The Evolution of Homeothermic Endothermy via Life History Optimization

AbstractEndothermy is an energetically expensive trait, yet it has posed an evolutionary advantage across different lineages-a paradox that remains puzzling to biologists. Here, I investigate whether endothermy can evolve through life history optimization using a model of the balance between energy assimilation and energy allocation to somatic maintenance, thermoregulation, growth, or reproduction. The model displays bistable strategies when assimilation rates and thermoregulatory costs increase, respectively, exponentially and linearly with body temperature: the "heterothermic strategy" consists of minimizing the costs of thermoregulation by maintaining body temperature close to ambient temperature, and the "homeothermic strategy" consists of increasing body temperature until the costs of thermoregulation are fully compensated by the increased assimilation capacity at higher temperatures. These strategies produce similar fitness outcomes and thus emerge as alternative stable states of the system, maintained by strong stabilizing selection preventing transitions between them. Using quantitative genetics simulations, I show that a drop in ambient temperature may push populations toward an evolutionary branching point, enabling the rapid radiation of homeothermic lineages coupled with body size reductions. I thus propose that life history optimization of energy balance can explain the radiation of homeothermic endothermy associated with either climate cooling or migration to colder regions by early endothermic lineages.