The big brown bat is aptly named, since it is one of our largest bats and is uniformly brown on the upperparts, ranging from dark brown to cinnamon brown. The underparts are lighter colored than the back. The wing and tail membranes, face, and ears are blackish. Only rarely is an individual grizzled or blotched with white.
The thick, naked ears are broad with rounded tips. The tragus (projection at the base of the ear hole) is less than half the length of the ear, is slightly curved forward, and has a broadly rounded or blunt tip.
For your safety and the safety of bats, do not handle bats. Biologists who study bats receive special training and pre-exposure rabies vaccinations. Bats are not aggressive toward people but will bite in self-defense if handled. If you absolutely must handle a bat, wear heavy leather gloves. For more information about bats, see the bats group page.
Similar species: The big brown bat is most likely to be confused with the evening bat, which is very similar in overall appearance (coloration) but is markedly smaller: the total adult length (tip of nose to tip of tail) not exceeding 95 mm (3¾ inches), and the weight not more than 14 g (0.5 oz.). Thus the largest evening bats are not larger than the smallest big brown bats.
The big brown bat might also be confused with our six species of mouse-eared bats (genus Myotis) and the tricolored bat. However, these species can be distinguished by noting differences in size, color and character of the fur, and shape of the ear and tragus.
In terms of size, big brown bats are almost as large as Missouri’s largest bat, the hoary bat; but that species looks quite different because of the frosty-looking (white) tips of its fur. Also, it is less frequently seen.
Total length: 95–127 mm (3¾–5 inches); tail length: 34–50 mm (1¼–2 inches); weight: 14–31 g (about 0.5–1.1 ounce); ear length: 15–19 mm (¼–¾ inch).
Statewide.
Habitat and Conservation
Because big brown bats are widespread in Missouri and North America — and because they are relatively large and frequently roost in people’s attics and walls, in urban, suburban, and rural areas — they are one of the most commonly encountered bats in the state.
In summer, the females establish nursery colonies of up to 2,000 individuals in hollow trees, attics, chimneys, lofts, or caves. Males generally roost solitarily in sheltered spots in human-made structures, including behind house siding or window shutters, or in trees.
As freezing weather arrives, big brown bats start to hibernate in a variety of well-protected sites. Overwintering places include deep crevices in rocky cliffs, caves and mines, storm sewers, road culverts, burial vaults, and cooler human structures. They either hang by their hind feet or become dormant in a horizontal position depending upon the nature of the resting place. They usually occur singly or in groups of 2 to 20, and rarely up to a couple hundred. When they use caves for hibernation, they select the coldest portions and are often found near the entrance in total daylight.
During winter, there are brief periods when big brown bats become active, fly about, lap up moisture, and occasionally mate. If they live in caves or buildings, they fly about their hibernating quarters and may go outside; occasionally they even move to a different site.
Like most bats, big brown bats are nocturnal, leaving their daytime roosts about sunset to feed. In foraging, they fly along watercourses at the forest edge and above cropland and roadways. They generally fly below or within the canopy of trees.
Big brown bats are guided in flight by means of their ultrasonic cries (inaudible to humans), which are reflected back from obstacles and received by the inner ear. Like all insect-eating bats, they hunt and pursue their prey by this same means.
Food
Big brown bats feed on insects, especially beetles and true bugs; examples include May beetles (June bugs), ground beetles, and green stink bugs. They also devour spotted cucumber beetles and several other types of crop pests. A single colony easily consumes hundreds of thousands of pest insects in a single summer.
These bats catch and eat most of their food on the wing. Sometimes, however, they bring large insects back to the roost to devour. They can consume one-third of their weight in food each day. Lactating females may consume a mass of insects equal to their own body mass.
Status
Native Missouri bat. Like all of our bats, considered beneficial for their role in reducing populations of insect pests.
Life Cycle
Compared to most other bats, big brown bats migrate only short distances (such as 10–50 miles) between their summer and winter spots, or between winter hibernating spots. This species is very hardy and is one of the last to fly about in fall.
Early in April the females leave their winter quarters and form nursery colonies of some 30–2,000 individuals. These may be in very protected places, but after a few weeks, when the young begin to fly, the whole colony may move to less-sheltered quarters. They apparently return to the same site(s) each year.
The males stay in their hibernating locations for some time after the females leave. During spring and early summer they are solitary, often roosting in a crevice. But after the nursery colonies have dispersed, they may be found in roosting sites with the females. Rarely do nursery colonies remain intact all summer.
Mating occurs in the fall and occasionally during winter when the bats become active, and again in spring. As with many other bats, the sperm received in the fall or winter matings remains dormant in the female during hibernation; the eggs become fertilized in about the first week in April. Only one or two of the fertilized eggs develop fully. After a gestation of about 60 days, one or two young are born sometime from the end of May to late June.
At birth the young are naked and have their eyes closed. They weigh about 2.8 g (0.1 oz.). The eyes open about the second day. The female leaves her young in the nursery colony when she goes out to feed. When she returns, she carefully selects her own young to nurse. If the colony moves to an alternate roost, the female carries her young in flight.
The young are able to fly by themselves at about 3–4 weeks of age. When two and a half months old, they are the same size as adults. All young males become sexually mature in their first fall, but only about three-fourths of the young females do so at this age.
Big brown bats may live to be nearly 20 years old.
Control
Human Connections
Big brown bats are a help to humanity because of their role in decreasing and controlling the numbers of insect pests: insects that damage crops and orchards, lawns and horticulture, and forest trees, and insects that pester humans.
A great example of how they reduce pests is their feasting on spotted cucumber beetles, one of their favorite foods. The cucumber beetle is a pest on a wide variety of crops: cucumbers, melons, and other cucurbits, plus beans and peas. The adult beetles eat tender young stems, leaves, buds, and flowers. The larvae, called corn rootworms, burrow in soil and attack the roots of corn. Cucumber beetles also spread bacterial wilt and cucumber mosaic virus.
- A single colony of 150 individual big brown bats, over the course of a single Midwestern summer, can consume 600,000 cucumber beetles as well as 335,000 stink bugs, 194,000 scarab beetles, and 158,000 leafhoppers. These insects are crop, garden, lawn, and/or orchard pests.
- More impressively, because that colony of 150 dines on so many adult cucumber beetles, they are estimated to prevent 33 million of the beetle’s larvae, the corn rootworm, from ever being produced.
Big brown bats are common around houses and neighborhoods. They are interesting to watch. Relax on your patio as the stars are coming out and look for them fluttering silently among the trees and rooftops, foraging for insects.
Although the populations of many bat species have declined due to human activities and habitat destruction, the big brown bat has benefited by the additional roosting habitat created by people’s homes, barns, vacant buildings, culverts, bridges, and so on.
Because big brown bats frequently enter people’s homes, they are one of the species that wildlife-control experts often deal with. See MDC’s Bat Control page for more information on this subject. All bats are protected by the provisions of the Wildlife Code of Missouri. Although the Code allows landowners to take action when wildlife is damaging property (see 3 CSR 10-4.130 Owner May Protect Property; Public Safety), nine bat species are listed as species of conservation concern, and three are classified as state endangered. Please exercise caution when addressing bat-related problems.
”Tally-ho! June bug at two o’clock” is the title of a section in Donald R. Griffin’s 1958 book Listening in the Dark. Griffin (1915–2003) was the researcher who, starting in the late 1930s, explored bats’ ability to navigate in flight by emitting ultrasonic “clicks” and sensing the echoes bouncing off of objects (including flying May beetles/June bugs). Griffin was the first to call this type of sonar “echolocation.” His “tally-ho” phrase highlighted the amazing hunting abilities of echolocating bats. The big brown bat and the little brown bat were two of the principal species he studied.
People often discount the value of biological research; “what practical value does it have?” Donald Griffin’s work on animals’ ability to detect wave echoes extended beyond bats — to insects, fish, whales, dolphins, nocturnal birds, and also humans. Yes, humans: sonar and radar were increasingly being used by instruments in navy vessels and airplanes, and Griffin was one of the first to demonstrate how blind people unconsciously develop their ability to hear soundwaves reflecting from nearby objects, helping them navigate as they walk.
This is an important story in the history of science: Donald Griffin began his bat research at Harvard University as an undergraduate. He used clever but fairly simple ways to test bats’ ability to perfectly navigate among closely spaced obstacles in complete darkness. Then, he moved his bats from the biology department to Harvard’s physics lab, which at that time was the only place in the world with ultrasonic microphones. There, he showed that the bats were making, hearing, and using lots of noises that humans cannot hear. But his scientific breakthrough wouldn’t have happened if he had not combined the cutting edges of two very different areas of science: biology and physics.
Griffin solved a century-old mystery that had been explored by a famous Italian scientist, Lazzaro Spallanzani. In the late 1700s, Spallanzani determined that bats could fly perfectly well in complete darkness, and in lit areas even when deprived of vision, but found that their navigation skills were devastated when they were deprived of their ability to hear. Spallanzani, not knowing that ultrasounds existed, was stumped to explain how the sense of hearing could be involved with the bats’ seemingly silent flights.
The big brown bat’s genus name, Eptesicus, is based on petomai, Latin for “house flier,” and refers to this species’ habit of living in houses. The second part, fuscus, is the Latin word for “brown” and describes the color. The common name, “big brown,” refers to the size and color.
Ecosystem Connections
Bats are also an important control on insect populations, and they are preyed upon by many kinds of animals. Western ratsnakes (black snakes), raccoons, and other predators eat bats as they are roosting, hibernating, or leaving a shelter to forage for an evening.
As the only mammals capable of true flight, bats are a unique and cherished life form on planet Earth.
Bat species can be identified by researchers by analyzing the frequencies and patterns of their echolocation calls. Bats that live in dense, cluttered forests or eat tiny prey click at higher frequencies, at fairly soft volume, for high resolution at close quarters. Bats that live in more open habitats have lower frequencies but more powerful voices, so they can detect objects that are farther away. Bats typically increase their pulse rate as they home in on a target; just as they dart in for the capture, the pulses speed up to a fast flutter.
The predator-prey relationship between bats and moths has led moths to evolve many fascinating ways to avoid being captured by bats. Several types of moths, upon sensing the sonar clicks of bats — and especially the “attack” flutter clicks of a bat moving in for the kill — will immediately, reflexively, engage in erratic flight patterns, or produce sounds that can confuse or jam the bats’ echolocation abilities.
Big brown bats are one of the few temperate-region bat species that are known to eat in wintertime. In the warm-blooded bodies of mammals, food equals energy for shivering, which warms the body and leads to survival in winter. Considering they frequently become active in winter, the additional food may be necessary for enduring the hibernation periods.
