One of the most widespread mammals in North America, the big brown bat is named for its large size and its fairly uniform brown fur. This species has a robust body and large, broad head, with relatively large eyes and short , thick, rounded ears, which are furred only at the base. The big brown bat’s wings are short and broad, and the tip of this specie’s tail extends to just beyond the edge of the tail membrane. The big brown bat’s fur is quite long and soft, and varies from dark brown to pinkish-tan or cinnamon-brown on the upper parts, with each individual hair having a dark base. The fur on the underparts is generally paler, while the face, ears, wings and tail membrane are blackish. Male and female big brown bats are similar in appearance, but females are slightly larger than males. Young big brown bats are darker and duller in color than the adults, and have slightly shorter fur. The big brown bat varies somewhat in appearance across its large range, and around 11 subspecies are currently recognized. Like most other bats, the big brown bat forages at night, usually leaving its roost within the second hour after sunset to feed. This species will forage in a variety of habitats, both over water and over land, and uses forests and clearings as well as rural and urban areas . The big brown bat has been shown to be able to use the Earth’s magnetic field to help it find its way back to its daytime roosting site. Like many other bats, the big brown bat also navigates and hunts using. When leaving its roost, it may also listen for the sound of chorusing frogs and insects to help it locate concentrations of prey. In addition to producing ultrasonic calls for echolocation, the big brown bat uses a variety of ‘social’ calls to communicate with other individuals. Over most of its range, the big brown bat hibernates during winter, usually alone or in a small group. This hardy bat is often one of the last species to be seen flying about in autumn, and some individuals do not begin to hibernate until November . Individuals often become active for brief periods during the winter months, sometimes even changing hibernation site, and big brown bats in Cuba may not hibernate at all, instead merely entering a state of torpor on cool winter nights. Female big brown bats emerge from hibernation in spring, usually around March or April, and form ‘maternity’ colonies in which to give birth and rear their young. These colonies vary in size from around 5 to 700 individuals and may be located in buildings, hollow trees or caves. During this time, the male big brown bats roost alone or in small all-male groups, only rejoining the females later in the summer. The big brown bat mates during the autumn, or sometimes during the winter and early spring, but fertilization does not occur until the spring. The female big brown bat usually gives birth to one or two young in May to July, after a gestation period of about two months. The young bats are born naked and blind, but their eyes open on about the second day. The female leaves the young in the colony while she feeds, and is able to recognize her own offspring when she returns, even retrieving them if they have fallen to the ground. Young big brown bats are able to fly at about three to five weeks old and reach adult size after about two and a half months. The male big brown bat reaches sexual maturity in its first autumn, but only some females reproduce at the end of their first year. The big brown bat may potentially live for 19 or 20 years in the wild, but many die in their first winter. A common cause of mortality in the big brown bat is failure to store enough fat for hibernation, although this species is also occasionally taken by predators such as owls, cats and snakes.