Thomas' Racer

Reptiles of Ecuador | Serpentes | Colubridae | Pseudalsophis | Pseudalsophis thomasi

Spanish common name: Culebra de Thomas, serpiente corredora de Thomas.

Recognition: ♂♂ 98.3 cm ♀♀ 84.8 cm. Pseudalsophis thomasi is one of two snake species known to occur on Santiago and Rábida islands, and probably the only one occurring on Bartolomé Island. The other species is the Santiago Racer (P. hephaestus), a smaller snake that has lateral black stripes.

Natural history: Frequent, about ten times as abundant as the co-occurring Santiago Racer (Pseudalsophis hephaestus). Pseudalsophis thomasi is a diurnal snake inhabiting volcanic rock areas, dry grasslands, and deciduous forests.1 Thomas' Racers are active on soil and rocks1 throughout the day, but usually not during hot midday hours. hey are mildly venomous, which means their bite is dangerous to small prey, but not to humans.2 These snakes are foraging predators that feed on Santiago Lava-Lizards (Microlophus jacobi), Mares Leaf-toed Geckos (Phyllodactylus maresi), and possibly also on invertebrates.1,3 Individuals of P. thomasi are preyed upon by rats.4


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Conservation: Near Threatened. We consider Pseudalsophis thomasi to be in this category following IUCN criteria5 because although the species is not undergoing a continuing decline in the extent and quality of its habitat, it is known from only seven localities and is facing the threat of predation by introduced rodents.4 Santiago Island has a long history of introduced species, but the island's conditions are improving. Pigs were eradicated from Santiago in 1999, followed by goats in 2003, and donkeys in 2005.

Special thanks to Robert A Thomas, our official protector of the Thomas' Racer, for symbolically adopting this species and helping bring the Reptiles of Galápagos project to life.

Distribution: Thomas' Racers are endemic to an estimated 459 km2 area in Bartolomé, Santiago, and Rábida islands. Galápagos, Ecuador.

Distribution of Pseudalsophis thomasi Distribution of Pseudalsophis thomasi in and around Santiago Island

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.6 The specific epithet thomasi honors Robert A Thomas, for expanding the knowledge of the systematics and taxonomy of New World snakes.7

See it in the wild: Thomas' Racers are secretive snakes, but, with some luck, they can be seen with ~40% certainty in some tourism sites in and around Santiago Island, including Bartolomé and Rábida islands. The best time to look for them is during the first hours after sunrise or right before sunset.

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 thomasi. In: Arteaga A, Bustamante L, Vieira J, Guayasamin JM (Eds) Reptiles of Ecuador: Life in the middle of the world. Available from:

Literature cited:

  1. Tropical Herping field notes.
  2. Thomas RA (1997) Galápagos terrestrial snakes: biogeography and systematics. Herpetological Natural History 5: 19–40.
  3. 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.
  4. Fritts TH, Fritts PR (1982) Race with extinction: herpetological notes of J. R. Slevin's journey to the Galápagos 1905–1906. Herpetological Monographs 1: 1–98.
  5. IUCN (2001) IUCN Red List categories and criteria: Version 3.1. IUCN Species Survival Commission, Gland and Cambridge, 30 pp.
  6. 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.
  7. Zaher H, Yánez-Muñoz MH, Rodrigues MT, Graboski R, Machado FA, Altamirano-Benavides M, Bonatto SL, Grazziotin F (2018) Origin and hidden diversity within the poorly known Galápagos snake radiation (Serpentes: Dipsadidae). Systematics and Biodiversity 16: 614–642.