If the cold nights around California have been bothering you lately, it’s time for some perspective. Wild animals in the cold, snowy Sierra Nevada have it a lot worse, and they have no warm hearths or electric blankets to retreat to.
How do birds survive long mountain winters? What about critters that live around water, like frogs? Or rodents, whose habitat gets buried by feet of snow? Do any of these animals hibernate? What is hibernation, anyway?
Will Richardson is fascinated by these questions. He’s been giving a series of lectures on the subject around the Lake Tahoe area lately. He has a doctorate in ecology and is co-founder of the Tahoe Institute for Natural Science. So The Bee asked him for some answers.
There’s a lot of things we don’t have to contend with as humans and sort of take for granted. It’s a really challenging, difficult time in the annual cycle for most creatures. A lot of the creatures that we’re accustomed to seeing in the summer, we don’t see in the winter. It’s good for people to consider all the challenges that animals have in the winter.
There are three main survival approaches. Some of those animals leave the area entirely in the winter. Or you can shut down: go dormant and save some energy; find shelter and basically take a long nap or some smaller naps, to conserve your energy by basically going into deep, deep rest. The third approach is to be tough and resourceful and stick it out. Those are the organisms that we do see. But when it’s stormy, even the hardiest of creatures have to kind of lay low for a while. For most organisms, it’s simply a matter of survival until next summer, when they can think about growth and making babies and all that kind of stuff.
They’re a minority. A great example are the voles – little meadow mice – which live under the snowpack. If you have a lawn around here you will see, when the snow melts in spring, that they have been very active. They are protected from predators and the cold by the insulating properties of a deep snowpack. They are comfortable and free to make babies all winter long. There’s even a bird species, red crossbill, which can lay eggs and raise young birds in the middle of winter if they find enough food. Their primary food is pine seeds from pine cones. If they find a good crop they will set up shop and lay eggs in the middle of winter. Most birds would never consider that.
Yes, but “hibernation” is kind of a tricky term. There are all these different terms that sort of overlap. You’ve got torpor, brumation, estivation. They all have slightly different meanings and they all kind of center around different variations on winter dormancy. But the fact is, yeah, a great number of our animals do sleep through the winter.
Most of our wild (black) bears, and particularly females, they definitely do go down for a nap of at least some period of time. It may only be a period of weeks, but usually much longer. They sleep the whole time. They’ll wake up and sort of circle around, the way a dog does, and lie back down again. Most of their metabolism is severely depressed. They don’t defecate, they don’t urinate, they don’t eat the whole time once they’ve committed to a period of dormancy. Whereas our chipmunks, they will store food underground and they will sleep for a period of time. Then they wake up, go to the bathroom and have a snack. And if it’s warm and accessible, they may poke their heads up and look around and then go back for another nap.
One of the most fascinating things is our Sierra chorus frog, which can freeze itself solid. Basically this frog pumps tons of glucose into its cells, which acts as an antifreeze, and puts protein nucleates in other areas that can withstand freezing. Their heart stops, their brain function stops. It’s really quite amazing. As snow falls and covers them up, they encounter a little bit of ice and that triggers this hormonal chain reaction. They just turn into a little block of ice. There’s no brain function, no heartbeat, nothing for weeks and possibly months.
Nobody knows, because it happens internally. There’s sort of a reverse process that starts inside. It starts to thaw from the middle out.