Bats are nocturnal mammals that use echolocation to navigate and hunt. As temperatures drop, many employ hibernation to survive periods when insects become scarce. This strategy is crucial for their survival in Wisconsin’s cold winters.
Hibernating Bat Species in Wisconsin
Yes, bats hibernate in Wisconsin. Four of the state’s eight bat species are “cave bats” that undergo hibernation during winter. These include the little brown bat, big brown bat, northern long-eared bat, and the tricolored bat. Hibernation is an adaptation allowing them to survive the lack of insect prey and cold Wisconsin winters.
The little brown bat, one of the most common species in Wisconsin, hibernates in caves and abandoned mines, often forming large clusters. Big brown bats are also widespread and can tolerate cooler conditions, sometimes even hibernating in buildings like attics in addition to mines and caves. Northern long-eared bats typically hibernate alone in stable-temperature mines and caves, unlike the clustering behavior of little brown bats. The tricolored bat, Wisconsin’s smallest species, is often among the first to enter hibernacula in the fall and the last to emerge in the spring.
The Mechanics of Bat Hibernation
Bat hibernation is a physiological process where the animal enters a state of deep torpor to conserve energy. During this period, their body temperature significantly lowers, often matching the surrounding environment, and their metabolic rate slows dramatically to less than 5% of their active state. This reduction in physiological functions, including heart rate and breathing frequency, enables bats to survive for months on stored fat reserves. They periodically awaken from this torpor, typically every few weeks, to regulate body temperature, excrete waste, or rehydrate.
In Wisconsin, bats typically enter hibernation in fall (October/November) and remain until late March, April, or May, depending on species and conditions. They seek specific locations called hibernacula, primarily caves and abandoned mines. These underground sites provide stable, cool temperatures and high humidity necessary for successful hibernation, protecting bats from freezing and dehydration. Avoiding disturbance is important, as waking prematurely can deplete their fat reserves, leading to starvation.
Threats to Wisconsin’s Hibernating Bats
A major threat to Wisconsin’s hibernating bat populations is White-Nose Syndrome (WNS), a devastating fungal disease. Caused by the cold-loving fungus Pseudogymnoascus destructans (Pd), WNS infects bats during hibernation, growing on their muzzles, ears, and wings. This infection irritates bats, causing them to awaken more frequently from torpor. Each arousal consumes significant fat reserves, which are necessary to sustain them through winter.
Increased arousal rapidly depletes energy, often resulting in starvation, dehydration, and death before spring. WNS was first identified in North America in 2006 and reached Wisconsin in 2014, spreading rapidly. All four of Wisconsin’s cave-hibernating bat species are susceptible to Pd infection. The disease has caused severe declines, with some species experiencing mortality rates exceeding 90% in infected hibernacula.
Other threats include disturbance or destruction of hibernacula. Human entry into caves and mines during hibernation can cause bats to prematurely awaken, expending vital energy. Habitat loss and fragmentation also indirectly affect the fat reserves bats need for successful hibernation. Conservation efforts focus on protecting these sensitive underground environments and researching ways to combat WNS.