Eptesicus fuscus (Beauvois, 1796)
Big Brown Bat
Basis for Listing
The Big Brown Bat (Eptesicus fuscus) is a common and wide-ranging bat found from Canada to South America (Kurta and Baker 1990). It is one of Minnesota’s four species of cave-hibernating bats and the second most common bat species found in the state. Secure winter hibernation sites, where the Big Brown Bat spends nearly half of each year, are critical to the survival of the species. Despite their abundance, the emergence of white-nose syndrome is threatening local populations throughout North America. Bat white-nose syndrome is a devastating disease affecting cave-hibernating bats caused by the fungus Pseudogymnoascus destructans. The syndrome is associated with high mortality in hibernating bats, with some sites documenting up to 90 or 100 percent mortality (Lankau and Rogall 2016). Hibernating bats observed to be affected by WNS display abnormal behaviors, such as daytime flights and clustering at the opening of the cave. These behaviors may result in stored body fat being depleted, with subsequent emaciation and death (Frick et al. 2010). White-nose syndrome first appeared in New York State in the winter of 2006. Since then, it has spread at an alarming rate across the continent and was confirmed in Minnesota by winter 2015-2016. Due to the slow reproduction rate of the Big Brown Bat, it is unlikely the species will recover quickly from this sudden and widespread mortality. While federal and state agencies are taking steps to slow the spread of WNS, its anticipated profound impact on cave-hibernating bats led to the Big Brown Bat’s designation as a species of special concern in 2013.
The Big Brown Bat is one of Minnesota’s larger bodied bat species, with long silky fur on the dorsal side. Color can vary based on geographic location. In Minnesota, Big Brown Bats are typically chocolate brown on the dorsal side, transitioning to a lighter hue or olive color on the ventral side. A large head, broad nose, fleshy lips, and larger eyes are characteristic of the Big Brown Bat, making it easy to distinguish from other species in the state. Ears are thick, short, and furred at the base. Exposed membranes on ears, wings, and snout are blackish brown. Size is 8.7-13.8 cm (3.4-5.4 in.) total length. Adults typically weigh 11-23 g (0.39-0.81 oz.) (Kurta and Baker 1990). The combination of size and facial characteristics makes the Big Brown Bat easy to distinguish from other bat species in the state. Little Brown Bats (Myotis lucifuscus) can sometimes be confused with big browns; however, the weight of Little Brown Bats, at 7-10 g (0.25-0.35 oz.), is almost half that of Big Brown Bats.
The Big Brown Bat is a year-round resident of Minnesota. Habitat use is influenced by time of year, sex, and reproductive status. Winter roosts are located in caves and mines, though this species also regularly hibernates in buildings, cellars, and tunnels. All these sites are sought for their high humidity levels, minimal airflow, and constant temperature in the winter season (Fitch and Shump 1979). These hibernacula consist of clusters of both sexes, with other species present. In Minnesota, although Big Brown Bats utilize the same cave systems as the Little Brown Bat and Tricolored Bat (Perimyotis subflavus), they prefer colder temperatures (Nordquist et al. 2006). Big Brown Bats are the first to emerge from hibernation in the spring and may be seen flying on warm days in April. Summer foraging areas are usually forested habitats (fire-dependent forests, mesic hardwood forests, and floodplain forests) near water sources (Kunz 1982). Warm season roosts can consist of human structures such as buildings and bridges; trees that are hollow, have crevices, loose bark, or cavities are also used. These resources are widespread throughout Minnesota, allowing bats to be dispersed widely throughout the state during summer foraging months.
Biology / Life History
In early fall, Big Brown Bats begin to gather at the openings of winter hibernation sites. This swarming behavior is thought to provide the opportunity for matin and an assessment of hibernaciula. Big Brown Bats are the last bats to hibernate. Bats remain in torpor (a state of physical inactivity) for most of the winter, occasionally becoming aroused, though very infrequently without disturbance. While mating occurs during swarming, ovulation and fertilization do not occur until arousal from hibernation (Kurta and Baker 1990). Movement from winter hibernacula to summer foraging grounds occurs in April or May (Phillips 1966). During this time, females form maternity colonies in which they give birth and raise their young. Big browns in Minnesota give birth to one or two pups between June and July. Young are completely reliant on their mother’s milk for the first three weeks of life. After three weeks, young start to develop flight abilities and transition to foraging for insects. Foraging occurs throughout the night in forest corridors, clearings, and near open water (Schnitzler et al. 2003). Echolocation is used by bats for spatial navigation and prey acquisition. Tonal signals are emitted from the bats that reflect off objects or prey. Information about location and identity are provided in the signals reflected back to the bat.
Conservation / Management
Since the first detection of WNS in 2006, unprecedented mortality has occurred among hibernating bats in the northeastern U.S.. The ability for the disease causing fungus (Pseudogymnoascus destructans) to spread rapidly prompted immediate action for research and monitoring. In 2008, a coordinated effort was made by the Department of Interior, Department of Agriculture, Department of Defense, and state wildlife management agencies to develop an effective national response to the disease. Elements of the plan included research on the fungus and monitoring of affected bat populations, education about the fungus and ecological importance of bats, reduction of environmental transmission to and from bats, and evaluation of the ecological and economic consequences of WNS (U.S. Fish and Wildlife Service 2011). Although much has been learned about the disease since onset, there are still gaps in knowledge, and a cure or method of preventing the fungus from entering other cave systems is as yet unknown.
Given the high mortality rates bats are experiencing in the face of white-nose syndrome (WNS), it is necessary to protect important habitats and mitigate potential impacts for this species. Mature trees utilized by bats for maternity colonies need protection and preservation, as successful reproduction will be critical in preserving bat populations affected by WNS. While Big Brown Bats use a range of tree species in varying decay stages, they all share the characteristic of large diameters (Betts 1996, Brigham 1991). Bats use multiple trees during the summer breeding season. These large trees, both dead and alive, are valued as lumber or other wood products and often removed from the landscape. It is important their value to bats as well as other species is recognized.
Conservation Efforts in Minnesota
The Minnesota DNR’s Minnesota Biological Survey (MBS) has been monitoring the health of hibernating bats since 2010. Collaborating with national research projects addressing the spread and possible control of white-nose syndrome, MBS continues to gather information on the status of Big Brown Bats through winter hibernacula surveys, summer acoustic surveys, and targeted population assessments. Education on the importance of bats and the effects of WNS is also a component of conservation efforts.
Melissa Boman (MNDNR), 2018
(Note: all content ©MNDNR)
References and Additional Information
Fitch, J. H., and K. A. Shump, Jr. 1979. Myotis keenii. Mammalian Species 121:1-3.
Frick, W. F., J. F. Pollock, A. C. Hicks, K. E. Langwig, D. S. Reynolds, G. G. Turner, C. M. Butchkoski, and T. H. Kunz. 2010. An emerging disease causes regional population collapse of a common North American bat species. Science 329:679-682.
Hazard, E. B. 1982. The mammals of Minnesota. University of Minnesota Press, Minneapolis, Minnesota. 280 pp.
Kunz, T. H. 1982. Roosting ecology of bats. Pages 1-55 in T.H. Kunz, editor. Ecology of bats. Plenum Press, New York, New York. 450 pp.
Kurta, A., and R. H. Baker. 1990. Eptesicus fuscus. Mammalian Species 356:1-10.
Lankau, E. W., and G. M. Rogall 2016. White-nose syndrome in North American bats ? U.S. Geological Survey updates: U.S. Geological Survey Fact Sheet 2016-3084. 4pp. <https://pubs.usgs.gov/fs/2016/3084/fs20163084.pdf>.
Nordquist, G. E., and E.C. Birney. 1985. Distribution and status of bats in Minnesota. Final report submitted to the Nongame Wildlife Program, Minnesota Department of Natural Resources. 64 pp.+ illustrations.
Nordquist, G. E., K. A. Lynch, and C. A. Spak. 2006. Timing and pattern of bat activity at Soudan underground mine. Final report submitted to the State Wildlife Grants Program, Minnesota Department of Natural Resources. 86 pp.
Phillips, G. L. 1966. Ecology of the big brown bat (Chiroptera: Vespertilionidae) in northeastern Kansas. American Midland Naturalist 75(1):168-198.
Rogall, G. M., and M. Verant. 2012. White-nose syndrome in bats: U.S. Geological Survey updates. U.S. Geological Survey Fact Sheet 2012-3076.
Rysgaard, G. N. 1942. A study of the cave bats of Minnesota with especial reference to the large brown bat, Eptesicus fuscus fuscus (Beauvois). American Midland Naturalist 28(1):245-267.
Schnitzler, H. U., C. F. Moss, and A. Denzinger. 2003. From spatial orientation to food acquisition in echolocating bats. Trends in Ecology and Evolution 18(8):386-394.
Szymanski, J. A., M. C. Runge, M. J. Parkin, and M. Armstrong. 2009. White-nose syndrome management: report on structured decision making initiative. U.S. Fish and Wildlife Service, Fort Snelling Minnesota. 51 pp.
U.S. Fish and Wildlife Service. 2011. A national plan for assisting states, federal agencies, and tribes in managing white-nose syndrome in bats, USFWS, Hadley, Maryland. 21 pp.