A recent citizen-science project through the North Lakeland Discovery Center and the Wisconsin Bat Program monitored the bat activity in my area, finding primarily Big Brown Bats.
Big Brown Bats are the largest species of bat in Wisconsin. They weigh between 1/2 – 1 ounce, but have a wingspan of a foot or more. Females typically grow to be larger than males. The bat’s body is covered in brown fur with hairless ears, nose, and wings.
Big Brown Bats are mammals that hibernate throughout the winter. Hibernation occurs from about November until April as a colony of individuals within a hibernaculum of a cave, abandoned mine, or manmade structure for protection from the winter weather. Emerging from hibernation, the females give birth to one or two pups. In a further adaptation to survive hibernation, females hold the sperm within their uterus until the spring, to hold off fertilization and pregnancy until after hibernation. Outside of the hibernaculum, bats sleep and raise their young from roosts. In forests these roosts are frequently tree cavities or under bark. These bats will frequently return to the same roost year after year. Through the summer, females form maternity colonies while the males are solitary. In the fall, all of the bats return to the hibernaculum for mating prior to hibernation.
While Big Brown Bats are widespread throughout North America, this member of the “vesper bat” family is Threatened in Wisconsin, and state populations are in decline. A primary source of decline is White Nose Syndrome (WNS), a fungal infection that can spread among bats in the hibernaculum. WNS was first identified in New York State in 2007 and first found in Wisconsin in 2014. The underlying fungus, Pseudogymnoascus destructans, thrives in the cool, damp conditions of a cave. Once present, the fungus is highly fatal to bats with a 95% mortality rate. Monitoring of disease spread and special precautions, like decontamination, must be taken by individuals entering hibernaculums in order to prevent and slow the spread of this disease.
Big Brown Bats are insectivores, and primarily feed upon beetles. A look inside one of their mouths reveals a set of ferocious, albeit tiny, teeth. These help them to break through the hard beetle exoskeletons. Despite becoming a beetle specialist as they age, all of their prey is caught mid-air as the beetles are flying. The bats use echolocation to track down their prey.
The bats hunt at night, usually emerging from their roosts soon after sunset. Bats primarily do this to avoid other predators, as bats will be preyed upon by hawks, grackles, bullfrogs, raccoons, weasels, feral cats, and owls. The bats use echolocation and the echolocation chirps are distinct across species. Researchers (like the ones in the citizen science project) often use echolocation monitoring to locate, identify, and count bats.
While not often seen, bats, including the Big Brown Bat, are important members of our ecosystems. So be on the lookout for bats in the evening and if you do encounter bats in a roost, leave them be.
Photo © Dave Redell, Wisconsin DNR