By Misi Stine, Environmental Educator, Member of the MN Herpetological Society
Photo credit: National Park Service, Public domain, via Wikimedia Commons
When you think of how animals adapt and survive winter, many strategies come to mind. Some migrate to locations south where conditions are more favorable, such as bird and bat species. Some animals take the "I am a winter warrior" approach and are active all year round. Depending on the winter, they tend to thrive more, and others struggle more. A great example is white-tailed deer. In mild winters, they can find more vegetation to forage and better maintain their winter body condition. In more severe winters where they have trouble getting to the remaining available food and deep snow, they find it challenging to make it to spring. Conversely, wolves thrive in more severe winters, when the ungulates make more effort to survive and are easier to make a meal out of them. Wolves' snowshoe-like feet help them stay on top of the snow better, using less energy to move around.
Then there are those animals that use hibernation to spend their winter months. Hibernation is thought of as a long winter sleep. When you look closer, you see a complex survival technique utilized to conserve energy during food and water shortages and unfavorable winter weather conditions. Over time, science continues to learn more about hibernation. Some hallmarks of hibernation involve physiological changes, including a decrease in body temperature, slower breathing, slowing of the heart rate, and slowing the creature's metabolism. Beyond that, there are variations on what we think of as hibernation. Although it is a necessary strategy for survival, there are downsides as well. Animals that hibernate are more vulnerable to predators and other dangers.
Hibernation is the general term used to describe inactivity in endotherms, "or warm-blooded" mammals. Some mammals are called true hibernators (i.e., some bat species, ground squirrels, woodchucks, and deer mice). They typically go into a longer and deeper inactive state. Then some are classified as going into torpor which is shorter in duration, and they are awoke/disturbed much easier. An example of an animal that would go into torpor is a bear. Their body temperature does not drop as much as the true hibernator, and during inactivity, the females give birth to young, nursing, and caring for the cubs during this time. Bears return to a more normal physiological state throughout their torpor and leave the den for short periods. However, they do not drink or eliminate during this time. (Price unknown, Griggs, et al. Unknown, Wikipedia 2021) (Biel and Gunther n.d.)
Brumation is a period of inactivity that has all the fundamental hallmarks discussed above. However, brumation occurs in three groups of ectotherms, or "cold-blooded" animals, specifically fish, reptiles, and amphibians. Because ectotherms cannot actively regulate internal functions like body temperature or metabolic rate, they rely on the environment to give them what they need to perform those functions, as a result they do not hibernate like mammals. During brumation, many species need access to a source of water to drink. One other significant difference is that hibernating mammals eat enough food to store as fat to help them survive the inactive period. They eat up until hibernation because although their metabolism slows, it is still active and needs that fuel to survive.
In contrast, those species that brumate stop eating before the inactivity period because their metabolic rate drops low enough to use almost no resources. Again, because ectotherms rely on the environment for those metabolic processes to operate efficiently, means they cannot digest a meal properly until the ambient temperatures increase. (Griggs et al. Unknown) (Tabatabai 2020) (Wikipedia 2021)
The third type of inactivity, diapause, is used to describe the inactive state of insects. Again, diapause has many of the fundamental hallmarks of hibernation, but insects will enter their dormant period and spend it in a single developmental stage (egg, larvae, pupae, or adult). They will do this before it gets too cold, and they will remain dormant until temperatures in the spring are favorable to resume their development. Insects also store up energy to help them survive, and their metabolic rate is reduced to the point that scientist can barely detect their use of oxygen. (Reynolds 2018)
Finally, some animals aestivate, which occurs in a warm climate, to escape the other extremes of heat and drought conditions.
As you explore the winter season in your woodland, you can now look around and wonder what creatures are spending their winter: in the leaf litter, the hollowed-out tree snag, deep in the rock crevices, in their den all snuggled up with their cubs, burrowed underground below the frost line and other essential winter hideaways. From my perspective, the saying, "don't poke a sleeping bear," is sound advice. (Price, unknown)
Works Cited and Resource List
Biel, M, and K Gunther. n.d. Denning and Hibernation Behavior. Accessed 9 2, 2021. https://www.nps.gov/yell/learn/nature/denning.htm.
Griggs, Gordon, Fritz Geiser, Stewart Nicol, and Christopher Turbill. Unknown. Australian Academy of Science. Unknown Unknown. Accessed September 25, 2021. https://www.science.org.au/curious/hibernation.
Price, Jo. unknown. Discover Wildlife. unknown unknown. Accessed September 25, 2021. https://www.discoverwildlife.com/animal-facts/what-is-hibernation/.
Reynolds, Julie. 20018. blogs.scienticamerican.com. October 15. Accessed September 25, 2021. https://blogs.scientificamerican.com/observations/how-insects-prepare-for-winter/.
Tabatabai, Sara. 2020. Effie Yeaw Nature Center. November 10. Accessed September 25, 2021. https://www.sacnaturecenter.net/visit-us/nature-blog/news.html?NewsID=70046.
Vinogradova, E. B. 2007. Diapause in Aquatic Insects, with Emphasis on Mosquitoes. Accessed 9 23, 2021. http://zin.ru/labs/expent/pdfs/vinogradova_2007.pdf.
Wikipedia, "Hibernation," Created August 2002. 2021. Wikipedia. Accessed September 25, 2021. https://en.wikipedia.org/wiki/Hibernation.