The big brown bat inhabits cities, towns, and rural areas, but is least commonly found in heavily forested regions. In presettlement times it is presumed the big brown bat roosted in tree hollows, natural caves, or openings in rock ledges. Occasionally groups of these bats are still found living in tree cavities. Total length is 110-130 mm of which the tail is about 38-50 mm. Pelage color depends on location and subspecies. Dorsally, it ranges from pinkish tans to rich chocolates. The ventral fur is lighter, being near pinkish to olive buff. Some have described it as being "oily" in texture. The bat's naked parts of the face, ears, wings, and tail membrane are all black. Female big brown bats form maternity colonies to rear young. The size of these colonies can vary from 5 to 700 animals. Males of the species roost alone or in small groups during this time. Both sexes will roost together again in the late summer. Baby bats who are separated from their mothers, either by falling from the roost, or by otherwise appearing lost, will squeak continuously. The squeaking can be heard from a distance of more than 30 feet. This communication is important for the baby's survival as it may help the mother locate and return them to a safer place. Eptesicus fuscus is an insectivorous bat. It preys primarily on beetles using its robust skull and powerful jaws to chew through the beetles' hard chitinous exoskeleton. It also eats other flying insects including moths, flies, wasps, flying ants, lacewing flies, and dragonflies. The big brown bat must confine its feeding activity to warm months when prey insects are active. Therefore it has to accumulate enough fat reserves, as much as one third of its body weight, before entering hibernation. Some estimate that these bats catch at least 1.4 grams of insects per hour. This bat utilizes echolocation to avoid obstacles and to capture flying insect prey. It can track insects into vegetation and intercept them while also avoiding the obstacles vegetation may present. The duration of each call and interval between calls varies depending on whether the bat is in search, approach, attack, or terminal phase. They are able to get acoustic images by integrating information from echoes in relation to their outgoing calls. Temperate North American bats are now threatened by a fungal disease called “white-nose syndrome.” This disease has devastated eastern North American bat populations at hibernation sites since 2007. The fungus, Geomyces destructans, grows best in cold, humid conditions that are typical of many bat hibernacula. The fungus grows on, and in some cases invades, the bodies of hibernating bats and seems to result in disturbance from hibernation, causing a debilitating loss of important metabolic resources and mass deaths. Mortality rates at some hibernation sites have been as high as 90%.
Source: Eptesicus fuscus Big Brown Bat, Animal Diversity Web, University of Michigan Museum of Zoology