Comp. Biochem. Physiol., 1975, Vol. 51A,pp. 449 to 455. Pergamon Press. Printed in Great Britain
SEASONAL SYMPATHO-ADRENAL AND METABOLIC RESPONSES TO COLD IN THE ALASKAN SNOWSHOE HARE (LEPUS AMERICANUS MACFARLANI) DALE D. FEISTAND MARIO ROSENMANN Institute of Arctic Biology, University of Alaska, Fairbanks, Alaska 99701, U.S.A. (Received 13 February 1974)
Abstract--1. Winter-acclimatized snowshoe hares achieved a significantly greater maximum metabolic response to cold (Mmnx)than summer hares. 2. Summer hares exposed to + 13°C and winter hares to -20°C excreted similar levels of urinary norepinephrine(NE) and epinephrine (E). 3. Cold exposure of summer hares to -20°C and winter hares to -45°C (conditions which elicit the same metabolic rate in both groups) caused significantly greater NE and E excretion in summer hares. 4. The results suggest that seasonal acclimatization involves enhanced non-shiveringthermogenesis, increased sensitivity to NE and increased Mmax in winter hares which enables retention of a constant annual metabolic range for activity.
INTRODUCTION SEASONAL acclimatization of mammals may involve both insulative and metabolic adjustments for winter survival. Large mammals (e.g. caribou) exhibit primarily insulative changes while small mammals (e.g. mice, rats) are believed to show primarily metabolic compensations (Hart, 1971; Irving, 1972). The snowshoe hare (Lepus americanus) a common mammal in central Alaska and the boreal forest of North America, survives the full impact of the Alaskan winter with a minimum reliance on microclimatic evasion (Morrison, 1964). Previous studies of the metabolism of snowshoe hares in Alaska (Irving et al., 1957) and in Canada (Hart et al., 1965) have shown this lagomorph to be quite resistant to cold. Because the increase in insulative capacity of the white winter pelage (which replaces the brown summer pelage) was found to be comparable to the decrease in thermal conductance of the animal from summer to winter (about 30 per cent change), Hart et al (1965) concluded that seasonal acclimatization in the snowshoe hare is largely insulative. Together with changes in thermal conductance, metabolic changes and enhancement of nonshivering thermogenesis (NST) could also play an essential role in seasonal cold resistance in the snowshoe hare. The findings of Rosenmann & Morrison (1965) of seasonal augmentation of myoglobin in winter suggest that snowshoe hares may undergo substantial seasonal changes in the biochemical capacity of tissues to provide a greater metabolic capability in the winter. No studies of sympatho-adrenal activity or of calorigenic responsiveness to catecholamines (CA) have been reported
for this species. However, several studies of the thermogenic effect of CA in cold-acclimated domestic rabbits (Cottle, 1963; Heroux, 1967; Kockova & Jansky, 1968) suggest only a minor role for CA-stimulated NST in cold-exposed rabbits. The studies reported here began as part of a broader project dealing with ftmetional aspects of snowshoe hares during phases of the 10 year population cycle. Our primary objectives were to determine whether seasonal acclimatization in the hares involved (1) changes in maximum metabolic response to cold (i.e. maximum oxygen consumption) and (2) changes in sympatho-adrenal responses to cold. MATERIALS AND M E T H O D S Animals
Snowshoe hares of both sexes were collected (with wire mesh Tomahawk live traps baited with pelleted rabbit chow) from an area of mixed birch and spruce taiga forest near Fairbanks, Alaska, from June 1971 to June 1973 in November, February, June and August. The animals were housed initially in individual metal cages in an environmental chamber at the prevailing mean ambient seasonal temperature (i.e. - 5 ° C to -10°C in November; -20°C in February; +13°C in June and Augus0 and seasonal photoperiod. They were given Albers rabbit chow and water or snow ad lib. The annual extremes of temperature and light to which hares are normally exposed outdoors in Fairbanks, Alaska, are shown in Fig. 1. Experiments
Two different sets of experiments involving separate hares were usually started on the first day after capture.
449
DALE D. FEIST AND M~.,~io ROSENMANN
450 50
22 Yearly Cycles in Ternperolure ond DrJyllght
40
I Monlhly ]-.------Max, and Min.
SEASONAL SYMPATHO-ADRENAL AND METABOLIC RESPONSES TO COLD IN THE ALASKAN SNOWSHOE HARE (LEPUS AMERICANUS MACFARLANI) DALE D. FEISTAND MARIO ROSENMANN Institute of Arctic Biology, University of Alaska, Fairbanks, Alaska 99701, U.S.A. (Received 13 February 1974)
Abstract--1. Winter-acclimatized snowshoe hares achieved a significantly greater maximum metabolic response to cold (Mmnx)than summer hares. 2. Summer hares exposed to + 13°C and winter hares to -20°C excreted similar levels of urinary norepinephrine(NE) and epinephrine (E). 3. Cold exposure of summer hares to -20°C and winter hares to -45°C (conditions which elicit the same metabolic rate in both groups) caused significantly greater NE and E excretion in summer hares. 4. The results suggest that seasonal acclimatization involves enhanced non-shiveringthermogenesis, increased sensitivity to NE and increased Mmax in winter hares which enables retention of a constant annual metabolic range for activity.
INTRODUCTION SEASONAL acclimatization of mammals may involve both insulative and metabolic adjustments for winter survival. Large mammals (e.g. caribou) exhibit primarily insulative changes while small mammals (e.g. mice, rats) are believed to show primarily metabolic compensations (Hart, 1971; Irving, 1972). The snowshoe hare (Lepus americanus) a common mammal in central Alaska and the boreal forest of North America, survives the full impact of the Alaskan winter with a minimum reliance on microclimatic evasion (Morrison, 1964). Previous studies of the metabolism of snowshoe hares in Alaska (Irving et al., 1957) and in Canada (Hart et al., 1965) have shown this lagomorph to be quite resistant to cold. Because the increase in insulative capacity of the white winter pelage (which replaces the brown summer pelage) was found to be comparable to the decrease in thermal conductance of the animal from summer to winter (about 30 per cent change), Hart et al (1965) concluded that seasonal acclimatization in the snowshoe hare is largely insulative. Together with changes in thermal conductance, metabolic changes and enhancement of nonshivering thermogenesis (NST) could also play an essential role in seasonal cold resistance in the snowshoe hare. The findings of Rosenmann & Morrison (1965) of seasonal augmentation of myoglobin in winter suggest that snowshoe hares may undergo substantial seasonal changes in the biochemical capacity of tissues to provide a greater metabolic capability in the winter. No studies of sympatho-adrenal activity or of calorigenic responsiveness to catecholamines (CA) have been reported
for this species. However, several studies of the thermogenic effect of CA in cold-acclimated domestic rabbits (Cottle, 1963; Heroux, 1967; Kockova & Jansky, 1968) suggest only a minor role for CA-stimulated NST in cold-exposed rabbits. The studies reported here began as part of a broader project dealing with ftmetional aspects of snowshoe hares during phases of the 10 year population cycle. Our primary objectives were to determine whether seasonal acclimatization in the hares involved (1) changes in maximum metabolic response to cold (i.e. maximum oxygen consumption) and (2) changes in sympatho-adrenal responses to cold. MATERIALS AND M E T H O D S Animals
Snowshoe hares of both sexes were collected (with wire mesh Tomahawk live traps baited with pelleted rabbit chow) from an area of mixed birch and spruce taiga forest near Fairbanks, Alaska, from June 1971 to June 1973 in November, February, June and August. The animals were housed initially in individual metal cages in an environmental chamber at the prevailing mean ambient seasonal temperature (i.e. - 5 ° C to -10°C in November; -20°C in February; +13°C in June and Augus0 and seasonal photoperiod. They were given Albers rabbit chow and water or snow ad lib. The annual extremes of temperature and light to which hares are normally exposed outdoors in Fairbanks, Alaska, are shown in Fig. 1. Experiments
Two different sets of experiments involving separate hares were usually started on the first day after capture.
449
DALE D. FEIST AND M~.,~io ROSENMANN
450 50
22 Yearly Cycles in Ternperolure ond DrJyllght
40
I Monlhly ]-.------Max, and Min.
