Eptesicus fuscus
(Big Brown Bat)

Order: Chiroptera
Order Description: Bats
Family Description:

The dorsal Click word for definitionfur, often oily, is a dark brown to cinnamon brown. The typically bare ears are broad with rounded margins that, if extended forward, do not reach significantly beyond the tip of the broad nose. The tragus Click word for definition, which is slightly curved forward, is less than one-half the length of the ear. The calcar Click word for definitionis keeled. The tail, enclosed in the tail membrane, usually extends several millimeters beyond the posterior margin of the membrane. Young of the year are usually darker and have duller fur than adults.

Broadly distributed from British Columbia east across southern Canada to Maine southward along the Atlantic coast to Florida and west to Baja California. This species extends south through Central America into Columbia. In Idaho, it has been collected in 15 counties but likely occurs everywhere in the state.

A common species in urban areas, the big brown bat often roosts in homes, especially behind shutters, in cool attics, and under eaves. In other buildings it may occur between enclosed walls. In forested areas, hollow spaces in snags or living trees are occupied. It is a common species near the entrances of caves and mines but usually does not cluster with other individuals in these colder locations. It appears to hibernate Click word for definitionfor a shorter period of time than members of the genus Myotis.

A predator on beetles, this species is important as a result of its consumption of cucumber beetles which are agricultural pests. A variety of other insects are consumed, especially along water courses and in forests. Food items may include up to ten orders of insects.

Dependent upon flying insects; in many areas, small beetles are most common prey. Species' large size, powerful jaw muscles, and robust teeth allow predation on larger insects with tough exoskeletons Click word for definition(i.e., beetles).

Forages over land or water, around clearings and lake edges, around lights in rural areas, or around trees or forest canopies. Common in towns and cities. Initial foraging period occurs within 5 hr after sunset, although most activity occurs within second hr after sunset (may subsequently retire to night roost). Flies less than 2 hr each night. Distance from day roost to foraging area averages about 1-2 km. Caves, mines, and human-built structures are used for hibernation Click word for definition. In temperate areas, many individuals do not appear at hibernacula Click word for definitionuntil November. Winter colonies rarely number more than a few hundred. Less gregarious in winter; usually roosts alone in crevice, but sometimes 2-20 may roost together. Summer roosts are generally in buildings, but hollow trees, rock crevices, tunnels, and cliff swallow nests may also be used; prefers roosting sites that do not get hot. Males are often solitary in summer, but may roost with females or in all-male colonies. When young are flying, males may join nursery groups to form large late-summer colonies. Individuals are capable of living at least 20 yr, but few attain old age.

Big-brown bats may select temporary roosting sites during bouts of foraging on warm, dry nights, although foraging usually occurs near their permanent roosts Click word for definition. Seasonal movements to hibernal areas are largely restricted to nearby sites. Females demonstrate strong fidelity Click word for definitionto nursery sites which are often used by a larger number of individuals as contrasted to hibernal sites. Bats taken from roosting areas and released elsewhere can return to the area of collection, in one study, return occurred over a distance of 450 miles.

Reproduction begins in the fall upon break-up of maternity roosts and may extend over winter and spring. Maternity colonies appear to be located in warm conditions and may move to alternate roosts by females that carrying their young in flight. Implantation Click word for definitionoccurs from April to May and birth about 60 days later. In the west, a single pup is born in late May or early June.

Breeds in fall and intermittently throughout winter. In temperate regions, ovulation Click word for definitionand fertilization Click word for definitionare delayed until after hibernation Click word for definition. Gestation Click word for definitionlasts 2 mo. Female produces 1 young in western range, 2 in east, in May-July. Lactation lasts 32-40 days; young fly at 4-5 wk. Males are usually sexually mature in first fall; not all females reproduce at end of first year. Nursery colony rarely numbers more than a few hundred.

Status: Unprotected nongame species
Global Rank: G5
State Rank: S4

Important State References:
Keller, B.L. 1987. Analysis of the bat species present in Idaho, with special attention to the spotted bat, Euderma maculatum. Dept. Biol. Sciences, Idaho St. Univ., Pocatello. 25pp.

Information written by Barry Keller,© 2000
Map image provided by
Stephen Burton,© 2000
Design by Ean Harker©1999, 2000.