This is a relatively nondescript, medium-sized, small-eared bat best recognized by eliminating the several more distinctive species that might be confused with it. The Yuma myotis is of similar size but is paler in color and the hairs lack metallic, burnished tips. The California myotis may be as dark but is considerably smaller; the long-legged myotis is a heavier bat with a keeled calcar (a spur of bone that projects inward from the ankle) and a wing that is furred beneath from body to elbow. The fringed myotis has longer ears (as has the long-eared myotis) and a distinctive fringe of stiff hairs on the trailing edge of the uropatagium. Average length is 97.9 mm; average length of forearm is 39.3 mm. Weights average about 7 g, and the wingspan is about 220-270 mm.
The little brown bat ranges across North America, from Alaska across Canada to Newfoundland and south, mostly in forested areas, to central Mexico. In Colorado the species may occur statewide in suitable habitat, ranging as high as 11,000 feet in Lake County. However, in the eastern two-fifths of the state, there are actual records only from Greeley and Pueblo.
This is a species of wooded areas -- including riparian woodland in the mountains and lower valleys -- urban areas, woodlots and shelterbelts. The little brown bat is one of the most tolerant of bats in terms of roost selection. Night roosts are located in tree hollows, beneath tree bark, in or under buildings, bridges, crevices in rock, behind shutters or beneath eaves. They may share roosts with other species of bats. Day roosts in attics may be used by large concentrations of bats. Hibernation sites include caves, mines and buildings. Some little brown bats hibernate in Colorado, but winter habits are poorly known here and elsewhere in the West. In Ontario, hibernation lasts from September to May. In some parts of the range, the animals may move several hundred kilometers from nursery colonies to hibernacula, but such long-distance movements have not been documented in the West. The animals can move 50 miles a night at speeds up to 19 miles per hour. Maximum longevity in these bats may be remarkably long; the current record is some 31 years. The average lifespan, however, is much shorter, as over half the young die in their first year. Predators include raccoons, mink, snakes and owls.
A great deal is known about the breeding habits of the little brown bat in the eastern part of its range, but the species has not been studied intensively in the West. Breeding takes place in autumn or early winter. There are two phases, an active phase in which males and females are alert and a passive phase in which males mate with torpid females. Breeding of both sexes is promiscuous. Sperm are stored by the female in the uterus until spring, when fertilization, implantation and gestation take place. Gestation lasts 50 to 60 days, depending on temperatures. The young are born almost always singly in nursery colonies from late May to early June. At birth, little brown bats are blind, but their eyes open in two days. The mother can fly with her offspring attached to a nipple, but the young usually is left in the roost during the mother's foraging flights. Young can fly on their own by 3 weeks and reach adult weight about a month after their first flight. About half the females breed their first autumn. Males breed first as yearlings. Nursery colonies of several hundred females are known. Non-breeding females and males roost away from nursery colonies.
Little brown bats emerge at dusk to feed, often following the same foraging route repeatedly through the night and on successive nights. They forage over water or among trees, 3 to 6 meters above ground. Foraging flight is erratic for a bat of this size. The diet consists largely of aquatic insects, including caddis flies and midges, but also includes moths, mosquitoes and other flies. The prey is knocked from the air with a wingtip, captured in the membrane between the legs and lifted to the mouth. Foraging behavior has to be learned, and adults are much more efficient feeders than are young; adults fill their stomach in as little as 15 minutes.
This is perhaps the most common, widespread bat of temperate North America, and it is one of the best studied. It deserves greater attention in Colorado, however, particularly because former colonies have disappeared or have been reduced in size.