Extreme seasonal variation in aerobic capacity in a hibernator

Energy budgets in endotherms are shaped by the interplay between metabolic costs and capacities, which vary with environmental and life-history demands. For endothermic animals, these costs and capacities are represented by the basal metabolic rate (BMR) and the maximal metabolic rate (V̇O2,max), respectively. Whereas BMR measures the minimum cost of euthermic maintenance, V̇O2,max represents the maximum, sustained capacity for aerobic work (=aerobic capacity). Here, we measured those variables in a field population of hibernating mammals, the marsupial Dromiciops gliroides. Hibernators live in a fast-slow pace-of-life contrast: they abandon endothermy during torpor in winter, and live intensely in spring and summer during reproduction and growth. Our results indicate that this contrasting lifestyle is accompanied by an extreme depression in aerobic capacity before hibernation in autumn, to reach a peak during breeding in summer, a 2.6-fold increase. This seasonal variation is primarily driven by increases in V̇O2,max and reductions in BMR. This evidence supports the 'aerobic flexibility hypothesis' and highlights the physiological plasticity of hibernators in adjusting energy expenditure across different seasonal demands. This is also a revisitation of the physiological capacity of marsupials in terms of sustained aerobic work.