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Big Brown Bat
(Eptesicus fuscus P. de Beavois)

From: Saunders, D. A. 1988. Adirondack Mammals. State University of New York, College of Environmental Science and Forestry. 216pp.

Order: Chiroptera

Family: Vespertilionidae

Description: The big brown bat is the Adirondack’s largest bat; only the hoary bat is larger.  The long, lustrous fur is brown. The ears, wing and tail membranes are black. Total length is 105-123 mm (4.1-4.8 in). Wingspread is 31-33 cm (12.2-13.0 in). Adults weigh 12-29 g (0.4-1.0 oz), and females are usually heavier than males.

Range and Habitat: The range is from the southern half of Canada south throughout the U.S., Mexico, and Central America.  Big brown bats frequently reside in attics, walls, and basements of buildings, in hollow trees, caves, and mines. Throughout most of its range, this species is relatively common. However, there are few records for this species during the summer months in the Adirondack Park. Where caves and mines occur in the Park, mainly along eastern periphery, the big brown bat is often present in small numbers, accounting for only one percent of the resident bats. Merriam declared this species “unquestionably the rarest bat found within the limits of its region,” (p 184). This is no longer true, and it may be that the increase in human dwellings and abandoned mines has enabled the big brown bat to expand its populations in the Adirondacks.

Food and Feeding Behavior: Big brown bats take wing late in the evening, although occasionally one may be active during the day.  They forage over fields and water, along the edges of forests, and over the forest’s canopy. Solitary individuals and entire colonies may feed in the same places from one night to the next, using a combination of vision, post-sunset glow, and landmarks to guide them to reliable sources of food. Beetles are the main food source, but they may consume many other flying insects as well. Foraging is most intense for the first few hours of darkness.

Activity and Movement: Big brown bats use mines, caves, hollow trees, buildings, and in urban areas, storm sewers as winter shelters. When using mines and caves, they tolerate cool, dry, draft, less humid perches and tend to be more abundant near entrances. This species is more tolerant of cold than most species of bats, and more abundant near entrances than most species of bats, even surviving short periods of sub zero temperatures. Another indicator of its hardiness is the relatively brief dormancy (November-April). During protracted mid-winter thaws, big brown bats may arouse, and forage on insects that also become active. This bat may return to the same summer and winter roosts which are seldom more than 48-97 km (30-60 mi) apart. Its flight is direst, steady, and slow (16-18 km/hr; 10-11 mph).

Reproduction: Adults mate in autumn, spring and during periods of winter arousal. Fertilization occurs at the time females begin to leave hibernating sites in April. The gestation period is approximately 60 days. Females bear two (occasionally one) young in June or July while roosting, most often in buildings. They produce one litter annually. The newborn weigh 3-4g (0.11-0.14 oz) and are nearly hairless, but develop fur by day 7. They are weaned and capable of flight at 21-28 days. Males and some females begin breeding in their first autumn. The average life span is 2-3 years, but potential longevity is at least 19 years for males and 16 years for females. 

Predators: American kestrels; great horned, barn, and screech owls; and snakes are some of the natural predators of the big brown bat.

Social Behavior:

  • Social system - This species is promiscuous, and compared to many species of bats, sedentary. In the summer, pregnant females roost in close proximity, forming maternity colonies of up to 200 individuals. Males are usually solitary at this time, but may join maternity colonies shortly before they disband when the young are weaned. Females tend to hibernate singly. Males hibernate singly or in small clusters.
  • Communication - Foraging bats produce audible squeaks and chattering notes which have a probable signal function. The young squeak as well which may aid an individual recognition and in maintaining parent-young bonds. Glands on the upper lips of adults produce a strong musky odor that may serve as chemical clues, e.g., to species identity and sexual status.

Additional References 

Buchler, E.R. and S.B. Childs. 1982 Use of the post-sunset glow as an orientation cue by big brown bats (Eptesicus fuscus). Journal of Mammalogy, 63;243-247.

Goehring, H.H. 1972. Twenty-year study of Eptesicus fuscus in Minnesota. Journal of Mammalogy, 43:201-207.

Mills, R.S., G.W. Barret, and M.P. Farrell. 1975. Population dynamics of the big brown bat (Eptesicus fuscus) is southwestern Ohio. Journal of Mammalogy, 56:591-604.


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