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Oxygen uptake and temperature regulation of young harbor seals (Phoca vitulina richardi) in water
Colllp, Bioe/It'm. PltJ'siul.. 1970, I~oL 54A. I'l" 105 1o 1117. I'l'rffamtnl t'rl'.~,~, l ' r l n h , d to Grca! //rilain
OXYGEN
UPTAKE AND TEMPERATURE REGULATION O F Y O U N G H A R B O R SEALS (PHOCA V1TULINA RICHARDI) IN W A T E R *
KEITH MILLER, MARIO ROSENMANNAND PETER MORRISON Institute of Arctic Biology, University of Alaska, Fairbanks, AK 99701, U,S.A. (Received 25 dune t975) Abstract--1. Oxygen uptake and body temperatures were measured in two young harbor se~ls at water temperatures between 5 and 37°C. 2. Basal metabolic rates of both animals were approximately 0.74u O z g - l h r - l or about 2.6 times the value expected for an adult terrestrial mammal of the same weight. Lower critical temperature was 12°C in one animal and 19°C in the other. 3. A surprising tolerenee to high water tempelatures was demonstrated and thermal and metabolic equilibrium could be established with only a T C difference between deep body and water temperature. 4. It is concluded that water at temperatures typically encountered by newborn of the northern harbor seal imposes only a modest cold stress requiring, at most. a 2-fold increase in metabolism over the basal rate.
INTRODUCTION
Because of its high heat capacity and thermal conductivity, water exerts a much greater thermal load on h o m e o t h e r m s than air at the same temperature. In spite o f this fact, m a m m a l s that spend most o f their lives in an aquatic habitat are limited to the same physiological mechanisms for maintaining body temperature as terrestrial mammals, namely, heat conservation, heat production, and heat dissipation. In cold northern waters the first two processes must be paramount, but in a pinniped such as the harbor seal, with its southern range extending into the waters off Baja California, the capability of dissipating h~at must become important. Little information exists o n the metabolism of seals in water. Irving & H a r t (1957); Hart & Irving (1959) measured oxygen uptake of subadult harbor seals in both air and water at different temperatures, and their studies have formed the basis for much o f the current knowledge o f temperature regulation and metabolism in pinnipeds. It has t-,"en assumed by many that their studies were c~rr~ed out on adult animals, but, as can be seen from their tables o f body weights, the seals used were in the 4-8 m o n t h range ( 2 0 - 4 0 k g). Although immature animals, the seals studied by Irving & H a r t did have well developed blubber layers and, except for a higher resting metabolic rate, their thermoregulatory responses would be expected to be similar to those o f adult seals. It seems likely that if the thermoregulatory capabilities of harbor seals are ever), seriously taxed by low temperatures it would be from the period shortly after birth until they acquire a significant layer o f insulating blubber. Since pups are born during late spring or early summer when air temperatures are not usually below freezing, the cold (0-d0°C) water they normally encounter in m a n y areas is probably
their most severe temperature stress. Aside from D a v i d o v & M a k a r o v a (1964), who measured oxygen consumption in newborn, white coat harp seals before a n d after a 3 0 m i n immersion in ice-water (at 0°C), no information exists o n the metabolic rate of newborn phocid seals in water. Following the completion of studies on metabolism and temperature regulation of young, dark coat (nonlanugo furred) h a r b o r seals in air (Miller & Irving, 1975), two young seals were obtained for tests in water. In spite o f the fact that only two seals were available to us for the aquatic studies, the resulting data provided an estimate o f basal metabolism in water and the responses of newborn animals to a wide range of water temperatures. Although o u r initial interest was primarily to describe the metabolic responses o f newborn, non-lanugo harbor seals to cold water, the seals demonstrated a surprising tolerance to high water temperatures, and this unexpected capability led us to include tests at and a b o v e the upper critical temperature. METHODS
Two seals, between 1 and 2 weeks of age, were captured at Izembek Lagoon near the tip of the Alaska Peninsula. They were immediately flown to Fairbanks where they were maintained in a temperature controlled room at about 10°C. which approximates their average natural environmental temperature during summer. They v'ere fed a homogenate of smelt, safflower oil, vitamins and water by stomach tube 3 or 4 times daily. The seals had continuous access to a 1000 liter tank of fresh water which was completely changed at least twice a day. Oxygen uptake was measured in a closed circuit system using a manometric technique (Morrison, 1951) with the seal in a temperature-regulated water bath as previously described for metabolic tests of sea otters (Morrison & Rosenmann, 1974). Chamber air was continuously circulated through Baralyme canisters to absorb carbon dioxide. The chamber was large enough to allow the seals to *This work was supported by USPHS. NIH Grant GMI0402 and Sea Grant 1-36109. maneuver easily, and they.could swim or dive at will with105
106
KEITIt
MILLER° MARIO
ROSENMANN AND PETEIt MORRISON
out restraint. To allow the animals an opportunity to recover between runs they were tested on alternate days. So that the seals would be in a postabsorptive state, food was withheld at least 6hr, and in some causes, up to 18hr before testing. Test runs usually lasted about 5 hr, during which time the water temperature was kept constant. Since it was desired to complete the series of tests before the seals could put on much insulating blubber, testing was limited to a 3 week period. Rectal temperatures were obtained before and after each test with a therlnistor probe inserted 20 cm, and surface temperatures were simultaneously measu:vd with a radiometer (Barnes IT4-A). A pressure transducer in the metabolism chamber provided a continuous record of respiration. Body weights were measured before and after each test with free water removed from the fur, and the average weight was used to calculate oxygen uptake.
RESULTS
AND
Newborn Hm'i~r Seal.L, /
~: O8 o~0G o.4 0.2
"--'~.
,.%
O.Ot
I
t
!....
0
5
JO
~5
1
,,.I
2 0 *C 2,5
I , 30
.1 ~35
f 40
Fig. 1. Oxygen uptake of resting young harbor seals at different water temperatures. Curves are eye-fitted with portions below lower critical temperature extrapolated to average rectal temperature (dashed lines).
DISCUSSION
After initial exploratory dives the seals adapted very readily to the metabolism chamber and spent much of the time resting quietly or sleeping. It was thus possible to o b t a i n both resting and sleeping metabolic rates over the temperature range from 5 to about 37°C. Water temperature, body weight, and initial and final rectal temperature for each test are given in Table 1. Average initial rectal temperature was 37.8 _+ 0-2°(2, which is the same as that reported for a group of seven young harFor seals previously tested in air (Miller & Irving, 1975). Surface temperatures in air were also similar to those previously obtained, but following prolonged water immersion, skin and hair covered surface temperatures were invariably within a degree or two of the bath temperature. Respiratory rates at rest averaged 29"5 +__4:4 brcaths/min, which is close to the average value (27.3) predicted by the equation of G u y t o n (1947) for a 13-6 kg mammal. Resting metabolic rates of the two seals at various water temperatures are plotted in Fig. 1. Basal rates averaged 0.74cm a O 2 g - l h r -~, which is a b o u t 2.6 times the value expected for.an adult terrestrial mammal of the same weight. Oxygen consumption of both animals was essentially the same within the thermoneutral region, but lower and upper critical temperatures differed considerably. Seal n u m b e r 1 had
Table I. Body weights and initial and final rectal tempera. lures for metabolism tests. Initlal
Test Temp.(°c)
BoW WL.(Kg.)
Seal No. I " -
5.0 14.0 19.7 ..o 32.5 36.7
!1.7 12.4 l,.~ 12.6 13.s 15.0
38,5* 37,4 3r.9 -37.s 37.8
Seal NO. ~2 " "
5.0 14.0 19.7 ZS,O 30.0 34.5 36.0 37.5
13.5 13.6 13.9 13.7 T4.2 14.4 13.9 13.5
37.9 37.7 38.1 37.7 37.7 37.7 38.3 37.7
"
~4
Tr(*C)
Final Tr{*C) 37.0 3;-5 37.8 3s.1
38.8 37.9 37.6 37.9' 37.8 37,8 38.3 38.6
:~.S
37.8tO. Z ' " * Obt~leed a f t e r constderable struggling, value not included t n mean. **Mean + S,0.
a lower critical of a b o u t 19°C a n d an upper critical of 25°C, while seal n u m b e r 2 had a lower critical near t2°C and a n upper critical of a b o u t 31°C. The narrower zone of thermoneutrality in seal n u m b e r 1 may partly be duc ' the Fact that it was generally more restless and ~ a b l e and that it responded to cooling and Iw:~, more vigorously than the obviously place, .'n sleepy seal n u m b e r 2. Seal number i also m e r than n u m b e r 2 during the first two low tc,,~. ,lure tests, and the lack of fat may Ilavc t'e,:ult~\: a narrower zone of thermoneutrality.-1 h~.rc 11o indication of hypothermia even in 5:(' "~r The most surprising feature of the metabolism tests was the tolerance exhibited to high water temperatures. Upper critical temperature in air is a b o u t 30°C (Miller & Irving, 1975), and we intuitively felt that upper critical in water would be considerably lower, with lethal limits not far above the critical. However, instead of becoming excited in the warm water the seals were quiet and slept a great deal. This response kept the metabolic rate down, and one seal tolerated 36.7°C water without difficulty while the other was able to reach a steady state with respect to both oxygen uptake and, apparently, body temperature at a water temperature o f 37-5°C. At this water temperature oxygen uptake reached a plateau and remained constant for 2 hr. Rectal temperature in tse 2 seals increased, respectively, I a n d 2°C, so that the temperature gradient from body core to water was just over 2°C. Data on metabolism a n d other physiological changes during sleep will be detailed elsewhere, but in the thermoneutral zone. oxygen uptake during sleep declined a b o u t 25% in seal n u m b e r 2 a n d 15~ in seal n u m b e r 1. T h e a m o u n t of time spent sleeping increased markedly in both seals at higher water temperatures, and when the water ~ipproached the normal deep body temperature, seal number 2 slept almost constantly. A decrease in activity and increased tendency to sleep was also seen in newborn h a r b o r seals at air temperatures above 25°C (Miller & Irving, 1975). Although harbor seals in warm air demonstrated increases in rectal temperature without a n y corresponding increase in metabolic rate (Miller & Irving,
Oxygen uptake and temperature regulation of young harbor seals
4C
Seots ~n
"-7-
Wotet,
,,. ~.5c
/ t
/
o Seol No i
ol
} 2,
I~ O5 0~--
J 5
l IO
I 15
Woter
......... I ............ I ~ ~5
"femperoture(%)
I 30
....I . . 35
40
Fig. 2, Thermal conductance values calculated for young harbor seals at different water temperatures. Tr indicates rectal temperature, Ta indicates water temperature.
1975), this response was not seen in the water tests, Conflicting information also exists on the metabolic response of the California sea lion to high air temperatures. Matsuura & Whittow (1973) report that metabolism increased with increasing rectal temperature, while South et aL (1973) have reported that in their calorimetry experiments metabolic rate remained unaltered even though body temperature underwent a significant rise. In spite of their somewhat different oxygen consumption curves, total thermal conductance of both seals was very similar over the full range of test temperatures (Fig. 2). Based on equations in the literature commonly used to describe conductance in a variety of mammals (Hart, 1971 ; Morrison, 1960; Herreid & Kessel, t967), the values obtained for young harbor seals below the lower critical temperature are about twice the expected value for small terrestrial mammals. Although evaporative heat loss was not measured, and could not be subtracted from the metabolic heat, values given for thermal conductance at the higher bath temperatures should not be greatly in error. This is due to the fact that the chamber air was saturated with water vapor at all temperatures, panting was not observed, and evaporative loss from the body could not occur. From the results of the present study it is apparent that exposure to 5 or even 0°C water for periods of several hours is readily tolerated~byqhe newborn harbor seal. A combination 9f high resting metabolic rate and peripheral tissure heterothermy apparently enables the pups to tolerate such low water temperatures. The rapid early growth phase in newborn harbor and other phocid seals thus serves the multiple
107
thennal advantages of providing increa.~d insulation by deposition of subcutaneous fat, rapid improvement in the surface to volume ratio, and an inherently I!igh heat production. The ability to reach thermal equilibrium at water temperatures equal to the normal deep body temperature was a surprising finding, and indicates the thermal versatility of the harbor seal in its aquatic habitat. Recent studies on the sea otter have demonstrat~ a similar, though not quite such an extreme, tolerance of high water temperatures (Morrison & Rosenmann, 1974). The increased thermal cqnductance in the seal at high water temperatures must involve large in, creases in peripheral blood flow especially t o the appendages, and this should provide an interesting area for future study.
REFERENCES
DAvtoov A. F, & MAKAROVAA. R. (1964) Changes in heat regulation and circulation in newborn seals on transition to aquatic form of life. Fedn Proc. Fed~ Am. Socs exp. Biol. 24, Trans. SuppL No. 4, Part II: T563-T566. GuYloN A. C. (1947) Measurement of the respiratory volume of lab. animals. Am. J. Physiol. 150, 70-77. HARTJ. S. {1971)In Comparative Physiofoqy t?f Thermore.qulation, (Edited by Wm'rrow G. C.). Chap. I. Academic Press, New York. HAkT J, S. &]RVtNG L. (t959) The energetics of harbor seals in air and water with special consideration of seasonal changes. Can, J. ZooL 3% 447-457. HERRV.tDC. F, II & KI':SSELR. (1967) Thermal conductance in birds and mammals. Comp. Biochem. PhysioL 21, 405--414. IRVlr~OL & HARTJ. S. (1957) The metabolism and insulation of seals as bare-skinned mammals in cold water. Can. J. Zool. 35, 497-511, Mh'rsuu~ D. T. & WltiTTOWG. W. (1973) Oxygen uptake of the California sea lion and harbor seal during exposure to heat, Am. d. Physiol. 225(3), 711-715. MILLER L. K. & laviNG "L. (1975) Metabolism and temperature regulation in young harbor seals. Am. J. PhysioL In Press, MortRtsoN P. R. (1951) An automatic manometric respirometer. Rev. scient, lnstrum. 22, 264-267. MORmSONP. R. (t960) Some interrelations between weight and hibernation function, Bull. Mus. comp. ZooL Harr: 124, 7,9-91. MoRmsor~P. R. & ROSE,~M.~NNM. (t974) Metabolism and thermoregulation in the sea otter. PhysioL Zoo/. 47. 218-229. SOUTHF. E., LUECKER., SHANKLINM. D. & ZATZMAIqM. L. (1973) Direct partitional calorimetry of the California sea lion in air. Tenth Am~. Coq~ opt Biol. Sonar and Diving Mare,, Stanford Res. Inst.
OXYGEN
UPTAKE AND TEMPERATURE REGULATION O F Y O U N G H A R B O R SEALS (PHOCA V1TULINA RICHARDI) IN W A T E R *
KEITH MILLER, MARIO ROSENMANNAND PETER MORRISON Institute of Arctic Biology, University of Alaska, Fairbanks, AK 99701, U,S.A. (Received 25 dune t975) Abstract--1. Oxygen uptake and body temperatures were measured in two young harbor se~ls at water temperatures between 5 and 37°C. 2. Basal metabolic rates of both animals were approximately 0.74u O z g - l h r - l or about 2.6 times the value expected for an adult terrestrial mammal of the same weight. Lower critical temperature was 12°C in one animal and 19°C in the other. 3. A surprising tolerenee to high water tempelatures was demonstrated and thermal and metabolic equilibrium could be established with only a T C difference between deep body and water temperature. 4. It is concluded that water at temperatures typically encountered by newborn of the northern harbor seal imposes only a modest cold stress requiring, at most. a 2-fold increase in metabolism over the basal rate.
INTRODUCTION
Because of its high heat capacity and thermal conductivity, water exerts a much greater thermal load on h o m e o t h e r m s than air at the same temperature. In spite o f this fact, m a m m a l s that spend most o f their lives in an aquatic habitat are limited to the same physiological mechanisms for maintaining body temperature as terrestrial mammals, namely, heat conservation, heat production, and heat dissipation. In cold northern waters the first two processes must be paramount, but in a pinniped such as the harbor seal, with its southern range extending into the waters off Baja California, the capability of dissipating h~at must become important. Little information exists o n the metabolism of seals in water. Irving & H a r t (1957); Hart & Irving (1959) measured oxygen uptake of subadult harbor seals in both air and water at different temperatures, and their studies have formed the basis for much o f the current knowledge o f temperature regulation and metabolism in pinnipeds. It has t-,"en assumed by many that their studies were c~rr~ed out on adult animals, but, as can be seen from their tables o f body weights, the seals used were in the 4-8 m o n t h range ( 2 0 - 4 0 k g). Although immature animals, the seals studied by Irving & H a r t did have well developed blubber layers and, except for a higher resting metabolic rate, their thermoregulatory responses would be expected to be similar to those o f adult seals. It seems likely that if the thermoregulatory capabilities of harbor seals are ever), seriously taxed by low temperatures it would be from the period shortly after birth until they acquire a significant layer o f insulating blubber. Since pups are born during late spring or early summer when air temperatures are not usually below freezing, the cold (0-d0°C) water they normally encounter in m a n y areas is probably
their most severe temperature stress. Aside from D a v i d o v & M a k a r o v a (1964), who measured oxygen consumption in newborn, white coat harp seals before a n d after a 3 0 m i n immersion in ice-water (at 0°C), no information exists o n the metabolic rate of newborn phocid seals in water. Following the completion of studies on metabolism and temperature regulation of young, dark coat (nonlanugo furred) h a r b o r seals in air (Miller & Irving, 1975), two young seals were obtained for tests in water. In spite o f the fact that only two seals were available to us for the aquatic studies, the resulting data provided an estimate o f basal metabolism in water and the responses of newborn animals to a wide range of water temperatures. Although o u r initial interest was primarily to describe the metabolic responses o f newborn, non-lanugo harbor seals to cold water, the seals demonstrated a surprising tolerance to high water temperatures, and this unexpected capability led us to include tests at and a b o v e the upper critical temperature. METHODS
Two seals, between 1 and 2 weeks of age, were captured at Izembek Lagoon near the tip of the Alaska Peninsula. They were immediately flown to Fairbanks where they were maintained in a temperature controlled room at about 10°C. which approximates their average natural environmental temperature during summer. They v'ere fed a homogenate of smelt, safflower oil, vitamins and water by stomach tube 3 or 4 times daily. The seals had continuous access to a 1000 liter tank of fresh water which was completely changed at least twice a day. Oxygen uptake was measured in a closed circuit system using a manometric technique (Morrison, 1951) with the seal in a temperature-regulated water bath as previously described for metabolic tests of sea otters (Morrison & Rosenmann, 1974). Chamber air was continuously circulated through Baralyme canisters to absorb carbon dioxide. The chamber was large enough to allow the seals to *This work was supported by USPHS. NIH Grant GMI0402 and Sea Grant 1-36109. maneuver easily, and they.could swim or dive at will with105
106
KEITIt
MILLER° MARIO
ROSENMANN AND PETEIt MORRISON
out restraint. To allow the animals an opportunity to recover between runs they were tested on alternate days. So that the seals would be in a postabsorptive state, food was withheld at least 6hr, and in some causes, up to 18hr before testing. Test runs usually lasted about 5 hr, during which time the water temperature was kept constant. Since it was desired to complete the series of tests before the seals could put on much insulating blubber, testing was limited to a 3 week period. Rectal temperatures were obtained before and after each test with a therlnistor probe inserted 20 cm, and surface temperatures were simultaneously measu:vd with a radiometer (Barnes IT4-A). A pressure transducer in the metabolism chamber provided a continuous record of respiration. Body weights were measured before and after each test with free water removed from the fur, and the average weight was used to calculate oxygen uptake.
RESULTS
AND
Newborn Hm'i~r Seal.L, /
~: O8 o~0G o.4 0.2
"--'~.
,.%
O.Ot
I
t
!....
0
5
JO
~5
1
,,.I
2 0 *C 2,5
I , 30
.1 ~35
f 40
Fig. 1. Oxygen uptake of resting young harbor seals at different water temperatures. Curves are eye-fitted with portions below lower critical temperature extrapolated to average rectal temperature (dashed lines).
DISCUSSION
After initial exploratory dives the seals adapted very readily to the metabolism chamber and spent much of the time resting quietly or sleeping. It was thus possible to o b t a i n both resting and sleeping metabolic rates over the temperature range from 5 to about 37°C. Water temperature, body weight, and initial and final rectal temperature for each test are given in Table 1. Average initial rectal temperature was 37.8 _+ 0-2°(2, which is the same as that reported for a group of seven young harFor seals previously tested in air (Miller & Irving, 1975). Surface temperatures in air were also similar to those previously obtained, but following prolonged water immersion, skin and hair covered surface temperatures were invariably within a degree or two of the bath temperature. Respiratory rates at rest averaged 29"5 +__4:4 brcaths/min, which is close to the average value (27.3) predicted by the equation of G u y t o n (1947) for a 13-6 kg mammal. Resting metabolic rates of the two seals at various water temperatures are plotted in Fig. 1. Basal rates averaged 0.74cm a O 2 g - l h r -~, which is a b o u t 2.6 times the value expected for.an adult terrestrial mammal of the same weight. Oxygen consumption of both animals was essentially the same within the thermoneutral region, but lower and upper critical temperatures differed considerably. Seal n u m b e r 1 had
Table I. Body weights and initial and final rectal tempera. lures for metabolism tests. Initlal
Test Temp.(°c)
BoW WL.(Kg.)
Seal No. I " -
5.0 14.0 19.7 ..o 32.5 36.7
!1.7 12.4 l,.~ 12.6 13.s 15.0
38,5* 37,4 3r.9 -37.s 37.8
Seal NO. ~2 " "
5.0 14.0 19.7 ZS,O 30.0 34.5 36.0 37.5
13.5 13.6 13.9 13.7 T4.2 14.4 13.9 13.5
37.9 37.7 38.1 37.7 37.7 37.7 38.3 37.7
"
~4
Tr(*C)
Final Tr{*C) 37.0 3;-5 37.8 3s.1
38.8 37.9 37.6 37.9' 37.8 37,8 38.3 38.6
:~.S
37.8tO. Z ' " * Obt~leed a f t e r constderable struggling, value not included t n mean. **Mean + S,0.
a lower critical of a b o u t 19°C a n d an upper critical of 25°C, while seal n u m b e r 2 had a lower critical near t2°C and a n upper critical of a b o u t 31°C. The narrower zone of thermoneutrality in seal n u m b e r 1 may partly be duc ' the Fact that it was generally more restless and ~ a b l e and that it responded to cooling and Iw:~, more vigorously than the obviously place, .'n sleepy seal n u m b e r 2. Seal number i also m e r than n u m b e r 2 during the first two low tc,,~. ,lure tests, and the lack of fat may Ilavc t'e,:ult~\: a narrower zone of thermoneutrality.-1 h~.rc 11o indication of hypothermia even in 5:(' "~r The most surprising feature of the metabolism tests was the tolerance exhibited to high water temperatures. Upper critical temperature in air is a b o u t 30°C (Miller & Irving, 1975), and we intuitively felt that upper critical in water would be considerably lower, with lethal limits not far above the critical. However, instead of becoming excited in the warm water the seals were quiet and slept a great deal. This response kept the metabolic rate down, and one seal tolerated 36.7°C water without difficulty while the other was able to reach a steady state with respect to both oxygen uptake and, apparently, body temperature at a water temperature o f 37-5°C. At this water temperature oxygen uptake reached a plateau and remained constant for 2 hr. Rectal temperature in tse 2 seals increased, respectively, I a n d 2°C, so that the temperature gradient from body core to water was just over 2°C. Data on metabolism a n d other physiological changes during sleep will be detailed elsewhere, but in the thermoneutral zone. oxygen uptake during sleep declined a b o u t 25% in seal n u m b e r 2 a n d 15~ in seal n u m b e r 1. T h e a m o u n t of time spent sleeping increased markedly in both seals at higher water temperatures, and when the water ~ipproached the normal deep body temperature, seal number 2 slept almost constantly. A decrease in activity and increased tendency to sleep was also seen in newborn h a r b o r seals at air temperatures above 25°C (Miller & Irving, 1975). Although harbor seals in warm air demonstrated increases in rectal temperature without a n y corresponding increase in metabolic rate (Miller & Irving,
Oxygen uptake and temperature regulation of young harbor seals
4C
Seots ~n
"-7-
Wotet,
,,. ~.5c
/ t
/
o Seol No i
ol
} 2,
I~ O5 0~--
J 5
l IO
I 15
Woter
......... I ............ I ~ ~5
"femperoture(%)
I 30
....I . . 35
40
Fig. 2, Thermal conductance values calculated for young harbor seals at different water temperatures. Tr indicates rectal temperature, Ta indicates water temperature.
1975), this response was not seen in the water tests, Conflicting information also exists on the metabolic response of the California sea lion to high air temperatures. Matsuura & Whittow (1973) report that metabolism increased with increasing rectal temperature, while South et aL (1973) have reported that in their calorimetry experiments metabolic rate remained unaltered even though body temperature underwent a significant rise. In spite of their somewhat different oxygen consumption curves, total thermal conductance of both seals was very similar over the full range of test temperatures (Fig. 2). Based on equations in the literature commonly used to describe conductance in a variety of mammals (Hart, 1971 ; Morrison, 1960; Herreid & Kessel, t967), the values obtained for young harbor seals below the lower critical temperature are about twice the expected value for small terrestrial mammals. Although evaporative heat loss was not measured, and could not be subtracted from the metabolic heat, values given for thermal conductance at the higher bath temperatures should not be greatly in error. This is due to the fact that the chamber air was saturated with water vapor at all temperatures, panting was not observed, and evaporative loss from the body could not occur. From the results of the present study it is apparent that exposure to 5 or even 0°C water for periods of several hours is readily tolerated~byqhe newborn harbor seal. A combination 9f high resting metabolic rate and peripheral tissure heterothermy apparently enables the pups to tolerate such low water temperatures. The rapid early growth phase in newborn harbor and other phocid seals thus serves the multiple
107
thennal advantages of providing increa.~d insulation by deposition of subcutaneous fat, rapid improvement in the surface to volume ratio, and an inherently I!igh heat production. The ability to reach thermal equilibrium at water temperatures equal to the normal deep body temperature was a surprising finding, and indicates the thermal versatility of the harbor seal in its aquatic habitat. Recent studies on the sea otter have demonstrat~ a similar, though not quite such an extreme, tolerance of high water temperatures (Morrison & Rosenmann, 1974). The increased thermal cqnductance in the seal at high water temperatures must involve large in, creases in peripheral blood flow especially t o the appendages, and this should provide an interesting area for future study.
REFERENCES
DAvtoov A. F, & MAKAROVAA. R. (1964) Changes in heat regulation and circulation in newborn seals on transition to aquatic form of life. Fedn Proc. Fed~ Am. Socs exp. Biol. 24, Trans. SuppL No. 4, Part II: T563-T566. GuYloN A. C. (1947) Measurement of the respiratory volume of lab. animals. Am. J. Physiol. 150, 70-77. HARTJ. S. {1971)In Comparative Physiofoqy t?f Thermore.qulation, (Edited by Wm'rrow G. C.). Chap. I. Academic Press, New York. HAkT J, S. &]RVtNG L. (t959) The energetics of harbor seals in air and water with special consideration of seasonal changes. Can, J. ZooL 3% 447-457. HERRV.tDC. F, II & KI':SSELR. (1967) Thermal conductance in birds and mammals. Comp. Biochem. PhysioL 21, 405--414. IRVlr~OL & HARTJ. S. (1957) The metabolism and insulation of seals as bare-skinned mammals in cold water. Can. J. Zool. 35, 497-511, Mh'rsuu~ D. T. & WltiTTOWG. W. (1973) Oxygen uptake of the California sea lion and harbor seal during exposure to heat, Am. d. Physiol. 225(3), 711-715. MILLER L. K. & laviNG "L. (1975) Metabolism and temperature regulation in young harbor seals. Am. J. PhysioL In Press, MortRtsoN P. R. (1951) An automatic manometric respirometer. Rev. scient, lnstrum. 22, 264-267. MORmSONP. R. (t960) Some interrelations between weight and hibernation function, Bull. Mus. comp. ZooL Harr: 124, 7,9-91. MoRmsor~P. R. & ROSE,~M.~NNM. (t974) Metabolism and thermoregulation in the sea otter. PhysioL Zoo/. 47. 218-229. SOUTHF. E., LUECKER., SHANKLINM. D. & ZATZMAIqM. L. (1973) Direct partitional calorimetry of the California sea lion in air. Tenth Am~. Coq~ opt Biol. Sonar and Diving Mare,, Stanford Res. Inst.