Annual body cycle, energy balance and hibernation in the alpine marmot (Marmota marmota) [Cycle corporel annuel, bilan énergétique et hibernation chez la marmotte alpine].

Résumé

During 6 month of hibernation alpine marmots live exclusively on their stored body fat. During the short vegetation period in summer marmots have to replenish their depleted body fat storages. As little is known about the influence of the naturally restricted food availability upon the annual body weight cycle and the onset of hibernation, we studied seasonal energetics of captive marmots under different environmental conditions and food regimes. Groups of 2-5 marmots were kept in outdoor enclosures in the Zoology Department, University of Marburg. During the hibernation season (October-April) the marmots were kept in climatic chambers maintained at an air temperature (Ta) of either 5°C (n=13) or 20°C (n=3). Animals kept at Ta=20°C were provided with food and water ab lib. those at Ta=5°C were not fed. The body weights were monitored regularly. Food uptake, faeces production, VO2 and CO2 production of 5 animals were measured simultaneously 5 times throughout the year at Ta =20°C. During winter body temperature and torpor bout length were recorded by implanted temperature transmitters. Captive marmots showed a pronounced annual body weight cycle with a maximum of about 5 kg in September (prehibernation) followed by a 30% weight reduction during hibernation. The start and the rate of the weight loss was independent from the food regime. Hibernation started between October and November but did not always coincide with the onsed of body weight loss. Hibernation started between October and November but did not always coincide with the onset of body weight loss. Hibernation bouts reached a maximum of 15 days in the 5°C-group but did not exceed 3 days in the 20°C-group. Those animals provided with food at Ta =20° C throughout winter terminated hibernation in February and recommenced gaining weight. The breeding season in these individuals occurred about 1.5 month earlier than in the wild. Energy expenditure and intake displayed an annual cycle with maximum values recorder in June/July. A 50% decrease in energy turnover was observed in normothermic animals in autumn and winter. Despite this large fluctuation in seasonal energy turnover, marmots maintained a relatively constant positive energy balance of about 285 kj/day from May to August. In September this extra energy intake was reduced and in October energy intake and expenditure became balanced. Even during the normothermic periods of hibernation in winter the animals exhibited a negative energy balance despite feeding. The massive body weight gain of about 9g/day during the activity season was achieved by a high extra energy intake. However, weight loss and hibernation during winter were not duced by reduced food availability. An endogenous rhythm appeared to be more likely, because neither feeding nor artificially high ambient temperatures (20°C) prevented hibernation and body weight loss. The annual cycle length and the pattern of hibernation were, however, modulated by the environment conditions experienced during the hibernation season.