Morphological differences between two semi-arboreal, bird-eating island pitvipers, Bothrops insularis and Gloydius shedaoensis, and their mainland relatives were investigated. Mitochondrial DNA sequence data for two genes show B. insularis to be rooted within B. jararaca. Bothrops insularis has a more anterior heart, a relatively longer tail, a longer head, and shorter fangs than B. jararaca. The greater head length is paralleled in the ecologically similar G. shedaoensis. Increased head size may represent an adaptation to the abundance of larger food items (migratory passerine birds), providing a selective advantage to snakes able to switch to larger prey at an earlier age. Furthermore, B. insularis and G. shedaoensis have converged on similar body sizes from opposite ancestral states. Other characters, including fang length, tail length and size of neonates do not show parallel variation in G. shedaoensis and B. insularis, suggesting that caution is required when interpreting character state shifts coinciding with ecological shifts in a single species only.
