Garter Snake Article Marin IJ David Herlocker

If you spend any amount of time on the trails of Marin County, chances are that you occasionally catch a glimpse of a snake. It is not uncommon to find a gopher snake sitting motionless in a fire road, or to see the last few inches of a sleek brown racer as it disappears into the trailside grass. If your travels take you near a lake, a stream, or any other freshwater habitat, chances are you will encounter one of the three species of garter snakes that live here.

The coastal marshes of Point Reyes are where you will usually find the gaudy red and turquoise common garter snake (our showy subspecies is called the California red-sided garter snake). In coyote brush and meadows you can expect to see our version of the western terrestrial garter snake (which is called the coast garter snake), and along our perennial streams and lakeshores you are most likely to encounter an aquatic garter snake. All of these snakes are characterized by the presence of one or more pale stripes that run the length of their body. When you see a garter snake against a plain background, this pattern looks anything but camouflaged, but when found among grasses or floating stems at the edge of a pond, the color scheme allows the snake to blend in perfectly. The name “garter snake” comes from this color pattern’s resemblance to the old fashioned striped garters that were used to hold men’s sleeves in place. Many people mistakenly refer to them as ‘garden snakes”; although they may be found in gardens, there is actually no species called the garden snake.

Garter snakes feed on a broad range of prey, from slugs and earthworms to birds and mammals. All three species will eat treefrogs (both adult frogs and tadpoles) and other amphibians. The western terrestrial garter snake probably has the broadest dietary range, shifting seasonally between lizards, baby birds, and small mammals. Some aquatic garter snakes eat adult treefrogs in late winter, tadpoles in early spring, and then gorge on newly emerged “froglets” as they emerge from the water. After that they may simply retire underground for the next eight months. The California red-sided garter snakes in some places may subsist on a diet composed exclusively of the endangered red-legged frog. This snake is also one of the only animals known to feed on the highly toxic adults of the rough-skinned newt. Newt skin contains a potent bacterium that deters virtually all predators. Some populations of garter snakes have evolved a physical resistance to these toxins; furthermore, after feeding on newts, these snakes are themselves toxic and are therefore avoided by potential predators.

Garter snakes have another defense mechanism to avoid being eaten, they produce a powerful musk. When molested, the snake will writhe violently and releases a foul smelling gray liquid that is smeared on its body (and your hand or muzzle depending on what kind of predator you are). By smearing itself with this musk, it becomes likely that a predator will get a taste of the musk before seriously injuring the snake. Some predators might conclude that garter snakes taste terrible and go look for a more palatable meal. This behavior may also have led to the belief that “snakes are slimy”. Slimy or not, there are many people that have an innate fear of snakes. This fear may seem irrational when you consider the fact that most of the snakes around here are harmless to humans, but this revulsion to snakes may be founded in our evolutionary past. Humans probably evolved in places where there were many deadly snakes, so those individuals that avoided snakes altogether may have enjoyed a survival advantage.

Garter snakes are not dangerous to humans, but it would be inaccurate to say that they are not venomous. Garter snakes have enzymes in their saliva that enter the skin of their prey and act as an anesthetic. It has long been known that garter snakes (and several other “non venomous” species) have toxic saliva, but recent research has revealed that virtually every living lizard and snake have extremely potent proteins in their saliva, but at relatively low concentrations. Those species that evolved higher concentrations of these compounds, and hypodermic needle-like teeth for injecting it, are the species that we consider truly venomous.

David Herlocker is the Naturalist for the Marin County Open Space District. He is responsible for the Naturalist Walks Program, which is offered by the Open Space District as an educational resource for the people of Marin County. For information on the program, contact David at (415) 499-3647, or by e-mail at: [email protected]. You can also get information on the walk schedule and the Open Space District on the web at: www.marinopenspace.org.This column will appear in the I.J. Weekend section on the last Thursday of each month.