Diet and snake venom evolution

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Abstract
VENOM composition within snake species can show considerable geographical variation1, an important consideration because bites by conspecific populations may differ in symptomatology and require different treatments2–5. The underlying causes of this phenomenon have never been explained. Here we present evidence that the variation in the venom of the pitviper Calloselasma rhodostoma (Serpentes: Viperidae) is closely associated with its diet. We also evaluated other possible causes of geographic variation in venom using partial Mantel tests6–10 and independent contrasts11, but rejected both contemporary gene flow (estimated from geographical proximity) and the phylogenetic relationships (assessed by analysis of mitochondrial DNA) among populations as important influences upon venom evolution. As the primary function of viperid venom is to immobilize and digest prey12–14 and prey animals vary in their susceptibility to venom15,16, we suggest that geographical variation in venom composition reflects natural selection for feeding on local prey.