Western Great Lakes Bird and Bat Observatory

Citizen Science

Little Brown Bats

By Kate Redmond

The little brown bat (Myotis lucifugus) comes by its name honestly. At 2 ½” to 3 ½” long (thumb-sized), with a wingspread of 8 ½” to 10,” and weighing in at less than a half-ounce (females are a bit larger than males), it is Wisconsin’s second-smallest bat. It ranges in color from dark to reddish to golden brown with a lighter-colored belly and dark wings. Bats are “four-legged” mammals, complete with fur, toes on their hind feet, and wings that are made up of membranes stretched over highly modified finger bones.

Although they prefer woodlands near wetlands, where the supply of drinking water and flying insects is good, little brown bats are found in a wide variety of habitats, including deserts.

Bats, famously, hang upside down in roosts, and they have roosts for different seasons and times of day. They sleep for about 19 hours out of 24, hunting during the few hours just after sunset and just before sunrise when their prey is most active. In warm weather, they use both day roosts and night roosts. Day roosts are oriented toward the setting sun, its light signaling the bats to wake up, but night roosts are snug, allowing bats to share body heat. Anyplace a bat can squeeze in will do – and they can squeeze through some amazingly small cracks. They use crevices in rocks and woodpiles, cavities in trees, and space under loose tree bark, but they’re also found in attics, garages, and under eaves and loose siding. Females that are giving birth and caring for young establish a separate nursery roost in a warm place.

Little brown bats hibernate in winter, generally tucking in during October and not reappearing until April, although a young bat may stay aloft for a few extra weeks in fall in order to build up its fat reserves. Their ideal winter roost (hibernaculum) is a cave or mine with a constant temperature and humidity. Although they are not considered migratory, little brown bats will travel to good overwintering sites.

They mate just before they go into hibernation, but fertilization is delayed until spring, and females bear a single pup (very rarely, twins) in June or July. Pups cling to their mother, and when they are small, will ride with her as she hunts. Larger pups are left in the nursery until she returns (she finds her offspring by its unique sound and smell), and mother and pup use sound to communicate. Young little brown bats are full-grown and pretty independent by four to six weeks. The most critical time in a little brown bat’s life is its first winter – most don’t reach their first birthday, but if they do, they commonly live for six to ten years.

Equally famously, bats find their food by a process called echolocation – using their larynx to create pulses of ultrasonic sound that bounce off of solid objects and come back as echoes. This echo allows the bat to compute the distance, size, altitude, direction and mobility of a targeted insect.

Echolocation also lets bats communicate with each other to avoid in-flight collisions and to detect objects before they bump into them. The upper limit of human hearing is about 20 kiloHertz (kHz), but high frequency bat vocalizations range from 14 to more than 100 kHz. Although there are similarities in the patterns of some calls, Wisconsin bats can be identified to species by their sounds using an acoustical monitor called an ANABAT.

And, contrary to the old saying “Blind as a bat,” bats can see in the daytime, and they orient using visual landmarks.

All Wisconsin bats are insect-eaters. Little brown bats prey on many species of flies, midges, mosquitoes, moths, mayflies, and wasps, and on beetles (which they grab using their sharp canines), and they are as adept at picking insects off of surfaces as they are at plucking them out of the air. They may trap their prey with their wings and tail before transferring them to their mouth, or they may grab insects with their teeth. A single little brown bat can eat 600 mosquitoes in an hour; a bat typically eats half its body weight daily, but a nursing female eats more (110% of her body weight), and larger insects.

Little brown bats have long been listed as the most common bat in Wisconsin and North America, but today, they are a Threatened species in this state. They have a variety of natural predators, parasites, and diseases. Hawks and owls catch them in the air, and weasels and other carnivores may eat bats that fall from their perch during hibernation. Roost predators like raccoons and rats pick them off in or near their roost sites, and researcher Dave Redell reported seeing at least 13 house cats lined up at one mine entrance to “catch the evening flight!”

Human-related mortality factors that chip away at bat populations include habitat destruction, roost disturbance, agricultural chemicals that build up in their fat reserves, barbed wire, and wind turbines. The final straw may be white nose fungus (WNF). First noted in New York in 2007 and now spread to 29 states and five Canadian provinces, WNF may have killed 94% of Eastern bat populations in less than a decade! Hibernacula have perfect growing conditions for the fungus, which somehow irritates hibernating bats, causing them to wake up and move around – even flying outside in winter - wasting precious fat reserves. Up to a month’s worth of fat can be metabolized in just minutes.