Little Brown Myotis

Myotis lucifugus
Chuck Fergus

Pennsylvania’s most common bat, the little brown, is found statewide. Length, including tail, is 3.1 - 3.7 inches; wingspread, 8.6 - 10.5 inches; weight ranges from 0.25 - 0.35 ounces.

Bats are the only mammals that fly. Their wings are thin membranes of skin stretched from fore to hind legs, and from hind legs to tail. The name of their order, Chiroptera, means “hand-winged.” Their long, slender finger bones act as wing struts, stretching the skin taut for flying; closed, they fold the wings alongside the body.  All Pennsylvania bats belong to family Vespertilionidae. They are insect eaters, taking prey on the wing. Often they feed over water, and some species occasionally land and seize prey on the ground. A bat consumes up to 25 percent of its weight at a single feeding. The eyes of bats are relatively small, but their ears are large and well developed. Bats can see quite well, but unique adaptations help them fly and catch prey in total darkness. While in flight, a bat utters a series of high-pitched squeaks (so high, in fact, they are almost always inaudible to humans), which echo off nearby objects — bushes, fences, branches, insects— and bounce back to the bat’s ears. These sound pulses may be only 2.5 milliseconds in duration. Split-second reflexes help the creature change flight direction to dodge obstructions or intercept prey. A bat will use its mouth to scoop a small insect out of the air. A larger insect is often disabled with a quick bite, cradled in a basket formed by the wings and tail, and carried to the ground or to a perch for eating. If an insect takes last second evasive action, the bat may flick out a wing, nab its prey, and draw the insect back to its mouth. Bats have sharp teeth to chew their food into tiny, easily digested pieces.

Most bats mate in late summer or early fall, although some breed in winter. The male’s sperm is stored in the female’s reproductive system until spring, when fertilization occurs. The young, born in summer, are naked, blind and helpless. They are nursed by their mothers and by six weeks of age, most are self-sufficient and nearly adult size. The reproductive potential of bats is low. Most bats, including the smaller species, usually bear a single young per year; the larger species may have up to four. There is only one litter per year. None of Pennsylvania’s bats fly during the brighter hours of daylight, preferring to make their feeding flights in late afternoon, evening and early morning. However, it’s not unusual to see a bat flying during the day. Roost disturbance and heat stress may cause bats to take wing during daylight hours. During the day, they roost—singly, in pairs, in small groups, or in large concentrations, depending on the species. They seek out dark, secluded spots such as caves, hollow trees and rock crevices. They may also congregate in vacant buildings, barns, church steeples and attics; some hide among the leaves of trees. They hang upside down, by their feet. In fall, winter and early spring, insects are not readily available to bats in Pennsylvania and other northern states. Throughout winter, they eat nothing, surviving by slowly burning fat accumulated during summer. A hibernating bat’s body temperature drops close to the air temperature; respiration and heartbeat slow; and certain changes occur in the blood. Most favor cave zones having the lowest stable temperature above freezing. During winter, bats may awaken and move about within a cave to zones of more optimum temperature. In many caves, bats of several species hibernate together. Perhaps because of their nocturnal nature, secretive habits and unique appearance — not to mention superstitions’ — bats have long been misunderstood and sometimes feared, and many misconceptions exist about them.


Pennsylvania Game Commission
Bureau of Wildlife Management
Attn: Mammal Atlas Coordinator
2001 Elmerton Avenue
Harrisburg, PA 17110
Pennsylvania Game Commission
Bureau of Wildlife Management