DIET AND SNAKE-VENOM EVOLUTION

VENOM composition within snake species can show considerable geographi cal variation(1), an important consideration because bites by conspeci fic populations may differ in symptomatology and require different tre atments(2-5). The underlying causes of this phenomenon have never been explained, Here we present evidence that the variation in the venom o f the pitviper Calloselasma rhodostoma (Serpentes: Viperidae) is close ly associated with its diet, We also evaluated other possible causes o f geographic variation in venom using partial Mantel tests(6-10) and i ndependent contrasts(11), but rejected both contemporary gene flow (es timated from geographical proximity) and the phylogenetic relationship s (assessed by analysis of mitochondrial DNA) among populations as imp ortant influences upon venom evolution, As the primary function of vip erid venom is to immobilize and digest prey(12-14) and prey animals va ry in their susceptibility to venom(15,16), we suggest that geographic al variation in venom composition reflects natural selection for feedi ng on local prey.