Life-History Evolution in Reptiles

Two consequences of terrestrial ectothermy (low energy needs and behavioral control of body temperatures) have had major consequences for the evolution of reptile life-history traits. For example, reproducing females can manipulate incubation temperatures and thus phenotypic traits of their offspring by retaining developing eggs in utero. This ability has resulted in multiple evolutionary transitions from oviparity to viviparity in cool-climate reptile populations. The spatial and temporal heterogeneity of operative temperatures in terrestrial habitats also has favored careful nest-site selection and a matching of embryonic reaction norms to thermal regimes during incubation (e.g., via temperature-dependent sex determination). Many of the life-history features in which reptiles differ from endothermic vertebrates—such as their small offspring sizes, large litter sizes, and infrequent reproduction—are direct consequences of ectothermy, reflecting freedom from heat-conserving constraints on body size and energy storage. Ectothermy confers immense flexibility, enabling a dynamic matching of life-history traits to local circumstances. This flexibility has generated massive spatial and temporal variation in life-history traits via phenotypic plasticity as well as adaptation. The diversity of life histories in reptiles can best be interpreted within a conceptual framework that views reptiles as low-energy, variable-temperature systems.