The Thirty-First Garter Snake

The 31st species of garter snake has a tiny range in western Mexico. Here’s what we know about it.


Taxonomy waits for no one. A decade after the publication of the definitive reference on garter snakes — The Garter Snakes: Evolution and Ecology by Rossman, Ford and Seigel, which listed 30 species of garter snake — there have been a few changes to the taxonomy set out in that book.

Those changes that have taken place among the species north of the U.S.-Mexico border are relatively easy to follow; they’re well documented online. Thanks to the Center for North American Herpetology’s web site, we know that Boundy and Rossman’s proposal to synonymize the San Francisco Garter Snake (Thamnophis sirtalis tetrataenia) with T. s. infernalis and rearrange the west coast subspecies of the Common Garter Snake has been rejected, and that the validity of both the Blue-striped Garter Snake (T. s. similis) and the Blue-striped Ribbon Snake (T. sauritus nitae) — both originally described by Rossman — has been challenged, as have most of the subspecies of the Western Terrestrial Garter Snake (Thamnophis elegans).1 On the other hand, a new subspecies of the Pacific Coast Aquatic Garter Snake (Thamnophis atratus), the Diablo Garter Snake (T. a. zaxanthus), was described in 1999.

But because Mexican herpetofauna is handled separately (and not as prominently), it’s easier for amateurs to miss taxonomic changes in species south of the U.S.-Mexico border. In 2000, a 31st species of garter snake, Rossman’s Garter Snake (Thamnophis rossmani), was described; I wondered about it for years before I finally read the article that named it.

Rossman’s Garter Snake is described in an article by the late herpetological legend Roger Conant2 in an 2000 article in the Occasional Papers of the Museum of Natural Science (Louisiana State University).3 Conant, who took many collecting trips to Mexico between 1949 and 1965, during which he collected a number of specimens of Mexican garter snakes, based his proposal on specimens collected in the Mexican state of Nayarit that were previously assigned to the Mexican Garter Snake, Thamnophis eques. Conant later determined that the Nayarit specimens — some collected by him during those trips, and some collected by Douglas Rossman in 1969 — were sufficiently morphologically different from T. eques to be recognized as a new species, which he named in Rossman’s honour.

Rossman’s Garter Snake is a brown snake with three “pale yellowish green” longitudinal stripes. As garter snakes go, it’s medium-sized: the largest specimen collected had a snout-vent length of 64.8 cm. It differs from T. eques in two ways, says Conant: the paired dark spots between the stripes are either very small or all but absent, whereas in T. eques they are prominent; and there is a narrow black line along each outer edge of the dorsal stripe.

Rossman’s Garter Snake is presumably a fish eater; Conant observed them catching small fish along spring rills.

Its known range is quite limited: “springs, seepage runs, and ditches near but not along the San Cayetano River, a small stream flowing northwestward to and beyond Tepic,” the capital city of Nayarit State; the length of this range is less than 10 km. While other suitable habitats may have existed previously, the area around Tepic has been heavily cultivated. Indeed, the areas where Conant collected his specimens may no longer be viable habitat: “The localities where we collected in 1959 and other meager, flat or arable terrain of the general region have been drastically altered,” Conant writes. “Available evidence, admittedly fragmentary, indicates that Thamnophis rossmani may be in grave danger, if not already extinct.”


  1. Those changes are not reflected in the Species Guide, partly because ignoring recently synonymized taxa may prove confusing, partly because I’m not sure how widely the changes are accepted; Collins’s snake taxonomist group has cut quite a swath through the list of North American snakes, and some of their other proposals have encountered strong resistance. It’s best to adopt a conservative approach until the dust settles.
  2. Conant died in 2003. His autobiography, A Field Guide to the Life and Times of Roger Conant, was published in 1997.
  3. My heartfelt thanks to Paul Hollander and Rick Bernhardt for helping me obtain a copy of this article.


Conant, Roger. 2000. “A new species of garter snake from western Mexico.” Occasional Papers of the Museum of Natural Science (Louisiana State University) 76: 1-7.

Rossman, Douglas. A., Neil B. Ford and Richard A. Seigel. 1996. The Garter Snakes: Evolution and Ecology. Norman OK: University of Oklahoma Press.

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