Heterothermy and cold acclimation in the arctic ground squirrel, Citellus undulatus

Comp. 8iochem."Physiol.

Vol. 67A, p p 447 to 452 9 Pergamon Press Lid 1980. Printed in Great Britain

0300-9629/80/1101-0447502.00/0

HETEROTHERMY AND COLD ACCLIMATION IN

THE ARCTIC GROUND SQUIRREL, CITELLUS UNDULATUS JOHN G. BAUST and ROBERT T. BROWN Department of Biology, University of Houston, Houston, TX 77004, U.S.A.

(Received 11 February 1980) Abstraet--l. One limb was exposed to room temperature (22 + 2~ while the contralateral limb was cooled to 0 + 1~ 2. Noncontrol individuals were acclimated at 5 ___ 1~ for periods up to 28 days. 3. Control animals responded to the cooling regimen (254)~ at 0.5 C/min) in a "poikilothermic" manner indicating local cold induced vasoconstriction (CIVC) was released when toepad temperatures reached 28.5 + 1.9~ 4. At this point, a transient cold induced vasodilation (CIVD) occurred. 5. After 28 days at 5~ limb exposure to 0~ resulted in a more gradual decline in toepad temperature. 6. This was indicative of an increased resistence to peripheral heterothermy as mediated by CIVD and initiated at 31.1 + 1.3~ 7. Animals maintained outdoors at 2 + 2~ with ample nest material exhibited responses intermediate to the experimental extremes. 8. CIVC were initiated at 29.8 + 0.7~ while toepad temperatures declined at a rate intermediate to non-acclimated controls and the 28-day acclimation group. 9. This data suggests the essential adaptative characteristics of peripheral heterothermy in this species.

INTRODUCTION Regional heterothermy in endotherms inhabiting cold environments is a well documented occurrence (Irving & Krog, 1955; Steen & Steen, 1965; Hammel, 1968; H e n s h a w et al., 1972). Indications of tissue and hence cellular adaptations have been reported (Chatfield et al., 1953; Miller & Irving, 1967; Miller, 1967, 1970). Due to the relatively high surface to volume ratio of limbs a n d the a t t e n d a n t high metabolic expense involved in maintaining limb temperatures near that of the body core, regional heterothermy provides the animal with a major heat conserving mechanism during cold exposure (Irving, 1972). Unlike the frequent oscillations in limb temperature that occur during repeated bouts of cold-induced vasodilation (CIVD) in temperate or tropical endotherms exposed to cold, adult arctic endotherms utilize a more efficient m e t h o d for preventing tissue freezing. A sustained and precisely regulated CIVD enables the animal to maintain tissue temperatures at just above their freezing temperature (Henshaw, 1978) even when ambient temperatures are - 3 5 ~ or colder (Scholander et al., 1950; Irving & Krog, 1955; Irving, 1964; Henshaw et al., 1972). The development of this heterothermic behavior in a n arctic m a m m a l during cold acclimation h a s received little attention. The purpose of this study was to evaluate the effect of cold acclimation on peripheral heterothermic behavior in the arctic ground squirrel (Citellus undulatus).

transported to the Naval Arctic Research Laboratory at Barrow, Alaska and maintained in isolation for 14 days. Specimens were then individually caged for air transport to Houston, Texas. In the laboratory all animals were individually caged (LD 12:12; T~ = 22 _+ 2~ and provided food and water ad libitum for a minimum of 14 days before being transferred to an environmental chamber (LD 12:12, T~ + 5 • I~ After 7-day intervals (7, 14, 21 and 28 days), specimens were removed and anesthetized with sodium pentobarbital (45 mg/kg i.p.). Supplemental i.p. injections of approx 5 mg each were administered during the course of the experiment to maintain a "light" anesthetic condition. Respiration and retinal reflex were monitored regularly as indicators of depth of anesthesia. Squirrels maintained in a sheltered outdoor colony at the Naval Arctic Research Laboratory, Barrow, were also used. The mean ambient temperature at Barrow during the experimental period was 0 _+ 2~ Needle thermocouple probes (28 gage copper constantan) were inserted subcutaneously at approx l-cm intervals along the footpad to an area just proximal to the stifle joint (Fig. 1). Throughout the experiment, rectal temperatures were monitored as were the subcutaneous temperatures in both hind limbs. The experimental protocol required that the individual specimen's torso be placed on a constant temperature aluminum plate (38~ The left hind limb was lowered into a vertically positioned circular chamber (4.5 • 9.5 cm) and secured. The air filled chamber was immersed in a programmable circulating bath at 25 + 1~ The contralateral limb was positioned horizontally to the torso with care taken to avoid contact with the warming plate. Following a 30-min stabilization period, the left hind limb was cooled at approx 0.5~ to 0~ and held there for up to 60 min.

MATERIALS AND METHODS RESULTS Arctic ground squirrels, Citellus undulatus (body wt = 590-1348 g)were captured at Driftwood, Alaska on the northern slope of the Brooks Range. Specimens were 447

The response of warm acclimated (WA) squirrels (n = 10) to the experimental protocol is illustrated in

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Fig. 2. As the bath temperature was lowered, toepad temperatures in the chilled limb decreased to 27 + 2~ at a rate approximately half that of the stimulus. At this time cold-induced vasoconstriction (CIVC) was initiated, and toepad temperatures dropped precipitously to 3.0 + I~ at a bath tempelature of 0 + 0.3~ The contralateral control limb exhibited a parallel though less severe decline in toepad temperatures to 23.7 ___0.5~ Temperature recordings from the control limb demonstrated a profound proximal to distal gradient of subcutaneous tissue temperatures. During the experiment, rectal temperatures increased slightly from 35.8 to 36.2 + 0.2~ After 7 days at 5~ (n = 4) toepad temperatures gradually decreased upon chilling to 22.8 + 1.1~ (Fig. 3) whereupon brief cold-induced vasodilation (CIVD) occurred followed by a sustained CIVC as the IO

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Fig. 4. Thermocouple temperature recordings from an arctic ground squirrel acclimated for 14 days at 5~ Number 1 denotes both, 2-5 chilled limb, 6-9 control limb and 10 rectal temperature. toepad temperatures fell to 2.7 ___0.5~ The contralateral control limb maintained relatively constant toepad temperatures of 34.5 + 0.4~ during the course of the experiment. The pronounced proximal to distal temperature gradient present in the contralateral control limb in WA squirrels was absent in the 7-day animals. Rectal temperatures increased from 36.3 ___0.5 to 37.2 ___0.3~ when CIVC was initiated in the chilled limb. Following 14 days of 5~ exposure (n = 5), toepad temperatures in the chilled limb decreased at half the rate of the bath temperature until the toepads reached 26.3 + 1.7~ (Fig. 4) whereupon a transient CIVD occurred followed by an intense CIVC causing tissue temperatures to fall to 5.7 _+ 0.5~ At this point, a second cyclical bout of CIVD occurred and raised the toepad temperatures to 32.5 + 1.5~ The contralateral control limb maintained constant toepad temperatures of 33.5 + 0.3~ until the second CIVD

when this temperature increased to 34.1 + 0.3~ Rectal temperatures showed gradual increase from 35.4 to 36.7 ___0.3~ At this time the second CIVD caused rectal temperatures to fall abruptly to 36.0 ___0.2~ Acclimation to 5~ for 21 days (n = 5) (Fig. 5) elicited a more gradual decline in toepad temperatures in response to the cooling regimen. There were an initial declines in toepad temperatures to approximately 28.5 + 1.4~ At this point low intensity CIVD occurred as manifested by a "shoulder" in the temperature recording. This CIVD was immediately followed by a steep fall in toepad temperature indicating intense CIVC. Toepad temperatures stabilized at 1.8 + 0.2~ The contralateral limb remained stable with toepad temperatures of 33.8 + 0.5~ during the initial part of the experiment. After 28 days of 5~ exposure (n = 4), the chilled limb reacted biphasically (Fig. 6). Initially toepad temperatures declined slowly until tissue temperatures

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Fig. 6. Thermocouple temperature recordings from an arctic ground squirrel acclimated for 28 days at 5~ Number 1 denotes both, 2-5 chilled limb, 6-9 control limb and 10 rectal temperature.

reached 27.0 ___ 1.5~ whereupon transient CIVD were followed by intense CIVC. Toepad temperatures stabilized at 5.7 + 0.4~ Contralateral limb toepad temperatures remained stable at 32.5 ___ 1.0~ until CIVC was initiated in the chilled limb. They then rose slightly to 33.5 + 0.2~ Rectal temperatures increased gradually during the course of the experiment from 33.5 + 0.2 to 37.0 ___0.1~ Ground squirrels (n = 22) maintained at ambient temperature (0 ___2~ at the Naval Arctic Research Laboratory in Barrow during the study period responded to the cooling regimen with a gradual decline in toepad temperatures interrupted by a series of transient low amplitude CIVD (Fig. 7). Toepad temperatures fell to 3.8 _+ 0.5~

DISCUSSION

These profiles of peripheral heterotliermic behavior illustrate the time course of cold acclimation,for an arctic endotherm. Unlike laboratory rats which maintain high peripheral temperatures when exposed to a similar experimental protocol (Brown & Baust, 1980), the arctic ground squirrel, as well as other arctic mammals (Irving & Krog, 1955; Steen & Steen, 1965), allows peripheral tissues to fall as much as 35~ below core body temperature. During these experiments and regardless of acclimation time, individual squirrels invariably allowed toepad temperatures to stabilize at 2-5~ when the air around the limb was 0~ Lower peripheral tissue temperatures decrease the heat lost through appendages exposed to low tem-

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Cold acclimation in Citellus

peratures (Henshaw, 1972). The benefit of such conservation is well recognized (Irving, 1972). The CIVC responsible for peripheral heterothermy is apparently mediated by an increased sympathetic tone (Swan & Henshaw, 1973). While conserving heat during exposure to sub-freezing ambient temperatures, the organism must also prevent its peripheral tissues from freezing. This is accomplished by CIVD. The metabolic economy of the sustained, well regulated CIVD common to arctic endotherms over poorly regulated, highly variable "hunts" (Lewis, 1930) common to tropical mammals is of prime importance to the arctic endotherm (Irving, 1972). A factor defined as the resistance to heterothermy (R) (Brown & Baust, 1980) was determined as follows: the ratio of the slope of the toepad cooling curve/the slope of the bath cooling curve. A low R value denotes a greater resistance to peripheral heterothermy during limb cooling. WA controls as well as acclimation times of 14 and 21 days showed little resistance to heterothermy (R = 0.92 _+ 0.04, 0.98 _+ 0.16, and 0.84 _+ 0.2, respectively) (Fig. 8). Squirrels acclimated to 5~ for 28 days had a lower R value (0.32 _+ 0.14) indicating a higher degree of resistence to tissue cooling. This reflects an adaptation of these animals by allowing a much more gradual cooling of peripheral tissues. There is also an anomalous low R value for the 7-day squirrels of 0.34 _+ 0.17. This may be due to a higher metabolic rate during the earlier acclimation period. The R value of 0.60 _+ 0.17 for those squirrels maintained in an outdoor group cage is intermediate to the other experimental extremes. Such a result underscores the differences in performance between acclimated and acclimatized animals (Heroux, 1961). As a small arctic mammal, the arctic ground squirrel does not have sufficient insulation to allow it to remain at an ambient temperature below 8-10~ without increasing its metabolism (Scholander et al., 1950; Erikson, 1956). Consequently even when the temperature is 0~ the arctic ground squirrel seeks shelter in a burrow and ventures out only rarely (Erikson, 1956). In such circumstances it is extremely difficult to accurately assess the thermal stimulus received by squirrels kept outdoors and supplied with nesting materials and artificial burrows. Since the temperature inside an inhabited burrow is considerably higher than that outside on a 0~ day, and since these squirrels leave their burrows very infrequently on such days, the actual amount of exposure to 0~ temperatures may be very small indeed. Each exposure period is short in duration (probably well under an hour). In addition, the arctic ground squirrel is a hibernator and avoids the extremely low temperatures of winter. Unlike the larger caribou and sled dog, the ground squirrel does not have to deal with temperatures 30~ or more below zero. In summary the arctic ground squirrel exhibits a radically different strategy for effecting acclimation to 5~ than the "tropical" rat. This perhaps reflects a basic evolutionary difference between a tropical and arctic species. Tropical species maintain elevated peripheral tissue temperatures and pay the price of tremendous heat loss to its environment. The arctic ground squirrel acts to conserve body heat, a precious commodity in the arctic, and allow peripheral tissues

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Acknowledgements--The authors wish to acknowledge

the helpful comments provided by Dr L. Irving, Institute of Arctic Biology, during the initial phases of this study and dedicate this manuscript to his memory. This work was supported by the Office of Naval Research, Contract No. 207-116, to JGB.

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STEEN I. & STEEN J. B. (1965) Thermoregulatory importance of the beaver's tail. Comp. Biochem. Physiol. 15, 267-270. SWAN K. G. • HENSHAW R. E. (1973) Lumbar sympathectomy and cold acclimatization by the arctic wolf. Ann. Surg. 177, 286-292.