Natural Histories

photograph of a Sunda Pangolin

Big Brown Bat

Eptesicus fuscus


The big brown bat is about 110-130 mm long (including the 38-50 mm tail) and has a wingspan of about 330 mm. Sexual dimorphism is present in this species (meaning you can differentiate males from females); female big brown bats tend to be larger than males. This species has a large skull with 32 sharp, heavy teeth that are capable of causing severe bites. The nose and lips of this bat are fleshy and the eyes are large and bright. Big brown bats have rounded ears and a broad tragus (inner foremost part of ear) with rounded tip. The color of this bat varies with location and subspecies. The dorsal side can range from a pinkish tan to rich chocolate color while the ventral side is lighter, pinkish tan to olive buff. The naked membranes of the face, ears, wings and tail are black.


This species can be found in ranges between southern Canada, through temperate North America, down through Central America to extreme northern South America and the West Indies. The big brown bat can thrive in cities, towns and rural areas. This species is not commonly found in heavily forested areas. These bats tend to roost in storm sewers, expansion joint spaces in concrete athletic stadiums, and copper mines. Occasionally this species will be found roosting in tree cavities or hibernating in caves. Big brown bats tend to hibernate in barns, silos, and churches because they do not require stable, intensely insulated environments like other bats.

Feeding Behavior and Diet

This species is insectivorous, meaning they only consume insects. Big brown bats prefer beetles but will also consume moths, flies, wasps, flying ants, lacewing flies, and dragonflies. Big brown bats do not typically feed during heavy rains or while air temperature is below 10 degrees centigrade. Foraging typically begins 20 minutes after sunset and then find a night roost where they can digest their meal and then return to the day roost before dawn.


Bats are the only species of flying mammals. Breeding begins in the fall, when females of maternity roosts break up into alternate roosts, and continues into winter and spring. Reproduction in temperate regions is delayed until after hibernation. Implantation is delayed after mating until April-May. The gestation period of a big brown bat is about 60 days, resulting in the birth of a single pup during late May or early June. Pups nurse from gland secretions on the belly fur of their mother and are weaned after 32-40 days. A mother big brown bat is able to recognize the squeaks of her young and carry them from roost to roost until they can fly at about 4-5 weeks old.

Months and Times of Activity

Groups of 5-700 female big brown bats form maternity colonies to rear young while males roost alone or in small groups. Both sexes can be found roosting together during late summer months. During the winter these bats hibernate in cavities unsuitable for other bat species because they are more tolerable to the cold. Big brown bats are capable of hibernating for 300-400 days.

Special Features, Stories, Relationships

  • Bats use echolocation to avoid obstacles and catch flying insects while in flight. Echolocation is the interpretation of echoes from volleys of calls that have been emitted from their mouths. Interpretation of echoes allows them to develop an acoustic image of what is in front of them.
  • Big brown bats can survive to be 19 years old in the wild. Male big brown bats tend to live longer than females. Most bats of this species die their first winter, the most common cause of death is not storing enough fat to survive winter hibernation.
  • The name Eptesicus  is derived from the Greek, meaning “house flyer”.
  • To read legends featuring this animal go to:
  • Children’s book featuring this animal: BORIS THE BAT by Robert Dickins


Myers, P., R. Espinosa, C. S. Parr, T. Jones, G. S. Hammond, and T. A. Dewey. 2014. The Animal Diversity Web (online). Accessed at