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Three species of snakes (Philodryas biserialis, Alsophis slevini and A. dorsalis) occur in the Galapagos Islands, all endemic. They are quite similar looking, about two to three feet long, brown with yellowish longitudinal stripes.

The snakes have arrived on the islands via at least two colonization events. The Alsophis species appear related to A. elegans in Peru and southern Ecuador whereas Philodryas biserialis has a Chilean relative, P. chamissonis.

Galapagos snakes can be slightly poisonous to humans, and may use venom to kill its prey. They first catch the prey with their mouths and mainly kill by constriction: wrapping around the victim and squeezing so it cannot breathe.

They hunt for small reptiles and mammals. Prey includes lava lizards, grasshoppers, geckos and marine iguana hatchlings. They also feed on finch nestlings. I was walking on the trail on Santa Fé Island and saw two finches considerably agitated, fluttering around their nest. I looked closer and saw a snake curled around the nest. It emerged after a couple of minutes with a distinct bulge in its mouth, from which protruded a couple of small legs — the finch chick. Michael Jackson suggests the globular shape of the finch nest is an adaptation to protect the nest from predation by snakes (Galapagos: A Natural History, p. 112). In turn, the snakes are preyed upon by the Galapagos hawk, their only natural predator. Feral cats also kill snakes and may indirectly affect snakes by making lava lizards (the snakes' main prey) more wary.

Although common and widespread, the snakes are not often seen as they are rather shy. Most islands have one or two of the species. A couple of northern islands have no snakes, presumably because they did not succeed in establishing viable populations or never reached there in the first place.


CAS Herpetology Holdings
Philodryas hoodensis (Van Denburgh, 1912)

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