Body and head size evolution in island snakes

Body size and head size are critical features throughout the radiation of snakes. Clutch size, clutch frequency, and longevity have a clear relationship with body size whereas prey size and shape are strongly related to head size. However, the extent to which variation in body and head size is a result of adaptation to local environments depends upon genetic differences between populations. To document whether these traits are locally adapted we must uncover the relative contributions of the environment and genetics to empirical variation; only after a heritable basis is established can we invoke adaptation.; Previous work revealed that island snakes become larger than or smaller than their presumed mainland ancestor but rarely stay the same size. Some snakes are more likely to become small on islands (viperids) whereas others are more likely to become larger (colubrids). However this work did not examine the effect of initial (i.e., mainland) size on island body size. This dissertation begins by testing the relationship between mainland and island body size and presents evidence on the existence of an optimal size in snakes. Surprisingly, patterns previously found for mammals were found for snakes: most notably, large species become small on islands and small species become large on islands. These results provided strong evidence for the existence of an optimal size in snakes and the concordance of these patterns in disparate vertebrate groups (mammals and reptiles) argued for the existence of an optimal size in all animal groups.; Field work on island and mainland populations of Boa constrictor in Belize documented that island boas consume mainly migratory passerine birds as adults and, while some islands support a high density of boas (16/ha), most islands support surprisingly small population sizes. A history of collection for the pet-trade has likely impacted at least one of these island populations. Compared to mainland boas, island boas were found to be half the length, one-fifth the mass, and produce lighter, shorter, and fewer offspring. Island boas also exhibit significantly different head shapes compared to mainland boas. Common-garden experiments tested the relative contributions of the environment and genetics to these differences and showed that while body size variation is determined in part by a genetic difference in growth rates and in part by a phenotypic response to prey availability, variation in head size appears to be determined mainly by genetic differences among populations. Lastly, island boas show variation in dorsal coloration and physiological color change both among islands and between islands and the mainland. The combined results from morphology and color variation imply that selection on morphology (e.g., head shape) may be relatively stronger than selection acting on color.