Brown bats, such as the little brown bat, are true hibernators. This adaptation allows them to survive harsh winters when their primary food source, flying insects, becomes scarce. By entering a prolonged state of inactivity, these bats conserve energy, enduring periods of cold and food scarcity. Hibernation is an important survival strategy, sustaining them until warmer weather returns.
The Biology of Brown Bat Hibernation
During hibernation, a brown bat’s body undergoes profound physiological changes to conserve energy. Their body temperature drops drastically, often reaching 2 to 10 degrees Celsius, accompanied by a dramatic slowing of their metabolic rate, which can decrease by as much as 98 percent. The heart rate of a little brown bat, normally around 210 beats per minute, can slow to 8 to 20 beats per minute during deep hibernation. Breathing also becomes incredibly infrequent, with some bats taking only one breath every few minutes. These adaptations enable the bats to rely almost entirely on fat reserves accumulated before winter.
The Hibernation Cycle: Timing and Location
Brown bats typically begin hibernation in late fall, usually between September and November, and emerge in early spring, from March to May. Before entering this state, they consume large quantities of insects, increasing their body weight by over 30 percent to build sufficient fat reserves. They seek out specific environments known as hibernacula for the winter, which are often natural caves, abandoned mines, or old tunnels. These sites are chosen for their stable conditions: consistent temperatures that remain above freezing, typically ranging from 2 to 10 degrees Celsius. High humidity, generally greater than 80 to 90 percent, is also crucial within these hibernacula to prevent dehydration, as bats can experience water loss even during torpor.
Why Undisturbed Hibernation is Crucial
Successful hibernation is important for the survival and population health of brown bats. While bats periodically arouse naturally, frequent or forced awakenings are energetically costly, as each arousal requires the bat to rapidly raise its body temperature and metabolic rate, consuming valuable fat reserves. For example, a single arousal can deplete 100 milligrams of fat, an amount that could sustain a bat in torpor for 67 days. Repeated disturbances quickly exhaust these limited fat stores, leading to starvation and mortality before spring arrives. The ability to complete hibernation without undue interruptions is also directly linked to their reproductive success, as females typically mate in the fall and store sperm, becoming pregnant only after emerging in the spring; therefore, minimizing any disturbance to hibernating brown bats is essential for their continued survival and the health of their populations.