San Francisco Garter Snake is, as we have seen, an endemic snake to San Mateo County and does not occur in San Francisco [except at the California Academy of Science's herpetarium where some of these rare snakes were stolen in a burglary a couple of years back]. It is also a subspecies [tetrataenia] of what is known as the Common Garter Snake Thamnophis sirtalis: a species found across the United States and into southern Canada. There are 13 recognized subspecies based on color pattern variation (Rossman et al. 1996), seven of which occur in the western U.S. (Stebbins 1966, 1985, 2003). Obviously, the San Mateo County subspecies tetrataenia is anything but "Common"! One of the rarest snakes in the world is a race of "Common Garter Snake."

As it happens, Common Garter Snake is decidedly unusual in several aspects. Not only is one of the subspecies exceedingly rare, but some western populations have evolved the ability to eat California Newts Taricha torosa, a salamander that exudes a poison on its skin that is toxic to most other snakes (see Brodie & Brodie 1999). The San Francisco Garter Snake will eat newts when encountered, but its principal food prey is small frogs, and particularly California Treefrog Hyla regilla and young Red-legged Frog Rana aurora. The latter is an Endangered Species (the California race draytoni is listed as Threatened on the Federal Endangered Species list) in its own right — so we have the unique situation of one Endangered Species eating another Endangered Species for its livelihood!

Name confusion is not limited to English common names; recent controversy has swirled around the scientific name of this beast. Boundy & Rossman (1995) pointed out some nomenclatural problems among Pacific coast populations and suggested that the San Francisco Garter Snake, previously known as T. s. tetrataenia, become T. s. infernalis, and that those populations known as Red-sided Garter Snake T. s. infernalis be included with T. s. concinnus[Red-spotted Garter Snake]. This proposed change was followed by Rossman et al. (1996) in their book about garter snakes. However, the change was bound to cause nothing but confusion. Barry et al. (1996) petitioned the International Commission on Zoological Nomenclature (ICZN) to suppress the changes proposed by Boundy & Rossman. In 2000, the ICZN agreed and voted to retain the historical taxonomic arrangement of subspecies within this evolutionary lineage. Accordingly, the subspecies tetrataenia was reinstated for San Francisco Garter Snake and the races concinnus and infernalis retain their historical definition (e.g., see Stebbins 1996, 2003).

As you can see in the photos below, San Francisco Garter Snake differs from California Red-sided Garter Snake by:

• having a prominent red stripe within the broad blackish stripe, rather than red spots along one border of the blackish stripe;
• having all the pale stripes washed with turquoise, rather than white; and
• having a pale blue-green belly, rather than white.

Focusing on the situation around the San Francisco Bay Area, they found that a group of three populations that circle the Bay Area (Sonoma Co., Contra Costa Co., and Santa Clara Co. [=Gilroy; apparently mistakenly attributed to Santa Cruz Co. in their text]) formed a divergent group ("the California clade") that was diagnosable and had an "elevated rate of molecular evolution." These are populations of Red-sided Garter Snake currently listed as T. s. infernalis.  Janzen et al. (2002) stated that it was "also remarkable that the populations in this group are only ~25-125 km away from [tetrataenia] in San Mateo Co., yet genetic distinctness from this nearby population is readily apparent." Despite the "genetic distinctness" of S. t. tetrataenia from the adjacent Common Garter Snakes, Janzen and colleagues concluded that "T. s. tetrataenia is not genetically unique from other California populations."  This was because the lone sample of tetrataenia grouped with snakes from El Dorado and/or Santa Barbara counties in the phylogenetic tree created from their data.

This seems to me to be an odd conclusion.  It has been quite some time since there was any genetic exchange between San Francisco Garter Snake and those in the Sierra Nevada or Santa Barbara. The intervening snakes (California Red-sided T. s. infernalis) have undergone genetic change.  If those in San Mateo County have not, this lack of genetic change means that either (a) S.F. Garter is a ancient remnant population now separated from its relatives or (b) something else is going on genetically. The S.F. Garter Snake may well be  "unique" -- especially considering just how different they look — but perhaps we don't yet have the evidence.  I suspect that mitochondrial DNA may have been an imprecise tool for the question under consideration: is T. s. tetrataenia a distinct and separate taxonomic unit? Janzen et al. (2002) explicitly leave this question open.  I wonder if newer techniques [e.g., sequencing nuclear DNA; see Barker et al. (2002) with respect to birds: nuclear DNA sometimes gave better estimates of relationships than mitochondrial DNA] might shed further light on this topic.

A final point of interest to me was learning how San Francisco Garter Snake managed to survive in habitats that host two other species of garter snake: Coast Garter Snake Thamnophis elegans terrestris [a subspecies of Western Terrestrial Garter Snake T. elegans] and Santa Cruz Aquatic Garter Snake Thamnophis atratus atratus [a subspecies of Aquatic Garter Snake T. atratus]. A quick glance through Stebbins (2003) or Rossman et al. (1996) shows that they all are listed as eating the same foods. But the sequence in which the prey is listed differs, and John Sullivan is quite right to quote herpetologist Sean Barry who said that where these species overlap, they divide up the habitat this way:

• San Francisco Garter eats mostly frogs;
• Coast Garter eats mostly slugs, and
• Santa Cruz Aquatic Garter eats mostly tiny fish and amphibian larvae.