How Do Turtles Survive a Frozen Winter?
Different animals use different strategies to get through the winter. Some are more obvious than others. What about turtles? The Masked Biologist gives us a glimpse into their lives in this week’s Wildlife Matters.
Recently a large snapping turtle garnered some attention on social media by doing what turtles do—swimming around slowly under water. What was a little unusual, at least for the photographer, is that the turtle swam right under where they were standing—there were a couple of inches of clear ice between them.
Every once in a while, it occurs to us to wonder what different animals do to survive the winter. I always tell people that animals have three options at their disposal, that they can use alone or in combination: migrate, hibernate, or mitigate. We probably know numerous animals that employ each strategy. It is well known that birds and some insects (like butterflies) migrate to warmer climes in the winter. Some make huge migrations, like arctic shorebirds that travel over 2,000 miles to South America. Others, like geese or woodcock might travel just south of the snow line to spend the winter.
Hibernation can take more than one form, from true hibernation, where animals basically enter a coma to torpor or brumation where animals slow their body temperature and metabolisms so much that they barely breathe and don’t eat but can still move about or awaken on warm days during the winter. It may include a hibernation chamber, or hibernaculum, established below the frost line. Some animals, like snakes, may bunch together in groups to share what body heat they can store, because they can’t generate their own heat. Both snakes and bears try to tie on the feed bag to store up calories before winter because they won’t be feeding until spring arrives.
Then there is mitigation. Animals get used to being cold; they have fluffy feathers or thick fur undercoats. They find warm thermal cover out of the wind on cold days, or maybe they den up in the deep snow that provides some insulation. These are the most visible animals for us right now—deer and chickadees frequent our yards and roadsides.
What about turtles? Turtles definitely do not migrate south for the winter, and they are not quite well equipped enough to excavate a den like a groundhog, chipmunk, or toad. All the rest of the year, other than maybe leaving to dig a nest a bury eggs, they stay in the water or very close to it. In the winter, they pack it in and take their chances, so to speak, with the water, mud, and ice. They use a combination of hibernation and mitigation. They go to the soft bottom of the lake, river, or pond where there is a soft enough substrate that they can sort of bury themselves in mud. In the summer, they wouldn’t be able to stay down there for more than a few minutes before surfacing for a gulp of air. That’s because the water is warm, and that makes their body warm, speeding up their metabolism. In the winter, the lake flips, or turns, the coldest water rising to the top and freezing and the warmer water moving to the bottom. As the water cools, their metabolism will slow by more than 90%, making life signs almost undetectable. When your metabolism is that slow, you don’t need to eat because you aren’t expending any energy. And you need far less oxygen. This is where mitigation meets hibernation. Turtles are specially adapted to breathe through some of their body parts, like skin, face, and cloaca or anal vent. It’s true; turtles can breathe through their butts if water is cold enough.
Like any survival strategy, there is an element of gambling here. If a turtle picks a shallow bay, the vegetation may die, start to decompose, and choke the oxygen out of the water. Turtles can handle this somewhat, although it raises the acid level of their blood. For some turtles, especially painted turtles, they can mitigate this too, using the calcium in their shells to balance their blood acidity. But if the winter is so cold that it freezes to the bottom, the turtles will die; they cannot survive freezing solid like some frogs can.
If the ice doesn’t get too thick, especially early in the season or in a mild winter, the sun can reach the plants and help them stay alive, keeping the water oxygen rich. During these times, bromating turtles can certainly emerge from the mud and do a little bit of exploring to see if spring has come. Since turtles can live thirty years, this survival strategy obviously serves them well. I think it would be a boring way to spend the winter, but then who am I to judge? My winter has been anything but eventful.