Fall is fully upon us here in Colorado, though it’s been pretty warm the last few days. When you think of fall and animal behavior, the first thing you might think of is birds migrating or certain mammals hibernating, but a couple of recent encounters have put snakes on my radar in a big way.
A few weeks ago I saw a pair of common garter snakes cavorting in the grass at Sawhill Ponds in Boulder. The common or red-sided garter snake (Thamnophis sirtalis) is a really interesting species. Unlike a lot of snakes, garter snakes bear live young instead of laying eggs, so the female is noticeably larger than the male in order to accommodate babies. The snakes I saw were almost certainly a male and female pair engaged in breeding because one was definitely smaller and they had their tails entwined. While spring is a common time for snakes to mate, according to the Idaho Museum of Natural History, common garter snakes may also breed in the fall.
This past Saturday I had another snake encounter. Coincidentally, it was with another snake that bears live young. I literally almost ran into my very first prairie rattlesnake (Crotalus viridis) in the wild at Chatfield State Park in Littleton. It was stretched out partially on the trail, perhaps sunbathing. It was certainly a beautiful day for it. Whatever it was doing, it definitely wasn’t happy to be disturbed, rattling and coiling up for a strike if I got too close. Needless to say, I kept my distance!
Both days I saw the snakes were really warm. So what do snakes like those garter snakes or that rattlesnake do when the weather turns cold? Snakes are reptiles and so they rely on soaking up heat from their surroundings in order to warm up enough to move or undergo other bodily functions like digesting food. This means they’re “cold-blooded,” or “ectothermic,” as opposed to “warm-blooded” or “endothermic” like humans and other mammals whose body temperatures and functions are related to internal metabolic processes. Therefore, snakes “hibernate” during the winter.
The scientific term for reptile hibernation is “brumation.” During brumation, the reptile’s bodily functions grind nearly to a halt, though they’re not technically sleeping like mammals undergoing hibernation. For instance, they still need to keep hydrated. Snakes and other reptiles will look for an insulated place away from potential intrusion called a “hibernaculum” (plural hibernacula) to spend the season. A hibernaculum may be a den, a pile of leaves, a crawl space under your house, under water, or anywhere else where temperatures are relatively stable. Sometimes, a bunch of snakes will enter the same hibernaculum and overwinter there together curled up into a ball.
Brumation only occurs in areas with distinct warm and cold seasons, however. In tropical areas, though there may be some physical changes over the year, reptiles don’t typically brumate because temperatures are pretty steadily warm. There’s no need for dormancy. In artificial environments like with pet reptiles, unless brumation is induced, there’s no need for the animal to undergo it either. But that’s an entirely different topic.