Hibernation, simply, is a state of inactivity in animals. It is typically characterized by a drop in metabolic activity—a physical slowing of the body—in addition to lower body temperature and slower breathing. Indeed, hibernation is a sleep deeper than most humans could imagine, but it is not impervious to interruption.
One animal that truly hibernates is the European hedgehog. Though hibernation periods among animals vary from a few days to a few weeks, most settle down during winter, when normal food supplies are limited. To survive this long period of inactivity, animals must spend the rest of the year building fat reserves that can supply energy during hibernation. However, if woken up early because of winters with unusual rain and high temperatures, the fat reserves will not be efficient enough for survival.
Interestingly, the animal most famous for hibernation, the bear, doesn’t truly hibernate. Instead, it enters a “winter sleep” state characterized by only a slightly slowed metabolism and stable body temperature, allowing them to quickly respond to any disturbances rather than if they were actually in hibernation. Regardless, this “winter sleep,” has been observed to be cut short in zoos, which only suggests the same is happening in the wild where there are no guaranteed meals.
Research showed that this lemur spends up to seven months of the year hibernating in tree holes during dry seasons. Before hibernating, the lemurs consume enough fruit to maintain fat in their tail, hence their name “fat-tailed” lemur. Madagascar’s winters are warm and the temperatures in a hibernating lemur’s tree hole can vary widely over its seven-month span. Research showed that not only is hibernation not dependent on low ambient temperatures, but also that reduced metabolism could occur in animals with a high body temperature. If there is too much weather variability during the dry season and the lemurs are woken up, their fruit sources may not have developed yet and they could starve.
Most species of bats hibernate—or at least enter a state of torpor—during the winter months. Though some rouse when warm weather causes insects to spawn, many spend six months or more in a state of complete hibernation.
Hibernating bats in North America are currently under siege by a deadly plague. “White Nose Syndrome” is caused by a fungal infection and once a cave is infected, it spreads rapidly through the sleeping population. It is not completely clear by what mechanism the fungus causes bats to die, but many believe it agitates hibernating individuals causing them to awaken and go searching for food, which is typically copious amounts of insects. The bats then may burn the small amount of stored fat they have reserved and can starve to death when there is not sufficient food supply during the colder months.
Western Diamondback Snake
It is not just mammals that hibernate. Western diamondback rattlesnakes have been known to enter states of hibernation in the summer, after finding cool caves to rest in. This process is sometimes referred to as estivation, or “summer hibernation.” Research suggests this is because they need cold temperatures to activate certain reproductive hormones. If temperatures continue to elevate, this could cause great risk to the rattlesnakes’ reproduction abilities.
Some species, like the hazel dormouse, both hibernate and estivate, or enter a similar stage of dormancy, depending on the weather and other conditions. This means that in any given year, a dormouse can spend most of the time sleeping. Its winter nest is often made beneath leaf litter on the forest floor. Climate change and disease are not the only things interrupting animal hibernation as human disturbances too can cause problems. Noise and vibrations from passing vehicles and light from towns and cities are increasingly common threats to these dormice in hibernation.
Even insects, notably ladybugs, spend some period of the winter dormant in a state of hibernation. After spending the summer and fall gorging on aphids and pollen, ladybugs cluster in buildings, under logs, or under piles of leaves to wait out the coldest months. If ladybugs deplete due to hibernation disturbances, many garden ecosystems could be harmed.
One species of bird, the common poorwill, hibernates in the winter instead of migrating to warmer climates. In addition to the risk of being woken up from their hibernation periods with little food supply, their typically dry, desert habitat is also subject to wild fires or heatwaves disrupting their breeding patterns.