Australian Snakes Do Not Follow Bergmann’s Rule

Abstract

Bergmann’s Rule (i.e., the tendency of body size to increase with decreasing environmental temperature) was originally explained by a mechanism that is unique to endotherms. Nevertheless, geographic variation of body size of ectotherms, including snakes, is increasingly studied, and some claim that the rule should apply to ectotherms, or to thermoregulating ectotherms. Such studies usually focus on assemblages or on species in a region, but mostly ignore species’ ecological and biological traits when seeking biogeographic patterns. We examined the relationship between environmental temperatures and body size of 146 Australian snake species. We examined this relationship while considering the effects of ecological traits (activity time and habitat use), climatic variables which are thought to influence snake body size, and shared ancestry. Our finding suggest that Bergmann’s Rule is not a valid generalization across species of Australian snakes. Furthermore, ecological traits greatly influence the relationship between snake body size and environmental temperature. Body size of fossorial species decreases with environmental temperature, whereas body size of nocturnal, surface active species increases. Body size of diurnal, surface active species is not related to environmental temperature. Our results indicate that lumping all species in a clade together is misleading, and that ecological traits profoundly affect the geographic variation of snake body size. Though environmental temperature generally does not exert a strong selective force on snake body size, this relationship differs for taxa exhibiting different ecological traits.