Quantifying aspects of the evolution of endothermy

Various conceptual or qualitative models have been proposed to explain the evolution of homeothermic endothermy from heterothermic ectothermy. Assessment of the feasibility of these models may benefit from quantitative analyses of hypotheses deriving from them. In this study, a quantitative approach was applied to a recent conceptual model which proposed a stepwise progression from ectothermy to endothermy in a medium sized vertebrate “evolving” in a warm, mesic thermal environment. By assuming that selection acted to maximise time available for activity, this study showed that endogenous heat, whether from activity or shivering thermogenesis, increased time within nominal preferred body temperatures only if low values of thermal conductance associated with insulation were already in place. Further, this study demonstrated that benefits were enhanced if the insulation was by-passable and associated with a wide range of thermal conductance. Because aerobic scope was incorporated as a constraint to thermogenic capacity, increases in standard metabolic rate were relatively ineffective until they were substantial and, at one stage, plesiomorphic daily torpor emerged, at least in the circumstances modelled. Also emerging from the modelling were the thermal neutral zone and basal metabolic rate characteristic of homeothermic endotherms. Additionally, benefits from rudimentary turbinal scrolls were quantified for this modelled, hypothetical animal in this particular environment. This initial exploratory study demonstrated how quantifying aspects of proposed models for the evolution of endothermy can provide insight into their feasibility. In so doing, this quantitative modelling highlighted avenues for further enquiry and demonstrated one tool for addressing debate about the evolution of endothermy.