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Smithsonian National Museum of Natural History, North American Mammals

  Chiroptera · Vespertilionidae · Eptesicus fuscus

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Eptesicus fuscus

Big Brown Bat

Order: Chiroptera
Family: Vespertilionidae

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Credit: New Mexico Bat Call Library, W. L. Gannon
Image of Eptesicus fuscus
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Conservation Status: Least Concern.

Big brown bats make their homes in rural areas, towns, and cities, sometimes choosing barns, houses, or other buildings as roosts. Males usually live alone; females gather in maternity colonies in the spring and summer to give birth and raise their young. A maternity colony may include 20 - 75 adults and their offspring. Females in the eastern United States usually give birth to twins; those in the West usually have a single pup each year. Females may return to the same colony year after year. On warm, dry evenings, the bats leave the roost shortly after sunset to forage for insectsespecially flying beetleswhich they catch and eat in the air. When the weather is cold or wet, they may stay in the roost, dropping their body temperature and living on stored fat. In the winter, they hibernate. Many migrate a short distance (less than 80 km) to find mines or caves for hibernation, but some spend the winter in attics or walls where the temperature is cool but stays above freezing.

Also known as:

Brown Bat

Sexual Dimorphism:

Females are larger than males.


Average: 112 mm
Range: 87-138 mm


Average: 16 g
Range: 11-23 g


Palisot de Beauvois, A.M.F.J., 1796.  Catalogue raisonne du muséum, de Mr. C.W. Peale, p. 18.  Parent, Philadelphia, 42 pp.


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Distribution of Eptesicus fuscus

Image of Eptesicus fuscus
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Skull of Eptesicus fuscus
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