Co-regulated plasticity between limits to energy budget in Dromiciops gliroides: a model for ancestral endothermy

Metabolic rate represents fundamental processes in ecology and evolution; it determines the rate at which organisms oxidize substrates to energy for growth, reproduction and survival. Metabolic limits, minimum energy use for self-maintenance and maximum expansion of aerobic performance define individuals' energy allocation capabilities. We studied the co-expression between metabolic limits in a species representing an ancient marsupial lineage and hypothetical plesiomorphic mammalian physiology. We partitioned covariation between basal (BMR) and maximum (V̇O2,max) metabolic rates to between- and within-individual components. Contrary to predictions of the aerobic model for endothermy evolution, we found a near-zero BMR-V̇O2,max correlation on the between-individual level, suggesting null genetic co-variance, or its cancellation by an among-individual maintenance-to-capacity trade-off. Instead, a substantial within-individual BMR-V̇O2,max correlation revealed plastic co-expression of the traits in the wild animals responding to environmental variation. Within-individual V̇O2,max-lean mass correlation reinforces the importance of this bioenergetic plasticity mechanism. We conclude that endothermic strategies can be sustained not only by fixed genetic coupling, but also by flexible, condition-dependent trait integration, hypothetically reflecting a physiological plastic property of the early stages of therian evolution.