736px Eptesicus nilssonii hibernating
 Northern Bat (Eptesicus nilssonii), a hibernating bat species. Photo: Magne FlåtenGNU Free Documentation License

Cold weather brings big shifts in nature. In many places, water sources freeze, plants cease blooming and drop their leaves, and the ground is covered in snow. These conditions mean diminished resources for animals, and difficulties for some to leave their shelters and find food. To deal with the change, some migrate to warmer climates, but for others who stick around, they have evolved various ways to get through these colder months.


Hibernation is sometimes misunderstood to be like a long restful sleep, but it’s actually quite a bit more dramatic, and happens in an effort for the hibernating animal to conserve energy. Hibernation is an involuntary biological process for some endotherms (warm-blooded animals), where the animal’s body temperature, heart rate, breathing, and other metabolic processes slow significantly. It’s thought that this process is triggered by both exogenous cues (changes in the environment like temperature, humidity, and barometric pressure) and endogenous cues (changes occurring inside the animal like circadian rhythms and hormonal alterations).  

While hibernation is almost exclusively seen in mammals, some birds like the common poorwill (Phalaenoptilus nuttallii) will hibernate. Other birds, however, typically utilize torpor, a state of decreased physiological activity.

Not all hibernating endotherms experience hibernation the same way. True or “deep” hibernators are incapacitated by their physical changes that make it just about impossible to get them to wake up. The animal may seem dead. Others experience less extreme forms of this and will wake up to eat, defecate and urinate.

To prepare for hibernation, endotherms build up fat reserves by gorging themselves, or becoming hyperphagic. You might be familiar with the annual October event, Fat Bear Week, at Brooks River in Katmai National Park. These brown bears that feast on salmon are an example of the amount of eating an animal can do to gain the fat needed to get them through cold periods. Smaller animals like chipmunks, however, horde food to eat during their intermittent periods of activity.


 Turtles at the UT turtle pond, getting some sun on a cool day.

Ectotherms, or cold-blooded animals including fish, reptiles and amphibians, cannot hibernate. The dormancy they exhibit is called brumation. The animal experiences physiological changes similar to hibernation, but on warmer days, these animals will move about. This is because they rely on the environment to regulate their body temperature. For example, you might spot a lizard out on a rock during January, getting some sun to warm itself.

Like mammals, reptiles will also eat more before the cold comes, but unlike mammals who build up fat, they build up high levels of glycogen in their blood. Glycogen is a form of polysaccharide or sugar that they use as energy for muscles. Another difference in the way ectotherms experience dormancy is how they tolerate low oxygen environments like mud where some may bury themselves. Ectotherms also don’t sleep during their dormancy as mammals do, and they also need to drink water.

You can see brumation in action on campus. On the rare cold days in Austin when the turtles in the UT turtle pond have seemed to disappear, they are in fact, brumating.


ice crawler
 Ice crawler on an alpine snow field in California (Grylloblatta sp. ice crawler) Photo: Alex Wild

Insects are not immune to changes in weather either. Many are short-lived and spend the warmer months mating and laying eggs before dying at the onset of cold weather. Some insects, like the Monarch butterfly for a well-known example, will migrate to warmer climates. Others experience diapause. Diapause is period of dormancy marked by reduction of metabolic activity, similar to hibernation, but without some of the bursts of activity seen during hibernation and brumation. There is also a cessation of development.

Diapause can happen at any stage of an insect’s life cycle. For example, the offspring of those insects that die at the end of mating season remain dormant during cooler months until warmth signals it’s time to emerge, mate, and start the cycle over again.

Some insects also have enough proteins and sugar alcohols like glycerol in their cells that act as antifreeze. For example, the woolly arctic moth caterpillar produces alcohols that allow them to survive temperatures as low at -70 degrees Fahrenheit. Others, like the Antarctic midge (Belgica antarctica), also dehydrate to the point where there is almost no water in their bodies to freeze.


“Ask a Naturalist: Hibernation vs. brumation vs. estivation.” January 13, 2016. Discovery Place blog. (https://nature.discoveryplace.org/blog/ask-a-naturalist-hibernation-vs.-brumation-vs.-estivation)

“Hibernating Mammals and Brumating Reptiles: What’s the Difference?” January 20, 2014. The Infinite Spider. (https://infinitespider.com/hibernating-mammals-brumating-reptiles-whats-difference/)

“Why do animals hibernate?” (https://www.teatown.org/hibernation/)

Bittel, Jason. “Ever Wondered: Where do bugs go in the winter?” The Washington Post, April 22, 2019. (https://www.washingtonpost.com/lifestyle/kidspost/ever-wondered-where-do-bugs-go-in-winter/2019/04/18/f09b3a10-563d-11e9-814f-e2f46684196e_story.html)

Panko, Ben. “What Do Insects Do in Winter?” Smithsonianmag.com February 15. 2017. (https://www.smithsonianmag.com/science-nature/what-do-insects-do-winter-180962183/)

Reynolds, Julie. “How Insects Prepare for Winter.” Scientific American, October 15, 2018. (https://blogs.scientificamerican.com/observations/how-insects-prepare-for-winter/)

Vaugh, Don. “Why Do Some Animals Hibernate?” Britannica (https://www.britannica.com/story/why-do-some-animals-hibernate)