Big Brown Bat

Scientific Name
Eptesicus fuscus
Also Known As
Big Brown Bat
All of Florida
Beetles, Flies, Cockroaches, Flying Insects
Life Expectancy
5 - 6 Year
The Big Brown Bat

Photo 133405235 © stephen_buckingham, CC BY-NC

Big Brown Bat conservation status - Vulnerable

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Brown Rats in Central Florida

The big brown bat (Eptesicus fuscus) is a common vespertilionid bat species found throughout much of North America. In central Florida, big brown bats thrive in urban and rural areas, roosting in buildings and foraging over water. This guide covers identification tips, biology, behavior, reproduction, ideal habitat, diet, health risks, and prevention and control of big brown bats in central Florida.

Appearance and Identification

Big brown bats can be identified by the following physical characteristics

Adult Big Brown Bat

Adult Big Brown Bats

  • Size: Adults reach 102 to 135 mm in length with wingspans between 32.5 and 35 cm. They weigh 14 to 21 grams.
  • Fur: The dorsal fur is glossy dark brown. The ventral fur is paler brown to gray.
  • Head: Broad, flattened head with small eyes and short, rounded ears.
  • Wings: The wing membranes are naked and leathery. The wings are broad with blunt tips.
  • Tail: The tail membrane extends beyond the hind legs, giving it a long tail appearance.
A Juvenile Big Brown Bat

Juvenile Big Brown Bats

  • Size: Newborns weigh 4 to 7 grams with a wingspan around 9 cm. They reach adult size in about a month.
  • Fur: Fur is grayish on the back and pale gray on the underside. The fur is short and soft.
  • Features: Large feet, dark facial skin, and underdeveloped canines.

Maturation Rate

Big brown bat pups grow rapidly after birth due to the high energy demand of flight. They are nearly adult sized in 3 to 4 weeks after birth. Pups become volant by 18 to 21 days old.

Sexual maturity is reached at 6 months to a year old. Average lifespans in the wild are 5 to 6 years for females and 2 to 3 years for males.

Habits and Behavior

Big brown bats are nocturnal, leaving roosts at dusk to forage. They utilize echolocation to navigate and hunt prey during flight. Big brown bats fly steadily and are fast, agile fliers.

They roost in hollow trees, rock crevices, bridges, barns, attics, and other man-made structures in both urban and rural areas. Big brown bats hibernate in winter in caves or buildings, entering torpor when temperatures drop below 10°C.

Reproduction and Lifespan

Mating occurs in autumn just prior to hibernation. Females store sperm over winter until spring. In May and June, females gather in maternity colonies of 20 to 300 bats.

Gestation is 60 days, after which a single pup is born in June or July. Pups begin flying by 3 weeks old. Lifespans in the wild average 5 to 6 years.