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Eptesicus fuscus - Big Brown Bat


Big brown bats range in color from pinkish tans to dark chocolate brown with a total body length of 110-130 mm. This species also exhibits sexual dimorphism, the females being slightly larger than the males (Kurta and Baker 1990).  Parts not covered with their “oily” fur (face, ears, and wings) are usually black in color.  They have a wing span of about 13 inches (Baker 1983) with a comparatively large skull for their size, which contains 32 teeth. Their forearms are 41-50 mm in length. They have large bright eyes and their ears are rounded with a round tipped tragus. In rare cases Big brown bats have been found with white patched on their wings and a few albinos have been found.


Big brown bats are found throughout the lower 48 states and are found through the entire state of Wisconsin.  They hibernate in caves, under eaves, or in abandoned mines. In Wisconsin they have been found hibernating in unheated attics, basements and wall spaces (Whitaker and Gummer 1999). One popular tree species used by E. fuscus is trembling aspen where the bats will reuse the same cavities year after year. E. fuscus prefers the aspen to their usual roost in conifers because of the loss of bark on conifers.  Less bark means more exposure to the elements (Willis et al. 2003).

The fossils of Eptesicus fuscus are found through out the Pleistocene era, and are the most widespread bat in America during this era. There are more than 30 sites were fossils of this bat have been found in the US and fossils have also been found in Mexico, Puerto Rico, and the Bahamas (Kurta and Baker 1990).


Big brown bats mate anywhere from November to March, but the female will hold the sperm in her reproductive tract until the beginning of April and that is when the fertilization takes place.  The female will give birth from 1-2 young after about 60 days.  The young are blind and naked and only weighs about 3.3 grams. The young will grow quite quickly, with their eyes opening after about 7 days. The young are nursed from the female’s two mammary glands and the young are weaned in late June to early July.  The females will form maternity colonies that range in size from 5 to 700 females.  Females as a group will pick maternity roosts for the group to use. Lausen and Barclay did a study to find if the roosts size, depth, and size of opening changed from pregnancy to post-lactation. Their study found that female E. fuscus picked roosts that were deeper during pregnancy and during lactation preferred roosts with larger openings.   The males will roost alone or in small groups and do not participate in the rearing of the young. Sometimes males will roost with the maternity group for a night or two but not very often (Lausen and Barclay 2002).

One interesting fact about the young and their mothers is that mothers can recognize their young but young cannot recognize their mothers and will try to latch onto the closest thing (Davis et al 1968).  When a mother comes back from feeding she will climb over the cluster of young and look for her young, licking around the face once she has found it.  If a youngster falls or gets lost from the group, it will squeak very loudly until found by its mother.  A young’s squeak can be heard from over 30 feet away.

Food Habits:

Big Brown bats are insectivores.  They eat stink bug, beetles, froghoppers, moths and other small insects.  Unlike other insectivorous bats, big brown bats do not eat as moth moths.  This may be contributed to the fact that moths can pick up the echolocation and thus can avoid the bat. (Agosta and Morton). Also the bats will eat certain orders of insects at different times of the year, dependent on what insects are abundant.  For example, in April the majority of the bats diet consists of insects from the family Hemerobiidae.  In July the bats will feed on more Coleoptera (Agosta and Morton). Males and females also forage for food differently.   It is estimates that a big brown bat can consume at least 1.4 grams of insects per hour of feeding time ( Baker 1983).Males, though, tend to fly farther and seem to pick areas with a poorer quality of food than females do (Wilkinson and Barclay 1997).  Females tended to stay closer to their roosts, especially during pregnancy and lactation.


As mentioned before, big brown bats hibernate like many other bat species.  The exact causes of what makes the bats hibernate is still a mystery.  Big brown bats hibernate in caves and other secluded spot that are partially insulated.  They can though withstand more temperature changes than most bats can.  They can also awaken during hibernation to move to a better spot and one bat was recorded to have moved over 400 yards to a new cave during the winter (Goehring 1972). They will also many times return to the same nesting roost as the year before (Whittaker and Gummer 1999).  Their heart will slow down to 4-62 beats per minute and their body temp will be around 5 degrees centigrade. One factor that can kill a bat during hibernation is the failure to accumulate enough body fat and thus they will run out of energy before it is time.  This happens to younger more inexperienced bats and is a major cause of death. The bats will lose up to 25 percent of their body mass over the hibernation period. This is also a great risk for reproductive females who already are being taxed by reproductive energy.

Male bats are also more likely to use daily torpor and would be in a deeper state of torpor than reproductive females (Hamilton and Barclay 1994). Even the bat’s roosting patterns can show the roosts warming pattern where the males roost at the end that warms up the fastest.

Big brown bats use mouth echolocation like many other bats do.  They will produce many pulses to determine their surroundings and when closing in on prey they will increase their pulses in what is called a “feeding buzz” (Nowak 1991).  Their use of echolocation if converted could be used to improve the sonar receivers of military and industrial receivers (Sanderson et al 2003).   Some data shows that bats move toward the loudest natural sounds, for the brown bats case, things like crickets and katydids.  That would mean with their echolocation capabilities, they could detect a 19mm ball at 5.1 meters and a 4.8 mm ball at 2.9 meters (Altringham 1996).

Literature Cited:

Agosta, S.J.  2002.  Habitat Use, Diet and Roost Selection by the Big Brown(Eptesicus fuscus) in North America: a case for conserving an abundant species.  Mammology Review 32(3): 179-198.

Agosta, S.J.and D. Morton.  2003.  Diet of Big Brown Bat, Eptesicus fuscus, from Pennsylvania and Western Maryland.  Northwest National 10(1): 89-104.

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

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

Brigham, R.M.  1991.  Flexibility in Foraging and Roosting Behavior by the Big Brown Bat.  Canadian Journal of Zoology 69(1): 117-121.

_____.  1987.  The Significance of Winter Activity by the Big Brown Bat (Eptesicus fuscus)- The Influence of Energy Reserves.  Canadian Journal of Zoology 65(5): 1240-1242.

Brigham, R.M. and M.B. Saunders.  1990.  The Diet of Big Brown Bats (Epesicus fuscus) in Relation to Insect Availability in Southern Alberta, Canada.  Northwest Science 64(1): 7-10.

Davis, W., R. Barbour and M. Hassell.  1968.  Colonial Behavior of Eptesicus Fuscus.  Journal of Mammalogy 4(1): 44-50.

Goehring, H.  1972.  Twenty-Year Study of Eptesicus Fuscus in Minnesota.  Journal of Mammalogy 53(1): 201-207.

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

_____.  1994.  Patterns of Daily Torpor and Roost Selection by Male and Female Big Brown Bats.  Canadian Journal Of Zoology 72(4): 744-749.

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

Kurta, A. and R. Baker.  1990.  Eptesicus fuscus.  Mammalian Species 356: 1-10.

Lausen, C.L. and R.M.R. Barclay.  2002.  Roosing Behavior and Roost Selection of Female Big Brown Bats (Eptesicus fuscus) roosting in rock crevices in Southeastern Alberta.  Canadian Journal of Zoology 80(6):  1069-1076.

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

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

_____.  2000.  Population Structure Dynamics of Big Brown Bats Hibernating in Buildings in Indiana.  American Midland National 143(2): 389-396.

Willis, C.K.R. et al.  2003.  Medium and Long term use of Trembling Aspen Cavities as Roosts by Big Brown Bats (Eptesicus fuscus).  Acta Chiropterlogica 5(1): 85-90.

Reference written by Andrea Fico, Biol 378: Edited by Chris Yahnke. Page last updated 4-23-04.

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