There are three species of garter snakes in the San Francisco Bay region. The terrestrial garter snake, the aquatic garter snake and then there are two snakes that are very closely related and are considered subspecies: the California red-sided garter snake and the famously endangered San Francisco garter snake. The latter incidentally is not found in the city of San Francisco but only on the Peninsula.
Garter snakes are the most commonly encountered snake in North America. Their range extends all the way into Canada and even Alaska and of course down into Mexico and beyond. They are found from the West Coast all the way east to the Atlantic Ocean. There are a lot of species and subspecies but they share many characteristics. None of them are particularly large (never more than four feet). They have longitudinal stripes down their body. They come in a wide range of colors -- red, orange, brown, blue, green and black. They are relatively gentle and rarely bite, though they can. And they have glands next to their cloaca that emit a foul smelling substance as a defense mechanism. By the way for years they were considered not poisonous but actually they do have mild venom. But it's not harmful to humans.
They are so successful because they can survive in a wide variety of habitats and have a very diverse diet. Basically anything that's smaller than they are is considered food. They are even immune to the powerful toxins created by toads and newts. Unlike many other snakes, they do not kill their prey first but swallow them alive.
That classic scene in Indiana Jones with all the writhing snakes actually occurs with garter snakes. I have had the good fortune to witness hundreds of them emerging from their den on a warm March day in Northern California's Tule Lake region. Males and females both emit characteristic pheromones and they find each other by following the scent trails. When the females emerge and are receptive to mating, more than 10 males will attempt to mate with them and form gigantic mating balls. That I'd love to see someday.
This is Michael Ellis with a Perspective