Conceptual Model for Thermal Limits on the Distribution of Reptiles

Recent climate change has re-invigorated scientific interest in the dynamics of geographic distributions of organisms. Climate responses and their biogeographical ramifications can be predicted indirectly by studying variation in fitness-related traits across environmental gradients in wide-ranging species. We review evidence for such variation in reptiles. Clinal variation in seasonal timing (onset) of nesting is common but may offer only minor compensation. In contrast, clinal variation in nesting behavior in two wide-ranging species suggests that reptiles can use nest site choice to counter climate differences. We suggest that when range boundaries located at climate extremes are determined by thermal conditions of embryos (vs. the adult stage) they cannot be predicted by the combination of environmental temperatures and thermal tolerances of embryos alone. We propose that nest site choice can reduce or eliminate the environmental variation that would be experienced by embryos in nests selected randomly with respect to temperature or factors influencing temperature. We also hypothesize that this compensation is limited at climate extremes; when temperatures are too cold for behavioral compensation (e.g., due to lack of suitably warm nest sites), reptiles can only invade colder climates by evolving viviparity. Animals with temperature-dependent sex determination (TSD) are more vulnerable to climate change than those with genetic sex determination (GSD) because the complete production of one sex in TSD species would occur before complete embryonic failure in GSD species. Collectively, research on reptiles suggests that range boundaries at climate extremes are determined by: (1) thermal limits and compensatory thermoregulation of free-living adults; (2) thermal limits of embryos; (3) the extent of compensatory nest site choice behavior; (4) mode of sex determination; and in cold climates (5) reproductive mode (oviparity vs. viviparity)