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big brown bat

Eptesicus fuscus

What do they look like?

Big brown bats are 110-130 mm in length and have a wingspan of 330mm (13 inches). Females tend to be slightly larger than males. They have sharp and heavy teeth that are able to bite down powerfully. This enables them to eat large, hard-bodied insects such as beetles. Color ranges from light brown to dark brown and can have reddish hues. The underside is lighter in color. The face, ears, wings and tail do not have fur and are all black.

  • Sexual Dimorphism
  • female larger
  • Average mass
    23 g
    0.81 oz
  • Range length
    110.0 to 130.0 mm
    4.33 to 5.12 in
  • Average wingspan
    330.0 mm
    12.99 in
  • Average basal metabolic rate
    0.113 W

Where do they live?

Big brown bats are native to the Neotropical and Nearctic regions. They live throughout North America, ranging from southern Canada, throughout the United states, south through Mexico, and along the Andean mountain chain into Colombia. They have colonized some islands of the West Indies including Cuba, Puerto Rica, Jamaica, and Hispaniola.

What kind of habitat do they need?

Big brown bats are the most common bat in cities, towns, and rural areas. These bats can live in many human dwellings, including homes, barns, churches, athletic stadiums, and storm sewers. They also roost in tree hollows and caves. Big brown bats forage in a variety of habitats including rivers and streams, forested areas, over open fields, and along city streets. Big brown bats are highly adaptable.

  • These animals are found in the following types of habitat
  • temperate

How long do they live?

Big brown bats can survive up to 19 years in the wild and males tend to live longer than females. Most big brown bats die in their first winter. If they do not store enough fat to make it through their entire hibernation period then they die in their winter roost.

How do they behave?

Big brown bats all roost together except when mothers are taking care of their young. During these times females will roost together in large groups and males will roost by themselves. Mothers can recognize their own young and will lick the baby before nursing it. Bats also hibernate in the winter. These bats utilize "echolocation" to avoid obstacles and to capture flying insect prey. They do this by making calls through their open mouths. The length of each call and time between calls varies depending on what activity the bats are doing: finding prey, attacking prey, or just flying. They use these calls by listening to the echoes of their calls and determining their position in relation to other objects. Typically, big brown bats will increase the rate of echolocation calls as they close in on prey. The calls end in what is called a "feeding buzz", a high pulse sound that signals that they are about to capture their prey.

How do they communicate with each other?

Baby bats who are separated from their mothers, either by falling from the roost, or by otherwise appearing lost, will squeak continuously. The squeaking can be heard from a distance of more than 30 feet. This communication is important for the baby's survival as it may help the mother locate and return them to a safer place. Bats also make a number of audible sounds, they squeak and hiss at each other in the roost

What do they eat?

Big brown bats are insectivores, eating mostly beetles. They will also eat other flying insects including moths, flies, wasps, and others. They use their strong teeth to chew though the hard outer shell of the insects. Big brown bats only eat in warm months when insects are alive. Therefore, they eat large amounts of insects in the summer and fall to prepare for hibernation. They will also not eat in heavy rain or if the temperature gets too low. They usually begin looking for food right after sunset, eat until they are full, then hang upside down to digest their meal. They return to the day roost during sunlight hours.

What eats them and how do they avoid being eaten?

Big brown bats choose secluded roosts to protect themselves from many predators. Young are often taken from maternity roosts by snakes, raccoons, and cats if they fall. Flying bats are sometimes captured by owls and falcons as they leave their roosts.

Do they cause problems?

Many people do not like sharing their homes with bats. The only way to keep them from entering homes or other buildings is to block the holes bats use as entryways. People also have concerns regarding bats and the virus that causes rabies. It is important to tell people not to handle any obviously sick wild animal but the risk of contracting rabies from bats is exaggerated.

How do they interact with us?

Big brown bats consume many insect pests, including common threats to crop plants. They eat the corn root worm which may be the single worst agricultural pest in the United States

  • Ways that people benefit from these animals:
  • controls pest population

Are they endangered?

Humans usually do not like bats living in their homes. Bats can be kept from re-entering a home if the holes used as entrances are blocked. This is best done at night once the bats have left to hunt for food. It should not be done during June or July when there may be baby bats remaining in the home. Since big brown bats are good at consuming agricultural pests, it has been suggested that farmers should actually encourage the bats to live in their barns. A further suggestion would be to design bridges to encourage bats to use them as roosts.

Big brown bats are fairly common and are not of any special conservation concern.

Temperate North American bats are now threatened by a fungal disease called “white-nose syndrome.” This disease has devastated eastern North American bat populations at hibernation sites since 2007. The fungus, Geomyces destructans, grows best in cold, humid conditions that are typical of many bat hibernacula. The fungus grows on, and in some cases invades, the bodies of hibernating bats and seems to result in disturbance from hibernation, causing a debilitating loss of important metabolic resources and mass deaths. Mortality rates at some hibernation sites have been as high as 90%. (Cryan, 2010; National Park Service, Wildlife Health Center, 2010)

Some more information...


Michael Mulheisen (author), University of Michigan-Ann Arbor, Kathleen Berry (author), University of Michigan-Ann Arbor, Phil Myers (editor), Museum of Zoology, University of Michigan-Ann Arbor.


Arlingham, J. 1996. Bats, Biology and Behaviour. Oxford University Press.

Baker, R. 1983. Michigan Mammals. Lansing, Michigan, USA: Michigan State University Press.

Buchler, E., S. Childs. 1981. Orientation to Distant Sounds by Foraging Big Brown Bats (Eptesicus Fuscus). Animal Behaviour, 29,2: 428-432.

Cryan, P. 2010. "White-nose syndrome threatens the survival of hibernating bats in North America" (On-line). U.S. Geological Survey, Fort Collins Science Center. Accessed September 16, 2010 at

Davis, W., R. Barbour, M. Hassell. February 1968. Colonial Behavior of Eptesicus Fuscus. Journal of Mammalogy, Vol 4, No. 1: pp. 44-50.

Fenton, B. 1985. Communication in the Chiroptera. Bloomington: Indiana University Press.

Fenton, B. 1983. Just Bats. Toronto: University of Toronto Press.

Goehring, H. February 1972. Twenty-Year Study of Eptesicus Fuscus in Minnesota. Journal of Mammalogy, Vol. 53, No.1: pp. 201-207.

Hamilton, I., R. Barclay. Aug 1998. Diets of Juvenile, Yearling, and Adult Big Brown Bats (Eptesicus fuscus) in Southeastern Alberta. Journal of Mammalogy, 79:(3): 764-771.

Knowles, B. Apr-Jun 1992. Bat Hibernacula On Lake Superiors North Shore, Minnesota. Canadian Field Naturalist, 106:(2): 252-254.

Kurta, A. 1995. Mammals of the Great Lakes Region. Ann Arbor: The University of Michigan Press.

Kurta, A., R. Baker. 26 April 1990. Eptesicus fuscus. Mammalian Species: No. 356, pp.1-10.

National Park Service, Wildlife Health Center, 2010. "White-nose syndrome" (On-line). National Park Service, Wildlife Health. Accessed September 16, 2010 at

Simmons, J., M. Ferragamo, T. Haresign, J. Fritz. Aug 1996. Representation of Perceptual Dimensions of Insect Prey During Terminal Pursuit by Echolocating Bats. Biological Bulletin, 191:(1): 109-121.

Whitaker, J. Oct 1995. Food of the Big Brown Bat Eptesicus-Fuscus From Maternity Colonies In Indiana And Illinois. American Midland Naturalist, 134:(2): 346-360.

Whitaker, J., S. Gummer. May 1992. Hibernation of the Big Brown Bat, Eptesicus-Fuscus, in Buildings. Journal of Mammalogy, 73:(2): 312-316.

University of Michigan Museum of ZoologyNational Science Foundation

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Mulheisen, M. and K. Berry 2000. "Eptesicus fuscus" (On-line), Animal Diversity Web. Accessed April 12, 2024 at

BioKIDS is sponsored in part by the Interagency Education Research Initiative. It is a partnership of the University of Michigan School of Education, University of Michigan Museum of Zoology, and the Detroit Public Schools. This material is based upon work supported by the National Science Foundation under Grant DRL-0628151.
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