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The effect of torpor on pulmo-cutaneous water loss in Perognathus parvus Guthrie, Donald Raymond 1971

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11221 THE EFFECT OF TORPOR ON PULMO-CUTANEOUS WATER LOSS IN Perognathus parvus. t>y DONALD RAYMOND GUTHRIE B.S., V i r g i n i a Polytechnic I n s t i t u t e , 1963 M.S., V i r g i n i a Polytechnic I n s t i t u t e , 1965 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY In the Department of ZOOLOGY We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA December, 1971 In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I f u r t h e r agree t h a t permiss ion fo r e x t e n s i v e copying o f t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s . It i s understood that copying 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 ga in s h a l l not be a l lowed without my w r i t t e n p e r m i s s i o n . Donald R. Guthrie Department of ZOOLOGY The U n i v e r s i t y of B r i t i s h Columbia Vancouver 8, Canada Date Maroh 6, 1972 1 ABSTRACT Studies were conducted to determine the effect of torpor on the pulmo-cutaneous water loss of a small, fossorial desert rodent, Perognathus parvus. Since the pulmonary and cutaneous components of the water budget are strongly affected by ambient temperature and humidity, these losses are extreme-ly Important In determining the a b i l i t y of an animal to maintain positive water balance. Simultaneous measurements of pulmo-cutaneous water loss and metabolic rate were made over a range of ambient temperatures of 1 0 - 3 5 C for both torpid and normothermlo animals. Additional experiments were conducted to determine the ratio of pulmonary to cutaneous water loss, and to deter-mine the relationship of these losses to ambient temperature. Models of energy budgets and water budgets were constructed to assist in determining the effect of variable amounts of torpor over the range of ambient temperature of 0 - 3 0 C. It was found that normothermic animals could main-tain positive water balance from 0 - 2 0 C at 0% relative humid-i t y , but torpid animals were always in negative water balance under the same conditions. The in a b i l i t y of torpid animals to maintain positive water balance i s attributed to the necessity of balancing the relatively fixed cutaneous loss against the much-reduced metabolic water production. It i s concluded that torpor cannot serve as a water-conserving mechanism in this species at 0% relative humidity. i i TABLE OP CONTENTS Page INTRODUCTION 1 MATERIALS AND METHODS ^ Experimental animals ^ Metabolic rate and pulmo-cutaneous water loss .. b Pulmonary:cutaneous r a t i o of evaporative water loss 5 Cutaneous water loss and t o t a l surface area .... 5 Metabolic rate during arousal from torpor ...... 6 Data analysis 7 Construction of energy budget „ 7 Construction of water budget 8 RESULTS 9 Energy requirements 9 Evaporative water loss 17 DISCUSSION 28 Energy requirements 28 1 . Torpor 37 2 . Entry into torpor 39 3 . Arousal from torpor *K) Pulmo-cutaneous water loss • ^2 1 . Cutaneous water loss ^8 2 . Separation of cutaneous and pulmonary losses , Energy and water budgets 52 1 . Energy budget model 52. 2 . Water-loss model 5 ^ H i TABLE OF CONTENTS - CONTINUED Page MAJOR FINDINGS OF THE THESIS ; 76 LITERATURE CITED 79 APPENDICES i v LIST OF FIGURES Figure Page 1 R e l a t i o n s h i p of metabolic r a t e to ambient temperature 10 2 R e l a t i o n s h i p of metabolic r a t e of t o r p i d animals to ambient temperature 11 3 Thermoneutral zone 12 k R e l a t i o n s h i p of r e s p i r a t o r y quotient to ambient temperature lMr 5 R e l a t i o n s h i p of r e s p i r a t o r y quotient of t o r p i d animals to ambient temperature 15 6 Change i n metabolic r a t e d u r i n g a t y p i c a l e n t r y i n t o t o r p o r a t 15 C 16 7 Metabolic r a t e s and r e s p i r a t o r y q u o t i e n t s during a r o u s a l a t 10 C 18 8 R e l a t i o n s h i p of pulmo-cutaneous water l o s s of non-torpid animals to ambient temperature 19 9 R e l a t i o n s h i p of pulmo-cutaneous water l o s s of non-torpid animals to ambient temperature 20 10 R e l a t i o n s h i p of r e l a t i v e pulmo-cutaneous water l o s s of non-torpid animals to ambient temperature 21 11 R e l a t i o n s h i p of pulmo-cutaneous water l o s s of t o r p i d animals to ambient temperature 22 12 R e l a t i o n s h i p of pulmo-cutaneous water l o s s of t o r p i d animals to ambient temperature 23 13 R e l a t i o n s h i p of r e l a t i v e pulmo-cutaneous water l o s s of t o r p i d animals to ambient temperature ... 24 V LIST OF FIGURES - CONTINUED Figure Page lk Relationship of cutaneous water loss of dead animals to ambient temperature 26 15 Relationship of body surface area to body weight 27 16 Relationship of BMR of fossorial rodents to body weight 30 17 Relationship of conductance of fossorial rodents to body weight . 31 18 Relationship of metabolic rate to body weight .. 3k 19 Relationship of metabolic rate to body weight at 20 C 35 20 Relationship of metabolic rate to body weight at 30 C 36 21 Minimum metabolic rates in torpor 38 22 Relationship of relative pulmo-cutaneous water loss of non-torpid animals to ambient temperature k$ 23 Minimum pulmo-cutaneous water loss for several species of small mammals k7 24 Comparison of observed pulmo-cutaneous loss with predicted loss, based on theoretical pulmonary loss + cutaneous loss from Fig. Ik (non-torpid) 50 25 Comparison of minimum pulmo-cutaneous loss of torpid animals (X) with min. predicted values, based on cutaneous loss (Fig. Ik) and on theoretical pulmonary loss 51 26 Estimated energy and water budgets (17 g animal) (7 torpor periods/week) 55 v i LIST OF FIGURES - CONTINUED Figure Page 27 Estimated energy and water budgets (17 g animal) (1 torpor period/week) 56 28 Estimated water production and pulmo-cutaneous loss of a 17 g mouse (0% r.h.; no torpor) 60 29 Estimated water production and loss f o r a torpid 17 g mouse {0% r.h.) 6l 30 Net water loss of a 17 g mouse i n the torpid and i n the active state 62 31 Energy conserved and water expended by torpor (17 g mouse; 0% r.h.) Gk 32 Cost of conserving energy by torpor (17 g mouse; 0% r.h.) 65 33 Estimated energy and water budgets (10 g animal) (7 torpor periods/week) 66 3^ Estimated energy and water budgets (10 g animal) (1 torpor period/week) 67 35 Estimated energy and water budgets (25 g animal) (7 torpor periods/week) 68 36 Estimated energy and water budgets (25 g animal) (1 torpor period/week) 69 37 Estimated water production and pulmo-cutaneous loss f o r a 17 g mouse under f i e l d conditions ... 73 38 Estimated net water loss of active and torpid mice under f i e l d conditions 7^ INTRODUCTION 1 The p h y s i o l o g i c a l a d a p t a t i o n s of d e s e r t a n i m a l s , p a r t i c u l a r l y the w a t e r - c o n s e r v i n g a d a p t a t i o n s , have i n t e r e s t e d s c i e n t i s t s f o r decades. S c h m i d t - N i e l s e n and S c h m i d t - N i e l s e n , and t h e i r a s s o c i a t e s , s t i m u l a t e d much a d d i t i o n a l r e s e a r c h i n t h i s f i e l d w i t h t h e i r work b e g i n n i n g i n 1 9 4 8 and c o n t i n u i n g t o the p r e s e n t . The e a r l i e r l i t e r a t u r e was a d e q u a t e l y r e v i e w e d by S c h m i d t - N i e l s e n and S c h m i d t - N i e l s e n ( 1 9 5 2 ) and w i l l n o t be r e p e a t e d here i n d e t a i l . Other good r e v i e w s of v a r i o u s f a c e t s of the problem of a d a p t a t i o n t o d e s e r t environments a r e those of S c h m i d t - N i e l s e n ( 1 9 6 4 a, b ) , Hudson ( 1 9 6 4 ) , Hudson and Bartholomew ( 1 9 6 4 ) , and Chew ( 1 9 6 l , 1 9 6 5 ) . The phenomenon of h i b e r n a t i o n - a e s t i v a t i o n has a l s o a t t r a c t e d much i n t e r e s t , and the l i t e r a t u r e on t h i s t o p i c i s t r u l y v o l u m i n o u s ; r e c e n t r e v i e w s of t h i s l i t e r a -t u r e a r e those of K a y s e r ( 1 9 6 1 ) , Lyman and C h a t f i e l d ( 1 9 5 5 ) . Lyman ( 1 9 6 l ) , and Hayward ( 1 9 6 7 ) . As S c h m i d t - N i e l s e n and S c h m i d t - N i e l s e n ( 1 9 5 1 ) p o i n t e d o u t , the w a t e r b a l a n c e of d e s e r t r o d e n t s i s h i g h l y dependent upon m e t a b o l i c r a t e , s i n c e the i n t a k e of w a t e r , p r o d u c t i o n of m e t a b o l i c w a t e r , and l o s s of wat e r t h r o u g h e v a p o r a t i o n and u r i n e a r e a l l n e c e s s a r i l y r e l a t e d t o m e t a b o l i c r a t e . I t i s a well-documented f a c t t h a t some d e s e r t r o d e n t s e x h i b i t a summer t o r p o r , o r 2 aestivation, which results in drastic reduction in metabolic rate. Many authors have assumed that aestivation i s useful for water conservation. This assumption i s exemplified by the comment of MacMillen ( I 9 6 5 ): "Thus i t appears certain that cactus mice aestivate in their burrows during the summer, employing torpor as a water-conserving device The nature of the problem i s stated clearly by Hudson and Bartholomew (196I+): "The correlation between hot dry seasons and the incidence of estivation generally has been used as ipso facto evidence that a lack of moisture may induce summer torpidity. "It may be argued that the reduction in the resting metabolism of estlvators i s an adaptation for coping with high ambient temperatures when the small gradient of body temperature to a i r temperature limits the rate of heat loss. This reduction in metabolism entails a reduction in metabolic water production which i s at least partly counteracted by a reduction in respiratory water loss. Since these factors are in unknown proportions, a reduced metabolism may or may not be particularly advantageous to the water economy This thesis occupies a position at the cross-roads of these two main lines of research, desert adaptation and torpor, and i t s main objective i s to determine the magnitude of the 3 various components of the water budget so that the e f f e c t s of torpor on the water budget can be evaluated. This information i s not available i n the e x i s t i n g l i t e r a t u r e . The experimental animal used f o r t h i s study-was Perognathus parvus, the Great Basin pocket mouse, which reaches the northern l i m i t of i t s geographical d i s t r i b u t i o n i n the Osoyoos A r i d Zone i n B r i t i s h Columbia. Like most heteromyids, P. parvus can survive i n d e f i n i t e l y on a d i e t of dry seeds. The members of the genus can produce a very concentrated urine, have a low insensible water l o s s , and have the a b i l i t y to become torpid a t environmental temperatures below body temperature. A comparison of phys i o l o g i c a l data f o r various species of Perognathus with s i m i l a r data f o r other desert rodents indicates that the sub-family Perognathinae i s as well adapted f o r desert environments as other nocturnal, f o s s o r l a l rodents, such as Dipodomys. Persons interested i n descriptions of the range, type of habitat, and gross morphology of the species are referred to Iverson ( 1 9 6 7 ) . This thesis w i l l be devoted to the c o l l e c t i o n of data of energy expenditure and insensible water loss (pulmo-cutaneous water loss) over the range of environ-mental temperatures encountered i n the f i e l d , i n order to test the following hypothesis: Torpor w i l l not reduce the net water l o s s of Perognathus parvus, a t y p i c a l small desert rodent, at ambient temperatures of 0 to 30 C and at 0% r e l a t i v e humidity. MATERIALS AND METHODS Experimental Animals The animals f o r t h i s study were obtained from the low areas at the north-east end of Lake Osoyoos. Longworth traps were set i n a grid pattern. Trap success varied from 5% to 50% with an average of about 30%. Animals were held i n mesh-covered metal jam t i n s stacked i n cardboard boxes i n an air-conditioned motel room u n t i l the c o l l e c t i o n was complete. Lettuce and sunflower seeds were provided d a i l y . There was l i t t l e or no mortality under these conditions. The animals were maintained i n the laboratory on a d i e t of sunflower seeds (ad l i b . ) . The photoperlod i n the animal room was 16 hr l i g h t and 8 hr dark. Humidity was about 50% and temperature was about 20 C. Metabolic Rate and Pulmo-cutaneous Water Loss Oxygen consumption, carbon dioxide production, and pulmo-cutaneous water loss were measured simultaneously i n a closed-system respirometer s i m i l a r to that used by Schmidt-Nielsen and Schmidt-Nielsen ( 1 9 5 0 ) . A pump cir c u l a t e d a i r through the system at approximately 500 ml per minute. Pulmo-cutaneous water was absorbed by two tubes of Dr i e r ! t e . Carbon dioxide was absorbed by a tube of Llthasorb. Both water loss and carbon dioxide production were determined by weighing the tubes to the nearest 0 . 1 mg. A spirometer recorded the oxygen consump-5 tion on a kymograph, the oxygen being replenished auto-matically by a solenoid valve on an oxygen cylinder. The animal chamber, a o n e - l i t e r Jar, was submerged i n a water bath to maintain constant temperature. In normal operation, the animal was placed i n the jar over a pan of mineral o i l and allowed to remain f o r at l e a s t one hour before data were co l l e c t e d . The length of a measurement was usually one hour, but one-half hour periods were used i n some cases. Oxygen and carbon dioxide values were converted to standard temperature and pressure before metabolic rate and respiratory quotient were computed. Pulmonary : Cutaneous Ratio of Evaporative Water Loss Ten measurements of the pulmonary:cutaneous water l o s s r a t i o were made. A spe c i a l double-compartment chamber was constructed with a rubber diaphragm separating the compartments. The animal was placed i n the chamber with i t s head through a hole i n the diaphragm, e f f e c t i n g ' a separation of "head skin + pulmonary" evaporative water loss from the "body skin" evaporative water l o s s . Two pumps and two separate water-absorbing tubes were used. Since these animals r e s i s t such confinement, i t was necessary to tr a n q u i l i z e them with Tranimal (Hoffman -LaRoche) p r i o r to the measurements. Other procedures were the same as f o r normal operation. Cutaneous Water Loss and Total Surface Area Cutaneous water loss was measured on 7 animals. 6 Technical problems involving the separation of pulmonary and cutaneous losses prevented the use of l i v e animals f o r t h i s experiment. The animals were k i l l e d by an overdose of Nembutol ( l . p . ) . Each animal was placed i n the metabolism chamber, and cutaneous water loss was measured at half-hour i n t e r v a l s . Each animal was tested at three d i f f e r e n t ambient temperatures, t y p i c a l l y at 1 0 , 2 0 , and 30 C. A f t e r the cutaneous loss was measured, th# animal was removed from the chamber and weighed. The h a i r was removed with a depilatory cream, and a thin layer of s i l i c o n e rubber was applied to the entire body surface. A f t e r the rubber had cured, i t was peeled off the body i n one piece. The rubber skin was traced on heavy cardboard, and surface area was determined by weighing the cardboard. Metabolic Rate During Arousal From Torpor A l l systems used i n t h i s experiment were as described previously. A torpid animal was selected, and a f t e r i t s deep body temperature had been taken with a thermistor probe through the abdominal wall, i t was placed i n the respirometer as quickly as possible. Oxygen consumption was recorded u n t i l the onset of shivering, usually-about 10 to 15 minutes. The oxygen consumption f o r the remainder of the 30-minute test period was recorded separately by switching to a new set of tubes when shivering was observed. At the end of the test 7 period the animal was removed and the deep body temperature was taken as described. The t o t a l c a l o r i e s required f o r arousal were calculated by multiplying the t o t a l amount of oxygen used, by the c a l o r i c equivalent of oxygen at the observed R. Q. (respiratory quotient). Data Analysis The data were punched on computer cards and examined f o r meaningful l i n e a r r e l a t i o n s h i p s by l e a s t squares regression. Construction of Energy Budget A model f o r weekly energy expenditure was constructed using two independent variables, "hours i n torpor per week" and "ambient temperature". The time required f o r entering torpor was assumed to be constant since the v a r i a t i o n with temperature seems to be r e l a t i v e l y small f o r a l l ambient temperatures between 10 and 30 C. The time required f o r arousal i s a function of ambient temperature. To simplify the model, i t was assumed that entry and arousal occur, even at zero hours of torpor. The least-squares regression equations f o r metabolic rate f o r the active and the torpid states were used i n con-structing the model. Tot a l time i n torpor per week was varied from 0 to 140 hours. Ambient temperature (T^) was varied from 0 to 30 C. Body weight and number of periods of torpor per week were introduced as constants. The computer 8 produced three-dimensional plots of the model f o r each selected combination of body weight and number of torpor periods per week. Construction of Water Budget The same Independent variables and constants used i n the energy budget were used also i n the construction of a model f o r weekly water l o s s . Metabolic water i s dependent upon d i e t , so i t i s necessary to construct a model f o r each d i e t considered. Since Schmidt-Nielsen's calculations ( 1 9 5 U were based on a d i e t of pearl barley, i t was desirable to use that d i e t i n the model to f a c i l i t a t e comparisons with h i s data. Three-dimensional plots of t h i s model were prepared by computer as described f o r the energy model. 9 RESULTS Energy Requirements A l l metabolic rates reported i n t h i s thesis are re s t i n g metabolic rates of post-absorptive animals. Thus, at thermal n e u t r a l i t y , the metabolic rate reported i s the "basal",or standard, metabolic rate. The l i n e a r regressions of metabolic rate over a range of ambient temperatures are shown f o r non-torpid (normothermic) animals i n F i g . 1 and f o r torpid animals i n F i g . 2 . The regression f o r non-torpid animals was based on data only up to 30 C to avoid the non-linear portion of the curve. The extrapolation of t h i s l i n e i n t e r s e c t s the X-axis at J6 C. The mean body temperature of 10 normothermic mice was 3 5 » ^ 0 . To obtain an accurate estimate of the thermoneutral zone, addi t i o n a l measurements of metabolic rate were made at small i n t e r v a l s of T^ (ambient temperature) over the range of 25 to 35 C. The re s u l t i n g curves f o r 8 i n d i v i d u a l s are shown i n F i g . 3 . The mean thermoneutral point (32 C) and the mean metabolic rate at 35 C from these data were used to extrapolate the metabolic rate curve above 30 G i n F i g . 1 . The slope of the metabolic rate regression i n F i g . 1 was s i g n i f i c a n t l y d i f f e r e n t from zero at the 0 . 0 1 l e v e l of si g n i f i c a n c e . The slope of the regression i n F i g . 2 was s i g n i f i c a n t l y d i f f e r e n t from zero at the 0 . 0 5 l e v e l . Since the p r o b a b i l i t y of obtaining these slopes by chance i s less than 5%, we may IG. I RELATIONSHIP OF rvCTABOLIC RATE TO ANCIENT \_ TEMPERATURE Y = 8 . 6 0 6 - 0,2.397 X N = 84 9 4-AMBIENT TEMPERATURE ( C) FIG. 2 RELATIONSHIP OF METABOLIC RATE OF TORPID ANIMALS TO AMBIENT TQtf^TURE Y = 0-3841 + 0-0144X N = 32 a 4. B 1 7 1 6 1 5 1 4 1 3 1 S ± 1 1 H 1 1 1 1 1 1 1 1 1 1 1 1 h 0 10 15 ao 2 5 30 3 5 40 AM3IENT TEIVPERATLJRE ( C) FIG. 3 T H E R M O N E U T R A L Z O N E (EACH CURVE R E P R E S E N T S ONE ANIMAL) 12 AMBIENT TEMP. (°C) 13 assume that they are true i n d i c a t o r s of the b i o l o g i c a l s i t u a t i o n s . :k To express metabolic r a t e i n c a l o r i c terms, i t i s necessary to know the r e s p i r a t o r y q u o t i e n t (R.Q.). R.Q. i s d e f i n e d as the r a t i o of CO2 produced to 0£ consumed. The R.Q. v a r i e s from 1.0 f o r carbohydrate metabolism t o about 0.7 f o r f a t metabolism. The c a l o r i c e q u i v a l e n t of oxygen corresponding to the v a r i o u s R.Q. values was obtained from Brody (19^5). F i g . 4 shows a l i n e a r r e g r e s s i o n of R.Q. over a range of T A f o r non-torpid animals, and F i g . 5 shows a s i m i l a r curve f o r t o r p i d animals. The p r o b a b i l i t y of the slope being zero i s 0.012 f o r non-torpid and 0.008 f o r t o r p i d . Again, we may assume that these slopes have v a l i d b i o l o g i c a l meaning. The s h i f t to a lower R.Q. i n d i c a t e s a trend towards metabolism of l i p i d s r a t h e r than carbohydrates or p r o t e i n s . Oxygen consumption d u r i n g entry i n t o t o rpor a t 15 C (T^) was measured a t short i n t e r v a l s of time f o r three animals. These curves had a s i m i l a r form. A l l animals r e q u i r e d about 1.5 hr to reach the t o r p i d s t a t e , and the area under the curve was approximately 35$ of the corresponding area p r i o r to e ntry (normothermic l e v e l ) . One such curve i s shown i n F i g . 6. Oxygen consumption d u r i n g a r o u s a l from t o r p o r a t 10 C (T^) was measured before and a f t e r the onset of s h i v e r i n g . Before s h i v e r i n g , the mean M.R. (metabolic r a t e ) and the S.D. (standard d e v i a t i o n ) f o r a group of 14 FIG. 4 RELATIONSHIP OF RESPIRATORY QUOTIENT TO AivGIENT TEMPERATURE Y = 6 3702 + .0 02G9X N = 64 is + 10 .8 ^ .6 1 4 1 .5 1 X H J - H 1 1 1 1 1 1 1 h — | 1 1 1 H O 5 10 1 5 3 0 E 5 30 35 40 AMBIENT TDvPERATURE ( C) 15 FIG. 5 RELATIONSHIP OF RESPIRATORY QUOTIENT OF TORPID - ANIMALS TO AMBIENT TEMPERATURE Y = .2 G074 + .0 2042X N = 20 ^ t x 4 1 1 1 1 1 1 1 1 1 1—H 1 1 1 1 h 0 5 1 0 1 5 B 0 E 5 3 0 3 5 40 AM3IENT TEMPERATURE ( C) 16 FIG. 6 CHANGE IN METABOLIC RATE DURING A TYPICAL ENTRY INTO TORPOR AT I5°C. E L A P S E D TIME (HOURS) 10 animals were 5.08 ± 0 . 7 7 ml 0 2/g/hr. The R.Q. f o r t h i s period was 0.51 * 0 . 0 5 9 . A f t e r shivering had commenced the mean M.R. and the S.D. were 7.28 i 2 .199* and the correspond-ing R.Q. values were 0.81 ± 0 . 1 5 1 . These data are shown i n F i g . 7 . The s h i f t i n R.Q. from 0.51 to 0.81 may indicate a s h i f t from l i p i d to carbohydrate substrate. A l t e r n a t i v e l y , i t may indicate a release of C0£ accumulated during torpor or a decrease i n CO2 f i x a t i o n . The rate of increase of T B (body temperature) was 0 . 5 3 C/min with a S.D. of 0.149. The mean t o t a l heaft expended during arousal was 0 .99 cal/g/G increase i n T B with a S.D. of 0 . 2 5 2 . Evaporative Water Loss Evaporative water losses can occur from the skin or from the lungs. In both cases, one could expect the losses to be related to body weight at any given temperature, since both surface area and metabolic rate are functions of body weight. These evaporative water loss data have been plotted i n three ways: mg water/hr, mg water/g body wt/hr, and mg water/ml 0 2 consumed. The data f o r non-torpid animals appear i n F i g s . 8, 9t and 10, and the corresponding data f o r torpid animals are shown i n F i g s . 11, 12, and 1 3 . The best f i t of the data from normothermic animals was obtained when plotted as mg/ml Og* i n d i c a t i n g that pulmonary loss i s the main component of water l o s s . However, the data from torpid animals had best f i t when plotted as mg/hr, in d i c a t i n g that pulmonary loss was not dominant over cutaneous l o s s . 1 8 FIG. 7 METABOLIC RATES AND RESPIRATORY QUOTIENTS DURING AROUSAL AT 10° C. CD CM O -J 2 UJ < cr o _j o CD UJ io <• 8 6 •• 4 2 -•1.0 T S. D. c > sz 1_ C O 0) MEAN c <D > .E co - . 0 . 8 " 0 . 6 UJ o " 0 . 4 - • 0.2 cr o H < cr a. co UJ cr 19 FIG. 8 RELATIONSHIP OF PUUvO-OiTANEOUS WATER LOSS OF NON-TORPID ANIMALS TO AMBIENT TEMPERATURE Y = 40-2818 + -0-155GX N = 5G BO 1 70 1 B O 1 ^ 50 1 40 30 1 ao 1 p 1 0 X X X H 1—H 1 1 1 1 H H 1 1 1 1 1» 0 10 15 3 0 2 5 3 0 3 5 40 AJvBIENT TEMPERATURE ( O 20 FIG. 9 RELATIONSHIP OF PULMO-CUTANEOUS WATER LOSS OF NON-TORPID ANIMALS TO AJvGIENT TEMPERATURE Y = 2-4263 + -O0099X N = 5G 4 + CD \ 1 AMBIENT TEMPERATURE ( C) 21 FIG. 10 RELATIONSHIP OF RELATIVE FlJLJviO-QJTANEOUS WATER LOSS OF NON-TORPID ANIMALS TO AMBIENT TEMPERATURE Y = -0 -1006 + 0-0389X N = 5G AMBIENT TEMPERATURE ( O 22 RELATIONSHIP OF PUU^-QJTAh€DUS WATER LOSS OF TDRPID ANIMALS TO Ah/BIENT TElvPERATLRE Y = 14-4909 + 0-2540X N = 12 AMBIENT TEtvPERATURE ( C) 23 FIG. 12 RELATIONSHIP OF PUUvO-QJTArCOUS WATER LOSS OF TORPID ANIMALS TO AM3IENT TOvPERATURE Y = 1-1341 + 0-0042X N = 12 4 t i AlvBIENT TEtvPERATLRE ( C) 24 FIG. 13 RELATIONSHIP OF RELATIVE FUJvO-QJTANEDUS WATER LOSS OF TORPID ANIMALS TO AMBIENT TEMPERATURE Y = 2-7208 + -0-0397X N = 12 4 + 4 — 1 1 1 1 1 1 1 1 1 1 1 1 h 10 15 40 AMBIENT TEh/PERATURE ( C) 25 The r a t i o of "pulmonary + head skin" l o s s : "body skin" loss, w i l l be referred to as Pulmonary: Cutaneous loss r a t i o . These r a t i o s ranged from 6 5 : 3 5 to 7 9 s 2 1 , with both mean and mode of 7 2 : 2 8 . The cutaneous loss ranged from 6 . 1 mg/hr to 2 0 . 0 mg/hr. The t r a n q u i l i z e r did not appear to a f f e c t the pulmonary or cutaneous components of water l o s s , since t o t a l evaporative loss remained e s s e n t i a l l y the same as f o r normal animals. The only noticeable e f f e c t was a very s l i g h t depression of metabolic rate. Cutaneous water l o s s , as determined on dead animals, varied from 6 . 0 mg/hr to 3 0 . 0 mg/hr. These data are shown i n F i g . 14. Surface area varied with body weight as shown i n the double-logrithmic plo t i n F i g . 1 5 . The re l a t i o n s h i p of body surface to body weight i s A = 2 0 . 2 5 W 0 , 3 ^ , where area (A) i s i n square centimeters and body weight (W) i s i n grams. The water loss data from t h i s experiment are i n agreement with the estimates of cutaneous water loss of l i v e animals from the previous experiment. 26 FIG. 14 RELATIONSHIP OF CUTANEOUS WATER LOSS OF DEAD ANIMALS TO AMBIENT TErvf€RATURE LOG Y = 0.B9G4 + 0.0120 X N=30 5 0 , 0 T Y= 7.88 • 1.03 4 0 . 0 1 3 0 - 0 1 5 0 . 0 1 01 LD a _ J LU tn z i 1 0 . 0 a 111 9 . 0 Z < B . O I— U 7 . 0 6 . 0 I 5 . 0 0 « 0 10*0 5 0 . 0 3 0 . 0 AMBIENT TEMPERATURE C C) 40*0 27 FIG. 15 RELATIONSHIP OF BODY SURFACE AREA TO BODY WEIGHT LOG Y = 100.0 90.0-. BO.O.. 70.0.. BO.O.. " 40.01 O in < 30-0 < < U. BO.O Z] in IO.O Y= 20.25 io.o 1-3064 + 0.3829 LOG X 0.38 N=7 X x x x X + 50.0 30.0 BODY WEIGHT (G) 40.0 50.0 28 DISCUSSION Energy Requirements The data obtained i n t h i s study can be compared w i t h data on s i m i l a r species by usi n g the " b a s a l " metabolic r a t e and the slope of the metabolic ra t e curve as standards of comparison. The slope of the metabolic ra t e curve can be taken as a measure of the r a t e of heat l o s s , and i t i s r e f e r r e d to as conductance (C). The data from t h i s study y i e l d a b a s a l M.R. of 1 . 7 5 ml 0 2 / g body wt/hr ( 1 2 . 2 5 c a l / g/hr) and a conductance of 0.24 ml 0 2/g/hr/C (1.128 c a l / g/hr/C). The average body weight was 1 7 . 0 g. These metabolic values are, of course, averages from a f a m i l y of curves. The BMR and Conductance can be expected to vary w i t h body weight, i n s u l a t i o n , season, and surface area. A number of workers have co n t r i b u t e d to our knowledge of en e r g e t i c s of the heteromyids. Dawson ( 1 9 5 5 ) gives a conductance = 0 . 1 7 6 and BMR = 1 . 2 f o r the kangaroo r a t Dlpodomys merrlami (average weight = 35 g) and s i m i l a r v alues of 0 . 1 5 8 and 1 . 2 f o r the l a r g e r D. panamintlnus (average weight = 60 g ) . Bartholomew and MacMillen ( 1 9 6 1 ) reported data f o r the kangaroo mouse, Miorodlpodops p a l l i d u s , (wt = 15 g) i n d i c a t i n g a BMR of 1 . 3 ml/g/hr and a conductance of about 0 . 1 ml/g/hr/C. However, higher values (BMR = 1 . 8 and conductance = 0 . 1 8 5 ) f o r the same species were reported r e c e n t l y by Brown and Bartholomew ( 1 9 6 9 ) . I t i s l i k e l y that the low values i n the e a r l i e r work were the r e s u l t of a tendency f o r 29 the animals to become hypothermic a t low ambient temp-e r a t u r e s . Tucker ( 1 9 6 5 a) provides a b a s a l r a t e of 0 . 9 7 and a conductance of 0.18 f o r Perognathus c a l i f o r n i o u s (wt = 22 g ) . Values f o r P. h i s p i d u s (wt = 40 g) are 1 . 2 5 and 0 . 2 0 1 (Wang and Hudson, 1 9 7 0 ) . Brody ( 1 9 4 5 ) and K l e i b e r ( 1 9 6 1 ) have produced equations, based on la r g e numbers of species , t h a t r e l a t e BMR to body weight. H e r r e i d and Kes s e l ( 1 9 6 7 ) have developed a s i m i l a r equation to r e l a t e conductance to body weight. I t i s i n t e r e s t i n g to compare the data from t h i s study and data from other small n o c t u r n a l , f o s s o r i a l rodents to these t h e o r e t i c a l curves. F i g . 16 shows the comparisons f o r BMR, and F i g . 17 shows the comparisons f o r conductance. Although the data from t h i s study are i n agreement w i t h the t h e o r e t i c a l p r e d i c t i o n s f o r a 1 7 g mammal, most other desert-adapted rodents have a lower BMR than p r e d i c t e d . Hayward ( 1 9 6 4 ) suggests that the tendency f o r heteromyids to stor e f a t , a t i s s u e t h a t has a low metabolic r a t e , could be re s p o n s i b l e f o r the lower BMR values reported f o r that f a m i l y of rodents. This would be c o n s i s t e n t w i t h the higher BMR f o r P. parvus reported here, since very few of the animals used i n t h i s study appeared to have l a r g e s t o r e s of f a t . S c h r e i b e r (pers. comm.) examined P. parvus throughout the year and found t h a t the f a t content was l e s s than 0.8 g, or about % of the f a t - f r e e body weight. Another 30 FIG. 16 RELATIONSHIP OF BMR LF FOSSORIAL RODENTS TO BODY WEIGHT LOG Y = OGG5G + -0-3633 LOG X N=20 m I \ CD \ a _ J UJ < Q: u a m < i — UJ < m 3*0^ E.5J> Y=4.63 -.36 D A T A F R O M V A R I O U S S O U R C E S — S E E A P P E N D I X 2 . 0 1 5-0 iO-0 50-0 BODY WEIGHT (G) 100«0 150.0 31 FIG. 17 RELATIONSHIP OF mvOJCTANCE OF FOSSORIAL RODEMTS TO BODY WEIGHT LOG Y = -0-1291 + -0-4GG4L0G X N=20 Y= 0.743 • X"*466 D A T A F R O M V A R I O U S S O U R C E S — S E E A P P E N D I X P. p a r v u s 0*10 5-0 —1 1 1 1 1 1 I C O 50-0 100-C 150-0 BODY WEIGHT CG) o 32 possible explanation f o r lowered BMR's i n desert rodents can be extracted from recent findings (Schmidt-Nielsen ejt a l . , 1970) that the counter-current heat exchange mechanism i n the kangaroo rat permits the saving of 8.8% to 16.1$ of the heat produced. This, of course, does not help to explain the higher values f o r P. parvus, since i t s counter-current heat exchange seems to be about as e f f i c i e n t as that of the kangaroo r a t . I t i s possible that s l i g h t differences i n body form, such as a smaller head size, with concomitant changes i n surface:volume r a t i o combine with lower f a t l e v e l s to produce the higher BMR f o r P. parvus. I t i s i n t e r e s t i n g to note that the BMR f o r Peromysous manlculatus from the same area as P. parvus i s 1.99 ml/g/hr (Hayward, 1964). These two species are almost i d e n t i c a l i n size and body form, and they are sympatric i n some areas. The s i m i l a r i t y i n t h e i r metabolic parameters suggests that the higher BMR has some adaptive value i n the northern part of the range, or that f a t storage i s of les s value. Hayward (1964) found that P. manlculatus from a southern desert habitat had a greater tendency to store f a t than the animals from the Okanagan V a l l e y . Although i t i s not possible to provide a d e f i n i t i v e answer to the question of why heteromyids i n p a r t i c u l a r , and desert rodents i n general, have lower BMR's than predicted by Brody's equation, i t seems very l i k e l y that P. parvus does f i t the th e o r e t i c a l predictions f o r BMR and conductance. I t would be 33 i n t e r e s t i n g to measure BMR, conductance, f a t l e v e l s , and body surface areas of f i e l d specimens of a l l species i n the family Heteromyldae. The data f o r normothermic animals (Fig. 1) show-large v a r i a t i o n . I have not calculated confidence l i m i t s f o r t h i s metabolic rate curve, since no s t a t i s t i c a l comparisons are made with other curves. However, i t i s necessary to determine the source of the v a r i a t i o n . P lots of metabolic rate over body weight at T^'s of 1 0 , 2 0 , and 30 C are shown i n F i g s . 18, 1 9 , and 2 0 . D e f i n i t e body weight e f f e c t s are indicated at 10 and at 30 C, but the slope of the curve at 20 C i s not s i g n i f i c a n t l y d i f f e r e n t from zero at the 5% l e v e l of confidence. These body weight e f f e c t s have contributed to the v a r i a b i l i t y of the data. Additional v a r i a b i l i t y probably has been introduced by seasonal changes i n the animals, since data were coll e c t e d throughout the year. Thus, the range of values shown i n F i g . 1 i s representative of the t o t a l v a r i a t i o n one could expect to encounter i n t h i s species. The respiratory quotient f o r normothermic animals has a tendency to increase with increasing T^. The average R.Q. f o r these data i s 0 . 7 2 , corresponding to the R.Q. f o r f a t metabolism. Since the animals were on a high-fat d i e t of sunflower seeds and were post-absorptive, these values are normal. The R.Q. of 0 . 7 2 corresponds to a c a l o r i c equivalent of oxygen of 4 . 7 c a l / 1 (Brody, 1 9 4 5 ) , and t h i s value was used i n converting oxygen consumption to c a l o r i e s . 3 ^ FIG. 18 RELATIONSHIP OF hCTABOLIC RATE TO BODY WEIGHT AT 10 C Y = 12-6481 + -0-3G7GX N = 13 BODY WEIGHT (G) 35 FIG. 19 RELATIONSHIP OF METABOLIC RATE TO BODY WEIGHT AT BO C -Y = 5-G792 + -0-1232X N = 27 BODY WEIGHT (G) 3 6 FIG. 2 0 RELATIONSHIP OF ivCTABOLIC RATE TO BODY WEIGHT AT 30 C Y = 5-2215 + -0.UB78X N = 15 a + BODY WEIGHT (G) 1. Torpor P. parvus, like most heteromyids, has the a b i l i t y to abandon homeothermy for the heterothermic state. Characteristically, the body temperature w i l l be 1 - 2 C above the ambient temperature. Obviously, this reduction in body temperature is accompanied by a depression of metabolic rate. Fig. 2 shows that the metabolic rate during torpor is a positive function of ambient temperature, the slope of the curve being 0.0144. Lasiewski (I963) observed a M.R. of 0.2 ml/g/hr in torpid hummingbirds at 16 C. The slope of the curve for humming-birds appears to be approximately 0.03. The general shape of the curve for hummingbirds i s quite similar to the corresponding curve for P. parvus. Chew et a l . (1967) report a minimum 0 2 consumption for torpid P. longlmembrls as follows: ml/g/hr = -0.177 + 0.027 T A T A below 22 C ml/g/hr = -0.686 + 0.048 T A T A above 22. C ( V Tucker (1965) has provided similar curves for torpid P. californlcus: ml/g/hr = 0.04 + 0.008 T B below 21 C ml/g/hr = -0.74 + 0.045 T B above 21 C If a smooth curve Is drawn through the minimum values shown for P. californlcus, and the curve re-plotted on a semi-logrlthmlc scale as Lasiewski (I963) has done for torpid hummingbirds, the result i s a straight line. Fig. 21 compares my data for torpid P. parvus with the FIG. 21 38 MINIMUM METABOLIC RATES IN TORPOR AMBIENT TEMPERATURE (°C) 39 curve for torpid P. californlcus and the curve for torpid hummingbirds. The curves for the two species of Perognathus have similar slopes, and the slopes are much less than the slope of the hummingbird curve. The approximate Q l 0 values for these curves, calculated for the interval of 20 - 30 C are: Hummingbird = 5.0, P. parvus = 1.7, and P. californlcus = 1.8. Thus, the metabolism of the hummingbird i s much more responsive to temperature than are the two species of Perognathus. Since chemical reaction rates are usually more than doubled by an _.o increase of 10 C (Q 1 Q greater than 2.0), i t i s possible that Perognathus regulates body temperature to some extent while in the torpid condition. Without data for more species, we cannot be sure that the similarity between the curves for Perognathus is not merely fortuitous. A comparative study of energetics of torpor in the heteromylds would be a welcome addition to the literature. 2. Entry into torpor Very few data were collected on metabolic rate during the entry phase of the torpor cycle, but the curve shown in Fig. 6 seems to be representative of the minimum M.R. during entry. As pointed out previously, this type of entry requires approximately 1.5 hr, and the area under the curve represents about 35$ of the normal homeothermic caloric expenditure. P. californlcus appears to require 1.5 - 2.0 hr to enter torpor (Tucker, 1 9 6 5 ) , and the general shape of the curves for body temperature and oxygen consumption are comparable to the curve in Fig. 6 . Chew et a l . ( 1 9 6 7 ) indicate that about 1 . 5 hr are required by P. longlmembrls for entry into torpor at 1 0 C. Thus, P. parvus enters torpor in a manner that i s essentially identical with that of the other pocket mice that have been studied. 3 . Arousal from torpor Arousal occurs in two relatively distinct stages a non-shivering stage and a shivering stage. Data for E. longlmembrls (Bartholomew and Cade, 1 9 5 7 ) f P» californlcus (Tucker, 1 9 & 5 ) » a n <* P» hlspldus (Wang and Hudson, 1 9 7 0 ) indicate that shivering begins after body temperature has risen to 2 0 C. Shivering did not appear, or was not intense, below 2 0 C (T_) in these species. Since chronically implanted thermistors were not used in this study, i t was not possible to obtain simultaneous measurement of oxygen consumption and body temperature. Thus, the rates of arousal are average rates of increase of body temperature over both stages of arousal. These rates of increase in T B varied from 0 . 2 1 to 0 . 7 1 C/min. The average rate was 0 . 5 3 C/min. Bartholomew and Cade ( 1 9 5 7 ) report an arousal rate of about 0 . 6 0 C/min for P. longlmembrls, and Tucker ( 1 9 6 5 ) gives values of 0 . 7 2 to 0 . 9 1 C/min for P. californicus, measured over small intervals of time. In fact, the value 41 reported by Tucker exceeded the th e o r e t i c a l values based on maximum heat production and minimum conductance. P. hlspldus aroused spontaneously at an average rate of 0.27 C/min, but maximum rate was; 0.50 C/min. When disturbed, the maximum rate of arousal was 0.8 C/min (Wang and Hudson, 1970). Hayden and Lindberg (1970) give an average value of 0.42 C/min f o r P. parvus which i s within the 95% confidence l i m i t s f o r my data. Oxygen consumption during the shivering phase of arousal i s 7.28 ml/g/hr, and the corresponding value before shivering commences i s 5.08 ml/g/hr. The average t o t a l cost of arousal i s 0.99 cal/g/C Increase of T_. Since the s p e c i f i c heat of mouse a tissue i s 0.83 cal/g/C (Hart, 1950), i t appears that P. parvus i s very e f f i c i e n t i n u t i l i z i n g i t s metabolic heat i n the arousal process. Examination of the data on R.Q. during torpor shows that an extremely low R.Q. i s associated with torpor at low temperatures. I t i s not l i k e l y that these low values r e f l e c t accurately the substrate being metabolized. Although i t would be possible, t h e o r e t i c a l l y , to obtain a long-chain f a t that could give values s i g -n i f i c a n t l y lower than 0.70, there i s no good evidence that t h i s i s the case. I t i s more l i k e l y that a reduced c i r c u l a t i o n to the s k e l e t a l muscles creates anaerobic conditions and enhances the g l y c o l y t i c pathway, leading to the accumulation of l a c t a t e . This hypothesis i s s u p p o r t e d by the f a c t t h a t the R.Q. I n c r e a s e s d r a m a t i c a l l y d u r i n g the s h i v e r i n g phase of a r o u s a l ( F i g . ? ) . A g i d and Ambid (1969) r e p o r t low R.Q. f o r the t o r p i d dormouse, and Yousef e t a l . (1967) r e p o r t v a l u e s a s low as 0.50 f o r t o r p i d h a m s t e r s . I n b o t h c a s e s the R.Q. i n c r e a s e d d u r i n g a r o u s a l , b u t i t i s n o t p o s s i b l e t o a s c r i b e t h e s e changes t o the h y p o t h e s i z e d a n a e r o b i o s i s . Twente and Twente (1968) found no I n c r e a s e i n t i s s u e l a c t a t e i n h i b e r n a t i n g C i t e l l u s . I t i s n o t p o s s i b l e , a t t h i s t i m e , t o e x p l a i n the e x t r e m e l y low R.Q. i n t o r p i d P. p a r v u s , b u t a low R.Q. seems t o be t y p i c a l f o r the t o r p i d s t a t e . H i b e r n a t o r s may have m e t a b o l i c pathways f o r f i x a t i o n of GO2 i n t o u s e f u l compounds. Pulmo-cutaneous Water L o s s S c h m i d t - N i e l s e n and S c h m i d t - N i e l s e n (195D computed the c a l o r i c i n t a k e , w a t e r i n t a k e , m e t a b o l i c w a t e r p r o d u c t i o n , and u r i n a r y w a t e r l o s s f o r kangaroo r a t s on a d i e t of d r y p e a r l b a r l e y . The n e t wat e r g a i n under these c o n d i t i o n s was 0.102 mg HgO/cal, i n d i c a t i n g t h a t a d r y d i e t would be s u f f i c i e n t i f no o t h e r w a t e r l o s s e s were i n c u r r e d . However, i t i s o b v i o u s t h a t some l o s s must o c c u r t h r o u g h the pulmonary and cutaneous r o u t e s . S i n c e a n e t g a i n i s p o s s i b l e a f t e r s u b t r a c t i n g w a t e r needed t o e x c r e t e t he i n g e s t e d n i t r o g e n and s a l t s , the pulmonary and cutaneous r o u t e s of w a t e r l o s s assume a p o s i t i o n of extreme importance t o the d e s e r t r o d e n t . The magnitude of these l o s s e s w i l l d e t e r m i n e the w a t e r balance of the animal. It is necessary to examine the manner in which these losses are related to environmental parameters, especially ambient temperature and humidity. The data from this study relate pulmo-cutaneous water loss to ambient temperature only, and they are valid only at 0% relative humidity. Pulmo-cutaneous water loss is expected to be controlled by several factors: ambient temperature, absolute humidity, metabolic rate, temperature of the expired a i r , and surface area of the skin. We can expect the respiratory, or pulmonary, water loss to be determined by the amount of moisture in the expired a i r , saturated at the temperature of the expired a i r , less the amount of moisture in the inspired a i r . The cutaneous loss should be a function of skin area and the humidity of the surround-ing a i r . Since heteromyid rodents do not have sweat glands, there should be l i t t l e or no Increase in cutaneous water loss with increased metabolic rate. The ratio of cutaneous loss:pulmonary loss was found to be 28:72 at 20 - 25 C As metabolic rate increases, with a con-comitant increase in pulmonary loss, the effect of cutaneous loss becomes relatively less Important to the total water economy of the animal. The plot of raw data (mg H^O/hr) for non-torpid animals in Fig. Stgives an extremely wide spread of points. The regression line has a slight negative slope. Since metabolic rate increases much faster with decreasing T A, than does the observed water loss, this indicates that the animals are not saturating the expired a i r at body temperature, but at some lower temperature. Unless the animals have some means of super-cooling the nasal passages, this saturation temperature w i l l be limited by T A. This question has been investigated by Getz (1968) , who found that the nasal temperature of Clethrlonomys gapperl and Peromyscus leucopus was 0 - 9 C above T A over the range of 5 - 30 C. More recently, Schmidt-Nielsen et a l . (1970) have demonstrated that a counter-cooling mechanism does exist in the nasal mucosa of birds and mammals. The kangaroo rat exhaled a i r at a temperature lower than T A, indicating that i t does "super-cool" the expired a i r . This enables the animal to recover to 83$ of the water in the expired a i r . A seml-logrithmic plot of the data from Pig. 10 produced a linear relationship as shown in Pig. 22. Since many factors can Increase the pulmo-cutaneous water loss of an animal (e.g. panting), i t i s easier to overestimate than to under-estimate this loss. Moreover, the minimum values are more useful in evaluating the potential per-formance of a species than are the average or maximum values. The minimum values for pulmo-cutaneous water loss are adequately described by the equation mg/ml = 0 . 1 5 • 0.0008 T A 2. This parabolic function, f i t t e d to the arithmetic plot of the data (Pig. 1 0 ) , 45 FIG. 2 2 RELATIONSHIP OF RELATIVE PLLMD-QJTANEOJS WATER LOSS OF NON-TORPID ANIMALS TO AMBIENT TEMPERATURE LOG Y = -0-G942 + 0-0242X N = 5G 4.0 , . Y= 0 . 2 0 2 • 1.06 0.0 10.0 50.0 30.0 AMBIENT TEMPERATURE ( C) 40-0 provides a plot that i s a straight line on a semi-log plot, over the pertinent range of temperatures of 10-30 C. The slope of this line on the semi-logarithmic scale i s almost Identical to the slope of the least squares re-gression. Pig. 23 compares the relationship of minimum pulmo-cutaneous water loss to ambient temperature for several other small mammals, as well as P. parvus. The curves for Notomys (a small, desert-adapted, Australian, Murid rodent) (MacMlllen and Lee, 1970) are essentially identical to the curve for P. parvus in both slope and level. The curves for P. californlcus and Peromyscus eremlous seem to d i f f e r from that of P. parvus in having less slope and a slightly higher level. Since both these species exhibit torpor only at temperatures above 15 G and inhabit more southerly deserts, the lower water loss for P. parvus at lower temperatures may indicate a superior adaptation for the colder areas that i t inhabits. Many more data would be needed to support such a hypothesis. Eutamlas (Wunder, 1970), in contrast to the other species, appears to have a change in slope at about 25 G. This coincides with a lower metabolic rate from about 25-35 C. Since the relative water loss (mg/ml Q2) increases when 0 2 consumption decreases, i t i s logical to assume that the cutaneous loss component i s higher in Eutamlas than in the desert species. Values for Dlpodomys venustus and D. merrlaml at 15 G (Church, 1969) and for D. spectabllls and D. merrlami at 13 C (Schmidt-Nielsen and Schmidt-Nielsen, 1950) 47 FIG. 2 3 MINIMUM PULMO-CUTANEOUS WATER LOSS FOR SEVERAL SPECIES OF SMALL MAMMALS AMBIENT TEMPERATURE (°C) a r e shown a l s o i n P i g . 23. These v a l u e s c o i n c i d e w i t h the d a t a from t h i s s t u d y . 1. Cutaneous w a t e r l o s s F i g . 1 4 shows t h a t minimum cutaneous l o s s has a l o g a r i t h m i c r e l a t i o n s h i p t o ambient t e m p e r a t u r e ; the e q u a t i o n f o r minimum v a l u e s i s Y = J.?6 • 0.0155 x , where Y i s mg H 20/hr and X i s T A i n C. The s l o p e of t h i s l i n e i s much l e s s t h a n the s l o p e of the a b s o l u t e h u m i d i t y c u r v e , i n d i c a t i n g t h a t the s k i n and f u r a r e v e r y e f f e c t i v e i n " i n s u l a t i n g " the a n i m a l from the d e s i c c a t i n g i n f l u e n c e of low h u m i d i t y . The f u r p r o b a b l y e x e r t s a major i n f l u e n c e on cutaneous l o s s by i n c r e a s i n g the d i f f u s i o n p a t h and r e d u c i n g the g r a d i e n t , b u t no d a t a were c o l l e c t e d on c l i p p e d a n i m a l s t o i n v e s t i g a t e t h i s p o s s i b i l i t y . 2. S e p a r a t i o n of cutaneous and pulmonary l o s s e s The curve r e p r e s e n t i n g 25 x a b s o l u t e h u m i d i t y a t T A , of 0 - 4 0 C d i d n o t p r o v i d e a s good a l o w e r l i m i t f o r the r e l a t i v e pulmocutaneous w a t e r l o s s d a t a as d i d the 2 c u r v e Y = 0.15 + 0.0008 T A . S i n c e the pulmonary e f f i c i e n c y of mammals i s about 20!^  (Hughes, 19&3) a n c * the a i r c o n t a i n s a p p r o x i m a t e l y 20^ 0 2, the a n i m a l would r e s p i r e 100 ml of a i r t o e x t r a c t 4 ml Og. I f the e x p i r e d a i r i s s a t u r a t e d a t T A, then 25 x a b s o l u t e h u m i d i t y s h o u l d e s t i m a t e the a c t u a l pulmonary l o s s . The d i s c r e p a n c y between the t h e o r e t i c a l and e m p i r i c a l c u r v e s i s e x p e c t e d , s i n c e b o t h pulmonary and cutaneous l o s s e s i n c r e a s e l o g a r i t h m i c a l l y 49 with T^, and the empirical curve is a summation of the pulmonary and cutaneous losses. If the minimum curves for cutaneous loss (Fig. 14) and pulmo-cutaneous loss (Fig. 22) are accurate, and i f the pulmonary loss i s described by the function, mg/ml = 25 * absolute humidity, the summation of minimum cutaneous loss and minimum pulmonary loss should approximate the curve fit t e d to p minimum pulmo-cutaneous loss (i.e. Y = .15 + .0008 T A ). Using the average metabolic rate curve and average body weight (17 g), I have computed the theoretical pulmonary and cutaneous losses over the range of T A of 0-35 C. These values were summed and expressed as mg/ml O2. Fig. 24 compares this summation with the line fi t t e d to the original data. The computed values correspond very 2 closely to the curve Y = 0.15 + 0.0008 T A , except at the higher values of T A. The computed values would • suggest that the slope of the fi t t e d line i s somewhat low. Similar values were computed for a 17 g mouse in torpor, with the values expressed as mg/hr. Fig. 25 shows that the computed values are in agreement with the minimum values for pulmo-cutaneous loss of torpid animals from Fig. 11. The general agreement between the minimum values from the data and the predicted values based on minimum cutaneous loss and the estimated mucosal temper-ature (mucosal temperature = T A ) , supports the r e l i a b i l i t y of the original data and the minimum curves fit t e d to them. 50 FIG. 2 4 COMPARISON OF OBSERVED PULMO-CUTANEOUS LOSS WITH PREDICTED LOSS, BASED ON THEORETICAL PULMONARY LOSS + CUTANEOUS LOSS FROM FIG. 14 (NON-TORPID) 51 FIG. 2 5 COMPARISON OF MINIMUM PULMO-CUTANEOUS OF TORPID ANIMALS (x) WITH MIN. PREDICTED VALUES, BASED ON CUTANEOUS LOSS (FIG. 14 ) AND ON THEORETICAL PULMONARY LOSS AMBIENT T E M P . ( ° C ) 52 Hopefully, more comprehensive studies of water loss in small desert rodents w i l l be forthcoming. It w i l l be most interesting to see comparisons between these data and data collected with newer, more sophisticated techniques, such as the isotope technique used by Mullen (1970). Energy and Water Budgets It i s easier to comprehend visual relation-ships than purely conceptual relationships. Thus, to transform these data to a visual form, I have constructed models which estimate the energy requirements and water losses of an animal at any given T A and at any given amount of torpor. This model may be useful as a ,.-predictive model, but i t cannot be so considered u n t i l i t i s tested. Thee limitations of the model are rather severe, since i t does not include-.activity and does assume Q% relative humidity. Neither of these restrictions i s true in the f i e l d , so any predictions to the f i e l d situation would require extremely careful study. The main advantage of the model Is the provision of a three-dimensional display of the data, although I hope that others w i l l construct refined versions for true predictive use under f i e l d situations. 1. Energy budget model In view of the large number of variables, i t was necessary to simplify the model as much as possible i n o r d e r t o produce a manageable e q u a t i o n . T h i s model, of c o u r s e , i s m e r e l y one of many p o s s i b l e , e q u a l l y v a l i d e s t i m a t e s of the energy e x p e n d i t u r e f o r t h i s s p e c i e s . To s i m p l i f y the model, the f o l l o w i n g a s s u m p t i o n s were made: 1. No component f o r a c t i v i t y i s i n c l u d e d . 2. E n t r y and a r o u s a l o c c u r , even when the time i n t o r p o r = 0 . 3. E n t r y time i s c o n s t a n t a t 1.5 h r . 4. E n e r g y consumption d u r i n g e n t r y a v e r a g e s 35$ of n o r m al consumption a t the e n t r y T^. 5. A r o u s a l time i s c o n s t a n t a t 1.1 h r . 6. M e t a b o l i c c o s t of a r o u s a l i s 1.00 c a l / g / C . 7. Maximum time i n t o r p o r i s l i m i t e d t o 140 hr/week. 8. M e t a b o l i c r a t e i n t o r p o r = 1.81 + 0.068 T^, w i t h M.R. i n c a l / g / h r . 9. Normothermic M.R. = 40.45 - 1.127 T A, w i t h M.R. i n c a l / g / h r . The f o l l o w i n g v a r i a b l e s were s e l e c t e d : X = h o u r s i n t o r p o r / b o u t ( l e n g t h of t o r p o r p e r i o d ) V a r y from 0 t o 140. Y = Ambient temperature V a r y from 0 t o 30 G. Z = T o t a l c a l o r i e s / w e e k N = Number of t o r p o r p e riods/week. W = Body w e i g h t . The e q u a t i o n , a f t e r s i m p l i f i c a t i o n , becomes: c a l o r i e s / w e e k = N W (21.5 - 0.6 Y ) + N W (35 - Y ) + N W X ( 1 . 8 1 + 0.068 Y ) + (w (40.45 - 1.127 Y ) ] [l68 - B X - 2.5 N ] T h r e e - d i m e n s i o n a l p l o t s of t h i s model f o r a 17 g a n i m a l a r e shown i n F i g . 26 f o r 7 b o u t s of torpor/week, and F i g . 27 f o r 1 bout o f torpor/week. The model i s a t i l t e d p l a n e , w i t h h i g h e s t p o i n t a t 0 C and 0 h r of t o r p o r , and l o w e s t p o i n t i n the o p p o s i t e c o r n e r a t 30 C and 1 4 0 h r of torpor/week. The maximum hours of t o r p o r p e r week was s e t a t 1 4 0 because t h i s i s the maximum c o n t i n u o u s t o r p o r observed f o r t h i s s p e c i e s by I v e r s o n (1967) o r by me. I n r e a l i t y , t h i s f i g u r e c o u l d be h i g h e r s i n c e an a n i m a l m i g h t arouse a f t e r 1 4 0 h r and r e - e n t e r t o r p o r i m m e d i a t e l y . Thus, the r e q u i r e m e n t t h a t an a n i m a l spend 2 8 hr/week i n the n o n - t o r p i d s t a t e tends t o i n c r e a s e the p r e d i c t e d energy e x p e n d i t u r e a t maximum t o r p o r above the v a l u e s t h a t c o u l d o c c u r i n a f i e l d s i t u a t i o n . 2. W a t e r - l o s s model S i n c e the main l o s s of wat e r i n P. parvus i s pulmo-cutaneous l o s s , and s i n c e the main source of wat e r I s m e t a b o l i c w a t e r , i t i s o b v i o u s t h a t the n e t l o s s i s some f u n c t i o n of d a i l y m e t a b o l i s m . Thus, the energy model, d e t a i l e d p r e v i o u s l y , becomes an i m p o r t a n t s t r u c t u r a l u n i t i n t h i s model. S i n c e c o m p o s i t i o n of d i e t i n f l u e n c e s the amount of m e t a b o l i c w a t e r produced, i t i s n e c e s s a r y t o s e l e c t a c o n s t a n t d i e t i n the c o n s t r u c t i o n of the model. 0 TORPOR Hours / W e e k TORPOR H o u r s / W e e k T O R P O R 57 I n t h i s model, the p e a r l b a r l e y d i e t d e t a i l e d by Sch m i d t -N i e l s e n and S c h m i d t - N i e l s e n (1951) w i l l be used. O t h e r d a t a p u b l i s h e d by thes e a u t h o r s , n o t a b l y maximum u r i n e c o n c e n t r a t i o n f o r P e r o g n a t h u s , w i l l be used i n c o n s t r u c t i n g the model. T h i s model, of c o u r s e , i s v a l i d o n l y a t 0% r e l a t i v e h u m i d i t y . The f o l l o w i n g a s s u m p t i o n s w i l l be made: 1. Pulmo-cutaneous w a t e r l o s s of n o n - t o r p i d a n i m a l s i s d e t e r m i n e d by M.R. • mg H 20 l o s t / m l 02 consumed. 2. Mg/ml = 0.15 + 0.0008 T A 2 3. Pulmo-cutaneous l o s s i n t o r p i d a n i m a l s i s e s t i m a t e d by: mg/hr =4.42 • 1.05 T a. 4. U r i n a r y w a t e r l o s s = 3.4 mg/100 c a l . 5. F e c a l water l o s s = 0 . 7 mg/100 c a l . 6. 20.4 ml 0 2 i s r e q u i r e d t o m e t a b o l i z e 100 c a l . 7. M e t a b o l i c w a t e r = 13.4 mg/100 c a l . 8. Preformed w a t e r i n f o o d = 0.9 mg/100 c a l . 9. The r e l a t i v e pulmo-cutaneous w a t e r l o s s (mg/ml 0 2) i s the same d u r i n g a r o u s a l and e n t r y as f o r the n o n - t o r p i d c o n d i t i o n . U s i n g these a s s u m p t i o n s , one can c o n s t r u c t the f o l l o w i n g components: E v a p o r a t i v e w a t e r l o s s i n t o r p o r = N X (4.42 • 1.05 T a). E v a p o r a t i v e w a t e r l o s s , n o n - t o r p i d = cal/week -[H X W (1.81 + 0.068 TA j] , n 2, *• £-9 • (0.15 + 0.008T A ) 58 H 20 L o s s H2O G a i n U r i n e = 3.4 mg/100 c a l M e t a b o l i c =13.4 mg/100 c a l Fec e s =0.? mg/100 c a l Preformed = 0.9 mg/100 c a l TOTAL = 4 . 1 mg/100 c a l TOTAL = 14.3 mg/100 c a l - 4.1 NET GAIN = 10.2 mg/100 c a l Combining t h e s e components w i t h the energy model produces a model f o r n e t w a t e r l o s s p e r week, where: X = h o u r s i n t o r p o r / b o u t V a r y from 0 t o 140 Y = ambient temperature V a r y from 0 t o 30 C Z = n e t w a t e r l o s s / w e e k (mg/week) N = number o f b o u t s of torpor/week W = body w e i g h t (g) The f i n a l e q u a t i o n becomes: mg H 20/week = N X (4.42 • 1.05TA) + [p.0306 + 0.000163 Y§[(l68 - N X) (W) (40.45-1.127Y]j - 0.102 [N W (21.5 - 0.6 Y) + N W (35 - Y) + N W X (1.81 + 0.068 Y) + (W) (168 - N X - 2.5 N)(40.45 - 1.127 Y)] T h r e e - d i m e n s i o n a l p l o t s of t h i s model f o r a 17 g a n i m a l a r e shown i n F i g . 26 f o r 7 b o u t s of t o r p o r p e r week and F i g . 27 f o r 1 bout of torpor/week. The model i s a c u r v e d , t i l t e d s u r f a c e , w i t h the l o w e s t p o i n t a t 0 C and 0 h r of torpor/week. A t 0 h r of t o r p o r , the a n i m a l p a s s e s from p o s i t i v e t o n e g a t i v e w a t e r b a l a n c e a t abou t 20 C. T h i s t r a n s i t i o n p o i n t i s r e a c h e d a t a p r o g r e s s i v e l y l o w e r T^ a s ho u r s o f t o r p o r i n c r e a s e , and the a n i m a l goes i n t o n e g a t i v e w a t e r b a l a n c e a t 9 C and 5 9 and 140 hr/week (20 hr/day) i n F i g . 26 and a t 13 C and 140 hr/week i n the continuous-torpor s i t u a t i o n of F i g . 27. Thus, the data i n d i c a t e that t o r p o r i s not a water-conserving mechanism. Torpor appears to provide a means of exchanging water f o r energy. The range of temperatures over which t o r p o r i s f e a s i b l e without encountering negative water balance i s increased considerably (from 9 G to 13 C) by reducing the number of bouts per week and simultaneously i n c r e a s i n g the l e n g t h of the torpor p e r i o d . In other words, one long p e r i o d of torpor i s more economical of water than i s the same amount of torpor d i v i d e d i n t o d a i l y p e r i o d s . The r e l a t i o n s h i p of water l o s s to energy conservation can be examined i n a s l i g h t l y d i f f e r e n t way. F i g . 28 shows the r e l a t i o n s h i p s of net water production and pulmo-cutaneous l o s s to ambient temperature f o r a 17 g, non-t o r p i d animal. As mentioned p r e v i o u s l y , the t r a n s i t i o n from p o s i t i v e to negative water balance occurs a t about 21 C. These same r e l a t i o n s h i p s are shown i n F i g . 29 f o r a t o r p i d animal of the same body weight. The water l o s s i s always g r e a t e r than water production under these c o n d i t i o n s , and torpor i s not conserving water. The d i f f e r e n c e s between production and l o s s f o r the t o r p i d and non-torpid c o n d i t i o n s are shown i n F i g . 30. This i n d i c a t e s that t o r p o r i s always more expensive, i n terms of water, than the non-torpid s t a t e , but the difference:; becomes minimal a t higher ambient temperatures. Thus, 60 FIG. 28 ESTIMATED WATER PRODUCTION AND PULMO-CUTANEOUS LOSS OF A 17-GM. MOUSE (0% r.h.No Torpor) 61 FIG. 2 9 ESTIMATED WATER PRODUCTION AND LOSS FOR A TORPID, 17-GM. MOUSE (0% r.h.) 7 0 6 0 5 0 + 4 0 + 3 0 AMBIENT TEMP. (°C) 6 2 FIG. 30 NET WATER LOSS OF A 17-GM MOUSE IN THE TORPID AND IN THE ACTIVE STATE AMBIENT TEMP. (°C) 63 we have a s i t u a t i o n i n w h i c h t o r p o r c o n s e r v e s energy b u t expends w a t e r . F i g . 31 shows the s e r e l a t i o n s h i p s of energy c o n s e r v a t i o n and w a t e r e x p e n d i t u r e t o ambient t e m p e r a t u r e . H a v i n g o b t a i n e d t h e s e c u r v e s , we can e x p r e s s c o n v e n i e n t l y the c o s t of t o r p o r by d i v i d i n g the w a t e r e x p e n d i t u r e by the energy c o n s e r v e d . The r e s u l t i n g c u r v e , F i g . 32, e x p r e s s e s the c o s t of t o r p o r as mg HgO/kcal o v e r ambient t e m p e r a t u r e . T h i s c u r v e i n d i c a t e s t h a t the c o s t of t o r p o r i s m i n i m a l a t 25 C w i t h a range of low v a l u e s e x t e n d i n g from 20 - 30 C. A l t h o u g h t o r p o r seems t o be e x p e n s i v e a t a l l ambient t e m p e r a t u r e s , " a e s t i v a t i o n " would a l m o s t c e r t a i n l y o c c u r w i t h i n t h i s l o w - c o s t r a n g e . T h i s i n d i c a t e s t h a t t o r p o r , i n P. p a r v u s , i s p r o b a b l y an e n e r g y - c o n s e r v i n g mechanism w i t h the c o s t , i n terms of w a t e r , a t a minimum d u r i n g the h o t t e r , d r i e r s eason. We can examine the e f f e c t of body s i z e on w a t e r c o n s e r v a t i o n d u r i n g t o r p o r by comparing the p l o t s f o r the 17 g a n i m a l ( F i g s . 26 and 27) w i t h s i m i l a r p l o t s f o r a 10 g a n i m a l ( F i g s . 33 and 34) and f o r a 25 g a n i m a l ( F i g s . 35 and 36). E x a m i n a t i o n of the z e r o - i s o l l n e s shows t h a t s m a l l e r a n i m a l s a r e a t a d i s a d v a n t a g e . The 25 g a n i m a l a p p e a r s t o be c a p a b l e of r e m a i n i n g i n p o s i t i v e w a t e r b a l a n c e d u r i n g l o n g - t e r m t o r p o r a t t e m p e r a t u r e s up t o 16 C, w h i l e the 10 g a n i m a l r e a c h e s I t s t r a n s i t i o n p o i n t a t 7 C under the same c o n d i t i o n s . T h i s advantage of l a r g e r o v e r s m a l l e r a n i m a l s can be a t t r i b u t e d t o the FIG. 31 ENERGY CONSERVED AND WATER EXPENDED BY TORPOR (17-GM MOUSE ; 0%r.h.) i i i i i i i 0 5 10 15 2 0 2 5 3 0 AMBIENT TEMP. (°C) 6 5 FIG. 32 COST OF CONSERVING ENERGY BY TORPOR (17-GM MOUSE ; 0 % r.h.) 80 -t 7 0 4 6 0 4 5 0 + 4 0 + 3 0 + 2 0 + 0 * AMBIENT TEMP. ( ° C ) 66 33 E S T I M A T E D E N E R G Y AHD^N/KTER BUDGETS ( I O G . A N I M A L ) ( 7 T O R P O R P E R I O D S / W E E K ) Hours / Week T O R P O R Hours / Week T O R P O R 140 Hours / W e e k TORPOR 140 69 TORPOR 69 16 k& lours / WeeK TORPOR Hours / Week TORPOR reduced s u r f a c e : v o l u m e r a t i o of l a r g e r a n i m a l s . The n e t m e t a b o l i c w a t e r p r o d u c t i o n i s a f u n c t i o n of mass, b u t cutaneous w a t e r l o s s i s a f u n c t i o n of s u r f a c e a r e a . As P i g . 15 shows, t h e r e i s a v e r y s l i g h t i n c r e a s e i n s u r f a c e a r e a w i t h i n c r e a s e d body w e i g h t . A 10 g a n i m a l has 70$ of the s u r f a c e a r e a b u t o n l y 40$ o f the mass of a 25 g a n i m a l . I t must be emphasized t h a t the model i s p r o b a b l y l e s s a c c u r a t e f o r body w e i g h t s above o r below the mean body w e i g h t (17 g ) . T h i s w i l l tend t o u n d e r - e s t i m a t e cutaneous l o s s f o r l a r g e a n i m a l s and o v e r - e s t i m a t e f o r s m a l l a n i m a l s , b u t the b a s i c r e l a t i o n s h i p s w i l l n o t be a f f e c t e d . A l t h o u g h t h e s e d a t a f a i l t o i n d i c a t e the often-assumed b e n e f i t of t o r p o r t o the w a t e r budget, t h e y suggest t h a t l a r g e r a n i m a l s , such as the ground s q u i r r e l s , c o u l d b e n e f i t from t o r p o r . T h i s would be the case o n l y i>f t h e i r cutaneous l o s s i s low and t h e i r s u r f a c e : v o l u m e r a t i o i s a l s o low. E x a m i n a t i o n of the c u r v e s i n F i g . 29 r e v e a l s t h a t the t r a n s i t i o n from p o s i t i v e t o n e g a t i v e w a t e r b a l a n c e would o c c u r a t 23 G i f t h e r e were no cutaneous l o s s . The cutaneous l o s s has the e f f e c t of s h i f t i n g the t r a n s i t i o n p o i n t t o a l o w e r T^. The e v o l u t i o n of low cutaneous w a t e r l o s s has e n a b l e d t h e s e d e s e r t a n i m a l s t o employ the e n e r g y - c o n s e r v i n g d e v i c e of t o r p o r a t h i g h e r ambient t e m p e r a t u r e s , w i t h o u t i n c u r r i n g a severe w a t e r l o s s . I t i s q u i t e p o s s i b l e t h a t energy i s a«.more se v e r e l i m i t i n g f a c t o r than w a t e r f o r f o s s o r i a l d e s e r t r o d e n t s . 71 A l l the data in this study, and therefore a l l the conclusions, were based on 0$ relative humidity and no< activity. Since these conditions rarely, i f ever, occur in a natural situation, It is essential to estimate the manner in which f i e l d conditions w i l l affect the conclusions. Burrow temperatures w i l l vary from 0 C in winter to 25 C in summer (Brown and Bartholomew, 1969). The humidity in the burrow i s expected to be 100$,\relative humidity (Schmidt-Nielsen and Schmidt-Nielsen, 1950). At 100$ relative humidity the animal w i l l lose l i t t l e or no moisture through the pulmonary route (Schmidt-Nielsen et a l . , 1970), and the cutaneous loss probably w i l l decrease to insignificant levels. Humidities greater than 0$ would reduce cutaneous loss due to a reduction in the moisture gradient. The pulmonary loss would be reduced considerably due to the inspiration of water. However, the temperature of the nasal mucosa w i l l increase since less evaporative cooling can occur. This w i l l reduce the efficiency with which water i s removed from the expired a i r . The total amount of water lost through the pulmonary route w i l l be much reduced, but » may be higher than expected i f the change in mucosal tempera-ture i s not considered. Total water loss at elevated humidi-ties w i l l probably decrease as some function of the Increase in humidity, beooming minimal at 100$ relative humidity. Activity can be estimated from data on metabolic rate for P. formosus under f i e l d conditions (Mullen, 1970). An isotopic method was used to estimate the metabolic rate of P. formosus. Since this species i s similar to P. parvus 72 In habits and size, i t Is reasonable to compare the data from this study with those for P. formosus. The f i e l d data indicate considerably higher values for metabolic rate than do lab data for resting metabolic rate. By subtracting the resting metabolic rate (this study) from the f i e l d metabolic rate (Mullen, 1 9 7 0 ) , we can obtain an estimate of activity. If the assumption i s made that a l l act i v i t y energy i s ex-pended within a 3-hr period at 0$ relative humidity, and that a l l other energy i s expended in the burrow at 1 0 0 $ relative humidity, i t i s possible to estimate the effects on water loss. Fig. 3 7 shows the estimated net water production for a 17 g animal on a diet of pearl barley and the estimated pulmo-cutaneous loss, based on the assumptions mentioned. The difference between net production and pulmo-cutaneous loss, representing the net gain or net loss, i s plotted in Fig. 3 8 . Assuming that a l l torpor oocurs at 1 0 0 $ relative humidity, the net water production becomes equivalent to a net gain. This relationship i s also shown in Fig. 3 8 . A comparison of the estimated net water gain in torpor with the estimated net gain of an active animal under the assumed f i e l d conditions, Indicates that torpor continues to be expensive of water, relative to the active state. These estimates show that the basic relationships between torpid and active states, under f i e l d conditions, do not d i f f e r from the relationships determined under laboratory conditions, and they reinforce the conclusion that i t i s not l i k e l y that torpor has evolved as a water-conserving mechanism. However, 73 FIG. 37 ESTIMATED WATER PRODUCTION PULMO-CUTANEOUS LOSS FOR A 17-GM. MOUSE UNDER FIELD CONDITIONS 9 0 8 0 7 0 • 6 0 «• ^ 5 0 t cr o cr UJ < 5 4 0 «• 3 0 2 0 1 0 " \ Loss I I I I I i i I I O 5 1 0 15 2 0 2 5 3 0 3 5 4 0 AMBIENT TEMP. ( ° C ) 74 FIG. 38 ESTIMATED NET WATER LOSS OF ACTIVE AND TORPID MICE UNDER FIELD CONDITIONS AMBIENT T E M P . ( ° C ) the animal i s in a positive water balance under both sets of conditions, so, as mentioned previously, i t i s unlikely that water balance i s a severe problem for small, fossorial desert rodents, under f i e l d conditions. The concept of water short-age for these desert rodents i s even less oredible when one considers that some green vegetation i s ut i l i z e d , and that seeds stored at 100$ relative humidity w i l l contain consider-able moisture. If there i s a water shortage for these animals, i t would be during the winter when the s o i l moisture i s in the form of Ice, and absolute humidity i s low. It i s also at this time that the animals may face an energy shortage. Although the cost, in terms of water, of conserve ing energy by torpor i s very high at the low temperatures expected during the winter (Fig. 32), the three-dimensional plots of the water budget model (Figs. 27t 34, and 36) indicate that even the smallest animals are capable of maintaining positive water balance at low ambient temperatures 76 MAJOR FINDINGS OF THE THESIS I t seems a p p r o p r i a t e t o summarize the main f i n d i n g s of t h i s t h e s i s and t o d i s c u s s some of the i m p l i c a t i o n s of these f i n d i n g s . The c o n c l u s i o n s t h a t may be drawn from t h i s s t u d y a r e : 1. T o r p o r i s e x p e n s i v e i n terms of w a t e r and cannot be a w a t e r - c o n s e r v i n g mechanism i n P. p a r v u s . F i g . 30 shows t h a t the n e t w a t e r l o s s of t o r p i d a n i m a l s i s g r e a t e r t h a n the l o s s of normothermic a n i m a l s over the e n t i r e range of ambient t e m p e r a t u r e s . F i g . 38, w h i c h shows the e s t i m a t e d w a t e r l o s s of normothermic and t o r p i d a n i m a l s under f i e l d c o n d i t i o n s , i n d i c a t e s a l s o t h a t t o r p i d a n i m a l s have a h i g h e r n e t w a t e r l o s s . 2. T o r p o r i s an e n e r g y - c o n s e r v i n g mechanism as i n d i c a t e d by a comparison of F i g s . 1 and 2. The energy c o n s e r v e d by t o r p o r a t 10 G i s a p p r o x i m a t e l y 90$ of the normothermic v a l u e . S i n c e t o r p o r c o n s e r v e s energy and expends w a t e r , i t i s p o s s i b l e t o r e l a t e the two parameters by e x p r e s s i n g the c o s t of energy c o n s e r v a t i o n as mg of w a t e r e x p e n d e d / k c a l of energy c o n s e r v e d , as shown i n F i g . 32. The c o s t of c o n s e r v i n g energy by t o r p o r i s l o w e s t a t 20-30 C. 3. P. p a r v u s i s c a p a b l e of m a i n t a i n i n g a p o s i t i v e w a t e r b a l a n c e under severe c o n d i t i o n s of c o l d and d r y n e s s . F i g . 30 i n d i c a t e s t h a t p o s i t i v e w a t e r b a l a n c e c o u l d be e x p e c t e d up t o 20 C a t 0% r e l a t i v e h u m i d i t y . I n a r e a l i s t i c f i e l d s i t u a t i o n , p o s i t i v e w a t e r b a l a n c e seems c e r t a i n a t a l l ambient t e m p e r a t u r e s up t o 30 G ( F i g . 38). S i n c e m e t a b o l i c w a t e r i s the major source of w a t e r , an adequate f o o d s u p p l y i s the main r e q u i r e m e n t f o r the maintenance of p o s i t i v e w a t e r b a l a n c e . S i n c e a d i e t of d r y p e a r l b a r l e y produces more m e t a b o l i c w a t e r t h a n i s r e q u i r e d by P. parvus t o e l i m i n a t e the u r i n a r y and f e c a l w a s t e s , i t i s c l e a r t h a t the problem of w a t e r b a l a n c e l i e s w i t h the pulmo-cutaneous w a t e r l o s s , w h i c h i s I n f l u e n c e d s t r o n g l y by h u m i d i t y and t e m p e r a t u r e . Measurements of pulmo-cutaneous w a t e r l o s s o v e r a range of e n v i r o n m e n t a l t e m p e r a t u r e s a t 0% r e l a t i v e h u m i d i t y show t h a t p o s i t i v e w a t e r b a l a n c e can be m a i n t a i n e d below 20 C by normothermic a n i m a l s . However, t o r p i d a n i m a l s e x p e r i e n c e a n e g a t i v e w a t e r b a l a n c e a t a l l t e m p e r a t u r e s from 10-30 C. T h i s i s a consequence of b a l a n c i n g a " f i x e d " l o s s (cutaneous l o s s ) a g a i n s t a much reduced m e t a b o l i c w a t e r p r o d u c t i o n . T h i s f i x e d l o s s makes i t i m p o s s i b l e f o r P. pa r v u s t o c onserve w a t e r by t o r p o r . S i n c e m e t a b o l i c water i s the u l t i m a t e source of w a t e r f o r d e s e r t r o d e n t s , i t i s more r e a s o n a b l e t o c o n s i d e r energy as the l i m i t i n g f a c t o r of g r e a t e s t s i g n i f i c a n c e t o d e s e r t r o d e n t s . T h e i r a d a p t a t i o n s f o r w a t e r c o n s e r v a t i o n have l e f t e n e r g y as the major l i m i t i n g f a c t o r . T o r p o r , as an e n e r g y - c o n s e r v i n g mechanism, i s a means of e x c h a n g i n g one l i m i t i n g f a c t o r (water) f o r a n o t h e r (energy) i n o r d e r t o a c h i e v e the most s a t i s f a c t o r y a d a p t a t i o n of the whole a n i m a l t o i t s environment a t a g i v e n t i m e . 79 LITERATURE CITED Agld, R. and L. Amibid. 1969. Effects of corporeal temp-erature on glucose metabolism in a homeotherm, the rat, and a hibernator, the garden dormouse. In; X. J. Musacchia and J. F. Saunders (ed.) Depressed metabolism. American Elsevier Publishing Co., Inc., New York. Bartholomew, G. A. and T. J. Cade. 1957. Temperature regulation, hibernation, and aestivation in the l i t t l e pocket mouse, Perognathus longlmembris. J. Mammal. 38: 60-72. Bartholomew, G. A. and R. E. MacMillen. I96I. Oxygen consumption, estivation, and hibernation in the kangaroo mouse, Mlorodlpodops pallidus. Physiol. Zool. 3*+: 177-1«3. Brody, S. 19^5. Bloenergetlos and growth. Hafner Publishing Co., Inc., New York. Brown, J. H. and G. A. Bartholomew. 1969. Periodicity and energetics of torpor in the kangaroo mouse, Mlorodlpodops pallidus. Ecology 50: 705-709. Chew, R. M. 1961. Water metabolism of desert-inhabiting vertebrates. B i o l . Rev. 36: 1-31. Chew, R. M. 1965. Water metabolism of mammals, p 43-178. In: W. V. Mayer and R. G. Van Gelder (ed.) Physiological mammalogy, Vol. II Academic Press, New York. Chew, R. M., R. G. Lindberg, and P. Hayden. 1967. Temperature regulation in the l i t t l e pocket mouse, Perognathus longlmembris. Comp. Biochem. Physiol. 21: 487-505. Church, R. L. I969. Evaporative water loss and gross effects of water privation in the kangaroo rat, Dlpodomys venustus. J. Mammal. 50: 514-523. Dawson, W. R. 1955. The relation of oxygen consumption to temperature in desert rodents. J. Mammal. 36: 543-553. Getz, L. L. I968. Relationship between ambient temperature and respiratory water loss of small mammals. Comp. Biochem. Physiol. 24: 335-3^2. Hart, J. S. 1950. Interrelations of daily metabolic cycle, activity, and environmental temperature of mice. Can. J. Res. D. 28: 293-307. 80 Hayden, P. and R. G. Lindberg. 1970. Hypoxia-induced torpor in pocket mice (Genus: Perognathus). Comp. Biochem. Physiol. 33: 167-179. Hayward, J. S. 1964. Aspects of temperature adaptation in Peromysous. Ph.D. Thesis. Univ. B r i t i s h Columbia. Hayward, J . S. 1967. (chairman). Cold thermogenesls. 9th Canadian Cold Physiology Conference. Univ. of Alberta, Edmonton. Hudson, J. W. 1964. Water metabolism in desert mammals. In: Thirst - Proc. 1st Internat. Symp. on Thirst. Pergamon Press, Ltd., Oxford. Hudson, J.W. and G. A. Bartholomew. 1964. Terrestrial animals in dry heat : estivators. p. 541 - 5 5 0 . In; D. B. D i l l (ed.) Handbook of physiology, Sec. 4 . Adaptation to the environment. Amer. Physiol. Soc, Washington Hughes, G. M. 1963. Comparative physiology of vertebrate  respiration. Harvard Univ. Press, Cambridge. Iverson, S. L. 1967. Adaptations to arid environemnts in Perognathus parvus (Peale). Ph.D. Thesis. Univ. Bri t i s h Columbia. Kayser, C. 1961. The physiology of natural hibernation. Pergamon Press, London. Kleiber, M. I96I. The f i r e of l i f e : an introduction to  animal energetics. Wiley, New York. Lasiewski, R. C. 1963. Oxygen consumption of torpid, resting, active, and flying hummingbirds. Physiol. Zool. 36: 122-129. Lyman, C. P. and P. 0. Chatfield. 1955. Physiology of hlbej^ation in mammajls. Physiol. Rev. 35s '403-425. Lyman, C. P. 1961. Hibernation in mammals. Circulation 2 4 : 434-445. MacMillen, R. E. 1965. Aestivation in the cactus mouse, Peromyscus eremicus. Comp. Biochem. Physiol. 16: 227-2 4 8 . MacMillen, R. E. and A. K. Lee. 1970. Energy metabolism and pulmocutaneous water loss of Australian hopping mice. Comp. Biochem. Physiol. 35: 355-369. Mullen, R. K. 1970. Respiratory metabolism and body water turnover rates in Perognathus formosus in i t s natural environment. Comp. Biochem. Physiol. 32: 259-65. 81 Schmidt-Nielsen, B. and K. Schmidt-Nielsen. 1950. Pulmonary water loss in desert rodents. Amer. J. Physiol. 162: 31-36. Schmidt-Nielsen, B. and K. Schmidt-Nielsen. 1950. Evaporative water loss in desert rodents in their natural habitat. Ecology 31: 75-85. Schmidt-Nielsen, B. and K. Schmidt-Nielsen. 1951. A complete account of. the water metabolism in kangaroo rats and an experimental verification. J. Cell Comp. Physiol. 38: 165-181. Schmidt-Nielsen, K. and B. Schmidt-Nielsen. 1952. Water metabolism of desert mammals. Physiol. Rev. 32: 135-166. Schmidt-SNielsen, K. 1964 a. Desert animals, physiological  problems of heat and water. Oxford Univ. Press, London. Schmidt-Nielsen, K. 1964 b. Terrestrial animals in dry heat: desert rodents. p 493-507. In: D. B. D i l l (ed) Handbook of physiology, Sec. 4. Adaptation to the environment. Amer. Physiol. Soc, Washington. Schmidt-Nielsen, K., F. R. Hainsworth, and D. E. Murrish. 1970. Counter-current heat exchange in the respiratory passages: effect on water and heat balance. Respiration Physiol. 9: 263-276. Tucker, V. A. 1965. Oxygen consumption, thermal conductance, and torpor in the California pocket mouse Perognathus  califomious. J. Cell Comp. Physiol. 65: 393-404. Twente, J . A. and J. W. Twente. 1968. Concentrations of L-lactate in the tissues of Cltellus l a t e r a l i s after known intervals of hibernating periods. J. Mammal. 49: 541-544. Wang, L. C.-H. and J. W. Hudson. 1970. Some physiological aspects of temperature regulation in the normothermic and torpid hispid pocket mouse, Perognathus hlspldus. Comp. Biochem. Physiol. 32: 275-293. Wunder, B. A. 1970. Temperature regulation and the effects of water restriction on Merriam's chipmunk, Eutamlas merrlaml. Comp. Biochem. Physiol. 33: 385-403. Yousef, M. K., D. Robertson, and H. D. Johnson. I967. Effect of hibernation on oxygen consumption and thyroidal ll31 release rate of Mesoorloetus auratus. Life Sciences 6: 1185-1194. APPENDIX I Source of data used In Pigs. Body Wt. Species 8.2 Perognathus longlmembrls 13.0 Mlorodlpodops pallldus 15.9 Peromysous orlnltus 17.4 Peromysous eremlous 19.1 Peromysous manloulatus 19.9 Peromysous e. eremlous 20.9 Peromysous orlnltus 21.5 Peromysous e. eremlous 22.0 Perognathus californlcus 24.2 Peromysous manloulatus 33.2 Peromysous ti, true! 33.3 Peromysous t. g l l b e r t l 34.7 Dlpodomys merrlaml 39.5 Perognathus hlspldus k5»5 Peromysous californlous 4 9 . 6 Peromysous californlcus 5 6 . 9 Dlpodomys panamlntlnus 110;0 Neotoma leplda 139.0 Neotoma leplda 1 4 5 » 0 Jaculus orientalis 16 and 1 7 . Source Chew et a l . 1 9 6 7 Brown & Bartholomew, 1 9 6 9 . MoNab & Morrison, 1 9 6 3 . MaoMlllen. I 9 6 5 . McNab Sc Morrison. 1 9 6 3 11 n fi Tucker. 1 9 6 5 . McNab & Morrison. 1 9 6 3 n n Dawson. 1 9 5 5 * Wang & Hudson. 1 9 7 0 . MoNab & Morrison. 1 9 6 3 11 Dawson. 1 9 5 5 . Lee. 1 9 6 3 . Lee. 1 9 6 3 . Klrmitz. 1 9 6 2 . APPENDIX II Computer Program for Three-Dimensional Energy and Water Budgets // JOB GUTHRIE // FOR *ONE WORD INTEGERS #IOCS(KEYBOARD »1132 PRINTER*UDISK»DISK) REAL N.NOR»NL DIMENSION Z(11)»ZEX(11),IHT(503)»XP(11)»YP(16) DEFINE FILE 1(176»2»U , IDN1)»2U76.2»U,IDN2) IN=6 10=3 IX=11 IY=16 Nl = l N2-2 NIHT=503 RYY=1.0 RZZ=0.3 ABOUT=30.0 ABOVE=30tO XSIZE=5tO YSIZE=5.0 C1-2I.5 C2=0.6 C3=35.0 C4=1.81 C5=0.068 C6=40.45 C7=1.127 C8=168.0 C9=2.5 C10=4.42 Cll=1.05 C12=0.0306 C13=0.000163 C14=168.0 C15=40.45 C16=1.127 C17=0.102 WRITE(IO*100)C1*C2.C3*C4»C5*C6*C7*C8»C9*C10»C11,C12 »C13,C14»C15»C1 16»C17 100 FORMAT(•1•»3F6.1»F6.2»F7.3»F7.2»F7.3»2F7.1»F7•2/ 1F7.3,F8.4»F10.6»F7.1»F7.2»2F7.3) 1 READ(IN)W»N IF<W*N)3»3»2 2 DX=14.0/N WRITE(10,101)W.N»DX 101 FORMAT(//F6.1»F6.0,F6.2/) IDN1=1 IDN2=1 Ya-2.0 DO 11 J=1»IY Y=Y+2iO X=-DX DO 10 1 = 1»IX X=X+DX ENT=N*W*(C1-C2*Y) ARO=N*W*<C3-Y) TOR»N*W*X*(C4+C5*Y) NOR=W*(C6-C7*Y)*(C8-N*X-C9*N) TCPW=ENT+ARO+TOR+NOR NL=N*X*(C10*C11**Y) + <C12+C13*Y*Y)MC14-N*X)*W*(C15-C16*Y) TNL=NL-C17*TCPW Z( I )=TCPW 10 ZEX(I)=TNL W R I T E ( I O » 1 0 2 ) Z , Z E X 102 F O R M A T ( 5 E 1 2 . 4 / 6 E 1 2 . 4 / 5 E 1 2 . 4 / 6 E 1 2 . 4 / ) CALL D I D L W ( N 1 , I D N 1 , Z . I X ) 11 CALL D I D L W ( N 2 » I D N 2 . Z E X . I X ) [ ( CALL S C A L F ( 1 . 0 . 1 . 0 , 0 . 0 . 0 . 0 ) CALL P E R S ( I X » I Y » R Y Y . R Z Z . A B O U T » A B O V E . X S I Z E » Y S I Z E . X P » Y P » Z . Z E X » N l » I D N H t l H T t N I H T ) CALL PENUP CALL F P L O T ( 0 , 0 . 0 . 5 . 0 ) CALL S C A L F ( 1 . 0 . 1 . 0 , 0 . 0 . 0 . 0 ) CALL P E R S ( I X , I Y . R Y Y . R Z Z . A B O U T . A B O V E . X S I Z E . Y S I Z E . X P . Y P . Z . Z E X . N 2 . 1 IDN2. IHT .NIHT) CALL PENUP CALL F P L O T f O . 9 . 0 . - 5 . 0 ) GO TO 1 3 CALL EXIT END // XEQ 

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