Western Galápagos Racer

Reptiles of Galápagos | Serpentes | Colubridae | Pseudalsophis occidentalis

English common names: Western Galápagos Racer, Fernandina Racer.

Spanish common names: Culebra occidental de Galápagos, culebra de Fernandina, serpiente corredora de Fernandina.

Recognition: ♂♂ 128.5 cm ♀♀ 114.2 cm. Pseudalsophis occidentalis is the largest snake in Galápagos, and one of two snake species known to occur on Isabela and Fernandina islands, as well as on two of their surrounding islets. The other species is the Darwin's Racer (P. darwini), a smaller snake that has a pattern of transverse black bands along the body.

Natural history: Uncommon to extremely common. Pseudalsophis occidentalis is a diurnal snake that inhabits volcanic rock areas, dry shrublands, dry grasslands, deciduous forests, and cattle pastures.1,2 Western Galápagos Racers are active during the daytime, but usually not during hot midday hours. They move on soil, rocks, and grass.1 Individuals of P. occidentalis are mildly venomous, which means their bite is dangerous to small prey, but not to humans.3 They are foraging predators that feed on Isabela Lava-Lizards (Microlophus albemarlensis),1 juvenile marine (Amblyrhynchus cristatus) and land (Conolophus subcristatus) iguanas,4 Simpson's Leaf-toed Geckos (Phyllodactylus simpsoni),5 and fish.6 Western Galápagos Racers have also been seen feeding on carrion (dead iguanas).5 Although generally solitary, Western Galápagos Racers congregate in groups of dozens of individuals during the hatching season of Marine Iguanas from May to June in some coastal localities of Fernandina Island. Individuals of P. occidentalis are preyed upon by hawks.


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Conservation: Least Concern.2 Pseudalsophis occidentalis is listed in this category because it is frequently encountered throughout its range and, presumably, is not undergoing population declines or facing major immediate threats of extinction.2 Fernandina is one of the most pristine islands in Galápagos and is currently free of introduced animals that may prey upon snakes. The threats that Western Galápagos Racers face in Fernandina are volcanic eruptions and the potential introduction of aggressive exotic species, mainly rats and cats. On Isabela, the largest island of the Archipelago, conservation challenges are more complex since the island has several introduced species, including humans, which may impact the P. occidentalis population.

Special thanks to Eric Osterman, our official protector of the Western Galápagos Racer, for symbolically adopting this species and helping bring the Reptiles of Galápagos project to life.

Distribution: Pseudalsophis occidentalis is endemic to Fernandina and Isabela islands, as well as two of their surrounding islets (Cowley and Tortuga), which collectively account for an area of 5,313 km2. However, Western Galápagos Racers probably occur throughout a much smaller area. Galápagos, Ecuador.

Distribution of Pseudalsophis occidentalis Distribution of Pseudalsophis occidentalis in western Galápagos

Etymology: The generic name Pseudalsophis, which comes from the Greek words pseudo (meaning “false”) and Alsophis (a genus of Caribbean snakes), refers to the similarity between snakes of the two genera.7 The specific epithet occidentalis comes from Latin and means “of the west”.8 It is a reference to the species' distribution on the western islands of Galápagos.9

See it in the wild: Western Galápagos Racers can be seen throughout the year with ~30% certainty at Punta Espinoza, Fernandina Island. However, dozens of Western Galápagos Racers may be seen daily at Punta Espinoza from May to June, the hatching season of the Marine Iguanas. The best time to look for the racers is during the first hours after sunrise or right before sunset.

Video. In 2016, BBC Earth filmed a thrilling footage of Marine Iguana hatchlings being chased down by dozens of Western Galápagos Racers. You can look it up here.

Authors: Alejandro ArteagaaAffiliation: Tropical Herping (TH), Quito, Ecuador. and Juan M GuayasaminbAffiliation: Laboratorio de Biología Evolutiva, Universidad San Francisco de Quito (USFQ), Quito, Ecuador.,cAffiliation: Galapagos Science Center, Galápagos, Ecuador.,dAffiliation: Centro de Investigación de la Biodiversidad y Cambio Climático, Universidad Tecnológica Indoamérica, Quito, Ecuador.

Academic reviewers: Robert A Thomas and Luis Ortiz-Catedral.

Photographers: Jose VieiraaAffiliation: Tropical Herping (TH), Quito, Ecuador.,eAffiliation: ExSitu, Quito, Ecuador.

How to cite? Arteaga A, Guayasamin JM (2020) Pseudalsophis occidentalis. In: Arteaga A, Bustamante L, Vieira J, Tapia W, Guayasamin JM (Eds) Reptiles of Galápagos. Available from: www.tropicalherping.com

Zoo de Quito logoThis species account of the Western Galápagos Racer (Pseudalsophis occidentalis) is available for free online thanks to the support of Zoo de Quito.

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Literature cited:

  1. Tropical Herping field notes.
  2. Márquez C, Yánez-Muñoz M (2016) Pseudalsophis occidentalis. The IUCN red list of threatened species. Available from: www.iucnredlist.org
  3. Thomas RA (1997) Galápagos terrestrial snakes: biogeography and systematics. Herpetological Natural History 5: 19–40.
  4. Werner DI (1983) Reproduction of the iguana Conolophus subcristatus on Fernandina Island, Galápagos: clutch size and migration costs. The American Naturalist 121: 757–775.
  5. Ortiz-Catedral L, Christian E, Skirrow MJA, Rueda D, Sevilla C, Kumar K, Reyes EMR, Daltry JC (2019) Diet of six species of Galapagos terrestrial snakes (Pseudalsophis spp.) inferred from faecal samples. Herpetology Notes 12: 701–704.
  6. Merlen G, Thomas RA (2013) A Galápagos ectothermic terrestrial snake gambles a potential chilly bath for a protein-rich dish of fish. Herpetological Review 44: 415–417.
  7. Zaher H, Grazziotin FG, Cadle JE, Murphy RW, Moura-Leite JC, Bonatto SL (2009) Molecular phylogeny of advanced snakes (Serpentes, Caenophidia) with an emphasis on South American Xenodontines: a revised classification and descriptions of new taxa. Papéis Avulsos de Zoologia 49: 115–153.
  8. Brown RW (1956) Composition of scientific words. Smithsonian Books, Washington, 882 pp.
  9. Van Denburgh J (1912) Expedition of the California Academy of Sciences to the Galápagos Islands, 1905-1906. IV. The snakes of the Galápagos Islands. Proceedings of the California Academy of Sciences 1: 323–374.