J. Comp. Physiol. I31,303-309 (1979) Journal of Comparative Physiology. B �9 by Springer-Verlag 1979

Water and Energy Metabolism in the Rock Hyrax (Procavia habessinica)

K. Rtibsamen, R. Heller, H. Lawrenz, and W.v. Engelhardt

Department of Zoophysiology, University Stuttgart-Hohenheim, D-7000 Stuttgart 70, Federal Republic of Germany

Accepted March 26, 1979

Summary. The influence of ambient temperature and water supply on water metabolism and Oz-consump- tion was measured in rock hyraxes (Procavia habes- sinica).

With ad libitum food and water (control), water turnover rates of hyraxes were significantly lower than the general eutherian mean; water turnover rates were 61.4, 44.1 and 55.1 m l . k g ~ 1 at 20, 27 and 35 ~ respectively. When greens were fed ad libitum but no drinking water was given, water turnover rate at 20 ~ was twofold higher, but at 27 and 35 ~ it was similar to that in control experiments.

Water turnover rates were significantly reduced when no drinking water and only 25 g greens per day were offered (25.8, 22.0 and 29.3 m l . kg -~ 24 h -~ at 20, 27 and 35~ respectively). Highest urine osmolality (3,200 m o s m . kg-1) was recorded at 20 ~

Oxygen consumption under control conditions was 43% below that predicted on the basis of body weight for most eutherian mammals . The thermoneu- tral zone ranged f rom 27 to 35 ~ and the basal metabolic rate was 165 kJ. kg - o.75. h - 1.

be a warming-up period after a slight hypothermia during the night. Continuous recording of the deep body temperature (Bartholomew and Rainy, 1971) confirmed these findings, the difference between the lowest values at night and the highest during the day amounting to 3 to 4 ~ Oxygen consumption, heart rate, and evaporative water loss were below values predicted on the basis of body weight for other euther- ian mammals (Taylor and Sale, 1969; Bartholomew and Rainy, 1971).

Little information is available on the water meta- bolism ofhyraxes. Hanse (1962) reported that a hyrax could survive without drinking for 134 days. This was confirmed by Meltzer (1976); due to an extremely low water demand hyrax can live without drinking even during the driest months of the year. The only water supply during such periods is from plants of relatively low water content. These findings indicate the presence of effective adaptation mechanisms to reduce water loss. The present studies were conducted to estimate water balance and energy metabolism in the rock hyrax under conditions simulating its natural habitat.

Introduction

Hyraxes live in many different climatic zones of Africa. The tree hyrax Dendrohyrax lives in moun- tains where the temperatures during winter may be well below freezing point, while the bush hyrax Hetro- hyrax and especially the rock hyrax Procavia are adapted to dry climatic conditions. Observations on the rock hyrax (Sale, 1966a) showed a behaviour pat- tern that appears unusual for desert animals. During daylight hours and especially in the morning the ani- mals prefer basking on rocks most of the time. Taylor and Sale (1969) supposed that this behaviour might

Materials and Methods

1. Animals, Feeding and Housing

Thirteen adult rock hyraxes (Procavia habessinica) were captured in Kenya and Abyssinia and adapted in our laboratory for at least 8 months. The body weight ranged from 1.6 to 2.9 kg, with a mean of 2.4_+0.3 kg. Four to 6 weeks prior to the experinaents all animals were held separately in wire cages (70 x 70 • 60 cm). The diet was a pelleted food for rabbits (Solikanin) mixed with flaked oats, and supplemented with greens. During the summer (experiments A and C) greens consisted of fresh leaves of dandeIion and grass and, during the winter (experiment B) carrots, cabbage, and apples. Food was given at 0830 hour; during experiments with ad libitum water supply the animals had continuous access to water.

0340-7616/79/0131/0303/$01.40

304 K. R/ibsamen et al. : Water and Energy Metabolism in the Rock Hyrax

2. Water Metabolism

a) THO-Turnover Rate. The total body water and the water turn- over rate were measured with the isotope dilution method. 10 to 50 gCi tritium labelled water (THO) were dissolved in 0.5 ml physiological saline and injected intramuscularly. The dose injected was calculated by weighing the syringe before and after injection. Due to the difficulties of obtaining blood samples from hyraxes THO activity was measured in urine water (Rfibsamen et aI., 1979). The animals were accustomed to use a small funnel mounted in the floor in a corner of the cage as a defaecation place. Urine and feces were separated by wire mesh and urine was collected quantitatively in one hour intervals with a fraction collector for at least 10 days. Water for THO-estimation was obtained from urine by vacuum sublimation, and THO activity measured with a liquid scintillation spectrometer. The logarithm of specific activity in urinary water was plotted against time of urination using the method of least squares. The THO space was assumed to be identi- cal with total body water (TBW), and was calculated from the THO activity extrapolated to zero time. The water turnover rate was estimated from the rate constant k of this curve multiplied by the THO-pool. To allow comparison between animals of differ- ent body weight water turnover rates are given in ml.kg- o.8z. 24 h- 1 (Richmond et al., 1962).

b) Water Balance. Daily food and water consumption were deter- mined by weighing. For the estimation of water loss from food and drinking water by evapor~ition a dish filled with food was kept in the room for a definite time, and food water and drinking water were corrected for these losses. Metabolic water was calcu- lated as total water turnover rate minus water intake as food and drinking water, and evaporative water was estimated as total water turnover rate minus urine and fecal water. Urine volume was measured by weighing the collecting vials, and subtracting urine dry matter from total urine weight. Osmolality of urine was determined by freezing point depression (Osmometer, Advanced Instruments). Fecai water loss was calculated from the dry weight of feces and the mean water content. The water content was esti- mated frequently from freshly sampled feces dried at 105~ to constant weight. The following experimental conditions were used :

A) Drinking water, concentrate and greens ad lib. B) No drinking water, concentrate and greens ad lib. C) No drinking water, concentrate ad lib., and 25 g greens

per day. Between one and five experiments were conducted with each

of the 13 hyraxes. There was no systematic schedule in selecting the animals for the different experiments. Room temperature was controlled to an accuracy of +0.5~ Humidity was not con- trolled; relative humidity was 57% (range 54 to 60) at 20~ 52% (range 42 to 67) at 27~ and 28% (range 25 to 31) at 35~

3. Oxygen Consumption

All measurements on O2-consumption were made between 0800 and 1200hours in an open flow system on animals denied of food for at least 12 h. The hyraxes were always adapted to the tempera- ture inside the perspex chamber for at least one hour. The double walled respiration chamber was temperature controlled by a water bath, and the temperature inside the chamber was measured with an accuracy of _+ 0.1 ~ Affluent air was dried by passage through two U-tubes, the first filled with calcium chloride, the second with phosphorus pentoxide. Water vapor in the air leaving the chamber was removed by means of sulfuric acid, calcium chloride, and phosphorus pentoxide. The air flow through the chamber was 41- min- 1 The air volume leaving the chamber was estimated using

a calibrated dry gas meter (Pipersberg). Oxygen was measured with a paramagnetic oxygen analyzer (Magnos, Hartmann & Braun) after CO2-absorption and additional drying with phos- phorus pentoxide. Oxygen consumption was measured for each animal at each ambient temperature (TA) for two to four 20-rain- periods. The measuring period was started only after steady state conditions were reached. Gas volumes were corrected for STPD. Estimates of error are given as standard deviations (S.D.); the significance of differences was assessed with Student's t-test.

Results

1. Influence of Ambient Temperature and Restricted Water Intake on Body Weight and Body Water Content

A t a d l i b i t u m f o o d i n t a k e ( e x p e r i m e n t s A a n d B)

b o d y w e i g h t r e m a i n e d c o n s t a n t t h r o u g h o u t the e x p e r -

i m e n t a l p e r i o d a t al l a m b i e n t t e m p e r a t u r e s . T h e b o d y

w a t e r c o n t e n t as a p e r c e n t a g e o f b o d y w e i g h t was

n o t s i g n i f i c a n t l y i n f l u e n c e d b y t he a m b i e n t t e m p e r a -

tu re , t he a v e r a g e d u r i n g exp. A a n d B was 74.0 _+ 5 . 7 %

(n = 20). W h e n d r i n k i n g w a t e r was w i t h h e l d a n d f o o d

w a t e r i n t a k e w a s r e s t r i c t e d (exp. C) b o d y w e i g h t de-

c r e a s e d b y 5 . 5 + 0 . 5 % a t a n a m b i e n t t e m p e r a t u r e o f

2 0 ~ b y 3 . 4 + 0 . 3 % a t 2 7 ~ a n d b y 8 . 1 + 0 . 2 % a t

35 ~ T h e m e a n p e r c e n t a g e o f b o d y w a t e r w a s

6 8 . 1 _ + 4 . 1 % a n d t h u s s i g n i f i c a n t l y l o w e r ( P < 0 . 0 1 )

t h a n t h e v a l u e s in e x p e r i m e n t s A a n d B.

2. Water Balance with ad libitum Food and Water (Experiment A)

T h e w a t e r t u r n o v e r r a t e d u r i n g e x p e r i m e n t A

d e p e n d e d h i g h l y o n t he a m b i e n t t e m p e r a t u r e (Fig . 1).

T h e h i g h e s t m e a n v a l u e was m e a s u r e d a t 20 ~

( 6 1 . 4 + 4 . 6 m l . k g - ~ h - ~, n = 3 ) , a n d t he l o w e s t

a t 2 7 ~ ( 4 4 . 1 + 1 0 . 9 m l . k g - ~ -1 , n = 3 ) ;

w a t e r t u r n o v e r r a t e a t 35 ~ was 5 5 . 1 + 17.0 m l - k g ~ 1, ( n = 3 ) . T h e g r e a t e s t

p r o p o r t i o n o f w a t e r g a i n was d r i n k i n g w a t e r , a n d

d i f f e r e n c e s in w a t e r t u r n o v e r r a t e w e r e m a i n l y d u e

to d i f f e r e n c e s in d r i n k i n g w a t e r i n t a k e . T h e c o n t r i b u -

t i o n o f m e t a b o l i c w a t e r to w a t e r t u r n o v e r was s m a l l

a n d n o s i g n i f i c a n t d i f f e r e n c e s in m e t a b o l i c w a t e r g a i n

a t t h e d i f f e r e n t a m b i e n t t e m p e r a t u r e s w e r e f o u n d .

W a t e r i n t a k e f r o m f o o d was c o m p a r a t i v e l y l ow a n d

d i d n o t c h a n g e s i g n i f i c a n t l y d u e t o t h e s m a l l c h a n g e s

in d r y m a t t e r i n t a k e a t 20, 27 a n d 35 ~ ( T a b l e 1).

E v a p o r a t i v e w a t e r loss was g r e a t e s t a t 35 ~ w h e n

it was 6 0 . 7 % o f t he t o t a l w a t e r t u r n o v e r r a t e (Fig . 2).

A t 20 a n d 27 ~ e v a p o r a t i v e w a t e r loss was r e d u c e d

to 4 2 . 4 % ( P < 0 . 0 0 1 ) a n d 5 3 . 7 % ( P < 0 . 0 1 ) , r e spec -

t ively. W a t e r loss w i t h u r i n e a n d feces as a p e r c e n t

K. Rfibsamen et al. : Water and Energy Metabolism in the Rock Hyrax 305

140

120 �84

100

4O :ii 20 27

v

exp : A

[ ] metabolic water

[ ] water in concentrate

[ ] water in greens

[ ] drinking water

~A .... w~ 35~ 20 27 350C 20 27 35~

i \ _ _ / ~ _ _ /

B C

Fig. 1. Water turnover rates and water gain of rock hyrax from metabolism, food, and drinking at 20, 27 and 35 ~ under different conditions of water deprivation. A, Drinking water, concentrate and greens ad lib. ; B, No drinking water, concentrate and greens ad lib. ; C, No drinking water, concentrate ad lib., and 25 g greens

per day. S.D. of the water turnover rate is given by the lines extending upwards; the lines facing downwards indicate the S.D. of modes of water gain. The black areas on top of the 3 columns on the right indicate additional water gained from the breakdown of body tissues when the animals were losing body weight

of water turnover were both higher (P<0.001) at 20 ~ than at 27 and 35 ~

3. Water Balance Under Conditions of Restricted Drinking Water Intake (Experiment B)

When no drinking water was offered but greens were given ad libitum, the water turnover rate at 20 ~ was increased twofold compared to experiment A (Fig. 1); 88% of the total water gained was water ingested with greens. The metabolic water gain and the water obtained f rom dry food were not signifi- cantly different f rom that of the control (exp. A). At 27 and 35 ~ water turnover rates and the portion of water gained f rom food and metabolism were not different f rom that of experiment A (food and water ad libitum). While a close relationship between dry matter intake and water turnover rate under most experimental conditions was found (Fig. 3), this was not true for experiment B at 20 ~ when only greens were fed. The rate of water turnover of all three ani- mals was above the common regression line.

During experiment B renal water loss at 20 ~ was significantly higher than in experiment A (P<0.001). Water content and dry weight of feces were both similar to those of experiment A (Fig. 4). At ambient temperatures of 27 and 35 ~ no differ- ences in renal, fecal or evaporative water losses be- tween control (A) and restricted water intake (B) were observed. Mean urine osmolality during experiment A and B was 1938_+ 594 m o s m . k g - I (n=23).

140

[ ] evaporative water

120 [ ] water in feces

[ ] water in urine

IO0

' ii i m o ~ 20 27 35~ 20 27 35~ 20 27 35~

v - - v v exp= A B C

Fig. 2. Water turnover rates and water loss by evaporation, urine, and feces at 20, 27 and 35 ~ under different conditions (see legend to Fig. 1) of water deprivation. Explanation of S.D. and of black areas as in Fig. 1

4. Water Balance Under Conditions of Restricted Drinking and Food Water Intake (Experiment C)

When no drinking water was offered and the supply of greens restricted in addition (exp. C), water turn- over rates at 20, 27 and 35 ~ were significantly dimi- nished (P < 0.001) compared to experiment A (Fig. 1). However, the absolute values obtained with the iso- tope dilution method must be too low because of the loss of body weight throughout experiment C. When this additional water gained f rom the break- down of body tissues is taken into account, water turnover rates are higher: 25.8 ml. kg- 0.82 24 h - 1 at 20~ 2 2 . 0 m l . k g - ~ 1 at 27~ and 29.3 ml . kg -~ h t at 35 ~ This reflects the in- crease in metabolism at 20 and 35 ~ as was also found during ad libitum food and water regimens. Due to the restricted intake of greens during experi- ment C, water gain f rom food water was similar at all ambient temperatures and accounted for 60% of total water turnover rate. While dry matter intakes

306

e-

120 o,'

80

E 40

o

y - 2 6 4 x - 0 . 7 8 '~

r - 0 . 9 0

n - 2 6

�9 exp. A �9 exp. B �9 exp. C

0 10 20 30 40

dry matter intake (g.kg-~ -~)

Fig. 3. Relationship between dry matter intake and water turnover rate under different experimental conditions. Open symbols repre- sent values obtained in exp. B at 20 ~ when water was removed and carrots, apples, and cabbage fed ad libitum. These three values were not included in the calculation of the common regression line

r

O~

. 0 1

60 1 70%

40

20

7 5 %

7 9 %

7 7 %

[ ] water content

[ ] dry weight

65%__ 6 6 % ,56%

0 ~

2O 27 ~ 2O 27 a S ~ 2O 27 35~ v v

exp: A B C

Fig. 4. Weight of feces and fecal water content at different experi- mental conditions�9 Numbers on top of each column indicate per- centage of water in feces. See legend to Fig. 1 for experimental conditions

Table 1. Dry matter intake in g-kg -~ h 1 during the experi- mental periods at various ambient temperatures; each value repre- sents the mean of 30 days

Experimental conditions a

Dry matter intake (g. kg- 0.75.24 h 1)

20 ~ 27 ~ 35 ~

A 28.7• 17.2• 19.8-+6.8 B 31.7• 17.4• 18.1_+2.4 C 9.8• 7.7_+1.6 6.3-+3.7

see Materials and Methods for details

at 20, 27 and 35 ~ were similar (Table 1), metabol ic water gain showed considerable var ia t ions due to the addi t iona l water available f rom b reakdown of body tissues. The reliabili ty of these values will be discussed later. Evaporat ive water loss, ur ine volume and water

K. Rfibsamen et al. : Water and Energy Metabolism in the Rock Hyrax

Table 2. Ratio between metabolic water gain and evaporative water loss under different experimental conditions

Experimental conditions a

Metabolic water gain/evaporative water loss

20 ~ b 27 ~ b 35 ~ b

A 0.53_+0.03 0.42+0.04 0.32_+0.05 B 0.41 +0.09 0 . 1 9 - + 0 . 0 8 0.27_+0.1i C 0.75 _+ 0.09 0.60 • 0.09 0.67 + 0.06

a See Materials and Methods for details b Relative humidity in the experimental room was 57, 52, and 28% at 20, 27 and 35~ respectively

t -

"7 Ell

v r

.o_

t - O

t - Q Ell

0.6

0.4

0.2

" ~ \ �9 Klementine �9 Mathilde �9 Ell•

�9 =~,x& �9 �9

�9 . " a . . " : - '

0 10 20 30 40

ambient temperature (~

Fig.5. Oxygen consumption of three hyraxes at different ambient temperatures

loss in feces were significantly lower compared to control experiments. This lower fecal water loss was no t due to drier feces bu t to the lower total fecal ou tpu t (Fig. 4). Fecal dry mat ter ou tpu t at 35 ~ was reduced by 60% compared to the value under control condi t ions (exp. A). The ur ine concent ra t ion under these condi t ions was significantly higher (P<0 .001 ) than in exper iment A, with a mean osmolal i ty of 2 ,495+ 462 m o s m - k g - 1 ( n = 23), and a m a x i m u m value of 3,200 m o s m . k g 1

The ratio of metabol ic water gain to evaporat ive water loss was lowest when no dr ink ing water bu t greens were offered ad lib. (exp. B, Table 2). It was

also inf luenced by the ambien t t empera ture ; in all experiments the value at 20 ~ was higher than at 27 or 35 ~ The highest ratio of metabol ic water gain to evaporat ive water loss was ob ta ined when dr ink ing water was withheld a nd greens were restricted (exp. C).

5. Oxygen Consumption

The oxygen consumpt ion was measured between 6 and 42 ~ (Fig. 5). In the the rmoneut ra l zone, which

�9 K. R~bsamen et al. : Water and Energy Metabolism in the Rock Hyrax 307

ranged from 27 to 35 ~ the metabolic rate expressed as oxygen consumption was 0.27 _+ 0.03 ml O z ' g - 1 . h - t . The relationship between oxygen con- sumption and ambient temperature below thermoneu- trality can be described by the formula y = 0.018 x + 0.786. The regression line does not extrapo- late to average body temperature; the x-intercept was found to be at 44 ~

Discussion

1. Body Water Content

Body water as a percentage of body weight during ad libitum feeding (exp. A and B) was 74% and thus in the range of most other mammals. During experi- ment C, total body water decreased to 68%, and the dehydrated hyraxes reduced their dry matter in- take considerably (Table 1). It may therefore be assumed that the relatively high gut to body weight ratio of hyraxes, which is comparable to that of sheep (Clemens, 1977), also was diminished as has been observed in macropod marsupials (Denny and Daw- son, 1975b). Due to the high gut water contents a reduction in gut volume would explain a large part of the percentage of water lost. This was also found in the camel, in which 52% of body water lost during dehydration was from the gut (Macfarlane et al., 1963).

2. Water Balance Under ad libitum Conditions

From the systematic position of the Hyracoidea, which is near the Proboscidea and Sirenia, eutherian levels of water and energy consumption may be expected. However, the mean rate of water turnover measured in hyraxes under conditions of food and water ad libitum was up to 60% lower than that of other eutherian mammals (Fig. 6) (Richmond et al., 1962). The water turnover of 44 ml �9 kg-0.a2 24h 1 is low, however, it was measured near thermoneutrality and under laboratory conditions. The water turnover of free living animals should be higher due to the additional energy expendi- ture associated with grazing and running. However, Meltzer (1976) showed that water intake of hyraxes in the wild during feeding of greens ad libitum did not exceed 170 ml. 24h 1. Related to a body weight of 3.5 kg this is equivalent to a water turnover rate of only 64 ml. kg- o. s 2.24 h- 1. This provides evidence that energy metabolism and, linked to it, water meta- bolism under free living conditions may be low, pos- sibly due to the energetically economic behaviour of

10 000

"T

~ ooo

O)

100 $

�9 Hulbert & Dawson,1974

* Denny & Dawson,1975 / /

�9 present study

�9 Do 7 *= M robustus ~ * - Me :ruf-a

~ J *-M.giganteus

* - M.eugenii

I~tridactylus - f ~ I. m a e r o u ~ / - �9 ==~ Proeavla habessinica

/ ~ nas~ta �9 - M. lagotis

. . . . . . . . i . . . . . . . . i . . . . . . . . i

1 10 100 body weight (kg)

Fig. 6. Compar ison of water turnover rates of hyraxes during ad libitum food and water regimen with values of marsupial mammals . Eutherian line from Richmond et al. (1962)

hyraxes (Sale, 1966b). Hoeck (1975) observed that the main feeding activities of Heterohyrax brucei were shortly after sunrise (0730 to 0930 hours), and in the evening between 1630 and 1830 hours; move- ments during the hot daytime hours were minimal. To our knowledge similar low rates of water turnover have been found only in macropod marsupials, espe- cially those adapted to desert habitats (Hulbert and Dawson, 1974; Denny and Dawson, 1975a; Haines et al., 1974).

The water turnover rate under conditions of food and water intake ad libitum was influenced by the ambient temperature. Because of the increased energy metabolism to maintain body temperature at 20 ~ (Fig. 5), and because of the close relationship between water metabolism and energy metabolism (Macfar- lane, 1976), the rate of water turnover at 20 ~ was also higher than at 27 ~ (Fig. 1). Although not statis- tically significant, the water turnover rate at 35 ~ was slightly higher than at 27 ~ the additional amount of water drunk was used for evaporative cool- ing (Fig. 2). Due to the greater amount of food eaten at 20 ~ than at 27 ~ urinary and fecal water vol- umes were also higher.

3. Water Balance Without Drinking Water

Deprivation of drinking water and offering greens ad libitum caused the hyraxes at 20 ~ to increase their rate of water turnover twofold compared to con- trols, while water turnover at 27 and 35 ~ were simi- lar to experiment A. As these values do not fit into the relationship between dry matter intake and water turnover rate (Fig. 3) we assume that the adaptation period of 7 days for this diet was too short. The water

308 K. Rfibsamen et al. : Water and Energy Metabolism in the Rock Hyrax

balances at 27 and 35 ~ and especially at 20 ~ indi- cate that hyraxes are able to maintain their normal rate of water turnover on a regimen of greens without additional drinking water. The animals thereby obvi- ously selected between greens and concentrate in such a way that dry matter intake as well as performed water intake were similar to control values (Table 1). The ratio of metabolic water gain to evaporative water loss during experiment B was significantly lower than under control conditions; this may show that there was no necessity for the animals to diminish evaporative water loss under these conditions.

A significant reduction in the rate of water turn- over was observed when the animals were deprived of drinking water and, additionally, the availability of greens was restricted (experiment C). It is, however, debatable to use the isotope dilution method in ani- mals losing body weight. In experiment C body weight decreased by 3 to 8% during the experimental period of 10 days. Thus water available from the breakdown of body tissue is not included in the values estimated with the isotope dilution method. If this additional body water is taken into account, water turnover rates are somewhat higher (Fig. 1). However, even if the water turnover rates are corrected for the tissue contribution they are still low compared to most other mammals. Haines et al. (1974) measured a similar low water turnover rate of 24 ml .kg ~ in Notomys alexis (an Australian desert murid) held un- der conditions of restricted water intake. The similar response of hyraxes to water restriction indicates that for shorter periods these animals are able to maintain normal body functions even on a rather dry diet. The maintenance of body weight, however, was not possible under these conditions, indicating that the level of minimum water intake must be above the offered limit.

Several factors are involved in this reduction of overall water turnover in the rock hyrax: (1) urinary water loss was reduced by the excretion of a highly concentrated urine. Urine volume was 6 . 2 m l . k g - ~ -1 and thus similar to the urine volume of 5 . 7 m l . k g - ~ calculated from Meltzer's (1976) results. Comparison with data of the South African hyrax (Maloiy and Sale, 1976) indicates that species differences in the concentrating ability occur, probably reflecting adaptation to different ha- bitats. (2) Water was also saved by a low evaporative water loss. Under conditions of food and water ad libitum at 20 ~ (exp. A) water loss by evaporation was 26ml . kg ~ and it was reduced to 1 4 m l . k g - ~ -1 in exp. C. The decrease in evaporative water loss was similar at 35 ~ when the need for evaporative cooling became greater; water evaporated at 35~ amounted to 33 m l - k g -~

24 h- 1 in exp. A, but was reduced to 18 ml �9 kg -~ 24 h- ~ in exp. C. Evaporative water loss as per- cent of total water turnover rate amounted to 42 to 70%. Evaporation therefore seems to be lower than in perameloid marsupials (Hulbert and Daw- son, 1974) which lost 70 to 80% of their water available by evaporation. This response to water de- privation suggests that a decrease in heat production during exp. C (as indicated by the low dry matter intake) is responsible for the low amount of water used for heat dissipation. On the other hand, the rise, in exp. C, in the ratio of metabolic water gain to evaporative water loss up to 0.75 (Table 2) indi- cates that the effect of the reduction in evaporative water loss is greater than that of the decrease in meta- bolic water gain. Schmidt-Nielsen et al. (1970) showed that evaporative water loss can be greatly reduced by counter-current heat exchange in the respiratory passages; this has not yet been studied in the hyrax. Thus, the magnitude of respiratory water recovery is unknown. However, it seems that the rock hyrax is not able to reduce evaporative water loss below the level of metabolic water gain and thus cannot be included in the group of animals which are inde- pendent from exogenous water.

4. Comparative Aspects' of Energy and Water Metabolism

Oxygen consumption of the rock hyrax measured at thermoneutrality was 0.28 ml O2"g- 1 .h- 1. Taylor and Sale (1969) found a somewhat higher basal meta- bolic rate of 0.4 ml 02" g- *" h- 1 in the same species. Heterohyrax brucei consumed 0.52 ml O2"g- l ' h - 1 (Bartholomew and Rainy, 1971) and showed an unu- sual decline in O2 uptake at ambient temperatures above 37 ~ The reason for the low 02-consumption measured in our animals may be the efforts undertaken to accustom the animals prior to the experiments to sit in the respiration chamber without excitation. Fur- thermore, our animals never showed a decline in O2- consumption at temperatures above thermoneutrality (Fig. 5) although the response in behaviour was simi- lar to that observed by Bartholomew and Rainy (1971). The animals laid flat on the floor and exposed the inner surface of their hind legs to the air while muscle tonus decreased. One possible explanation for the different oxygen consumption results might be the higher relative humidity (23.5%) in Bartholomew and Rainy's experiments. This would reduce the effi- ciency of evaporative cooling more than in our experi- ments in which the relative humiditiy inside the chamber never exceeded 18%.

K. Riibsamen et al.: Water and Energy Metabolism in the Rock Hyrax 309

The basal metabolic rate of most eutherian mam- mals follows the expression M=293 W-0.75 (Kleiber, 1961) where M is the metabolic rate in joules per day and W is the body weight in kilograms. The basal metabolic rate of the rock hyrax deviates consid- erably from the eutherian value. Assuming a heat production of 19.7kJ/1 02 consumed, the mean metabolic rate was 165kJ.kg-~ -1, only 5"7% of the value predicted on the basis of its body weight. The low metabolic rate of some rodents has been shown by McNab (1966) to be related to their fossorial existence. This relationship may also hold for the rock hyrax, which spends long periods in rock shelter. However, there is evidence (Taylor and Sale, 1969) that the basal metabolic rate of all Hyracoidea is below the mean eutherian value, even though the different genera occupy a wide range of habitats with different ecological requirements. This suggests that the basal metabolic rate of the Hyracoidea is deter- mined by their phylogeny rather than by their ecol- ogy.

The water turnover rate of an animal is closely related to its energy metabolism. From the preceding considerations we conclude that the low overall water consumption represents a secondary effect, reflecting the low energy metabolism more than adaptation to desert environments. On the other hand the high urine concentrating ability and the lower than predicted evaporative water loss (due perhaps to a countercur- rent heat exchange in the respiratory passage similar to that found in desert adapted mammals by Schmidt- Nielsen et al., 1970) are mechanisms that have devel- oped to enable the animal to survive dry periods. However, despite these attributes, our findings indi- cate that the rock hyrax is not able to live without exogenous water for extended periods.

The present work was supported by grant EN 65/8 of the "Deutsche Forschungsgemeinschaft".

We are grateful to Miss U. Nolda for expert technical assis- tance and to Dr. P. Hoppe for providing most of our hyraxes. We thank Dr. W. Schneider for his advice and the permission to use his laboratory equipment, and we thank Dr. I.D. Hume for his aid in preparing the manuscript. We are also grateful to the International Atomic Energy Agency (IAEA), Vienna, for help in starting this project.

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