When the air has turned cold and crisp, and the insects have gone away for the winter, what do bats do to make it through?
02.25.25
By Alyson Brokaw
When the ground outside is covered in a layer of snow and you have to scrape the ice off your windshield every morning before heading into work, it is easy to feel a little jealous of the critters like bears and bats who get to just sleep through the winter. But are they really “sleeping”? When the air has turned cold and crisp, and the insects have gone away for the winter, what do bats do to make it through?
Torpor versus Hibernation
For many bats living in temperate climates like North America, surviving the cold winter means tucking themselves in a cave or other safe place and hibernating. They are not alone ̶ ground squirrels, hedgehogs, chipmunks, and bears all also hibernate through the winter. During hibernation, bats are in a physiological state called torpor. Use of torpor is not limited to the winter, with many bat species temporarily reducing their body temperatures and metabolism when conditions are not ideal for hunting throughout the year. However, when animals enter and stay in a state of torpor for a period of weeks to months, it is considered hibernation.
Hibernating big brown bats have been shown to reduce their breathing rates to just one breath every two hours.
Long Winter Sleep?
Colloquially, hibernation is often referred to as “winter sleep,” but these animals aren’t actually sleeping. It’s almost the opposite ̶ many hibernating animals actually emerge from hibernation sleep deprived!
During sleep, the body is in a state of rest. Breathing slows, body temperature decreases and the unconscious brain cycles through different phases of activity. While some parts of the body are taking a break, cells also use the rest time to repair cell damage, grow, and other maintenance. In contrast, animals in hibernation experience extreme decreases in body functions, including immune and cell repair functions. The body temperature of hibernating bats falls, coming close to the ambient temperature of their cave or hibernation roost, in some cases even hovering close to freezing (usually between 35 and 40 degrees F). Hibernating little brown bats (Myotis lucifugus) reduce their breathing rate to one breath every 47 minutes, while big brown bats (Eptesicus fuscus) have been shown to reduce their breathing rates to just one breath every two hours!
Energy Savings
Despite their small size, bats have voracious appetites ̶ some insect-eating bats consume as much as 33 percent of their body weight in insects every night. Their high energy needs are linked to both their small size and the demands of flight. Small animals lose heat quickly due to their high surface area to volume ratio. Cold weather also means less insects flying around, reducing the ability for bats to meet their energy needs. By hibernating, bats are minimizing the amount of energy they spend over the winter. Still, bats in hibernation mode take periodic “awake breaks,” where they return to normal metabolism and breathing rate. Warming their bodies back up uses up a lot of energy ̶ so why don’t they just stay in torpor the whole winter?
Due to more frequent arousals, WNS-infected bats expend twice as much energy over the course of a winter as an uninfected bat.
Remember that while in torpor, almost all cellular processes are significantly slowed down. This means reduced energy use, but it also means the processes responsible for cleaning up cellular waste and other maintenance processes are also down. By waking up periodically, bats (and other small animals) can clear out the build of these waste products. With their small size and lots of surface area, bats are also susceptible to water loss via evaporation. During these periods of arousal, bats will replenish some of that water, usually by licking the droplets of water that have accumulated on their fur or wings. Long periods of time of reduced activity can also lead to decreased muscle mass. Occasional periods of arousal may help bats prevent this kind of muscle atrophy, especially for important wing muscles.
White-Nose Syndrome and Hibernating Bats
Each arousal costs a bat precious fat reserves, and if a bat runs out of energy before the winter is over they are in trouble. And as if surviving winter wasn’t already tough enough, many bats now face an even greater challenge—white-nose syndrome (WNS).
White-nose syndrome is a disease caused by a cold-loving fungus that lives in the soils and on the surface of caves. This fungus takes advantage of the suppressed immune systems of hibernating bats, forming a fuzzy, white growth on the wings, ears, and noses of bats. Since the fungus was first detected in the United States in 2006, little brown bats, northern long-eared bats (Myotis septentrionalis), and tricolored bats (Perimyotis subflavus) populations have declined by as much as 90 percent in just the last 10 years.
Infection of the fungus results in a range of physiological and behavioral changes in hibernating bats, notably causing dehydration and increasing the number of times a bat wakes up from hibernation. Due to these more frequent arousals, WNS-infected bats expend twice as much energy over the course of a winter as an uninfected bat. Research by BCI scientists has found that heavier bats have a better chance of surviving a winter, even with WNS. Providing bats a healthy habitat and making sure they have access to the insects they need to fatten up, especially near hibernacula, is one approach bat conservationists are trying in this face of this devastating disease.
As we wait out the last weeks of winter, we humans may be able to take some lessons from hibernating animals like bats; sometimes slowing down is a good way to recharge and prepare for the seasons ahead. It is also a reminder of what bats do to survive, the challenges they face and how we can ensure they thrive for many seasons to come.
