mammals Species Profile

Big Brown Bat

Big Brown Bat (Eptesicus fuscus). Photo by Sherri and Brock Fenton, used with permission.

Last year, I observed 2 bat species while on a night hike with the Norfolk Field Naturalists (for more about this hike, go here). The 2 bat species I observed were Eastern Red Bats and Big Brown Bats. I’d like to explore their biology and natural history, specifically within Ontario. This first post will be focused on the Big Brown Bat and another will focus on the Eastern Red Bat. I will be pulling most of my information from The Natural History of Canadian Mammals (2012), by Donna Naughton, unless otherwise indicated.

Big Brown Bats (Eptesicus fuscus):

Meaning Behind the Name: Eptesicus is from Greek which means “I fly” and “house” because Big Brown Bats like to roost in houses, and the species name fuscus is Latin for “dusk” (Etymologia 2005).

Biology and Natural History:

At 13 cm long and with a wingspan of up to 39 cm, this is Ontario’s second largest bat (the largest being the Hoary Bat (Lasiurus cinereus), and is fairly common in southern Ontario. Their global range extends all the way south to South America, and at the northern end there are scattered reports from Alaska. With such a wide range, there are differences in their habits across it. For example, Big Brown Bats in Ontario hibernate through the winter in “caves, mines, and deep rock crevices, as well as heated buildings” (Naughton 2012), but in more southern regions with plentiful insect food throughout the winter, they are active year-round. The list above of hibernation sites are specific permanent locations bats will find to spend the winter. During the day, however, Big Brown Bats will use a variety of roost locations, including tree hollows and beneath bark*.

*A curious note describes a surprising discovery of a male Big Brown Bat that had been roosting beneath loose bark in a Michigan wetland. While the author of the note was interacting with a data logger in the wetland, “a strip of bark about 1 m in length fell from one of the trees and crashed into the water about 3 m away from me. Mixed in with the bark fragments and covered with duckweed (Lemna sp.) was a half-submerged bat that I eventually identified as an adult male big brown bat.” (Kurta 1994). I was glad to read that the bat was “torpid but unharmed” and after warming up “the bat flew away” (Kurta 1994).

Big Brown Bats are generalist insectivores, consuming basically any insects they can catch. Their diet of hard-bodied insects wears down their large teeth but apparently worn teeth don’t affect their feeding habits. They feed at night, if conditions are favourable (such as not rainy, and sufficiently warm night temperatures). On cooler nights, some bats will undergo torpor (a sort of mini-hibernation state) to save energy and forgo foraging. When they are out hunting, Big Brown Bats use echolocation to find insect prey. Although we think of echolocation calls as strictly for feeding, they inevitably function as signals, sometimes unintentionally. It has been demonstrated that Big Brown Bats are attracted to the echolocation calls of another species of bat (the Little Brown Bat, Myotis lucifugus) and the other species is attracted to Big Brown Bat calls as well (Barclay 1982). This is likely because echolocating bats represent an area with foraging opportunities or food sources.

Big Brown Bat (Eptesicus fuscus). Photo by Sherri and Brock Fenton, used with permission.

Pups are born in June-July in Canada, and begin flying at 21 days or later. In Eastern North America, most Big Brown Bats give birth to twins, while single pups are most often born in Western regions. Although the pups’ wings are the same size as adults, their weight is much smaller, providing them with an advantage while learning to forage. After about a month, the young are able to hunt for themselves (ie. are no longer dependent on nursing from their mothers), but will stick with their mothers for their first few hunts. Some male Big Brown Bats have lived more than 20 years (the demand on females of pregnant foraging and nursing is high and reduces their maximum lifespan).

Big Brown Bats are fascinating, and I was happy to hear and observe them last year. Next up will be the Eastern Red Bat!


Barclay, R. M. R. 1982. “Interindividual use of echolocation calls: eavesdropping by bats.” Behavioral Ecology and Sociobiology, 10: 271-275. cited in: Altringham, John and Fenton, M. Brock, 2003. “Sensory Ecology and Communication in the Chiroptera” in: Kunz, Thomas and Fenton, M. Brock (eds.). 2003. Bat Ecology. University of Chicago Press.

Etymologia: Eptesicus fuscus. Emerg Infect Dis []. 2005, Dec [date cited: February 11, 2023].

Kunz and Lumsden, 2003. “Ecology of Cavity and Foliage Roosting Bats” in: Kunz, Thomas and Fenton, M. Brock (eds.). 2003. Bat Ecology. University of Chicago Press.

Kurta, Allen. 1994. “Bark Roost of a Male Big Brown Bat Eptesicus fuscus.” Bat Research News. Volume 35: no. 2,3.

Naughton, Donna. 2012. The Natural History of Canadian Mammals. University of Toronto Press.

For other mammal-focused posts, see:

Flying Creatures of the Night

Moose (Alces alces) Family

Swimming Squirrels