Pt. Reyes Garter Snakes

Posted on September 9, 2012 by Tim 5 Comments

The aquatic garter snakes, Thamnophis atratus, in Marin County are an intergrade between the northern and southern subspecies.

The third Pt. Reyes (and SF Bay region) garter snake is the aquatic garter snake, T. atratus. There are two subspecies locally, from San Francisco southward is the Santa Cruz garter, T. a. atratus and north up the coast is the Oregon aquatic garter, T. a. hydrophilus. Immediately to the north of SF, along the Pt. Reyes region there is an intergrade zone between these two subspecies. The southern subspecies has a more defined dorsal stripe, which can be a bright yellow to orange color while the northern subspecies can have the stripe lacking with the dorsum becoming more blotched in darker coloration. This species can be mistaken for coast garters lacking red coloration and in those less common cases you would have to revert to scale counting. Luckily there are usually enough color cues in our local snakes to separate the species fairly readily. The aquatic garter snake prefers to spend its time in or about waterways. This includes the ponds and marshes around Pt. Reyes but can also include lakes, streams, creeks, and rivers where you generally won’t see the red-sided garter, T. sirtalis. The aquatic garter is often seen basking on shorelines where it will readily flee into the water if disturbed. This likely warms the snake enough to engage in its cool water hunting strategies where it prefers to take fish, tadpoles, and amphibians. It will feed on other prey but generally this feeding strategy coupled with its location will identify the species.

It is not often you find three closely related species of snake living in sympatry. While garter snake taxonomy shows at least two distinct clades within the genus, their ecological roles greatly overlap such that these three species definitely compete with each other. However, the astounding wildlife diversity surrounding the San Francisco Bay Area, especially the Pt. Reyes region allow each species to utilize its generalist lifestyle and carve out its own natural history niche. If you are a San Francisco Bay resident head out to your favorite park or open space preserve and look for these harmless beauties. Otherwise, if you are a living or visiting the Western Hemisphere, it shouldn’t be too hard to locate any one of the many widespread species of spectacular garter snakes as they are found almost anywhere. And if you happen to be around Pt. Reyes marveling at the great bird and mammal diversity don’t forget to look down for one of our three species of garter snake. Maybe you’ll be adventurous enough to experience the wonder of stinky snake poop on your hands.

A striking specimen of the coast garter snake, Thamnophis elegans terrestris. Note the absence of red coloration on the head; this helps identify this species from the red-sided garters, T. sirtalis.



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  • shirley paine says:

    I’m truly impressed and enjoying your new site. Looking forward to extended education in viewing amphibios.

  • Brenden says:

    Came across a beautiful little red checkered garter snake along the ridge between Tomales Bay and McClure Beach near Point Reyes. No red head, more like blackish and no sky blue dorsal stripe, does this sound like the more common T. elegans? I’ve long been a fan of garters, but had normally enountered them closely associated with fresh water, which in this case was about a km away, in the form of tule marsh along the bay shoreline. Are these guys more terrestrial than common garters? Maybe feeding more on mole crickets and juvenile mice than tadpoles and tree frogs?

    • Tim says:

      Brenden, I would say you saw the terrestrial garter, T. elegans. T. sirtalis is less common (despite the name) in our area. Both species have a huge overlap in habitat and diet preference. In our area T. sirtalis tends to be more associated with waterways; ponds especially. And they tend to favor frogs and fish more, but will take small mammals and the occasional invert I’m sure. Both these species will eat just about anything they can get their mouths around. Which is one of the reasons they are so widespread.

  • P. J. Perring says:

    Saw a large brillant blue sided garter on trail at Abbott’s Lagoon March 26 about 5:30 PM. Never seen one that large – at least a yard long.

    • Tim says:

      Very nice. The big ones aren’t seen very often but they are out there. My recent outing to Abbott’s has only produced T. atratus and T. elegans. Most of the time they are missed because they are usually aware of us before we are of them. And they can move quickly and silently away before we get a chance to see them.

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