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U.S.A. Threatened California Threatened
Giant garter snakes are endemic to the Central Valley of California. They hibernate in subterranean retreats and typically emerge to forage and breed in April dependent on local weather conditions. Upon emergence, they utilize small mammal burrows, crevices, and other surface objects for nocturnal retreats. Typically, males will begin to search for mates immediately upon emergence from hibernation, and a secondary breeding season has been known to occur during September. Females are viviparous and bear 10 to 46 young from late July to early September in hidden sites such as densely vegetated riparian zones or in organic matter near streams. Young tend to seek refuge in dense cover immediately after birth where they absorb the yolk sac before foraging on their own. Breeding potential is reached at about 3 years for males and 5 years for females.
Giant garter snakes are highly aquatic and the diet reflects this mode of life. Typical prey includes carp, minnows, mosquitofish, Pacific tree frogs, and bullfrogs. Historically they preyed upon thick-tailed chub (Gila crassicauda, now extinct) and the California red-legged frog (which has been extirpated from the Central Valley floor.
Ideal habitat would be characterized as having dense emergent vegetation for escape from predation, deep and shallow pools of water (which persist throughout the seasonal cycle of activity) in which to forage and seek cover, open areas along the margins to allow for basking, and upland habitat with access to structures suitable for hibernation and escape from flooding. Rice fields often possess these very requirements and are therefore readily utilized by this species.
Historically, the species probably ranged throughout the central valley near major rivers and tributaries where spring and summer flooding had occurred, and in freshwater marshes and larger flood basins. The exact distribution is not known but is thought to have included the valley floor from Buena Vista Lake in Kern County, north to near Gridley in Butte County. Current distribution is limited to 13 separate populations: Butte basin, Colusa basin, Sutter basin, American basin, Yolo basin/Willow slough, Yolo basin/Liberty farms, Sacramento basin, Badger creek/Willow creek, Caldoni Marsh, East Stockton Diverting Canal and Duck Creek, North and South Grasslands Waterfowl Easement areas ((U.S. Fish and Wildlife easements, Merced Co.), Mendota State Wildlife Area, and Burrell/Lanare. Unfortunately, these populations are isolated from one another and stochastic events as well as genetic processes may prove to be major threats to the giant garter snake's continued existence.
Class REPTILIA, Order SQUAMATA, Family COLUBRIDAE, Subfamily NATRICINAE, Genus Thamnophis, Species gigas
None
Thamnophis ordinoides gigas, Thamnophis elegans gigas, Thamnophis couchii gigas
None
Brode J. and G. Hansen. 1992. Status and future management of the giant garter snake ( Thamnophis gigas) within the southern American Basin, Sacramento and Sutter Counties, California. California Department of Fish and Game, Inland Fisheries Division; Fitch, H.S. 1940. A biogeographical study of the ordinoides artenkreis of garter snakes (genus Thamnophis). University of California Publications in Zoology 44:1-150.; Fox, W. 1951. Relationships among the garter snakes of the Thamnophis elegans rassenkreis. University of California Publications in Zoology 50:485-530; Hansen G.E., and J.M. Brode. 1980. Status of the giant garter snake, Thamnophis gigas (Fitch). California Department of Fish and Game, Inland Fisheries Endangered Species Program, Special Publication Report No. 80-5:1-14; Stebbins, R.C. 1985. A field guide to western reptiles and amphibians, Second edition. Houghton Mifflin Company, Boston, MA, 336 pp.; U.S. Fish and Wildlife Service. 1999. Draft recovery plan for the giant garter snake ( Thamnophis gigas). U.S. Fish and Wildlife Service, Portland, Oregon, 192 pp;
To 1.2+ m (4+ ft)
Giant garter snakes often reach 1.2 m (4 ft) or more in length. They may be recognized by well-separated spots on the dorsal surface, a dull yellow, cream or orange dorsal stripe and olive-brown, cream orange or pale blue coloration on the ventral surface. Ventral markings may or may not be evident. Background coloration, patterns, and striping vary geographically. Individuals in the southern portion of their known range (the San Joaquin Valley) may lack the dorsal stripe entirely. They may be differentiated from the terrestrial garter snakes (Thamnophis elegans) by the lack of a prominent dorsal stripe (which is very conspicuous in the terrestrial garter snake), the shape of the internasal scales (which tend to be wedge-shaped in the giant garter snake, they are not pointed at one end in the terrestrial garter snake), the size of the chin shields (the rear pair are generally longer than the front pair in giant garter snakes, whereas they are approximately the same size in the terrestrial species), and by comparison of the upper labial scales (both species usually have 8 upper labials but in the giant garter snake the 6th and 7th are not higher than they are wide as is generally the case in the terrestrial species).
Darren P. Newman