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.
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