Most bat species are not active during winter, largely disappearing from view. This period presents a difficult time for bats due to the scarcity of their primary food source, insects, and plummeting temperatures. Their specific behaviors vary depending on the species and geographical location, but activity is typically minimal.
How Bats Survive Winter
Bats employ two main strategies to endure winter: hibernation and migration. These adaptations are necessary because their primary insect diet becomes scarce when temperatures drop. Without a reliable food source, bats cannot maintain the high metabolic rates and body temperatures needed for active flight and foraging.
Many bat species, particularly in temperate regions like North America, enter deep hibernation. This strategy allows them to conserve energy by significantly reducing bodily functions. Examples include the Little Brown Bat and Big Brown Bat. Other species, especially tree-roosting bats like the Hoary Bat and Silver-haired Bat, opt for migration. These bats travel to warmer climates where insects remain available, often flying south to areas like Mexico or the southern United States. Some species may even utilize both strategies, depending on environmental conditions.
The Hibernation Process
Bat hibernation is a complex physiological state known as torpor. During torpor, a bat’s body undergoes profound changes to conserve energy. Its body temperature can drop to nearly freezing, often mirroring the ambient temperature of its surroundings. A bat’s heart rate drastically slows, from an active rate of 200-300 beats per minute to as low as 10-20. This reduction is accompanied by a dramatic decrease in metabolic rate and oxygen consumption, sometimes by as much as 98%.
Bats seek specific locations, known as hibernacula, which provide stable, cool temperatures and humidity levels necessary for successful hibernation. Common sites include natural caves, abandoned mines, rock crevices, and human-made structures like attics, barns, or old buildings. While hibernating, bats periodically awaken for short periods, typically every few weeks. These arousals allow them to drink water, urinate, or move within the hibernaculum. Each awakening is energetically costly, consuming fat reserves accumulated during the fall; a single arousal can use energy equivalent to several weeks of torpor.
Encountering Bats in Winter
If a bat is encountered during winter, its presence can indicate several possibilities. It might have been disturbed from its hibernation, perhaps due to temperature fluctuations or human activity in its roost. Occasionally, a bat may become disoriented or sick, leading it to appear in an unusual location. Remember that bats should never be touched with bare hands due to the potential risk of rabies, even if they appear healthy.
For an indoor bat, open a window or door in the room to allow it an exit, provided it can fly. Turning off lights and sounds can guide the bat towards the opening. If the bat cannot fly, or is found outdoors distressed, injured, or on the ground, contact a local wildlife rehabilitation center or animal control. These professionals are equipped to handle bats safely and provide appropriate care.