To endure colder months when insect prey is scarce, many bat species in Pennsylvania employ a survival strategy known as hibernation. This deep, sleep-like state allows them to conserve energy during periods of limited food availability.
The Timing of Bat Hibernation in Pennsylvania
Bats in Pennsylvania typically begin hibernation in late fall, around October or November. Species like the Indiana bat may start entering hibernacula as early as mid-September, with most settled by early November. Big brown bats are often among the last to enter, sometimes as late as December.
Hibernation extends throughout the winter months, when insects, their primary food source, are unavailable. This allows bats to survive until warmer temperatures return and insect populations rebound. Their emergence usually occurs in early spring, often in March or April.
The precise timing of entry and emergence can vary slightly depending on the specific bat species and environmental conditions. For instance, little brown bats typically emerge in April or May, and tri-colored bats in April or early May. Temperature fluctuations and food availability influence these seasonal patterns.
Key Hibernation Sites for Bats in Pennsylvania
Bats in Pennsylvania seek specific locations for hibernation, known as hibernacula, which provide stable environmental conditions. Natural caves are frequently used, offering consistent temperatures and high humidity. These underground environments shield bats from harsh winter weather.
Abandoned mines also serve as significant hibernation sites. These man-made structures often replicate the stable, dark, and undisturbed conditions found in natural caves. Some abandoned transportation tunnels also provide suitable hibernation habitat.
Ideal hibernacula maintain temperatures ranging from 2 to 10 degrees Celsius, or just above freezing. These sites help bats conserve energy throughout their long winter sleep. In a survey, over 91% of wintering bats were found utilizing abandoned man-made structures.
The Biological Process of Bat Hibernation
Hibernation is a profound physiological adjustment bats undergo to survive winter. During this state, known as torpor, a bat’s body temperature drops significantly, often nearing the ambient temperature. This reduction is accompanied by a dramatic decrease in heart rate and metabolic activity.
A bat’s metabolic rate can fall to less than five percent of its active rate during torpor, and respiration slows considerably. This deep, sleep-like condition allows the bat to conserve energy by slowly burning the fat reserves accumulated during the warmer months. These fat stores are needed for sustaining the bat through winter without needing to forage.
Bats periodically arouse from torpor throughout hibernation. These awakenings are energetically expensive, as the bat must raise its body temperature to normal levels to perform essential functions like drinking and eliminating waste. Sufficient fat reserves are needed not only for torpor but also for these arousals and for the initial period after spring emergence.
Protecting Hibernating Bats in Pennsylvania
Hibernating bats in Pennsylvania face a significant threat from White-Nose Syndrome (WNS), a disease caused by a cold-loving fungus. This fungus, Pseudogymnoascus destructans, grows on bats’ muzzles and and wings, causing skin lesions. The infection frequently causes bats to awaken prematurely from hibernation, forcing them to expend valuable fat reserves.
The depletion of these energy stores leads to dehydration, starvation, and ultimately, death for many affected bats. Since its confirmation in Pennsylvania in 2008-2009, WNS has caused severe population declines, with some species experiencing losses of 90 to 99 percent.
Disturbing hibernating bats can have serious consequences, as each unplanned awakening uses up valuable energy reserves. Wildlife agencies recommend avoiding entry into bat hibernacula during the hibernation season, typically from October 1 to April 30. Conservation efforts include installing bat-friendly gates at hibernacula entrances to prevent human disturbance while allowing bat access.