What is hibernation?
There are numerous ways that animals respond to winter: they migrate, adjust or hibernate. But hibernation isn’t as easy as going to sleep for a couple of months. Although there are various degrees and period, hibernation always involves certain changes for animals. Their body temperature decreases, their breathing slows, and their metabolic rate drops.
It can be a lot of work, getting prepared to hibernate. Many animals have to find or create that ideal, safe spot to bed down for months at a time, whether it’s a cave, a hollow tree, or a den dug into the ground. Generally, before they go into hibernation, the animal has to enhance its body fat to survive, which means eating much more than usual in the months leading up to winter. Although hibernation constantly happens in winter, many dissimilar things can act as the real trigger for animals to start, including temperature drops, reduce in food accessibility, changes in day length and hormone changes.
What animals hibernate?
There are several animals that hibernate– skunks, bees, snakes, and groundhogs to name a few– but bears and bats are the most well-known.
Bears enter their dens for hibernation based on changes in the weather. They usually begin hibernating in September or October and come out six to seven months later around April. During hibernation, bears don’t reduce their body temperature as much as some other hibernating species. This gives them the aptitude to warm up more promptly in reaction to danger. But they do cut their breathing rate from 6-10 breaths per minute to one breath about every 45 seconds, and their heart drops rate from 40-50 to just 8-19 beats per minute. While they’re hibernating, bears live off fat that they build up during the summer and fall months. For female bears that level of fat is particularly significant because they will give birth in their dens through hibernation, and spend months nursing their cubs without eating anything themselves. In the spring, bears begin to come out from their dens as the temperature warms up and food becomes more accessible. Having not eaten for months, these newly-awoken bears are hungry, and some have new cubs to feed as well. Since they are desperate for food both before and after they hibernate, fall and spring are the times where bears are most at risk of getting into clash with humans over what they see as simple food sources.
Some bats also hibernate, mainly those in northern areas where insects are scarce in winter. They hibernate in caves, cavities of trees, old mine shafts and wells, or even in attics. Like bears, bats store up body fat before they begin to slow down their activity and go into hibernation. While they hibernate, they can survive on just a few grams of stored fat for five to six months. But unlike bears, bats go into true deep sleep hibernation with a heart rate as low as ten beats per minute (it’s normally closer to 300-400 beats per minute!). Hibernating bats may not even take a breath for up to an hour. Their metabolism slows down and their body temperature is just a few degrees over air temperature. Bats physically wake up a few times each winter to drink and execute other bodily functions, but the majority of the time they’re out like lights.
Hibernating animals at risk
Spending a few months asleep may be a good way to get through the winter, but it’s not without its risks. If an animal isn’t able to store up enough fat, or find enough food after it awakens, it may not survive. And if a hibernating creature wakes up too early, they can burn through their fat reserves far too quickly, and die.
Few animals know the danger of this as well as bats. One of the major causes of the decline of hibernating bats is white-nose syndrome, a disease caused by a fungus that is transmitted from bat to bat during hibernation.
Perhaps a larger issue is that animals’ hibernation patterns themselves may be at risk. New studies have found that as the winter temperatures heat up due to climate change, chipmunks in these warmer areas are less likely to hibernate. This raises a big question about how climate change is affecting hibernation, migration and other ways animals cope with the changing seasons.
Some Animals That Hibernate
Bats are interesting for so many reasons. They’re the only true flying mammals, they navigate using echolocation, have been around since the dinosaur days and they hibernate. Bats go into a true hibernation, meaning they are in such a deep sleep that they may appear to be dead. During hibernation, a bat’s heart rate drops from 400 to 25 beats per minute, and its breathing slows so much that it might not take a breath for up to an hour.
When people think of animals that hibernate, bears are often the first animals they think about. Ironically, bears aren’t true hibernators. They are more like light hibernators. Instead of hibernating, bears go into what is called a torpor. The main difference between a torpor and a hibernation is that during a torpor the animal is easily awoken. So tread lightly near that bear’s den; it will wake up.
If you saw a frozen frog that had ice crystals on it, that wasn’t breathing and didn’t have a heartbeat, you would probably think it was dead. If it was a wood frog, it would most likely be in hibernation.
Wood frogs hibernate inside logs or burrows or under rocks or leaf piles. During the cold of winter, when in hibernation, they actually stop breathing, their heart stops and ice crystals form in their blood.
Deer mice have short lives and short torpor (light hibernation) periods. During cold weather, they torpor from morning to late afternoon bundled up with their buddies and then spend the night searching for food. Between their daily torpor and the shared heat from snuggling up together, deer mice are able to save valuable amounts of energy.
An interesting fact about ground squirrels is that they go into hibernation as a response to a change in their blood. Scientists have been able to get an active ground squirrel to go into hibernation by injecting it with blood from a hibernating squirrel.
So what can we humans do to help?
Besides continuing to fight climate change, the best thing we can do is leave these animals and their habitat uninterrupted. If you spend time outdoors in the winter, know what animals in your area may be hibernating, and stay away from their most likely denning sites like burrows, dens and caves. We can also work year-round to defend significant wildlife habitat from being destroyed or fragmented. Without healthy habitat to find food and denning sites, wildlife can have a much harder time surviving the winter.