Eptesicus fuscus
big brown bat

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).

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).

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).

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.

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 (Nowak 1994).

One published study focused on maternity colonies roosting in buildings in Kentucky. The data came from a group of 40 adult females. They found that when the bats were in late pregnancy, the females were reluctant to fly. Also, newborn young are not carried by the mothers during feeding flights, instead they are left behind in a cluster. Apparently, the mother only moves the young to transport them from one day roost to another (Davis et al 1968).

A mother Eptesicus fuscus can recognize her own young after returning to the cluster. In this study, the young bat tried to grab at any adult that came near it. The mothers crawled around among the group looking for their own young. Once found, the mother would lick the baby around the lips and face prior to nursing (Davis et al 1968).

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 (Davis et al 1968).

The big brown bat hibernates in various structures, either man-made or natural environments. They prefer cool temperatures and can tolerate conditions many other bats cannot. They may become active during their winter hibernacula and can move to an optimum habitat. One banded bat was recorded to have moved to a different cave 400 yards away, during the same winter (Goehring 1972).

The stimuli that cause mammals to hibernate are not well understood, however, in bats it seems that decreasing ambient temperature is the primary factor. The failure to accumulate enough fat for a long winter is a major mortality factor particularly for younger, less experienced bats (Kurta 1995). The bat's circulation system slows considerably and oxygen consumption and heart rate are greatly reduced. Heart rate in torpor is 4 to 62 beats/minute, at 5 degrees centigrade. Heart rate during arousal from hibernation increases from about 12 to 800 beats per minute (Kurta and Baker 1990). By the end of this hibernation period, the bat may have lost up to 25% of its weight prior to hibernation (Fenton 1983).

One study of the big brown bat found it was capable of hibernating for 300-340 days. It was kept in a refrigerator at a constant low temperature, provided only with water (Hill and Smith 1984).

In the 1930s, echolocation pioneer Donald R. Griffin took some bats, including E. fuscus, into a lab which had a microphone sensitive to ultrasonic sound. He used a battery of experiments to show that the bats used the echoes of their calls to locate obstacles (echolocation) (Fenton 1983). Since then it has been found that echolocation calls in bats often include frequency modulated (FM) and constant frquency (CF) components. In E. fuscus there is a short FM signal (1-5 ms) that sweeps downward from 50 kHz to 25 kHz, the CF portion is often deleted or shortened (Hill and Smith 1984).

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 (Simmons et al 1996). These bats emit volleys of calls through their open mouths. 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 (Nowak 1991).

Typically, insectivorous bats will increase the rate of echolocation calls as they close in on prey. The calls terminate in what is described as a "feeding buzz", a high pulse repetition rate associated with an attack on prey (Nowak 1991).

Some data suggest big brown bats forage orient toward the loudest natural sound fields. This tends to correlate with high densities of insect prey. They were able to detect chorusing cricket frogs and katydids over hundreds of meters away (Bucher and Childs 1981). Another study showed these bats were first able to detect 19 mm diameter spheres at 5.1 meters and 4.8 mm spheres at 2.9 meters (Altringham 1996). Numerous studies have been done in connection with the big brown bat's echolocating capacities and it is worth further reading to understand the depth and sensitivity of their abilities.

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

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).

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.

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.

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).

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.