Bergmann's Clines in Ectotherms: Illustrating a Life-History Perspective with Sceloporine Lizards

The generality and causes of Bergmann's rule have been debated vigorously in the last few years, but Bergmann's clines are rarely explained in the context of life-history theory. We used both traditional and phylogenetic comparative analyses to explore the causes of latitudinal and thermal clines in the body size of the eastern fence lizard (Sceloporus undulatus). The proximate mechanism for larger body sizes in colder environments is delayed maturation, which results in a greater fecundity but a lower survival to maturity. Life-history theory predicts that a higher survivorship of juveniles in colder environments can favor the evolution of a Bergmann's cline. Consistent with this theory, lizards in colder environments survive better as juveniles and delay maturation until reaching a larger body size than that of lizards in warmer environments. We expect similar relationships among temperature, survivorship, and age/size at maturity exist in other ectotherms that exhibit Bergmann's clines. However, life-history traits of S. undulatus were more strongly related to latitude than they were to temperature, indicating that both abiotic and biotic factors should be considered as causes of Bergmann's clines. Nonetheless, analyses of the costs and benefits of particular body sizes in different thermal environments will enhance our understanding of geographic variation.