A Giant in a Genus of Small Snakes


Atractus gigas
Perhaps the most specious genus of snakes is Atractus (Family Dipsididae) with about 130 species.  Atractus are commonly known as ground snakes, and tend to be small to medium snakes that feed on earthworms, arthropods, and mollusks. They are distributed from Panama to Argentina and overall knowledge about their natural history is at best very incomplete. They occur from sea level to 4,500 m in elevation and many, if not most of them, are known only from the type specimens. They are closely related to the Middle American and northern South American fossorial and cryptozoic genera Adelphicos and Geophis. Atractus range in size from snakes that are less than 200 mm at maturity to the giant, Atractus gigas that exceeds 1000 mm. Myers and Schargel (2006) described Atractus gigas from a single specimen collected on the west side of the Ecuadorian Andes, Tolhurst et al. (2010) recently reported a second specimen of A. gigas about 50 km from the type locality on the basis of photographs.  Now, Passos et al (2010) discovered additional specimens of this poorly known snake in museum material and collected new specimens during fieldwork in the northeastern Peruvian Andes. The largest female was 1040 mm in body length, while the largest male was 255 mm in body length (presumably this was a subadult). The female’s body size makes this species the largest in the genus. The authors describe the juvenile and adult color patterns and detail the sexually dimorphic scale counts. They encountered this snake at Santuario Nacional Tabaconas Namballe, San Ignacio, Peru, on a coffee plantation and in nearby montane forest. Individuals were observed from early morning to late afternoon. Thus, Atractus gigas inhabits primary and secondary cloud and montane forest as well as coffee plantations between 600 and 2300 m on both sides of the Andes. The diurnal activity of this snake is also unusual for the genus, since many others ground snake species are known to be nocturnal. One female contained 12 oviductal eggs, this is an exceptionally large clutch size for an Atractus [other Atractus with known clutch sizes, i.e. A. reticulatus and A. trilineatus, have 1-6 eggs]. Many Atractus show sexual dimorphism with large females and smaller males. Thus this clade of snakes may make an excellent study group to test hypotheses about sexual dimorphism in snakes - given the number of species it is likely one or more has males that are larger than females.

Literature
Myers, C. W. and W. E. Schargel. 2006. Morphological extremes – Two new snakes of the genus Atractus from northwestern South America (Colubridae: Dipsadinae). American Museum Novitates, 3532: 1‑13.

Passos, P., M. Dobiey, and P. J. Venegas  2010. Variation and Natural History Notes on Giant Groundsnake, Atractus gigas (Serpentes: Dipsadidae). South American Journal of Herpetology 5(2):73-82.

Tolhurst, B.; M. Peck; J. N. Morales; T. Cane and, I. Recchio. 2010. Extended distribution of recently described dipsadine colubrid snake: Atractus gigas. Herpetology Notes, 3: 73‑75.

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