Little Brown Bat (Myotis lucifugus): Facts and Habitat

Appearance and identification

The little brown bat has glossy, rich brown fur on the back, ranging from pale tan or reddish to dark brown, and paler buff or grayish fur on the belly. The wings and ears are dark brown to black and almost hairless. It has a short snout, a gently sloping forehead and small rounded ears with a blunt tragus, the small flap of skin in front of the ear opening that helps a bat judge the direction of echoes.

Its name in Latin means “light-shunning mouse-ear,” a good description of a nocturnal bat of the mouse-eared genus Myotis. More than 100 species of Myotis live around the world, and several look very similar. In North America the little brown bat is most easily confused with:

• Indiana bat (Myotis sodalis): duller fur and a keeled calcar, a ridge on the spur that supports the tail membrane.
• Northern long-eared bat (Myotis septentrionalis): clearly longer ears with a long, sharply pointed tragus.
• Yuma myotis (Myotis yumanensis): a western species with duller, shorter fur.

The little brown bat also has long hairs on its toes that extend past the tips of the claws, a detail biologists use when they examine bats in the hand. Despite the similar name, it is not closely related to the larger big brown bat.

How big is a little brown bat?

This is a very small mammal. Adults weigh about 5.5–12.5 g (0.2–0.4 oz) and measure 8–9.5 cm (3.1–3.7 in) from nose to tail tip. The forearm, the standard measurement for bats, is only 36–40 mm (1.4–1.6 in), and the wings span about 22–27 cm (8.7–10.6 in).

Females are slightly larger than males on average. Weight changes a lot through the year: bats are heaviest in autumn after fattening up for winter and lightest in spring, when they emerge from hibernation having lost around a quarter of their body mass.

Habitat and range

The little brown bat lives across much of North America, from Alaska and most of Canada east to Labrador and Newfoundland, south through the northern and central United States. It reaches southern California and the northern parts of Arizona and New Mexico but is scarce or absent along the Gulf Coast and in much of the southern Great Plains and southern Florida. The largest winter gatherings were historically in the limestone cave regions of the eastern US.

It uses three kinds of places through the year:

• Day roosts: warm, dark spaces in attics, barns, behind siding, in tree hollows, under loose bark, in wood piles and rock crevices.
• Feeding areas: the edges of forests, rivers, ponds and lakes, where insects rise from the water.
• Winter sites (hibernacula): caves and old mines that stay above freezing, with humidity above about 90%.

Some females travel hundreds of kilometers between their summer roosts and the caves where they hibernate. To see where other bats spend their days, read Where do bats live?.

Diet and echolocation

The little brown bat is an insectivore that hunts at night. It leaves its roost around dusk, feeds for one to three hours, rests, and often feeds again before dawn. It hunts mostly over water and along woodland edges, catching insects in flight with its mouth or scooping them up with its wings or tail membrane.

Its prey includes midges, mosquitoes, caddisflies, mayflies, moths, beetles, true bugs, lacewings and the occasional spider. Many of these insects hatch from water, which is why the bat so often feeds over ponds and rivers. A study of colonies in Wisconsin found mosquito DNA in most droppings, but mosquitoes are only one part of the menu.

To find prey in the dark, the bat makes short, very high-pitched calls that sweep down from about 80 to 40 kHz, far above the range of human hearing, and listens for the echoes. It calls about 20 times a second while searching and much faster in the final rush toward an insect. Learn more in How do bats use echolocation?.

Energy needs are high for such a small body. Pregnant females eat around 60% of their body weight in insects each night, and nursing mothers eat even more, close to 85% of their weight, with demand peaking when the pup is a few weeks old.

Behavior and social life

Little brown bats are social and colonial. In spring and summer, females gather in maternity colonies, usually of several hundred bats, in warm places such as attics, barns and large tree hollows. Males and non-breeding females roost alone or in small groups elsewhere.

In late summer and autumn, bats of both sexes fly to caves and mines and gather at the entrances at night, a behavior called swarming. Swarming is when most mating takes place, and it probably also helps young bats learn where hibernation sites are.

Before white-nose syndrome, little brown bats hibernated in dense clusters, sometimes of many thousands. Single hibernation sites once held up to about 180,000 bats. Today far more bats hibernate alone or in small groups, a change that may reduce the spread of the disease.

Hibernation

From late autumn to spring, the little brown bat hibernates. Its body temperature falls to just above that of the cave, and its heart rate drops from up to about 210 beats per minute at rest to as few as 8. A bat may remain in uninterrupted torpor for weeks, and in some cases up to about 90 days.

Waking up is the most expensive part of hibernation, because the bat must warm its body back up using stored fat. Even with careful energy saving, a bat loses roughly 25% of its body weight over the winter. This tight energy budget is why disturbance by people, and the extra waking caused by disease, can be deadly. Read more in Do bats hibernate?.

Reproduction and life cycle

Mating happens in autumn and sometimes during the winter. Females store the sperm in their bodies through hibernation, and fertilization takes place only in spring. Pregnancy then lasts about 50–60 days.

Each female gives birth to a single pup in late spring or early summer. The newborn weighs about 2 g, up to 30% of its mother’s weight, a huge baby for a mammal. Pups grow quickly: they start to fly at about three weeks old and are fully weaned by about 26 days. Females can breed in their first year, while males usually mature in their second year.

How long do little brown bats live?

For its size, the little brown bat is extraordinarily long-lived. The average lifespan is around 6–7 years, but banding studies have recaptured individuals more than 30 years after they were marked, with a record of about 34 years in the wild. Most mammals this small, such as mice and shrews, live only one to three years. See How long do bats live? for the reasons bats age so slowly.

Predators and threats

Healthy little brown bats have few predators. Owls such as the great horned owl, eastern screech owl and northern saw-whet owl take some, and raccoons sometimes pick bats off cave walls or the floor of hibernation sites.

The biggest threat by far is white-nose syndrome, a disease caused by the fungus Pseudogymnoascus destructans. It was first recorded in New York in 2006 and spread rapidly. The fungus grows on the skin of hibernating bats, makes them wake too often and burn through their fat, and many starve before spring. More than a million little brown bats died in the northeastern US by 2011, and counts at hibernation sites in the region fell by an average of about 90%. Other threats include the loss or disturbance of roosts and hibernation sites, wind turbines and road traffic.

Conservation status

The little brown bat is listed as Endangered on the IUCN Red List (2021 assessment), a dramatic change from 2008, when it was considered Least Concern and one of the most common bats in North America. Its population trend is decreasing. Canada listed it as Endangered in 2012 after an emergency assessment. In the US it is not federally listed, but several states protect it as endangered or threatened.

Some colonies in the northeast have survived the disease, and a few have begun to stabilize, giving scientists hope. Conservation work includes protecting and gating important caves and mines, decontaminating caving gear and testing possible treatments. Find out more in Why are bats dying in North America?.

Little brown bats and people

Little brown bats often live in buildings, and nursery colonies in attics and barns are a common sight in summer. They can enter through gaps as small as about 3.8 by 0.6 cm (1.5 by 0.25 in). A colony is noisy and leaves droppings, but unlike rodents the bats do not gnaw wood or wiring.

Rabies is uncommon in this species: of several hundred little brown bats tested in the US in 2015, under 3% were positive. Even so, bats should never be handled with bare hands. If someone is bitten or scratched by a bat, seek medical help immediately.

Many people put up bat boxes to give these insect-eaters a place to roost. Like other bats, the little brown bat is a mammal, not a flying mouse; compare its body plan with a bird’s in bat vs bird.