaviour affects juvenile s u r v i v a l . Consequently most of the remaining experiments are concerned with males only. The following experiment was designed to test a fundamental prediction of the hypothesis, namely that aggressive males have a greater ef f e c t than d o c i l e males upon juvenile s u r v i v a l . Methods The colony maze was used as i n the preceeding experiment except that now columns 2, 5, and 7 each contained a d o c i l e male and columns 1, 3 and 6 each contained an Table IV S u r v i v a l and growth of j u v e n i l e s i n the maze with t e r r i t o r i a l a d u l t males or a d u l t females. T e r r i t o r y No. Released No. S u r v i v i n g X Wt. Increase S.E. Male Female 8 8 3 8 1.3 g. 2.1 g. 2.0 1.17 C o n t r o l 4 4 1.1 g. 0.76 14 aggressive male. Aggressive and doc i l e males were selected on the basis of t h e i r performance i n encounters with members of the graded s e r i e s . The method for conducting encounters with the graded series has been described i n the experiment on the cycle i n aggressiveness. The same index of aggres-sion was used. Docile males rated <5 on the index and aggressive males rated >20 on the index. Two days after the males were released into the maze, pairs of randomly chosen juveniles were introduced into the columns. Juvenile weights were recorded each day for seven days. Results Juveniles i n the columns with d o c i l e adults grew almost twice as fast as juveniles i n the columns with aggres-sive adults (Fig. 3); the slopes of the lines regressed on weight are s i g n i f i c a n t l y d i f f e r e n t (t = 2.074 P<0.05). Clearly, aggressive adults are capable of exerting a greater eff e c t on juveniles than d o c i l e adults are. In fact there was no difference between the growth of juveniles with d o c i l e adults and the growth of juveniles alone i n columns 4 and 8. Spontaneous A c t i v i t y in Males Even though aggressive males are capable of F i g u r e 3. Weight changes of j u v e n i l e s i n the maze with a g g r e s s i v e males ( s o l i d c i r c l e s ) and j u v e n i l e s i n the maze with d o c i l e males (open c i r c l e s ) . 15 influencing juvenile growth i n the enclosed laboratory system, p r a c t i c a l l y nothing i s known about the way aggression occurs i n the natural habitat. Data from the experiment on seasonal changes i n aggression suggest that aggressive males might be more spontaneously active than d o c i l e males. If t h i s i s true then aggressive males should be capable of occupying larger home ranges or of p a t r o l l i n g t h e i r home range more e f f i c i e n t l y . I decided to explore further the p o s s i b i l i t y that aggressive males are more active. Methods Experimental animals were f i r s t bouted against members of the graded ser i e s to assess t h e i r l e v e l s of aggressiveness. Highly aggressive or do c i l e males were s e l -ected by the c r i t e r i o n previously noted. Before each test the animal to be tested was given 24 hours to become accus-tomed to a small (8 i n . square) wire mesh cage. After 24 hours the cage was suspended by three e l a s t i c bands so that i t bounced f r e e l y each time the animal moved. The cage was connected to a kymograph pen and the bounches were recorded as j i g g l e s on a moving ink trace. After the cage was con-nected to the kymograph the mouse was l e f t i n the dark and a c t i v i t y was recorded f o r approximately 15 min. A c t i v i t y was scored by measuring the periods of a c t i v i t y to the nearest l/10th of an inch along 12 i n . of 16 the ink trace. Measurement was begun one inch from the s t a r t of the trace i n order to reduce any bias i n the f i r s t part of the trace due to connecting the kymograph and shutting off the l i g h t . The length of the trace measured represents about 12 min. of time. Results Ten aggressive males averaged 6.29 i n . of a c t i v i t y , while eleven d o c i l e males averaged 3.15 i n . The data were transformed into logs and treated i n a Student's t test. Aggressive mice were s i g n i f i c a n t l y more active than d o c i l e mice (P<0.05). Lagerspetz (1964) found a s i m i l a r r e l a t i o n -ship between aggressiveness and motor a c t i v i t y i n a s t r a i n of white mice selected f o r aggressiveness and d o c i l i t y . In a complex environment l i k e our coastal forests the number of s o c i a l contactsa mouse makes must depend, at least i n part, on how active i t i s . In t h i s case the aggressive mice would be more l i k e l y to encounter and threaten a strange juvenile. FIELD EXPERIMENTS Isolated Plot Experiments The laboratory tests showed that juveniles grew poorly when placed with aggressive adults. However, i t may be argued that t h i s r e s u l t was a laboratory a r t i f a c t 17 r e s u l t i n g from crowded conditions and confinement. Therefore the prediction was retested i n natural habitat at normal population d e n s i t i e s . Methods Two is o l a t e d woodland plots were used to test the prediction that aggressive adults reduce juvenile s u r v i v a l . The f i r s t was a plot of about 3 1/2 acres ( S a d l e i r 1 s 1965 plot B). A grid of 19 traps (three l i n e s of f i v e traps each and one l i n e of four traps) was set on th i s plot for f i v e days and a l l captured animals were removed. Then four aggressive male adults were released onto the plo t . One week l a t e r the plot was retrapped to census the surviving adults, and 13 juveniles were released. The plot was retrapped on days 4, 7, 11, and 14 after the juveniles were released, to census the surviving juveniles and measure t h e i r growth rates. After the 14th day the plot was intensively trapped to capture any juveniles that had been missed. Whatever adults had survived were retested at the end of the experi-ment to be sure they had retained t h e i r aggressiveness. After a l l the surviving mice had been removed a duplicate experiment was performed using d o c i l e adults. Six experiments (three of each type) were performed on t h i s plot between May and October 1964, and f i v e between these months i n 1965. The second plot was much smaller, about 1 1/2 acres (Sadleir's 1965 plot A). 18 It was used only i n 1965, and f i v e experiments were performed on i t concurrent with those on plot B. The experimental technique used on t h i s plot was the same as that used on plot B except that three adults and ten juveniles were released i n i t i a l l y . On plot B adults were released at trap s i t e s 1, 6, 11, 16, and on plot A (seven trap s i t e s , two traps at each si t e ) at s i t e s 1, 4, and 7 (Fig. 4). On plot B juveniles were released at s i t e s 2, 3, 5, 7, 8, 9, 10, 12, 13, 14, 15, 17, 19, and on plot A one juvenile was released at each of s i t e s 1, 3, 5, 7, and two juveniles at each of s i t e s 2, 4, 6. Nest boxes from Longworth l i v e - t r a p s , supplied with food and cotton bedding were used as release boxes. Both plots A and B o r i g i n a l l y supported a resident population of deermice so the habitat was sui t a b l e . Four i s an average summer complement of males f o r a plot the siz e of plot B; three was perhaps an overestimate f o r plot A (average for a l l plots i n the summer of 1962 and 1963 was s l i g h t l y more than one male per acre). S i m i l a r l y 13 juveniles represent a reasonable juvenile production from four females on plot B (average l i t t e r s i z e 4.5), and ten a reasonable number for three females on plot A. Figure 4. Diagram of plots A and B showing approximate locations of trap s i t e s . Table V Survival of juveniles on plots A and B when adult aggression was constant. Plot B: Aggressive Adults No. of Adults No. of Juveniles Date Surviving to Surviving out of 13 day 14 day 4 7 , 11 May '64 1 _ 6 _ 5 Jun.-Jul. '64 2 7 4 4 2 Aug. '65 2 6 4 4 4 X i.75 6.5 4. 7 4 3 Plot B: Docile Adults No. of Adults No. of Juveniles Date Surviving to Surviving out of 13 day 14 day 4 7 11 Jun. '64 2 10 7 3 3 J u l . '64 3 12 11 10 " 9 Sep. '64 3 12 12 12 12 Sep. '65 3 13 10 10 8 X 2.8 •11.8 10 8.8 8 14 14 Plot A: Aggressive Adults No. of Adults No. of Juveniles Date Surviving to Surviving out of 13 day 14 day 4 7 11 14 J u l . *65 3 7 4 3 1 Plot A: Docile Adults No. of Adults No. of Juveniles Date Surviving to Surviving out of 13 day 14 day 4 7 11 14 Jun. '65 1 8 4 2 1 Aug. '65 3 6 5 4 4 Oct. '65 3 8 7 7 6 X 2.3 7.3 5.3 4.3 3.7 Table VI Growth r a t e s of j u v e n i l e s on p l o t s A and B when ad u l t a g g r e s s i o n was constant. P l o t B: A g g r e s s i v e A d u l t s J u v e n i l e Growth (X g.) Date day 4 7 11 14 May '64 -0.7 0.0 - J u l . '64 -0.9 -0.5 1.1 3.0 Aug. '65 -0,8 1.0 1.8 2.2 X -0.85 -0.1 1.5 1.7 P l o t B: D o c i l e A d u l t s J u v e n i l e Growth (X g.) Date day 4 7 11 14 Jun. *64 -0.2 0.9 2.1 3.7 J u l . '64 -0.1 0.1 1.1 2.6 Sep. '64 0.3 0.3 2.5 2.8 Sep. '65 -0.1 0.7 0.6 •1.1 X -0.02 0.5 1.6 2.6 P l o t A: Aggressive A d u l t s J u v e n i l e Growth (X g.) Date day 4 11 14 J u l . '65 -0.6 0.0 0.7 2.6 P l o t A: D o c i l e A d u l t s J u v e n i l e Growth (X g.) Date day 4 7 11 14 Jun. '65 0.1 0.4 0.0 1.0 Aug. '65 -0.1 -0.5? 2.2 Oct. '65 0.3 0.25 0.5 2.0 X 0.1 0.05 0.25 1.7 Table VII Survival of juveniles on plots A and B when adult aggression changed. Plot B: Aggressive—Docile Adults No. of Adults No. of Juveniles Date Surviving to Surviving out of 13 day 14 day 4 7 11 14 Aug. '64 3 11 11 11 11 May '65 2 11 7 3 3 Jun. - J u l . '65 4 11 8 6 3 X 3 11 8.7 6.7 6 Plot B: Docile—Aggressive Adults No. of Adults No. of Juveniles Date Surviving to Surviving out of 13 day 14 day 4 7 11 14 J u l . '65 1 7 4 3 1 Plot A: Aggressive—Docile Adults No. of Adults No. of Juveniles Date Surviving to Surviving out of 13 day 14 day 4 7 11 14 Sep. '65 3 8 7 7 7 Table VIII Growth r a t e s of j u v e n i l e s on p l o t s A and B when adu l t a g g r e s s i o n changed. P l o t B: A g g r e s s i v e — D o c i l e A d u l t s J u v e n i l e Growth (X g.) Date day 4 7 11 14 Aug. '64 -0.4 0.3 1.4 2.5 May '65 0.3 0.7 1.7 2.8 Jun. - J u l . '65 0.5 1.7 2.2 2.7 X 0.1 0.9 1.8 2.7 P l o t B: D o c i l e — A g g r e s s i v e A d u l t s J u v e n i l e Growth (X g.) Date day 4 7 11 14 J u l . '65 -0.9 0.1 -0.2 2.5 P l o t A: A g g r e s s i v e — D o c i l e A d u l t s J u v e n i l e Growth (X g.) Date day 4 7 11 14 Sep. '65 -0.6 -0.8 1.0 1.6 19 Results The s u r v i v a l and growth of juveniles released onto the plots when d o c i l e adults were present was considerably better than the s u r v i v a l and growth of juveniles released onto the plots when aggressive adults were present (Tables V and VI). In four experiments adults which were aggressive when released were d o c i l e when retested at the end of the experiment. In one experiment adults which were d o c i l e when released were aggressive when retested. These experiments were not included i n the s t a t i s t i c a l analysis. Survival and growth i n these experiments were intermediate (Tables VII and VIII) . Differences i n juvenile s u r v i v a l rates on plot B were compared i n an analysis of variance; treatments were s i g n i f i c a n t (F = 140.6 P<0.001). Because few experiments were done on plot A and the r e s u l t s varied widely I f e l t that separate s t a t i s t i c a l treatment of the data from plot A would be meaningless. However, s u r v i v a l rates from plots A and B may be lumped and tested together i n an analysis of variance. The lumped data s t i l l show that aggressive males reduce juvenile s u r v i v a l s i g n i f i c a n t l y more than d o c i l e adults do (F = 18.5 P< 0.001), Differences i n juvenile growth rates between tr e a t -ments on plot B are also s i g n i f i c a n t (F = 18.3 P<0.025). 20 Growth rate data from plots A and B may again be lumped. When t h i s i s done the differences are s t i l l s i g n i f i c a n t (F = 4.50 P<0.05). The most s t r i k i n g thing about these growth rates i s the great loss of weight shown during the f i r s t four days by juveniles released onto plots with an aggressive male population. This weight loss i s i n marked contrast to the mild loss or gain shown by juveniles released onto the plots when d o c i l e adults were present. In experiments with d o c i l e adults, juveniles sur-vived better i n the l a t t e r part of the summer. The probabil-i t y that t h i s improvement i n s u r v i v a l was due to chance i s low (F = 9.95 P<0.01). However, as w i l l be pointed out l a t e r , the b i o l o g i c a l s i g n i f i c a n c e of such a r e s u l t may be questioned. As f a r as juvenile s u r v i v a l and growth are con-cerned, experiments i n which aggressive adults became do c i l e are more l i k e experiments with d o c i l e males (Tables VII and VIII). S i m i l a r l y the one experiment i n which d o c i l e males became aggressive resembles an experiment with aggressive adults. Probably the changes i n aggressiveness occurred very quickly. Loss of data from these experiments might have been avoided, therefore, i f the adults had been retested just before the juveniles were released, as well as at the end of the experiment. 21 Despite the fact that densities on the isolated plots were lower than normal, owing to the loss of some of the adults, the r e s u l t s of the laboratory experiments were confirmed. It seems j u s t i f i a b l e to conclude from these experiments that the aggressiveness of adults i n summer reduces the s u r v i v a l of early l i t t e r s , and that d e c l i n i n g aggressive-ness i n the f a l l permits the rapid recruitment of juveniles at t h i s time. A r t i f i c i a l Immigration onto Trapped Out and Natural Plots Methods The hypothesis also predicts that, besides reducing juvenile s u r v i v a l , aggressive resident animals should hinder the s e t t l i n g of adult immigrants. The following experiments were designed to test t h i s prediction. The experiments were performed on two trapping grids located i n the Endowment forest around the University campus. Each grid consisted of four l i n e s of f i v e traps with about 15 paces between each trap. Eight experiments were performed on these grids, three i n 1964 and f i v e i n 1965. Early i n 1965 one of the grids used i n 1964 was bulldozed and a new grid had to be set up i n a d i f f e r e n t part of the fores t . In addition to these eight experiments three more were done i n 1964 on grids half the s i z e . 22 At the beginning of each experimental series the grids were trapped f o r f i v e days. On one grid the trapped animals were marked and released, and on the other they were removed. At the end of the f i v e days, s i x adult deermice (three males and three females) were released at the centre of each g r i d . The grids were retrapped four and seven days l a t e r and the s u r v i v a l rates of these a r t i f i c i a l immigrants recorded. After the seventh day trapping ceased for one week, then a new experiment was begun. With each new experiment, the gri d that had been trapped out i n the previous experiment became the grid with the resident population and vice versa. Three adults were released instead of s i x i n the experiments using half s i z e grids. Instead of using the same grids each time and alternating the grid which was trapped out, new grids were set out f o r each experiment. Otherwise the procedure on the smaller grids was the same as that out-lined above. Results The released mice were more successful i n estab-l i s h i n g themselves on trapped out plots than on plo t s with resident animals (Table IX). For both trapping days the d i f -ferences i n the a b i l i t y of mice to establish themselves on the two plots were s i g n i f i c a n t (day 4, t = 1.935 P<0.05; CQ o o i d 00 CD to to CO w co to > CH CH C _ S M C C C C P CD iq P 3 S"< cn • v j CD co cn cn cn id co >d 01 C5 C5 05 G5 id co t o M co CO id Cn CO 05 to cn co co cn H H M O H »d co t o <l CO CO id id ts3 05 tO I-1 CO CO CO M CO M M CO > C C H > C H C H C C C C C C Oq M M O P M £3 . V J V J • V J CD M CD Oi id 05 00 <1 05 id C5 CO CO CO C5 05 05 O r i g i n a l No. of Resi d e n t s No. of Immigrants Released id cn CO cn cn to to M co M cn t o i - 1 o t o t o M O H H H H 1 ^ co cn co cn t o cn t o t o \~> CO r-> co 10 O O N tO M O M H H H M R e s i d e n t s Immigrants to Unoccupied Area Immigrants to Occupied Area D i f f e r e n c e R e s i d e n t s Immigrants t o Unoccupied Area Immigrants to Occupied Area D i f f e r e n c e P cr r - 1 CD HH X P CQ S3 C 1 a ^ < o O H -• 3 < c t p O t-> C •a o < o < c t P co » a OQ < 3 H" P c t c t H 0 to a P CD CD cn i- i H- CD id a P CD CO S CD c t a •a o O 53 o •a c t • c 0 t—1 CQ P c t C c t 4 H- P <5 O t 3 13 13 < CO CD • a S3 OQ 0 c c t c t o •o a r - 1 p o v ; c t CO < i 23 day 7, t = 2.972 P<0.01, paired sample t t e s t ) . Despite the fact that the mice were more successful i n e s t a b l i s h i n g themselves on the trapped out plots, a con-siderable number did s e t t l e down on the plots with resident populations. Not only that but some of the normal resident animals moved away after the a r t i f i c i a l immigrants were introduced (Table IX). Flooding the grids with a r t i f i c i a l immigrants may have caused a breakdown i n the s o c i a l r e l a t i o n -ships of the residents, allowing the population to reassort i t s e l f . Andrzejewski e_t a l (1963) observed that introducing a large number of strangers into an established colony of white mice caused disruption of the e x i s t i n g population structure. Residents fought with each other as well as with the newcomers. When the colony f i n a l l y s e t t l e d down some of the new animals had been accepted and some of the o r i g i n a l colony members were dead. Homing i n Peromyscus In the 1964 tests of immigrant success, animals were sometimes moved from one plot to another. When t h i s was done several instances of homing were observed. In fact two experiments had to be terminated because of homing. In 1965 t h i s problem was avoided by releasing only animals which had been kept i n the laboratory over the winter. The nine 24 instances of homing observed i n 1964 are reported i n Table X. The most in t e r e s t i n g part of these observations i s that s i x of the nine mice homed afte r being released onto trapped out p l o t s . The stimulus that makes a mouse home when there are no residents to hinder s e t t l i n g i n the release area presents a perplexing problem. This sort of homing may shed some l i g h t on the s o c i a l organization of natural populations and w i l l be discussed l a t e r . Neighbour and Stranger Responses One of the more recent discoveries i n ornithology i s that a t e r r i t o r i a l bird i s more tolerant of his immediate neighbours than he i s of complete strangers (Stenger and F a l l s 1959, F a l l s and Brooks 1965). It seemed worthwhile to test the p o s s i b i l i t y that there i s reduced antagonism between male Peromyscus on adjacent home ranges. Any reduction i n antagonism would indicate that deermice are able to recognize at l e a s t . t h e i r immediate neighbours. Methods Grids of l i v e - t r a p s were set i n the forest around the campus. Males captured at the same s i t e or adjacent s i t e s were placed together i n a neutral arena, and t h e i r behaviour recorded f o r 10 min. Subsequently the i n d i v i d u a l Table X Nine i n s t a n c e s of homing i n P. m. auste r u s . Days Between Sex I n i t i a l Capture And Release m m m m m m f f m 12 12 1 3 3 3 2 2 3 D i s t a n c e Homed 300 yd 300 yd 600 yd 600 yd 600 yd 600 yd 600 yd 600 yd 1 m i l e P l o t Type Homed From To Time Days (Max) 14 14 4 6 4 4 4 4 4 - Unoccupied p l o t + Occupied p l o t Table XI Mean number of aggressive responses of neighbours and strangers dur-ing 10 min bouts i n a neutral arena. Neighbours Strangers N Threat Chase Agg/Groom Fight N Threat Chase Agg/Groom Fight 11 1.73 0,45 0.00 0.27 12 6.7 0.75 -.1.1 0.67 Range 0-5 0-3 0-3 0-15 0-7 0-5 0-4 25 males were placed i n the arena with a strange member of the laboratory stocks, and behaviour again recorded f o r 10 min. Encounters between neighbours and strangers were randomly ordered to eliminate any e f f e c t s of experience i n the arena. Acts recorded were: threat, chasing, f i g h t i n g , aggressive grooming, grooming ( f r i e n d l y ) , naso-nasal, naso-anal, washing, exploring, and mutual upright. Results When encounters between neighbours were compared with encounters between strangers, i t was found that only the aggressive acts showed large and consistent differences. Means and ranges f o r each of these acts are presented i n Table XI. The aggressive acts were summed for each animal and the differences between the two groups were tested by means of a Mann-Whitney U test f o r ranked scores. The d i f -ferences were s i g n i f i c a n t (U = 27 P<0.01). DISCUSSION On the basis of Sadleir's (1965) des c r i p t i v e study, and the experimental work described i n t h i s paper i t seems reasonable to conclude that s o c i a l i n t e r a c t i o n determines population densities i n Peromyscus maniculatus austerus. There can be l i t t l e doubt that males show seasonal changes 26 i n aggressiveness, and that these changes are regulated by changing environmental conditions. Increasing aggressiveness in the spring a f f e c t s population densities through i n t o l e r -ance fo r strange animals and the establishment of i n d i v i d u a l home ranges. As Sadleir (1965) suggests, early l i t t e r s survive poorly because they must compete with aggressive adults f o r a place to l i v e . It might be argued that juveniles seldome die from encounters with aggressive adults i n the laboratory experi-ments, and that t h i s r e s u l t refutes the hypothesis that aggressive adults a f f e c t juvenile s u r v i v a l i n the f i e l d . It i s easy to reconcile low mortality i n laboratory experiments with high losses i n f i e l d experiments when one remembers that laboratory experiments do not permit d i s p e r s a l . In the f i e l d competition between adults and juveniles f o r space would probably r e s u l t i n the emigration of a l o t of juveniles. Those juveniles that persisted i n the f i e l d experiments grew poorly for the f i r s t few days, but l a t e r appeared to have been accepted into the population, and thereafter grew well. Only the e f f e c t of adult aggression on growth was measured i n the laboratory experiments, and i n t h i s respect they agree well with the f i e l d experiments. I n t r a s p e c i f i c aggression i n the f i e l d between small nocturnal rodents l i k e deermice i s d i f f i c u l t to demonstrate 27 u n e q u i v o c a l l y . The high l e v e l of p o s s e s s i v e n e s s shown by some males f o r a home cage, and the poor success of immigrants i n t o n a t u r a l p o p u l a t i o n s are good i n d i r e c t evidence f o r t e r -r i t o r i a l defence i n the f i e l d , at l e a s t between s t r a n g e r s . Terman (1961) made a few d i r e c t o b s e r v a t i o n s of the response of a r e s i d e n t animal to a strange animal i n f i e l d e n c l o s u r e s . H i s g e n e r a l c o n c l u s i o n was that i n t r a s p e c i f i c a g g r e s s i o n was r a r e . However, he was u s i n g l a b o r a t o r y bred s t o c k s , and I have some i n d i c a t i o n s that my own l a b o r a t o r y bred mice behave d i f f e r e n t l y from w i l d mice. Whatever the method of communi-c a t i o n between animals i n the f i e l d , an animal's chance of s e t t l i n g i n an area i s a f f e c t e d by the a g g r e s s i v e n e s s of the r e s i d e n t animals. The way that a g g r e s s i o n and the o t h e r b i t s of d a t a presented above f i t i n t o the complex problem of animal i n t e r r e l a t i o n s i n the f i e l d w i l l be c o n s i d e r e d next. The S o c i a l System i n Peromyscus The r e l a t i o n s h i p s between i n d i v i d u a l s i n a Peromyscus p o p u l a t i o n appear d i f f e r e n t i n winter and summer. Some authors have suggested that i n winter the mice l i v e t o g e t h e r i n s m a l l f a m i l y or s o c i a l groups (Howard 1949, McCabe and Blanchard 1950). The evidence f o r such clumping i s s l i g h t . However, home ranges are about the same s i z e i n winter and summer (Healey unpub.), h i g h e r numbers are present i n w i n t e r , and i n t r a s p e c i f i c a g g r e s s i o n i s p r a c t i c a l l y non-28 existent, so clumping i s not u n l i k e l y . In spring the s i t u -ation changes. Generally more mice survive the winter than can l i v e together compatibly i n the breeding season, so that in spring there i s a d i s p e r s a l period (Sadleir 1965, McCabe and Blanchard 1950, Howard 1949). Some animals take up home ranges or t e r r i t o r i e s where they overwintered, and the excess moves away. Probably the animals which s e t t l e down at once are either the adults resident there i n the f a l l of the preceding year, or t h e i r progeny. Success i n s e t t l i n g no doubt depends on an animal's a b i l i t y to achieve s o c i a l dominance over i t s winter mates. The d i s p e r s a l period produces a s e t t l e d breeding population and a wandering group. Wanderers probably do not take part i n breeding and are more vulnerable to predators. The maintenance of the balance between residents and wanderers presents an i n t e r e s t i n g problem. We know from laboratory studies that several factors impart a s o c i a l advantage to an animal. These are: (1) past successes i n i n t r a s p e c i f i c encounters (Scott and Frederickson 1951), (2) sexual maturity (Beeman 1947), (3) f a m i l i a r i t y with the area where the encounter takes place (Petrusewicz 1959, Barnett 1964, t h i s study), (4) presence of f a m i l i a r animals (Petrusewicz 1959). The s e t t l e d animal possesses a l l four of these. By winning the right to s e t t l e he has presumably been successful more often than not i n i n t r a s p e c i f i c combat. 29 He has the advantage of f a m i l i a r surroundings, sexual matur-i t y , and f a m i l i a r neighbours. Wanderers have only sexual maturity i n t h e i r favour, and so are at a disadvantage i n competition with residents. Weanlings are at the bottom of the l i s t , possessing no s o c i a l advantage at a l l . If a ter-r i t o r y becomes vacant, therefore, a wanderer would have a better chance of taking over than a weanling. On the other hand, i n the absence of a wanderer i t should be possible for a weanling to set up permanent residence i n a vacant t e r r i -tory. In t h i s scheme wanderers represent genes dispersing through the population which can contribute to the gene pool only i f they f i n d a place to s e t t l e . Because an animal i n a resident population posses-ses a psychological and reproductive advantage over wandering members of the population, gaining resident status must be very desirable to the wanderer. Also the resident animal displaced from his normal population niche should suff e r anxiety and show appetitive behaviour directed toward regaining f a m i l i a r surroundings. Such appetitive behaviour would explain the homing from trapped out plots observed in 1964. There i s some additional evidence i n support of the s o c i a l system outlined above f o r Peromyscus maniculatus. Dr. Paul Anderson (pers. com.) has suggested that i n the 30 p o p u l a t i o n o f house mice he s t u d i e d on Great G u l l I s l a n d (Anderson e t a l 1964) j u v e n i l e s were more l i k e l y t o be r e p r o d u c t i v e l y s u c c e s s f u l i f they s e t t l e d near t h e i r b i r t h p l a c e . Rasmussen (1964), has e v i d e n c e t h a t gene f l o w t h r o u g h a c o n t i n u o u s p o p u l a t i o n of P. m a n i c u l a t u s i s r e s t r i c -t e d , and t h a t t h e a c t u a l p a n m i c t i c u n i t i s s m a l l (10-75 a n i m a l s ) . T h i s u n i t i s t i n y compared w i t h t h e d i s p e r s a l c a p a b i l i t i e s o f d e e r m i c e , and i n d i c a t e s t h a t an a n i m a l ' s chances of b r e e d i n g a r e s e v e r e l y l i m i t e d when i t moves any d i s t a n c e from i t s b i r t h p l a c e . I t i s i m p o r t a n t f o r an a n i m a l to g a i n r e s i d e n t s t a t u s , but e s t a b l i s h e d a n i m a l s r e s i s t t h e s e t t l i n g o f s t r a n g e r s , so t h a t some s o r t o f c o m p e t i t i v e i n t e r a c t i o n must o c c u r between r e s i d e n t s and a n i m a l s which a r e l o o k i n g f o r a p l a c e t o s e t t l e . However, a h i g h l e v e l o f i n t e r a c t i o n among e s t a b l i s h e d a n i m a l s would waste energy. The measure-ment of i n t e r a c t i o n between n e i g h b o u r a n i m a l s and s t r a n g e r s i n d i c a t e d t h a t i n t e r a c t i o n between n e i g h b o u r i n g a n i m a l s i s i n h i b i t e d . The s o c i a l u n i t t h e n c o m p r i s e s an a n i m a l and i t s n e i g h b o u r s , among whom mutual a g g r e s s i o n i s r e d u c e d , and whose range b o u n d a r i e s a r e m a i n t a i n e d by h a b i t and mutual a v o i d a n c e . The s o c i a l system d e s c r i b e d does not e x c l u d e the p o s s i b i l i t y o f a wanderer s u p p l a n t i n g a r e s i d e n t ; i t merely 31 makes i t e x t r e m e l y u n l i k e l y . I n e x p e r i m e n t s where i m m i g r a n t s were r e l e a s e d onto a g r i d w i t h a r e s i d e n t p o p u l a t i o n , the sudden i n f l u x of a l o t of s t r a n g e a n i m a l s caused a p o p u l a -t i o n r e s h u f f l e w i t h some of the e s t a b l i s h e d r e s i d e n t s moving out and some of t h e r e l e a s e d a n i m a l s s e t t l i n g i n . These e x p e r i m e n t s p r o b a b l y r e p r e s e n t extreme c a s e s of i n t e r a c t i o n between r e s i d e n t s and wanderers, s i n c e wanderers seldom a r r i v e i n l a r g e groups. The r e s h u f f l e s d i d show t h a t t h e r e i s a degree of v a r i a b i l i t y i n t h e optimum summer p o p u l a t i o n , making i t even more l i k e l y t h a t summer p o p u l a t i o n s are con-t r o l l e d by b e h a v i o u r r a t h e r than some q u a l i t y o f the e n v i r o n -ment . A l t h o u g h a d u l t a g g r e s s i o n i s i m p o r t a n t i n r e g u l a -t i n g p o p u l a t i o n d e n s i t y , o t h e r f a c t o r s may be o p e r a t i n g as w e l l . When j u v e n i l e s are r e l e a s e d onto p l o t s w i t h d o c i l e r e s i d e n t s one would e x p e c t t h a t chance m o r t a l i t y f a c t o r s would cause wide v a r i a b i l i t y i n s u r v i v a l . The s u r v i v a l of j u v e n i l e s r e l e a s e d onto p l o t s w i t h a g g r e s s i v e r e s i d e n t s s h o u l d be u n i f o r m l y low. T h i s i s what happened. However, on both p l o t s A and B i n e x p e r i m e n t s w i t h d o c i l e r e s i d e n t s t h e s u r v i v a l o f j u v e n i l e s improved p r o g r e s s i v e l y from s p r i n g t o f a l l . P oor j u v e n i l e s u r v i v a l i n t h e s p r i n g even w i t h d o c i l e r e s i d e n t s s u g g e s t s t h a t o t h e r f a c t o r s a s s o c i a t e d w i t h season may be o p e r a t i n g t o enhance the e f f e c t s o f a g g r e s s i v e b e h a v i o u r ; f a c t o r s which may o r may not be a s s o c i a t e d w i t h 32 behaviour. Up to t h i s point females have been ignored. No car e f u l quantitative study was made of the e f f e c t s of females on juveniles. However, there i s some evidence that t h e i r responses to strange animals are s i m i l a r to the responses of the males. Females occupy i n d i v i d u a l home ranges i n the summer, as do the males. If male and female home ranges are plotted separately, male home ranges are mutually exclusive, and so are female home ranges. How-ever, a male and a female home range may overlap completely (Burt 1940, S t i c k e l 1960). There i s probably competition within sexes for space, then, but no, or very l i t t l e com-p e t i t i o n between sexes. Like males, female P. m. austerus w i l l defend a home cage against a stranger of the same sex. This contradicts Eisenberg's (1962) observation that during induced t e r r i t o r i a l c o n f l i c t between pairs of Peromyscus the females play a passive r o l e while the males f i g h t . Nevertheless I have observed that when females are alone many show a high l e v e l of cage possessiveness. Female aggressive postures are quite s i m i l a r to those of the male. The resident s o c i a l group probably behaves as a unit then, with males repulsing strange males and females repulsing strange females, but with intragroup aggression inh i b i t e d i n the intere s t of economy. 33 The S e l e c t i v e Advantage of the S e l f - R e g u l a t o r y System The f a c t t h a t s e l f - r e g u l a t o r y systems have evolved suggests t h a t they must g i v e some advantage to the s p e c i e s . However, what advantage t h e r e i s i n a system t h a t r e g u l a r l y d e s t r o y s most of the y e a r l y p r o d u c t i o n of new animals p r e s -ents a problem. Weather, p r e d a t o r s , p a r a s i t e s , i n f a c t the whole m i l i e u of a p o p u l a t i o n a f f e c t s numbers and s u r v i v a l . But are these alone s u f f i c i e n t to regulate p o p u l a t i o n d e n s i t y w i t h i n . p e r m i s s i b l e l i m i t s ? I doubt i t . To be s u c c e s s f u l a s p e c i e s must be a b l e to s u r v i v e the bad years as w e l l as the good y e a r s . In o r d e r to do t h i s the r e p r o d u c t i v e p o t e n t i a l of the animals must be geared to the i n f r e q u e n t c a t a s t r o p h i c event . The p o p u l a t i o n cannot p r e d i c t i n advance a harsh w i n t e r , o r a sudden i n f l u x of p r e d a t o r s , o r an outbreak of d i s e a s e . Consequently the p o p u l a t i o n s must p r o v i d e a b u f f e r of excess animals on the o f f chance that a random c a t a s t r o p h e w i l l o c c u r . However, the p o p u l a t i o n must not over e x p l o i t i t s h a b i t a t d u r i n g the good years e i t h e r . I s h a l l c o n s i d e r o n l y the s i t u a t i o n i n Peromyscus, , a l though the example p r o b a b l y h o l d s f o r s m a l l mammals i n g e n e r a l ( C h i t t y 1964, K i n g 1955), and w i t h m o d i f i c a t i o n f o r a l l p o p u l a t i o n s (Chapman 1962, Wynne-Edwards 1962). D u r i n g most of the b r e e d i n g season numbers are low, and the b u f f e r to any l o c a l c a t a s t r o p h e i s the cont inuous p r o d u c t i o n of 34 new a n i m a l s . But towards t h e end o f the b r e e d i n g season a r a p i d r e c r u i t m e n t o f j u v e n i l e s t a k e s p l a c e . T h i s r e c r u i t -ment p r o v i d e s the b u f f e r f o r any random c a t a s t r o p h e o v e r the l o n g n o n - b r e e d i n g s e a s o n . Depending on t h e w i n t e r and chance t h e r e a re t h r e e p o s s i b l e s i t u a t i o n s which may e x i s t i n l o c a l p o p u l a t i o n s i n the s p r i n g . (1) I n most y e a r s w i n t e r m o r t a l i t y i s low so t h a t many more a n i m a l s a re p r e s e n t i n s p r i n g than can be s o c i a l l y c o m p a t i b l e i n t h e b r e e d i n g season. ( 2 ) M o r t a l i t y may reduce t h e p o p u l a t i o n t o some number which can form a s o c i a l l y c o m p a t i b l e b r e e d i n g p o p u l a -t i o n . (3) M o r t a l i t y may reduce the p o p u l a t i o n t o t h e p o i n t t h a t too few a n i m a l s a re p r e s e n t t o e x p l o i t t h e h a b i t a t e f f i c i e n t l y . I f s i t u a t i o n (1) o b t a i n s d i s p e r s a l must o c c u r , presumably w i t h t h e most s o c i a l l y dominant a n i m a l s s e t t l i n g down and t h e i r l e s s f o r t u n a t e w i n t e r mates moving away. I t i s t h i s s p r i n g d i s p e r s a l p e r i o d which e n s u r e s t h a t a n i m a l s never become abundant enough t o o v e r - e x p l o i t the e n v i r o n -ment . I n s i t u a t i o n ( 2 ) no d i s p e r s a l would o c c u r , and t h e r e s i d e n t s would a l l o w no i m m i g r a n t s t o s e t t l e . 35 If s i t u a t i o n (3) occurs then either immigrants w i l l come i n from adjacent areas or early l i t t e r s w i l l survive to bring the number of animals back up to a l e v e l which can e f f i c i e n t l y exploit a l l the habitat. The point i s , that i n order to prevent extinction during years of high mortality the reproductive pot e n t i a l of the animals must be high, and some sort of mechanism must be evolved to get r i d of the excess during years of low mortal-i t y . This mechanism must be i n t r i n s i c to the population. In deermice i t i s dis p e r s a l and presumably the death of most of the excess. The mechanism could well be exclusion of the excess from breeding s i t e s (Carrick 1963), or some physio-l o g i c a l mechanism to prevent breeding and so allow natural mortality to reduce numbers (Crowcroft and Rowe 1958). Whatever the mechanism i t must operate through s o c i a l i n t e r -action. SUMMARY 1. In a recent paper Sadleir (1965) proposes that the su r v i v a l rate of juvenile Peromyscus maniculatus i s deter-mined by adult aggressiveness. 2. Sadleir's data on the seasonal changes i n male aggressiveness were retested and confirmed. In addition, laboratory experiments showed that adults on home ground 36 i n h i b i t j u v e n i l e growth w h i l e a d u l t s on n e u t r a l ground do n o t ; t h a t males have a g r e a t e r e f f e c t i n t h i s r e g a r d t h a n f e m a l e s ; and t h a t a g g r e s s i v e males i n h i b i t j u v e n i l e growth but d o c i l e males do n o t . 3. A g g r e s s i v e males were shown t o be s p o n t a n e o u s l y more a c t i v e than d o c i l e males. T h i s t r a i t may s e r v e t o i n c r e a s e t h e number o f c o n t a c t s between a g g r e s s i v e males and w e a n l i n g j u v e n i l e s i n t h e f i e l d . 4. The s u r v i v a l o f j u v e n i l e s i n c o m p e t i t i o n w i t h a g g r e s s i v e o r d o c i l e males was s t u d i e d e x p e r i m e n t a l l y i n the f i e l d . A g g r e s s i v e males had a s i g n i f i c a n t l y g r e a t e r e f f e c t on j u v e n i l e growth and s u r v i v a l than d i d d o c i l e males. 5. The s u r v i v a l o f a r t i f i c i a l i m m i g r a n t s onto t r a p p e d out p l o t s was compared w i t h the s u r v i v a l o f a r t i f i c i a l i m m i g r a n t s o n t o p l o t s w i t h r e s i d e n t p o p u l a t i o n s . S u r v i v a l was s i g n i f i c a n t l y b e t t e r on t r a p p e d out p l o t s , i n d i c a t i n g t h a t r e s i d e n t a n i m a l s r e s i s t t h e e s t a b l i s h m e n t o f newcomers. 6. Nine i n s t a n c e s o f homing were n o t e d ; i n s i x o f the n i n e i n s t a n c e s a n i m a l s homed from t r a p p e d out p l o t s . 7. The amount o f a g g r e s s i o n between a n i m a l s from a d j a c e n t home ranges was much l e s s than the a g g r e s s i o n between a n i m a l s which had never e n c o u n t e r e d each o t h e r . A n i m a l s from a d j a c e n t home ranges a re p r o b a b l y f a m i l i a r w i t h 37 one another, and maintain home range boundaries through mutual avoidance r a t h e r than o v e r t a g g r e s s i o n . 8. I t i s proposed that, d u r i n g the b r e e d i n g season an animal and h i s immediate neighbours act as an o r g a n i z e d s o c i a l u n i t . Aggression between members of the u n i t i s reduced to conserve energy, but each animal shows aggres-s i o n toward any strange animal attempting to s e t t l e w i t h i n h i s home range. The f u n c t i o n of such a g g r e s s i o n would be to keep numbers of mice w i t h i n c e r t a i n l i m i t s . The s e l e c -t i v e advantage of such a s e l f - r e g u l a t o r y mechanism i s d i s c u s s e d . ACKNOWLEDGMENTS T h i s study owes much to the s t i m u l a t i n g guidance of Dr. Dennis C h i t t y . The course of my t h i n k i n g was a l s o i n f l u e n c e d by Dr. R.M. S a d l e i r and Dr. J . E i s e n b e r g . In 1965 Miss G a i l O'Hagan pr o v i d e d t e c h n i c a l a s s i s t a n c e . To a l l o t h e r s who have o f f e r e d a d v i c e and help I am g r a t e f u l . T h i s study was supported by r e s e a r c h and t e a c h i n g a s s i s t a n t -s h i p s from the U n i v e r s i t y of B r i t i s h Columbia, and by a bursary and s t u d e n t s h i p from the N a t i o n a l Research C o u n c i l of Canada. 3 8 REFERENCES Anderson, P. K., L. C. Dunn, and A. B. Beasley. 1 9 6 4 . Introduction of a l e t h a l a l l e l e into a f e r a l house mouse population. Am. Nat. 9 8 , 5 7 - 6 4 . Andrzejewski, R., K. Petrusewicz, and W. Walcowa. 1 9 6 3 . Absorption of newcomers by a population of white mice. Ecologia Polska, Seria A. j L l , 2 2 3 - 2 4 0 . Barnett, S. A. 1 9 5 8 . An analysis of s o c i a l behaviour i n wild r a t s . Proc. Zool. Soc. Lond. 1 3 0 , 1 0 7 - 1 5 2 . . 1 9 6 4 . Social Stress. Viewpoints i n biology 3 . Ed. J. D. Carthy and C. L. Duddington. pp 1 7 0 - 2 1 8 . Beeman, E. 1 9 4 7 . E f f e c t of male hormone on aggressive behaviour i n mice. Physiol. Zool. J 2 0 , 3 7 3 - 4 0 4 . Burt, W. H. 1 9 4 0 . T e r r i t o r i a l behaviour and populations of some small mammals i n Southern Michigan. Misc. Pub. Mus. Zool. Univ. Mich. No. 4 5 , 1 - 5 8 . Carrick, R. 1 9 6 3 . E c o l o g i c a l s i g n i f i c a n c e of t e r r i t o r y i n the Australian Magpie, Gynnorhina t i b i c e n . Proc. XIII Intern. Ornithol. Congr. pp. 7 4 0 - 7 5 3 . Chapman, D. W. 1 9 6 2 . Aggressive behaviour i n juvenile coho salmon as a cause of emigration. J. Fish. Res. Bd. Canada. 19, 1 0 4 7 - 1 0 8 0 . Chitty, D. 1 9 6 4 . Animal numbers and behaviour, i n : Fish and W i l d l i f e : A Memorial to W. J. K. Harkness. Ed. J. R. Dymond. Longmans. Canada, pp. 4 1 - 5 3 . 39 Crowcroft, P. and F. P. Rowe. 1958. The growth of confined colonies of the wild house mouse (Mus musculus): The effect of d i s p e r s a l on female fecundity. Proc. Zool. Soc. Lond. 131, 357-365. Eisenberg, J. F. 1962. Studies on the behaviour of Peromyscus maniculatus gambelii and Peromyscus c a l i -fornicus p a r a s i t i c u s . Behaviour 19, 177-207. F a l l s , J. B. and R. J. Brooks. 1965. Studies on i n t r a -specif i c i n d i v i d u a l recognition by songbirds (abstract). Am. Zool. 5_, 225. Grant, E. C. and J . H. Mackintosh. 1963. A comparison of the s o c i a l postures of some common laboratory rodents. Behaviour 2_1, 246-259. Howard, W. E. 1949. Dispersal, amount of inbreeding, and longevity i n a l o c a l population of p r a i r i e deermice on the George reserve Southern Michigan. Contr. Lab. Vert. B i o l . Univ. Mich. No. 43, 1-50. King, J. A. 1955. Social behaviour, s o c i a l organization, and population dynamics of a b l a c k - t a i l e d prairiedog town i n the Black H i l l s of South Dakota. Contr. Lab. Vert. B i o l . Univ. Mich. No. 67, 1-123. Lagerspetz, K. 1964. Studies on the aggressive behaviour of mice. Annales. Acad. Scientiarum Fennecae Ser. B. 131, 1-131. 40 McCabe, T. T. and B. Blanchard. 1950. Three s p e c i e s of Peromyscus. Rood A s s o c i a t e s . Santa Barbara, C a l i f . 136,pp. Montgomery, K. C. 1955. The r e l a t i o n between f e a r induced by novel s t i m u l a t i o n and e x p l o r a t o r y behaviour. J . Comp. P h y s i o l . P s y c h o l . 48, 254-260. Petrusewicz, K. 1959. Research on r a p i d i t y of the forma-t i o n of i n t e r p o p u l a t i o n r e l a t i o n s and the sense of ownership of cages i n mice. B u l l . Acad. P o l o n a i s e S c i . CI I I , 7, 323-326. Rasmussen, David I. 1964. Blood group polymorphism and i n b r e e d i n g i n n a t u r a l p o p u l a t i o n s of the deermouse Peromyscus maniculatus. E v o l u t i o n 18, 219-229. S a d l e i r , R. M. S. F. 1965. The r e l a t i o n s h i p between a g o n i s t i c behaviour and p o p u l a t i o n changes i n the deer-mouse Peromyscus maniculatus (Wagner). J . Anim. E c o l . 34, 331-352. S c o t t , J . P. and E. F r e d e r i c k s o n . 1951. The causes of f i g h t i n g i n mice and r a t s . P h y s i o l . Z o o l . 24, 273-309. Stenger, J . and J . B. F a l l s . 1959. D i f f e r e n t i a l responses of male oven b i r d s to recorded songs of neighbouring and more d i s t a n t i n d i v i d u a l s . Auk 7^ 6, 343-351. S t i c k e l , L. F. 1960. Peromyscus ranges at hig h and low d e n s i t i e s . J . Mammal. 41, 433-441. 41 Terman, C. R. 1961. Some dynamics of s p a t i a l d i s t r i b u t i o n within semi-natural populations of prairie deermice. Ecol. 42, 288-302. Wynne-Edwards, V. C. 1962. Animal dispersion i n r e l a t i o n to s o c i a l behaviour. Hafner. New York. 653 pp. 42 Appendix I The graded s e r i e s i s a group of males o r i g i n a l l y e s t a b l i s h e d by Dr. S a d l e i r t o be used as a s t a n d a r d i n measuring t h e a g g r e s s i v e n e s s o f h i s f i e l d - c a u g h t a n i m a l s . E v e r y member of the s e r i e s was matched a g a i n s t e v e r y o t h e r member i n a n e u t r a l a r e n a , and t h e number of a g g r e s s i v e a c t s performed i n 10 min. was r e c o r d e d . These d a t a were used t o rank t h e males i n o r d e r o f a g g r e s s i v e n e s s . T e s t s o f a g g r e s s i v e n e s s i n t h i s s t u d y g e n e r a l l y i n v o l v e d an e x p e r i m e n t a l male on home ground w i t h a member of t h e graded s e r i e s used as an i n t r u d e r . F o r such a t e s t t o be r e l i a b l e t h e members of the graded s e r i e s must main-t a i n a c o n s t a n t l e v e l o f aggressiveness and t h e r e s p o n s e s o f t h e e x p e r i m e n t a l male must not be i n f l u e n c e d by t h e b e h a v i -our o f t h e i n t r o d u c e d a n i m a l . I t i s i m p r o b a b l e t h a t e i t h e r o f t h e s e two c o n d i t i o n s i s f u l f i l l e d by t h e graded s e r i e s . The a g g r e s s i v e n e s s o f the graded s e r i e s no doubt changes as t h e a n i m a l s age and s u f f e r more and more^ s o c i a l d e f e a t s i n t h e e x p e r i m e n t a l e n c o u n t e r s , t o say n o t h i n g o f p o s s i b l e endogenous s e a s o n a l and d i u r n a l changes i n b e h a v i o u r . Any b i a s o f t h i s s o r t was p a r t l y o f f s e t by p e r i o d i c a l l y r e t e s t -i n g t h e members of t h e graded s e r i e s a g a i n s t one a n o t h e r . They m a i n t a i n e d t h e i r r e l a t i v e p o s i t i o n s o f dominance, a l t h o u g h t h e amount of a g g r e s s i o n each male showed f l u c t u -43 ated c o n s i d e r a b l y . As the o r i g i n a l members of the s e r i e s aged they were r e p l a c e d by l a b o r a t o r y - r e a r e d males. The range of aggressiveness w i t h i n the s e r i e s of l a b o r a t o r y -r e a r e d males was l e s s than w i t h i n the s e r i e s of w i l d -caught males. I f e e l the l a b o r a t o r y - r e a r e d animals main-t a i n e d t h e i r i n d i v i d u a l l e v e l s of aggressiveness b e t t e r too. I t i s i m p o s s i b l e to e l i m i n a t e the e f f e c t s of the i n t r o d u c e d animal on the behaviour of the experimental animal. A f t e r one became f a m i l i a r with the graded s e r i e s , though, i t was p o s s i b l e to r e c o g n i z e such e f f e c t s and compensate f o r them wit h a d d i t i o n a l encounters. M u l t i p l e encounters added l i t t l e to the p r e c i s i o n with which a g g r e s s i v e o r d o c i l e animals were s e l e c t e d , however. In more than 90% of the t e s t s performed the r e s u l t s of the f i r s t encounter were con-firmed i n subsequent encounters. Appendix II In the b e h a v i o u r a l r e p e r t o i r e of the male, t h r e a t behaviour, c h a s i n g , f i g h t i n g , and a g g r e s s i v e grooming may be regarded as good i n d i c a t o r s of a g g r e s s i v e i n t e r a c t i o n . The q u e s t i o n i s , which of these a c t s , o r what combination of them, i s the most s e n s i t i v e i n d i c a t o r of aggressiveness i n males? T h i s q u e s t i o n was p a r t l y answered by p l o t t i n g each act against the sum of a l l f o u r a c t s f o r each bout. Data from the c o n t r o l s e r i e s i n the experiments on s e a s o n a l 09 c CD O CD rt- 1—' P P M rt-P TO O 3 TO CO 4 cr CD H-co »o CO » CD I rt-< CD CD 3 H) o c p aq CP9 CD CO CO < CD P a <-+ CO p a o in o o — ro o a O CM 10 2 0 3 0 4 0 5 0 T o t a l A g g r e s s i o n 6 0 7 0 9 C e o k_ o • « I .» 24 co ^ cn < - 8- , 1 0 2 0 3 0 4 0 5 0 T o t a l A g g r e s s i o n 6 0 7 0 o in o o to <D te o •= O <-> CM' O J I 8 0 10 2 0 3 0 4 0 5 0 6 0 T o t a l A g g r e s s i o n 7 0 8 0 ? o. i II • , » * . 8 0 10 2 0 3 0 4 0 5 0 T o t a l A g g r e s s i o n 6 0 7 0 8 0 44 changes i n aggressiveness were used. The four r e s u l t i n g scatter diagrams are shown i n Fi g . 5 . Only threats and chases show a consistent r e l a t i o n to t o t a l aggression, hence the sum of threats and chases was chosen as an index of aggressiveness. 

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