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Integrade Gartersnake

Thamnophis sirtalis tetrataenia

Intergrade Gartersnake

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The San Francisco gartersnake (T.s. tetrataenia) and red-sided gartersnake (T.s. infernalis) are two distinct subspecies of the common gartersnake (Thamnophis sirtalis). Both of these subspecies are found on the San Francisco Peninsula. At the present time the gartersnakes found at Stanford are considered intergrade between these two subspecies.

The dorsal surface of the head of Intergrade gartersnakes is red to orange. The basic color pattern of the body is a series of four stripes (light blue dorsal, followed by black, then red, then black, with a bluish lower body and underside). In some individuals the red/orange stripe is partially interrupted by black markings. The interruption of the red/orange stripe is particularly evident at the anterior end of some individuals.

Red-sided gartersnakes also have red to orange heads. Their basic color pattern is a series of three stripes (light blue dorsal, followed by black, then a red and black checkered stripe, with the lower body and underside bluish in color). In some individuals the red markings dominate and nearly form a more-or-less solid red stripe (with minor black markings), particularly along the posterior part of the body.

There is individual and population-level variation in color patterns for both subspecies.

The color pattern of individuals from intergrade populations can be quite variable, but individuals from these populations generally exhibit at least some characteristics of both the San Francisco gartersnake and red-sided gartersnake, and some individuals from intergrade populations can look very similar to either of the two subspecies. The color patterns of intergrade individuals are also often asymmetrical. In general, populations in the northern portion of the intergrade zone have more individuals that are partially or completely striped, which is more similar to the patterns that are diagnostic of San Francisco gartersnakes. In the southern portion of this zone, which includes Stanford, most of the individuals exhibit the alternating red and black markings that are characteristic of red-sided gartersnakes.

These snakes can reach 4 feet in length, but most individuals are less than 3 feet in length.

The common gartersnake is one of the most widely distributed snake species in North America. It is found from coast to coast, from mid-Canada to the Mexican border, being absent from only the most extreme dry and cold areas. On the San Francisco Peninsula there is a fairly well documented intergrade zone between the San Francisco gartersnake and red-sided gartersnake located on the eastern flank of the Santa Cruz Mountains, from Upper Crystal Springs Reservoir to Boronda Lake in Foothills Park. Stanford is within this intergrade zone. The intergrade populations are not considered to be either the red-sided gartersnake subspecies or the San Francisco gartersnake subspecies.

San Francisco gartersnakes are found west of the crest of the Santa Cruz Mountains, along virtually the entire coast of San Mateo County, north to San Francisco County. East of the crest of the Santa Cruz Mountains, the San Francisco gartersnake is found from the City of South San Francisco and the San Francisco airport, south to Crystal Springs Reservoir (all San Mateo County). The San Francisco gartersnake may stray south into extreme northern Santa Cruz County.

Red-sided gartersnakes are currently recognized as having a disjunct distribution, with populations being found from coastal Humboldt County south to coastal Monterey County (surrounding the distribution of San Francisco gartersnakes). The gartersnakes that have been found in Santa Clara County have been identified as red-sided gartersnakes. After being replaced by another subspecies (T. sirtalis fitchi) in southern Monterey, red-sided gartersnakes are absent from coastal California until Santa Barbara County. From Santa Barbara County, red-sided gartersnakes are found south along the coast to northern San Diego County.

Populations of the local subspecies are typically associated with permanent or nearly permanent bodies of water, usually areas of shallow water and heavily vegetated shores. However, they are known to occur, at least temporarily, in grassland, riparian woodland, oak woodland, and coniferous forest. Sag ponds in the San Andreas Fault rift zone and freshwater coastal marshes are considered prime habitats for the San Francisco subspecies.

Community relationships and behavior
The local subspecies feed on a wide range of animals, including frogs, salamanders, small fishes, and invertebrates. Small rodents and birds may also be consumed. The San Francisco gartersnake is often considered a specialist on ranid frogs, and California red-legged frogs are a major component of the diet of adult snakes in many locations. Juvenile San Francisco gartersnakes will prey heavily on Sierran treefrogs (Pseudacris sierra) metamorphs. Prey is often captured in wetlands, either in the emergent vegetation or in areas of shallow water.

The local subspecies are prey for a number of species, including raccoons, skunks, dogs, foxes, coyotes, cats, fishes, raptors, herons, and egrets.

The local subspecies mate in the late winter to early spring, and the young are born in summer to early fall. They generally range from 5 to 8 inches in length. Clutch size varies with size of female and year, but generally ranges from eight to 20 young.

In the Bay Area, the local subspecies are generally dormant during the coldest part of winter and may also have a dormancy period during prolonged periods of exceptionally hot and dry weather.

Loss of habitat and the subsequent isolation of formerly interacting populations are likely the biggest threats to the gartersnakes on the San Francisco Peninsula. Urbanization of the eastern flank and bay shore portions of the Peninsula, in particular, has been pervasive and many snake populations have been lost. Those surviving individuals and populations face an array of human-related threats, including being killed on roads, trapped in drains/sewers, poisoned by biocides or pollutants, or any of a myriad of other factors associated with the built environment.

Overcollecting may also be a threat, particularly for the San Francisco gartersnake. Gartersnakes are relatively easy to maintain in captivity and are very popular as pets. Given the vibrant color of the San Francisco gartersnake and the allure of keeping a rare specimen, these snakes have been collected, illegally since 1967, for the pet trade for decades.

The large number of captive specimens also presents another problem for the conservation of the subspecies. The release of specimens from captive bred lineages could be problematic for several reasons, including having a genetic make-up not typical of wild stocks (captive breeding invariably introduces an element of artificial selection or genetic drift) or by transmitting disease.

Gartersnakes at Stanford
Stanford is within the southern portion of the red-sided/San Francisco gartersnake intergrade zone. As such, the individuals found at Stanford exhibit color patterns that are generally more characteristic of red-sided gartersnakes.

The intergrade populations have been studied at Stanford and the vicinity sporadically for nearly 100 years. At the present time, T. sirtalis is not a common species at Stanford. A few individuals are encountered at Lagunita most years, but specimens from other locations at Stanford are only very infrequently observed. Given the number of museum records and mentions in the scientific literature, it is likely that historically these snakes were more common in the area.

References (PDF, 36 KB)