I recently took Grantham Elementary students outside to investigate the phenomenon of hibernation. Each child had a grape that they needed to keep warm and safe from freezing for 25 minutes outside. They could bury it in snow, put it in a crevice in a tree, or even keep it on their person. After the 25 minutes of grape hibernation, they came back to find most of them frozen!
Now, we know that grapes and animals are very different, but it does beg the question, What do animals do to prepare for hibernation and where do they go to make sure they are warm and safe all winter long?
In a previous naturalist notes, we learned that many insects produce antifreeze in their cells in winter and burrow down under the soil or into a log. While this might be a good strategy for small, cold-blooded animals, I doubt it would work for large, warm-blooded ones. So what strategies do they use?
Let's take a look at bears as an example. They eat high fat foods in the fall (nuts and seeds) to put on a layer of insulating fat and they grow thicker fur to keep the heat in their bodies. They find sheltered places to hibernate - digging holes under the roots of downed trees, climbing a large dead tree and crawling in a hole, or squeezing into shallow caves. They add leaves and other soft materials to better insulate their hibernation dens.
Once in their dens, bears don't move very much and go to sleep. Their metabolism drops so they aren't burning many calories and their digestive system slows down so much, they don't even go the bathroom. Amazingly, the females give birth and nurse during hibernation. The newborn cubs are very small and stay warm by keeping close to mom.
In Northern latitudes, bears may spend 5-7 months hibernating. The duration depends on how much food is available. In the Northeast, where there is abundant food in the forest, some bears will wake up during fall and spring thaws and continue to forage.