Among- and within-individual mechanisms of energy budgets in a wild mammal

Energy is an essential ecological resource, needed to fulfill all organismal processes. In seasonal environments, the energy budgets of animals are temporally restrained, exposing individuals to allocation trade-offs. To test between allocation and performance bioenergetic mechanisms in a wild endotherm (Apodemus sylvaticus), we repeatedly determined individuals' limits to energy budgets, aerobic capacity (V̇O2,max) and maintenance costs (BMR), and estimated their within- and among-individual correlations. We consistently found that intra-individually, V̇O2,max (τ=0.29-0.59) reduced in winter (by ∼15%), whereas BMR (τ=0.40-0.44) was invariable in females (<1.2%) and increased in males in winter (by 7.7%). In females, the budget components were positively correlated at the among-individual level (rind=0.56) and negatively at the within-individual level (re=-0.32), reflecting simultaneous operation of the performance and allocation mechanisms, respectively. In males, V̇O2,max and BMR were expressed independently (r≈-0.05), with their budgets 41% more plastic than in females. Our results corroborate the performance mechanisms and parental provisioning model for the evolution of endothermy, but only in females and conditioned by seasonal trade-offs. We conclude that bioenergetic mechanisms are not only condition dependent, but also that their ecological context is sexually dimorphic, and their synchronous operation may mask overall phenotypic covariation among components of energy budgets.