M. californicus M. leibii M. septentrionalis M. auriculus M. sodalis M. volans M. lucifugus M. occultus M. thysanodes M. evotis M. keenii The little brown bat was described as a new species in 1831 by American naturalist John Eatton Le Conte.

[6] Based on a 2007 study using mitochondrial and nuclear DNA, it is part of a Nearctic clade of mouse-eared bats.

However, a 1983 study by Herd and Fenton found no morphological, genetic, or ecological evidence to support the notion that the two species hybridize.

[16] The little brown bat is a small species, with individuals weighing 5.5–12.5 g (0.19–0.44 oz) with a total body length of 8.0–9.5 cm (3.1–3.7 in).

[2] A variety of fur colors is possible, with pelage ranging from pale tan or reddish to dark brown.

[2] The little brown bat is dichromatic and its eyesight is likely sensitive to ultraviolet and red light, based on a genetic analysis that discovered that the genes SWS1 and M/LWS were present and functional.

[22] In fall through spring, the little brown bat enters torpor, a state of decreased physiological activity, daily.

The exception to this rule is females at the end of pregnancy, which no longer have the ability to thermoregulate, and therefore must roost in warm places.

The two can be differentiated by the little brown bat's lack of a keeled calcar—the cartilaginous spur on its uropatagium (the flight membrane between its hind legs).

Additionally, the little brown bat can be distinguished by the presence of hairs on its toes and feet that extend beyond the length of the digits.

[2] The northern long-eared bat (M. septentrionalis), another similar species, can be distinguished by its much longer ears, and tragi that are long and sharply pointed.

[26] While they have a small absolute mass, they are enormous relative to their mothers, weighing up to 30% of her postpartum body weight at birth.

They exhibit rapid growth; at around three weeks old, the young start flying, begin the weaning process, and are of a similar size to adults in forearm length but not weight.

Historically, individuals within these colonies were highly aggregated and densely clustered together, though the disease white-nose syndrome is making solitary hibernation more common.

Prey species include beetles, flies, mayflies, true bugs, ants, moths, lacewings, stoneflies, and caddisflies.

However, there is no assurance that individuals forage with such high efficiencies for long periods of time, or that prey is dense enough in natural settings to allow capture rates observed in enclosed areas.

[47] The little brown bat is also susceptible to the disease white-nose syndrome, which is caused by the fungus Pseudogymnoascus destructans.

[51] The growth of P. destructans on bats erodes the skin of their wing and tail membranes, muzzles, and ears.

White-nose syndrome causes affected bats to burn through their energy reserves twice as fast as uninfected individuals.

[50] Some individuals are more likely to survive based on their genetics, which predisposes them to remain in torpor longer and have larger fat reserves.

[53] Little brown bats are most affected by white-nose syndrome when they exhibit social, grouping behavior when hibernating, as P. destructans is transmitted by direct contact.

In hibernacula where bats exhibit more solitary behavior, colonies are more prone to avoid infections of white-nose syndrome.

In some colonies where grouping behavior was common before exposure to white-nose syndrome, bats now hibernate in a more solitary fashion.

[2] In one study in the Canadian province of Alberta, its foraging activity was significantly higher in old-growth forest than would be expected based on its relative availability.

[55] As of 2021, the little brown bat is evaluated as an endangered species by the IUCN, a dramatic change from 2008 when it was designated as the lowest conservation priority, least concern.

[48] From 2006 to 2011, over one million little brown bats died from the disease in the Northeastern United States, with winter hibernacula populations declining up to 99%.

[59] In 2010, Kunz and Reichard published a report arguing that the precipitous decline of the little brown bat justified its emergency listing as a federally endangered species under the U.S.

Once inside a building, a colony of little brown bats can disturb human inhabitants with their vocalizations and production of guano and urine.

Large accumulations of guano can provide a growth medium for fungi, including the species that causes histoplasmosis.

Some install bat houses in an attempt to negate the effects of removing a colony from a human structure ("rehoming" them into a more acceptable space).