Big Brown Bat: Eptesicus Fuscus

Image of a big brown bat found in the attic

Big brown bats are found in most of the United States. These bats have been forced to adapt to humans changing their environment. They are often seen roosting in human-made structures.

This species of bat is often seen as a beneficial animal because the big brown bat eats an extraordinary amount of insects. The big brown bat will load up on insects before winter in preparation for hibernation.

Big brown bats have distinctive mating/roosting habits that set them apart from other bat species. They also tend to be larger than most bats found in the U.S.

Learn More: North American Bat Species 

Physical Characteristics Of The Big Brown Bat

Big brown bats resemble Myotis bats found in Montana but can be distinguished by size. Except for large Fringed Myotis, even juvenile big brown bats are larger than almost all Myotis.

There are typically dark brown membranes on adult big brown bats and uniform pelage that varies from dark brown to blond.

Typically, adults have forearm lengths of 43 to 49 mm and weigh between 14 and 25 g.

The ears are relatively short, ranging from 11 to 15 mm. The forearm is used to distinguish this species from the myotis family.

Pillage patterns can be used to identify big brown bats. The size, color, and fur patterns are all used to recognize the big brown bat. 1Go To Source -“Big Brown Bat – Eptesicus Fuscus”

Big Brown Bat Habitat

Big brown bats are primarily associated with human-made structures, as they have adapted well to the changes brought to the landscape by humans.

Consequently, they are the bats that humans most often encounter. Common maternity sites include buildings, under bridges, caves, and hollowed trees.

Unless the temperatures get too extreme, these hardy bats also roost in buildings’ eaves during winter. 2Go To Source -“Big Brown Bat”

Big Brown Bat Range

The big brown bat’s overall geographical distribution extends from southern Canada through the entire United States and down into South America through Middle America. Some of the Bahama Islands, Cuba, Hispaniola, Jamaica, Puerto Rico, Dominica, and Barbados are also home to the big brown bat. 3Go To Source -“Big Brown Bat”

Diet Of The Big Brown Bat

Image of big brown bat roosting on a tree

Small beetles are their most frequent prey, yet large brown bats will consume a wide range of night-flying insects. Their foraging behavior and habitat choices are very general and seemingly show little preference for feeding on water vs. land or in forests vs. clearings.

In addition to numerous species of moths and grasshoppers, numerous feeding studies of large brown bats indicate that they consume a significant amount of crop and forest pests.

Reproductive females can often consume their body weight in insects each night. In fact, in one summer, a colony of 150 adult large brown bats can consume enough cucumber beetles to prevent egg-laying that would produce a significant corn pest problem.

Large brown bats contribute to a healthy environment, like all insect-eating bats, and are vital players in insect pest checks and balances. 4Go To Source -“Big Brown Bat”

Big Brown Bat Behavior

Big brown bats typically roost together, with the exception of mothers taking care of their young. When new bat young are born, females in large groups will roost together, and males will roost by themselves. Before nursing, mother brown bats will lick the young to make sure it’s their young they are handling.

In the winter, bats also hibernate. To navigate obstacles and to capture flying insect prey, these bats use “echolocation.” They accomplish this by making calls through their open mouths. Depending on what the bats are doing, each call’s length and the time between calls vary.

By listening to the calls’ echoes, big brown bats can sense objects or other bats near them.

Typically, as they close in on prey, large brown bats will increase the rate of echolocation calls. The calls end in what is known as a “feeding buzz,” a high pulse sound that signals their prey is about to be captured. 5Go To Source -“Big Brown Bat”

Big Brown Bat Life Expectancy And Reproduction

Some big brown bats can live up to 20 years, but the average life span is around 7-9 years.

Mating occurs primarily in fall and winter, but females do not become pregnant until spring, just after hibernation ends. In May through June, the young are born, and twin babies are typical in the east. Non-twin babies are common in the western United States.

Young big brown bats are fed milk for approximately 4 to 5 1⁄2 weeks, learn to fly for 3 to 5 weeks, and stay to forage with their mother for another 2 to 2 1⁄2 weeks.

Maternity colonies commonly contain between 20 and 300 bats, consisting of pregnant females, young nursing females, and fully-developed females.

Maternity roosts are typically located under loose tree bark, in buildings, within tree cavities, or within cliff-face crevices. During the roost of impregnated females, males will roost on their own. 6Go To Source -“Big Brown Bat”

Hibernation Habits For The Big Brown Bat

Photograph of big brown bat showing it's teeth

In caves and human-made structures, big brown bats hibernate. The only bat species known to roost in buildings during winter are big brown bats.

Structures in which large brown bats hibernate remain above freezing and typically range from 48-57 ° F throughout the winter.

Large brown bats hibernate in colder, drier areas and more exposed to airflow compared to other bat species.

During hibernation, this species occasionally forms clusters and can also be found hanging separately from the ceiling or wall. More research is needed to find what features make caves and mines suitable for big brown bat hibernation. 7Go To Source -“Big Brown Bat (Eptesicus fuscus) Species Guidance”

Predators Of The Big Brown Bat

Big Brown Bats will choose secluded roosts with the intent to ensure safety from predators.

Predators such as snakes, feral cats, and raccoons often target bat roosts with newborns. If a predator cannot reach the roost, it may wait below for a young bat to fall.

Flying bats can easily be captured by predators such as owls and falcons or other predatory birds when leaving their roosts. Flying predators are often faster than big brown bats, thus making the bat an easy meal.

White Nose Syndrome Affecting Big Brown Bats

White-nose syndrome is an emerging hibernating bat disease that has spread at an alarming rate from the northeast to the central United States.

Millions of bats in 33 states and counting have died from this devastating disease since 2007-2008. The condition is named after Pseudogymnoascus destructans, a white fungus that infects the skin of hibernating bats’ muzzles, ears, and wings. 8Go To Source -“What is White-nose Syndrome?”

Before winter ends, bats with WNS use up their fat reserves, probably the result of increased arousal frequency during hibernation. Starving bats may fly outside the cave and die because in cold weather, their main food source, insects, are not active.



  1. Big Brown Bat — Eptesicus fuscus. Montana Field Guide. Montana Natural Heritage Program and Montana Fish, Wildlife and Parks. Retrieved on December 8, 2020, from
  2. “Kentucky Department of Fish & Wildlife Big Brown Bat.” Kentucky Department Of Fish & Wildlife Resources, Accessed 8 Dec. 2020.
  3. Fort Hays State University. “Kansas Mammal Atlas: Big Brown Bat.” Kansas Mammal Atlas, Sternberg Museum of Natural History, 21 Aug. 2018,
  4. “Big Brown Bat – Shenandoah National Park (U.S. National Park Service).” National Park Service, Accessed 8 Dec. 2020.
  5. Mulheisen, Michael, and Kathleen Berry. “BioKIDS – Kids’ Inquiry of Diverse Species, Eptesicus Fuscus, Big Brown Bat: INFORMATION.” The Regents of the University of Michigan, University Of Michigan, Accessed 8 Dec. 2020.
  6. National Park Service,,spring%2C%20just%20after%20hibernation%20ends. Accessed 8 Dec. 2020.
  7. Wisconsin Department of Natural Resources. “Big Brown Bat (Eptesicus Fuscus) Species Guidance.” Wisconsin DNR, Bureau of Natural Heritage Conservation, 2012,
  8. “What Is White-Nose Syndrome?” USGS, Accessed 8 Dec. 2020.