Big brown bat (Eptesicus fuscus)

Synonyms: Vespertilio fuscus
GenusEptesicus (1)
SizeTotal length: 8.7 - 13.8 cm (2)
Tail length: 3.4 - 5.2 cm (3) (4)
Wingspan: 32.5 - 35 cm (4)
Weight11 - 25 g (2) (4)
Top facts

The big brown bat is classified as Least Concern (LC) on the IUCN Red List (1).

One of the most widespread mammals in North America (5), the big brown bat (Eptesicus fuscus) is named for its large size and its fairly uniform brown fur (3) (4). This species has a robust body and large, broad head, with relatively large eyes and short, thick, rounded ears (2) (3), which are furred only at the base (2) (4). The big brown bat’s wings are short and broad, and the tip of this species’ tail extends to just beyond the edge of the tail membrane (2).

The big brown bat’s fur is quite long and soft, and varies from dark brown to pinkish-tan or cinnamon-brown on the upperparts (2) (3), with each individual hair having a dark base (3) (4). The fur on the underparts is generally paler (2) (3) (4), while the face, ears, wings and tail membrane are blackish (2) (3).

Male and female big brown bats are similar in appearance, but females are slightly larger than males (2) (3). Young big brown bats are darker and duller in colour than the adults (2) (3), and have slightly shorter fur (2).

The big brown bat varies somewhat in appearance across its large range, and around 11 subspecies are currently recognised (2) (6).

The big brown bat is widespread across North and Central America, and its range also extends into north-western South America and the Caribbean (1) (2) (4) (6). This common species is found across southern Canada, through most of the United States and Mexico, south to Colombia, Venezuela and Brazil. It has also been recorded in the Bahamas, the Greater Antilles, and Dominica and Barbados in the Lesser Antilles (1) (2) (6).

The big brown bat is found in a wide variety of habitats, although it is generally more abundant in areas dominated by deciduous forest (6) (7). This species has adapted well to the presence of humans, now commonly occurring in towns and even large cities (1) (4) (6). It also often forages over cultivated fields (4).

In winter, the big brown bat hibernates in a range of different structures, both natural and man-made. A hardy species, it prefers cool temperatures and can tolerate conditions other bats cannot (1). The big brown bat now mostly hibernates in houses, as well as barns, churches, storm sewers and mines, but some individuals still use tree cavities, rock crevices and caves (1) (3) (4).

The big brown bat feeds mainly on beetles (1) (2) (3) (6) (7), using its powerful jaws and strong teeth to bite through their hard wing cases (1) (6). Other prey items include flies, flying ants, lacewings, caddisflies and moths, as well as other flying insects (1) (3) (4) (8). The big brown bat is considered to be an important predator of insect pests such as the spotted cucumber beetle (Diabrotica undecimpunctata), scarab beetles, stinkbugs and leafhoppers (8).

Like most other bats, the big brown bat forages at night, usually leaving its roost within the second hour after sunset to feed (2) (6). This species will forage in a variety of habitats, both over water and over land, and uses forests and clearings as well as rural and urban areas (2) (6) (7).  The big brown bat has been shown to be able to use the Earth’s magnetic field to help it find its way back to its daytime roosting site (9).

Like many other bats, the big brown bat also navigates and hunts using echolocation (2) (3). When leaving its roost, it may also listen for the sound of chorusing frogs and insects to help it locate concentrations of prey (10). In addition to producing ultrasonic calls for echolocation, the big brown bat uses a variety of ‘social’ calls to communicate with other individuals (11).

Over most of its range, the big brown bat hibernates during winter, usually alone or in a small group. This hardy bat is often one of the last species to be seen flying about in autumn, and some individuals do not begin to hibernate until November (2) (3) (4) (6). Individuals often become active for brief periods during the winter months, sometimes even changing hibernation site (1) (2) (3) (4), and big brown bats in Cuba may not hibernate at all, instead merely entering a state of torpor on cool winter nights (6).

Female big brown bats emerge from hibernation in spring, usually around March or April (3) (4), and form ‘maternity’ colonies in which to give birth and rear their young (1) (2) (3) (4). These colonies vary in size from around 5 to 700 individuals (1) (2) and may be located in buildings, hollow trees or caves (2) (3) (4) (6) (7). During this time, the male big brown bats roost alone or in small all-male groups, only rejoining the females later in the summer (1) (2) (3) (6).

The big brown bat mates during the autumn, or sometimes during the winter and early spring, but fertilisation does not occur until the spring. The female big brown bat usually gives birth to one or two young in May to July, after a gestation period of about two months (2) (3) (4) (6). The young bats are born naked and blind (2) (3), but their eyes open on about the second day (3). The female leaves the young in the colony while she feeds, and is able to recognise her own offspring when she returns, even retrieving them if they have fallen to the ground (2) (4).

Young big brown bats are able to fly at about three to five weeks old (2) (3) (6) and reach adult size after about two and a half months (3). The male big brown bat reaches sexual maturity in its first autumn, but only some females reproduce at the end of their first year (2) (3) (6). The big brown bat may potentially live for 19 or 20 years in the wild (1) (2) (3) (4) (6), but many die in their first winter (1). A common cause of mortality in the big brown bat is failure to store enough fat for hibernation (1), although this species is also occasionally taken by predators such as owls, cats and snakes (2) (4).

The big brown bat is a fairly common bat species with a widespread distribution, and is not currently considered to be at risk of extinction (1). This species adapts well to the presence of humans, and may even have increased in numbers and range due to its ability to inhabit buildings and other man-made structures (1) (4) (6).

However, the big brown bat’s habit of roosting in buildings can sometimes bring it into conflict with humans, and it is often the subject of control measures, such as the sealing of roost entrances (2) (6). It is also vulnerable to the accumulation of toxins in its body, such as pesticides, which could potentially cause health problems or even death (6).

The big brown bat is one of several bat species in North America affected by white-nose syndrome, a fungal disease which has already killed millions of bats in the eastern United States. However, the fact that the big brown bat commonly hibernates in relatively warm buildings may mean it is less vulnerable than bats which predominantly hibernate in the cool caves and mines where the fungus thrives (6).

No specific conservation measures are currently known to be in place for the big brown bat, but this widespread species occurs in a number of protected areas across its range (1). The big brown bat is considered to be beneficial to humans as it eats significant numbers of agricultural pests (4) (7) (8), and some scientists have argued that it therefore deserves legal protection (8).

Suggestions for the conservation of the big brown bat include research into the effects of pesticides and of disturbance at its roosts (5) (6). Although this species is one of North America’s most common and widespread bats, studies into its populations and life-history requirements would be a useful tool in assessing the needs of other bat species which are rare and endangered (5).

Find out more about the big brown bat and other bat species at:

More information on bat conservation:

This information is awaiting authentication by a species expert, and will be updated as soon as possible. If you are able to help please contact:

  1. IUCN Red List (August, 2013)
  2. Kurta, A. and Baker, R.H. (1990) Eptesicus fuscus. Mammalian Species, 356: 1-10. Available at:
  3. Schwartz, C.W. and Schwartz, E.R. (2001) The Wild Mammals of Missouri. Second Revised Edition. University of Missouri Press, Columbia, Missouri.
  4. Whitaker Jr, J.O. and Hamilton Jr, W.J. (1998) Mammals of the Eastern United States. Cornell University Press, Ithaca.
  5. Agosta, S.J. (2002) Habitat use, diet and roost selection by the big brown bat (Eptesicus fuscus) in North America: a case for conserving an abundant species. Mammal Review, 32(2): 179-198.
  6. NatureServe Explorer - Eptesicus fuscus (August, 2013)
  7. Bat Conservation International - Eptesicus fuscus (August, 2013)
  8. Whitaker Jr, J.O. (1995) Food of the big brown bat Eptesicus fuscus from maternity colonies in Indiana and Illinois. American Midland Naturalist, 134(2): 346-360.
  9. Holland, R.A., Thorup, K., Vonhof, M.J., Cochran, W.W. and Wikelski, M. (2006) Navigation: bat orientation using Earth’s magnetic field. Nature, 444: 702.
  10. Buchler, E.R. and Childs, S.B. (1981) Orientation to distant sounds by foraging big brown bats (Eptesicus fuscus). Animal Behaviour, 29(2): 428-432.
  11. Wright, G.S., Chiu, C., Xian, W., Wilkinson, G.S. and Moss, C.F. (2013) Social calls of flying big brown bats (Eptesicus fuscus). Frontiers in Integrative Physiology, 4: 214.