Eptesicus fuscus, also known as the Big Brown Bat, ranges from southern Canada, through temperate North America, down through Central America to extreme northern South America, and the West Indies (Nowak 1991).

Biogeographic Regions: nearctic (Native ); neotropical (Native )

occurs (regularly, as a native taxon) in multiple nations

Canada

Origin: Native

Regularity: Regularly occurring

Currently: Present

Confidence: Confident

Type of Residency: Year-round

United States

Origin: Native

Regularity: Regularly occurring

Currently: Present

Confidence: Confident

Type of Residency: Year-round

Global Range: Southern Canada (including all provinces bordering the U.S.) south to northern Colombia, northwestern Venezuela, and northern Brazil; all Mexican states except those of Yucatan Peninsula (in northern Mexico most prevalent in eastern and western Sierra Madre bordering arid midlands of Mexican Plateau); in and along central mountain chain in Central America; Greater Antilles; Bahamas; Dominica and Barbados in Lesser Antilles; perhaps Alaska (Honacki et al. 1982; Kurta and Baker 1990; Jones 1989; Koopman, in Wilson and Reeder 1993).

Total length is 110-130 mm of which the tail is about 38-50 mm. Forearm length is 41-50 mm; hindfoot length is 10-14 mm. Height of the ears from the notch is 16-20 mm (Kurta 1995). The wingspan is about 330 mm (13 inches) (Baker 1983). This species is sexually dimorphic in size, females being slightly larger than males (Kurta and Baker 1990)

The skull is comparatively large and contains 32 teeth. The teeth are sharp, heavy, and were described as capable of causing severe bites. The bat's nose is broad and the lips are fleshy; the eyes are large and bright. The ears are rounded and the tragus is broad with a rounded tip (Baker 1983)

The tail is less than half the total body length and the tip projects slightly beyond the uropatagium. This bat also has a cartilaginous calcar which articulates with the calacaneum, and has a keel-shaped extension (Baker 1983)

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 (Kurta and Baker 1990). Occasionally, E. fuscus has been found with white blotches on the wings, and some albino specimens are known as well (Baker 1983).

Range length: 110.0 to 130.0 mm.

Average wingspan: 330.0 mm.

Other Physical Features: endothermic ; heterothermic ; bilateral symmetry

Sexual Dimorphism: female larger

Average mass: 23 g.

Average basal metabolic rate: 0.113 W.

Length: 13 cm

Weight: 18 grams

• Terrestrial

The big brown bat inhabits cities, towns, and rural areas, but is least commonly found in heavily forested regions (Kurta 1995).

Some bats require stable, highly insulated environments in order to hibernate. Eptesicus fuscus has a more tolerant constitution so it can winter in less substantial structures. Besides human dwellings, it has been found to take up residence in barns, silos, and churches. Also, this bat has been found roosting in storm sewers, expansion joint spaces in concrete athletic stadiums, and copper mines (Baker 1983).

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 (Baker 1983). Recently, some were found hibernating in caves in Minnesota (Knowles 1992).

The generic name Eptesicus is derived from the Greek, meaning "house flyer". All this bat needs is a small hole or warped, loose siding to gain entry into a home. Once inside, it prefers to roost in double walls or boxed-in eaves rather than attics. It is reasonable to speculate that populations of the big brown bat have increased with an increasing number of human habitations (Baker 1983).

Habitat Regions: temperate

Terrestrial Biomes: forest ; rainforest ; scrub forest

Comments: Various wooded and semi-open habitats, including cities. Much more abundant in regions dominated by deciduous forest than in coniferous forest areas. Summer roosts generally are in buildings; also hollow trees, rock crevices, tunnels, and cliff swallow nests; prefers sites that do not get hot. Typically roosts in twilight part of cave. Maternity colonies form in attics, barns and occasionally tree cavities; often in rock crevices or dead ponderosa pines in western Canada; in deep crevices and holes in limestone caves in Puerto Rico (Anthony 1925). Most adult females return to same maternity roost site in successive years. About 10-30% of volant immature females return to natal site (Kurta and Baker 1990). Caves, mines, and especially buildings and manmade structures are used for hibernation (e.g., see Whitaker and Gummer, 1992, J. Mamm. 73:312-316).

See Vonhof and Barclay (1996) for information on characterisitcs of roost trees in British Columbia. In Alberta, reproductive females selected warmer diurnal roost sites than did males and nonreproductive females (Hamilton and Barclay 1994).

Non-Migrant: Yes. At least some populations of this species do not make significant seasonal migrations. Juvenile dispersal is not considered a migration.

Locally Migrant: No. No populations of this species make local extended movements (generally less than 200 km) at particular times of the year (e.g., to breeding or wintering grounds, to hibernation sites).

Locally Migrant: No. No populations of this species make annual migrations of over 200 km.

Fairly sedentary. Probably remains within 50 km of birthplace (Barbour and Davis 1969). Rarely moves more than 80 km between summer and winter roosts, though there is evidence that some individuals in the Midwest migrate south for winter.

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 (Baker 1983). One study indicated that juvenile E. fuscus ate a greater range of softer food items in their diets, compared to adults. The same study also indicated that bats having survived their first winter (yearlings), did not differ significantly in diet from the adults (Hamilton and Barclay 1998).

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 (Baker 1983). Another study identified a single adult which gorged on food at a rate of 2.7 grams per hour (Davis et al 1963).

Like most other bats, E. fuscus does not feed in heavy rain or when the air temperature dips below 10 degrees centigrade. In good weather they will begin foraging 20 minutes after sunset. They eat until full, and then often make use of a "night roost". This means the bat will hang under a porch or in a barn to rest while digesting its meal. It returns to its day roost before dawn (Kurta 1995).

Primary Diet: carnivore

Comments: Dependent upon flying insects; small beetles are the most common prey in many areas. Large size, powerful jaw muscles, and robust teeth allow predation on larger insects with tough exoskeletons (e.g., beetles). Basically a generalist in foraging habitat; forages over land or water, clearings and lake edges; may forage around lights in rural areas. According to Schmidly 1991, seems to prefer foraging among tree foliage rather than above or below the forest canopy. Distance from day roost to foraging area averages about 1-2 km (Brigham 1991).

Big brown bats choose secluded roosts to protect themselves from many predators. Young are often taken from maternity roosts by snakes, raccoons, and cats if they fall. Flying bats are sometimes captured by owls and falcons as they leave their roosts.

Known Predators:

• owls (Strigiformes)
• snakes (Serpentes)
• raccoons (Procyon lotor)
• domestic cats (Felis silvestris)
• falcons (Falconidae)

Note: For many non-migratory species, occurrences are roughly equivalent to populations.

Estimated Number of Occurrences: > 300

Comments: This species is represented by a very large number of occurrences (subpopulations).

100,000 to >1,000,000 individuals

Comments: Total adult population size is unknown but presumably exceeds 100,000. See Arita (1993) for information on population size in Mexico.

Males most often solitary in summer, or may roost with females or in all-male colonies. Winter colonies rarely number more than a few hundred. Less gregarious in winter; usually solitary in crevice, sometimes in small groups. When young flying, males may join nursery groups to form large late-summer colonies (Barbour and Davis 1969). Capable of living at least 20 years, though few actually attain old age.

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. Bats also make a number of audible sounds, they squeak and hiss at each other in the roost

Comments: Initial foraging period occurs within 5 hours after sunset; most activity within second hour after sunset; subsequently may retire to night roost. Flies less than 2 hours each night. In Alberta, both sexes exhibited daily torpor during the gestation, lactation, and postlactation periods; males were torpid more frequently and used deeper torpor than did reproductive females (Hamilton and Barclay 1994).

In temperate areas many do not appear at hibernacula until November (Barbour and Davis 1969). Apparently does not hibernate in Cuba; may become torpid on cool winter nights (Kurta and Baker 1990).

Big brown bats can survive up to 19 years in the wild and males tend to live longer than females. Most big brown bats die in their first winter. If they do not store enough fat to make it through their entire hibernation period then they die in their winter roost.

Range lifespan

Status: wild: 19.0 (high) years.

Average lifespan

Status: wild: 19.0 years.

Big brown bats tend to mate right before they go into hibernation, but the female does not become pregnant until the spring, in the beginning of April. 60 days after the female becomes pregnant, she gives birth to one or two babies. The average weight of a pup is 3.3g. The babies are born blind and with no fur, but they grow quickly and are able to fly by early July.

Eptesicus fuscus employs delayed fertilization as a reproductive strategy. In males, spermatazoa are produced beginning in October. This species mates just prior to entering winter hibernacula, however, copulation can take place if individuals wake during hibernation. Sperm is stored in the female tract until the beginning of April, at that time ovulation and fertilization take place (Baker 1983).

The female gives birth to one or two young after a 60-day gestation period. At birth, the young is blind, naked, has closed eyes, and weighs about 3.3 g (Kurta 1995). The young grow rapidly, their eyes opening after about seven days. Female big brown bats have two mammary glands to nurse the young which are then weaned and ready to fly by late June or early July. They may reach adult size in August (Baker 1983). Males are not involved in parenting.

Breeding season: Late fall

Range number of offspring: 1.0 to 2.0.

Average gestation period: 60.0 days.

Range weaning age: 18.0 to 35.0 days.

Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; fertilization (Internal ); viviparous ; sperm-storing

Average birth mass: 3.9 g.

Average gestation period: 35 days.

Average number of offspring: 2.

Average age at sexual or reproductive maturity (male)

Sex: male: 730 days.

Average age at sexual or reproductive maturity (female)

Sex: female: 547 days.

Females must eat at least their body weight in insects each night when they are nursing young. They leave the young in the roost while foraging.

Parental Investment: altricial ; female parental care

Copulates in fall and intermittently throughout winter. In temperate regions, ovulation and fertilization delayed until after hibernation. Gestation lasts 2 months. Young are born May-July, with slight trend toward earlier parturition in lower latitudes (Barbour and Davis 1969); mostly late May to June in Texas (Schmidly 1991). Litter size usually is 1 in western North America, 2 in eastern North America and Cuba. Lactation lasts 32-40 days; young fly at 4-5 weeks. Males usually are sexually mature in first fall; not all females reproduce at end of first year. Nursery colony rarely numbers more than a few hundred (mostly 25-75 adults in the eastern U.S.).

Tactile hairs in wing membranes of bats serve as flight control organs by providing immediate sensorimotor feedback.

Sonar of big brown bats enables navigation of dense or cluttered landscapes via frequency shifting.

Conflicts with humans can occur when the bats enter dwellings. Bats can be kept from re-entering a home if the holes used as entrances are blocked. This is best done at night once the bats have left to hunt for food. It should not be done during June or July when there may be flightless young bats remaining in the home (Kurta 1995).

Since big brown bats are beneficial in consuming agricultural or nuisance pests, it has been suggested farmers should actually encourage the bats to form maternity colonies. A further suggestion would be to design bridges to encourage bats to use them as roosts (Whitaker 1995).

Big brown bats are fairly common and are not of any special conservation concern.

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%.

US Federal List: no special status

CITES: no special status

State of Michigan List: no special status

IUCN Red List of Threatened Species: least concern

Canada

Rounded National Status Rank: N5 - Secure

United States

Rounded National Status Rank: N5 - Secure

Rounded Global Status Rank: G5 - Secure

Comments: On a range-wide scale, no major threats have been identified. Locally, the following factors may be significant.

Because E. fuscus appears to be a habitat generalist, readily uses human-made structures as roosts, and takes advantage of insect concentrations near lights, habitat is probably a less important conservation component than it is for other bats. However, current forestry practices may have a negative impact on tree-roosting bat species, and foraging activity has been shown to decrease with increasing urbanization, possibly because of lower insect abundance (Agosta 2002).

Big brown bats and other species that roost in buildings are often perceived as a nuisance and are vulnerable to exclusion and eradication attempts (Pierson 1998, Agosta 2002).

Big brown bats are vulnerable to the bodily accumulation of toxins (e.g., pesticides) because of their high trophic rank and longevity. Adverse effects of organochlorine pesticides (e.g., DDTs) on bats are well documented; pesticides can cause mortality, altered behavior, and can be transferred to nursing young. Although banned in the U.S., residues still persist in soils and accumulate in some bat populations. Pesticide exposure is likely an important cause of decline for some insectivorous bat populations (Agosta 2002).

Management Requirements: See Greenhall (1982) for information on house bat management. Introducing lights in roost area in building may reduce population by 41-96% (see Kurta and Baker 1990).

Biological Research Needs: The following biological research needs were identified by Agosta (2002): Research is needed to assess risks to reproduction and survival from pesticide exposure. Additional studies are needed to better understand roost selection by bats including both tree and building roosts. The level and effect of disturbance at roosts needs study, particularly in buildings that house maternity colonies. Studies that identify sources of population decline and important life-history requirements of abundant bats like E. fuscus could be useful in directing research for rare and endangered bat species.

Many people do not like sharing their homes with bats. The only way to keep them from entering homes or other buildings is to block the holes bats use as entryways.

People also have concerns regarding bats and the virus which causes rabies, all mammals are susceptible to the disease. However it is important to caution that people should not handle any obviously sick wild animal. Also, the risk of contracting rabies from bats is exaggerated.

Negative Impacts: injures humans (carries human disease); causes or carries domestic animal disease ; household pest

Big brown bats are insectivorous. They consume many insect pests, including common threats to crop plants. They eat the corn root worm which may be the single most important agricultural pest in the United States (Whitaker 1995).

Positive Impacts: controls pest population

Comments: Koopman (1989) included both E. fuscus and E. lynni in E. serotinus. Jones et al. (1992) used the name E. fuscus for this species. Koopman (in Wilson and Reeder 1993) and Simmons (in Wilson and Reeder 2005) listed E. fuscus and E. serotinus as separate species but noted that the two may be conspecific; Koopman and Simmons both included E. lynni in E. fuscus.

Eptesicus fuscus exhibits significant morphological variation across its range and is represented by 11 subspecies (Agosta 2002). Subspecies fuscus and pallidus apparently intergrade in northwestern Texas (Jones and Manning 1990).

The relationships of the genera Eptesicus and Pipistrellus are unclear; for several Old World species there is some uncertainty as to which is the appropriate genus; the species of Eptesicus that are chromosomally characterized by 2n=50 and FN=48 form a genetically homogeneous group, despite the included taxa coming from different continents (Hill and Harrison 1987, Morales et al. 1991).