Snakes are cold-blooded animals, or ectothermic, because they get their temperature from their surroundings and cannot generate their own body heat. While this can come in handy, the downside of being a cold-blooded animal is the struggle to survive in cold environments. Incidentally, if the outside temperature rapidly drops, their physical temperature can drop to temperatures that are life-threatening, too.
Species that live in habitats where winter months are inhospitable stay safe in the form of hibernation. Although reptilian hibernation is different from mammal hibernation on a physiological level — in fact, it is often referred to by scientists with the term “brumation,” instead — it follows almost all the same principles: If it’s too cold to prey and mate, you need to rest.
“Why waste energy? If you're not able to feed and you're not mating, you might as well take advantage of the fact that you're an ectotherm and that you can get really cold and still survive,” says Matt Goode, an assistant research scientist at the University of Arizona School of Natural Resources and the Environment.
Snakes Hibernate According to Their Habitat and Geography
Not all species of snakes must go into hibernation — it depends on where they live. Species living in tropical areas don’t usually hibernate, according to Goode, although they might have periods of lower activity and dormancy associated with other environmental factors such as droughts or dry seasons. That’s the case for anacondas, for instance, who are native to warm, tropical climates.
“But in temperate areas, snakes can spend many, many months underground,” says Goode. His first research focused on prairie rattlesnakes in Wyoming, where temperatures can drop to 5 to 10 degrees Fahrenheit in the winter and the ground can freeze solid 5 feet deep.
“Snakes have to get below that,” Goode says. “If they don't, they could freeze and not survive through the winter.” Tiger rattlesnakes start brumation from late October to early December, while Western diamondback rattlesnakes, which are slightly larger, will go into their dens a little later than that.
How do Snakes Hibernate?
Since snakes get their warmth from external factors, like sunshine, when temperatures start to drop, they will physiologically experience a drop in their body temperature, too. As colder days become more frequent, snakes will start eating less and will slow their metabolism and heart rate to save up on energy expenditure. They’ll move less and stay as still and relaxed as possible, almost grinding their body to a halt.
“Because snakes spend so much time completely not moving around, their heart rates go way, way down, and their respiration rates, of course, go way down,” says Goode.
Read More: Why Do Snakes Eat Themselves?
Then, they find a spot to stay safe and warm throughout the cold and take a nap for as long as the frost continues. In fact, they take more of a rest and a nap than a deep slumber, because they might still make appearances outside the den if needed – like basking in the sun to warm up or fight an infection. Their internal storage of glucose will help them spring back into action if needed in case of an emergency.
Where do Snakes Hibernate?
Snakes usually hibernate in what scientists call a hibernaculum, aka a den. “[A snake is] hiding anywhere that has the ability to buffer those cold temperatures on the outside,” says Goode.
That is usually in burrows deep underground, under the frost and the portion of the ground that is frozen. But snakes can also hibernate in tree hollows and rotting logs, tree roots, sinkholes, tunnel systems excavated by other animals, railroad embankments, water-filled cisterns, houses, basements and sheds meant for humans. Smaller snakes might like to spend the winter deeper underground than larger snakes in some cases, and some snakes like to return to their den over the years.
Oftentimes, these dens can house hundreds, if not thousands, of snakes at the same time throughout the winter — adult snakes and baby snakes alike, sometimes even from different species. In Canada, for example, Goode explains, garter snakes are known to hide out in dens with over 20,000 garter snakes in one spot.
“It’s a limiting feature of their environment to find that really good place that allows them to get under the ground and avoid freezing,” says Goode. It’s also helpful to hibernate in large groups because that helps produce heat, since a bunch of snakes intertwine and retain warmth in the group.
Scientists Speculate Dens Serve a Social Function, too
“There may be some social function to [hibernating in a den] as well,” says Goode. He argues that the evolution of snake social systems, for example, might have come from the fact that large groups of snakes are essentially forced together under the ground.
“It's not like snakes want to hang out with each other, right,” says Goode. “But some species actually, when they emerge from the den, they remain at the den site above ground, and then you'll see social interactions happen.”
Diamondback rattlesnakes do this, for example. But other snakes, like tiger rattlesnakes, overwinter completely in solitary. They have no interaction with other species or other individuals of their species until later, after they've moved away from the den.
Especially for snakes that live in places where it gets frigid for a long time, hibernation becomes a crucial part of their lifecycle. It underlies many of the hormonal mechanisms that later allow them to reproduce successfully (that’s why snake breeders will put their snakes in brumation too, even if they don’t necessarily need to).
For instance, brumation comes as an advantage for ectotherms’ longevity. The rattlesnakes Goode studies are long-lived compared to their body size especially because they’ve evolved to be great at conserving energy.
“Being able to go down for extended periods of time like that really allows them to live longer lives and to reproduce more,” says Goode. “If we were ectotherms, with our large body sizes, we probably would live to be a couple 100 years old.”