Illinois Natural History Survey - University of Illinois

Species Spotlight: Little Brown Bat

The little brown bat (Myotis lucifugus) is one of the most common of the 12 bat species that occur in Illinois. It lives throughout the state during the summer and is the most abundant bat found hibernating in Illinois caves and mines during the winter. Little brown bats have glossy brown fur and wingspans of 9 to 11 inches. They may resemble mice with wings, but their life history is very different from that of rodents. Little brown bats (and bats in general) have a much lower reproductive output than most rodents. They also have an unusually long lifespan for a small mammal and may live 20 to 30 years. The longevity record for a little brown bat is 34 years.

Like all Illinois bats, little brown bats are strictly insectivorous and consume large quantities of night- flying insects. An adult male eats half its body weight in insects each night; a lactating female will eat more than her body weight nightly. Little brown bats often forage above lakes, ponds, and streams, sometimes zipping back and forth repeatedly through swarms of aquatic insects. Their prey include midges, moths, caddisflies, mayflies, small beetles, and mosquitoes which they find by echolocation--the perception of objects using the reflection of sound waves emitted by the bats.

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The little brown bat (Myotis lucifugus).

During the summer female little brown bats form maternity colonies. A colony typically consists of a few hundred females, but may include several thousand individuals. Maternity colonies typically roost in the attics of buildings or the rafters of barns. They may also take up residence in bat houses. Little brown bat females choose roost sites where the temperature can exceed 100deg.F; such high temperatures promote the pre- and postnatal development of their young. Each female gives birth to a single pup, usually during June. During birth, the female reverses her normal roosting position and hangs by her thumbs in order to catch the pup in her tail membrane. At birth the pups are relatively large, weighing 25-30% as much as their mothers. When the females leave the roost at night to forage, they leave their pups in the roost. Upon her return each female is able to recognize her offspring by its unique vocalizations and odor. Pups develop quickly and are able to fly at three weeks of age. While females gather in maternity colonies, male little brown bats roost separately, either singly or in small bachelor colonies. Males roost in a variety of cooler sites including buildings, caves, mines, bridges, and trees.

During late summer and autumn, little brown bats accumulate large amounts of body fat, which will be their winter energy supply, and migrate up to 200 miles to a suitable cave or abandoned mine for hibernation. Mating occurs before the bats enter hibernation; however, because of a rare reproductive process called "delayed fertilization," females do not ovulate and become pregnant until the following spring. Little brown bats choose hibernation sites with temperatures several degrees above freezing, minimal air flow, and very high humidity. There they hang singly or in small clusters. During hibernation a bat's body temperature drops to about 40deg.F, drastically slowing all of its body functions. For example, the heart rate of a little brown bat in flight is 1,000 beats per minute, but the rate of a hibernating bat is only 5 beats per minute. Bats remain in hibernation from November until March or April, but occasionally arouse to drink, urinate, or change locations. In the spring they return to their summer quarters, completing the annual cycle.

Joyce Hofmann, Center for Biodiversity



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