Diet and hibernation of yellow-bellied marmots: Why are marmots so fat? [Régime alimentaire et hibernation des marmottes à ventre jaune : pourquoi sont-elles si grasses?].

Résumé

Yellow-bellied marmots, Marmota flaviventris, are hibernators that must gain sufficient mass to insure overwinter survival. Marmots may be limited by food availability because of their short growing season. I provided two social groups of yellow-bellied marmots (Marmota flaviventris) with supplemental food to test hypotheses about the effects of food abundance and quality on life history characteristics. Food addition did not affect weaning mass, however, supplemented young had significantly higher growth rates (24.1-48.7 grams per day) than reference young (24.1-33.3 g/day). Supplemental food only increased growth rates of adults after young were weaned. There was no clear effect of food addition on survival rates, female recruitment, age of first reproduction, or reproductive effort, such as increased litter size. Life-history characteristics are probably influenced by social structure, but not food availability. Diet selection was examined using multiple-choice feeding preference experiments, in which animals are offered multiple food items simultaneously. Marmots were offered the seven plants most commonly found in their foraging areas. Using Hotelling's T2, a multivariate statistical test, I rejected the null hypothesis that the proportions of plant species consumed were equal to the expected value of 1/7 (T2 = 180.63, p = 0.025). Cafeteria-style feeding trials indicated that dandelions (Taraxacum officianale) were selected significantly more often than expected by a random distribution (p = 0.001), whereas grasses (Bromus spp. and Poa spp.) and cinquefoil (Potentilla gracilis) were avoided (p = 0.002). Marmots reduce energy expenditures by active metabolic suppression and not as the result of change in temperature (Q10). Q10 changed with decreasing VO2 between normothermia and torpor, but was generally well above 2.0. Therefore, marmots actively reduce their metabolism to enter torpor and decrease TB faster than with Q10 effects alone. During recorded torpor bouts, TB ranged from 10-14°C (TA = 10°C) and 6-9°C (TA = 6°C) and we observed circadian TB rhythms. Circadian rhythms are maintained in the yellow-bellied marmot during deep torpor and may play a role in the timing of arousals from hibernation.