Big Brown Bat

Big Brown Bats

The Big Brown Bat (Eptesicus fuscus) is a common flying mammal found in North and Central America. They live in cities, towns, rural areas, and heavily forested regions.

They like temperate climates, but colonies who live in colder regions hibernate in caves, mines, attics, and other buildings.


Kingdom: Animalia (Animals)
Phylum: Chordata (Chordates)
Subphylum: Vertebrata (Vertebrates)
Class: Mammalia (Mammals)
Order: Chiroptera (Bats)
Family: Vespertilionidae (Evening Bats / Vesper Bats)
Genus: Eptesicus (Brown bats and Forest Bats and their relatives)
Species: Eptesicus fuscus (Big Brown Bat)

The Eptesicus genus is divided into species groups. The Big brown bat belongs to the serotinus group which includes, the Argentine brown bat (E. furinalis), Brazilian brown bat (E. brasiliensis), and the Long-tailed House bat (E. hottentotus).

The serotinus group morphology is defined as having a large elongated skull, flat braincase, and a long snout.

Physical Description

They have a brown to glossy copper-colored fur. The belly fur tends to be lighter. They have small rounded black ears, fleshy lips, and black wings.

It has a tail that is as long as half the total body length. Pelage color depends on the location and subspecies. It ranges from pinkish tan to rich dark chocolates.

Its large skull can contain up to 32 teeth for adult bats. The teeth are sharp and heavy and have been known to cause severe bites.

Female big brown bats are slightly larger than males of the same species. Their average weight is 0.5-1.2 oz (14-21 gm’s) and their wingspan is 12-16 inches (32-40 cm).

Life Span

Big brown bats are known to survive up to 18-20 years in the wild. They have a high infant mortality rate and most young bats die hibernating in their first winter.

A study of a big brown bat population in Colorado, USA. shows their life expectancy is a little over 6.5 years. It is much shorter than those documented in the wild.

The species is widespread and common. It is currently not on any endangered or conservation list.


They are primarily insectivorous. Their diet consists of many kinds of insects. Big Brown Bats are a common species spread across two continents and their insect diet varies depending on the prey availability in their location.

They prefer eating beetles over other insects. Their sharp teeth and strong jaws are enough to bite into the beetle’s hard exoskeleton. Their secondary diet consists of small flying insects like moths, flies, and wasps that they catch mid-flight.


The big brown bat is nocturnal. They rest in their shelters during the day and hunt at night. They are also semi-solitary. They roost in groups, but with the exception of pregnant and lactating females they hunt and travel alone.

Like most bats, the big brown bats use echolocation to avoid obstacles and hunt while flying. They are known to be one of the fastest bat species reaching flight speeds up to 40 mph.

Big brown bats in colder climates hibernate during winter months. Some subspecies and colonies are known to migrate to different warmer roosts.

They communicate by making human audible squeaking sounds and hissing noises. Baby bats are known to squeak loudly when in peril. It can be heard from a distance of over 30 feet.

Life Cycle

Big Brown Bats breed during autumn and winter months. After the mating season in early spring, pregnant females roost together. It takes 50-60 days for young bat pups to be born from their mothers. Pregnant females normally bear just one offspring. Like most bat species, the big brown bat has only two nipples. It is unlikely that more than two offspring can survive.

Maternity colonies rear the young pups and mothers can recognize their own offspring in the colony. They mark their young by licking them before nursing. It is assumed this is how they are recognized in the future. The pups are left behind together in the nursing colony while mothers hunt food at night.

Newborn young are not able to fly. Mothers can transport them between roosts. Young bat pups stay clustered together in the nursery roosts until they learn to fly. It normally takes 2-3 months before young bat pups can fly on their own.

Young bats that fall from their nursery are preyed upon by other animals such as dogs, cats, and snakes. Older flying bats are predated by Owls.

It takes 12-14 months before young big brown bats mature into adulthood and become capable of mating.