Body temperature and torpor bout duration in Alpine marmots: the benefit of joint hibernation [Température corporelle et durée des bouts de torpeur chez la marmotte alpine : Bénéfice de l'hibernation en groupe].

Résumé

All marmot species except woodchucks (Marmota monax) show a high sociality and hibernate in family groups with up to 20 individuals. Ambient temperature (Ta) in their burrows does not remain constant but decreases exponentially from approximately 15 0C in October to 0 0C ln late March. Alpine marmots (Marmota marmota) rely exclusively on their fat stores during the six month hibernating season and these fat stores are most limited in young marmots which cannot reach adult weight during their first summer. To evaluate the extent of energy cost reduction we kept six marmots in climate chambers either solitary in plexiglas metabolic boxes (1 male, 2 female with 3540 to 4100 g prehibernation weight) or in a wooden box (1 female with her offspring of that year 1 m, 1 f, 2685 to 3525 g prehibernation weight). Body temperature (Tb) was recorded continuously via temperature sensitive transmitters from November 11 th 1991 through March 3rd 1992 for singly hibernating animals and April 5 th 1992 for the group respectively. Ta within the climate chamber was decreased stepwise from 15 0C in November to 0 0C in March Ta remained constant until each individual showed at least two entire hibernation bouts at a given temperature. Torpor bout duration varied with Ta and was significantly different between both experimental groups. At 15 0C Ta the average bout duration was 51.7 hours for individually hibernating animals but 90. 1 hours for group members. At 11 0C mean values were 89.8 versus 151.4 hours, at 7 0C 90.2 vs. 234.5 hours, at 5 0C 75.8 vs. 210.9 hours, at 2.5 0C 38.0 vs 138.7 hours, at 0 0C 31.2 vs. 65.6 hours for single and group animals respectively. Arousals of the family group members were not perfectly synchronized. The arousal-pattern was more or less the same within the youngs whereas the adult female showed a different pattern. Solitary hibernating marmots maintained higher body temperatures than group members: minimal Tbs of 7.9, 8.6, 10.7 0C versus 5.8, 5.2, 4.9 0C. This preference of high body temperatures evokes a greater Tb-Ta-gradient in singly hibernating marmots and as a consequence as increase in metabolic heat production due to additional cold load. In summary the benefits of social thermoregulation e.g. prolongation of torpor bout duration, smaller Tb-Ta-gradients and reduced thermal conductance due to larger body mass in the nest, allow the reduction of hibernation weight loss to approximately 50% of the weight loss of singly hibernating marmots. The mean weight loss was 0.17% per day in family group members versus 0.33% per day in solitary hibernating marmots. The young male in the group lost 23.7 % of his prehibernation weight in 143 days whereas one of the single individuals with a comparable prehibernation weight lost 33.6 % in 94 days. The results of this study show that joint hibernation is above all indispensable for the survival of young marmots which have the most limited fat reserves.