Understanding Hibernation in Animal Species

A Long Winter's Nap

With the unusually cold winter we’ve had so far, sometimes it is difficult in the mornings to awaken from our warm beds and venture out into the cold and wind. That makes it understandable then that many wildlife species choose to spend their winter asleep and inactive during those cold months. Although we typically call this behavior hibernation, many of the animals who exhibit these lethargic behaviors are not actually true hibernators.

Hibernation is typically when an animal prepares for being inactive or “asleep” for a long period of time, the animal has a slowed down metabolism and lowered body temperature by several degrees from normal and slows its heart rate as well, and the animal is not very easily awakened – some say the animal appears dead. Even true hibernators may sometimes arouse from this sleep and restore their body temperature, but only when conditions are favorable, and they are not typically very easily awakened and most can easily go back into hibernation after being woken up. Hibernation can last for weeks or months, depending on physiology and weather conditions. Animals that are true hibernators include some species of bat, ground squirrels, rodents, and woodchucks.

In contrast, some animals exhibit a behavior known as “torpor” which is slightly different from hibernation. Torpor is the same in that the animal experiences a drop in body temperature and heart rate, but they are typically not asleep for as long, this may even be a daily occurrence, although it can last for longer periods of time. Torpor is also more generally a response to conditions than something that is prepared for, and most animals can relatively easily be awakened from this condition. Birds and some other mammals experience torpor daily during cold months, where their temperature and metabolism drops at night to allow them to deal with colder temperatures.

One of the animals we think of in particular when we think of hibernation are bears. However, bears do not truly hibernate – their body temperature does drop in colder months but only slightly. Bears do typically remain in their dens and do not eat during this period, which is why some people call them “super-hibernators,” but they are usually pretty easily aroused. Since it is not true hibernation though, some call what bears do “denning” or “winter lethargy.”

And hibernation or torpor behaviors are not just for cold weather either. Some animals experience a kind of hibernation in the hot summer months, and this is typically called estivation or “summer sleep.” These species experience a similar decrease in metabolism, and a reduction in the loss of water, to deal with hot and dry conditions. As such, this is a more common occurrence in species that live in desert regions. A number of aquatic species, such as some land snails, and reptiles and amphibians such as crocodiles or western diamondback rattlesnakes aestivate. Animals can also become inactive when conditions are unfavorable, such as low oxygen or low food supply. It is pretty remarkable how adaptable organisms are to harsh environmental conditions, which makes it even more important for us to try our best to not add other stresses, such as pollution and habitat loss, to the list of stressors they must face.

-- Crystal Cockman