Big Brown Bat

Eptesicus fuscus
Chuck Fergus

Second in size to the hoary bat, the big brown is 4.1 - 4.8 inches long; wingspread, 12.1 - 12.9 inches; weight, 0.42 - 0.56 ounces. The fur is dark brown, and the face, ears and flight membranes are blackish. This common bat ranges throughout the state in diverse habitats: attics, belfries, barns, hollow trees, behind doors and shutters, in city and country. Big brown bats fly at dusk, and generally use the same feeding grounds each night. They fly in a nearly straight course 20 - 30 feet in the air, often emitting an audible chatter. Major foods include beetles and true bugs (junebugs, stinkbugs and leafhoppers) many of which are major agricultural pests. A colony of 150 big brown bats can eat enough cucumber beetles during the summer to protect farmers from 18 million rootworm larvae.

Among the last bats to enter hibernation, big brown bats seek out caves, buildings, mines and storm sewers in October, November or December. They hang close to the mouths of caves, and emerge in March and April. Females bear young in June, usually two per litter. As young mature and leave the nursery colony, adult males enter and take up residence. Big brown bats have lived up to 19 years in the wild. 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. Pennsylvania bats range in size from the hoary bat (length, 5.1 - 5.9 inches; wingspread, 14.6 - 16.4 inches; weight, 0.88 - 1.58 ounces) to the pygmy bat, or pipistrelle (length, 2.9 - 3.5 inches; wingspread, 8.1 - 10.1 inches; weight, 0.14 - 0.25 ounces).

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 our 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.


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