The Little Brown Bat (Myotis lucifugus) is a small, once abundant species across North America. Weighing 5 to 14 grams, this bat has glossy brown fur and a wingspan up to 27 centimeters. Its survival is closely tied to the specific environments it uses throughout the year. The bat’s habitat requirements are precise and seasonal, necessitating different types of shelter for its annual cycle of feeding, reproduction, and winter dormancy.
Broad Geographic Range
The Little Brown Bat historically had one of the largest distributions of any bat species on the continent. Its range spans from the boreal forests of Alaska and northern Canada, south across the contiguous United States, reaching into the higher elevation forested regions of central Mexico. The species is largely absent from the extreme southeastern United States, including much of Florida and the southern Great Plains. Within this geographical area, the presence of these bats is dictated by the availability of surface water, such as streams and lakes, which provide the abundant flying insects necessary for nightly foraging.
Summer Roosts and Maternity Colonies
During the warmer months, typically May through September, Little Brown Bats seek sheltered day roosts to rest between nocturnal foraging bouts. These summer sites must offer protection and fall into two categories: natural and human-made. Natural roosts include cavities within trees, spaces beneath exfoliating bark, and rock crevices. Anthropogenic structures are also commonly used, particularly in northern parts of the range, as they provide superior thermal conditions. These structures include:
- Attics
- Barns
- Sheds
- Spaces within wall voids
Maternity Colonies
A subset of these summer locations become specialized maternity colonies, occupied almost exclusively by reproductive females and their young. These nursery roosts require high, stable temperatures, often reaching 23 to 34 degrees Celsius, to promote the rapid growth of the pups. Females cluster tightly together in these warm microclimates, frequently found under the eaves or in the hottest parts of human-made buildings like attics, where heat accumulates.
Winter Hibernation Locations
The Little Brown Bat must find a specific shelter, known as a hibernaculum, for its winter dormancy period, which can last six to eight months. The preferred structures are deep limestone caves and abandoned mines that extend far enough underground to maintain constant conditions. A suitable hibernaculum must be cool but remain consistently above freezing, generally ranging between 2 and 12 degrees Celsius. An extremely high relative humidity, ideally over 90%, is also necessary to prevent dehydration during their prolonged state of torpor. Any significant fluctuation in temperature or humidity can cause the bats to prematurely wake up, rapidly burning the fat reserves needed to survive until spring.
In the late summer and early fall, both male and female bats congregate near the entrances of these potential winter sites in a behavior known as swarming. This period is when mating occurs and is thought to be a time when younger bats learn the location of suitable hibernacula. Following swarming, individuals enter the deep recesses of the cave or mine to cluster densely together for the winter.
Conservation Status and Habitat Threats
The viability of the Little Brown Bat’s habitat has been severely impacted by the emergence of a specific fungal pathogen. This species was once the most abundant bat in many regions, but its population has declined dramatically, especially in the eastern half of North America. The primary cause of this collapse is White-Nose Syndrome (WNS), a disease caused by the cold-loving fungus Pseudogymnoascus destructans.
The fungus thrives in the stable, cold, and humid conditions of the winter hibernacula. WNS grows on the exposed skin of the bats, causing irritation that forces them to wake up from torpor more often than usual. Each arousal burns stored fat reserves, resulting in starvation and death before the end of the winter season. The disease has caused a functional loss of habitat in many areas, even though the physical caves and mines remain. In affected regions, population declines have exceeded 90 percent in some hibernacula, compromising the ability of these sites to support large, healthy populations.