Vespertilionid Bats: Vespertilionidae

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Although this group of bats is large and contains many different-looking species, they do share several characteristics. Compared to many other bats that have what might be described as bizarre noses with flaps and other fleshy decorations, the vespertilionid (ves-per-TILL-ee-on-id) bats have plain faces. They are even known as the "plain-faced bats." Some species have noses shaped like tubes, however, with nostrils at the end of the tube.

The "webbed tail," known as a patagium (pah-TAY-jee-um), is actually a membrane or a thin bit of skin that stretches between the hind legs and aids the bat in flight. In these bats, the patagium is hairless. Their ears are noticeable and sometimes quite large, and they also have tails at the middle of the patagium that can be as long as the body. All have an obvious outgrowth, called a tragus (TRAY-gus), arising from the bottom of the ear. Most of them have small eyes. Overall body length ranges from about 1.4 to 5.5 inches (3.5 to 14 centimeters) and weight from 0.01 to 1.6 ounces (2.5 to 45 grams).


Vespertilionid bats live in temperate to tropical climates worldwide. They are absent from far northern North American and Eurasia, as well as Antarctica.


The habitat varies in this large group of animals. Many of them spend the day resting in caves, or in tight little places, like cracks in a house or a barn, underneath bark or in the hollow of a tree. Some even rest during the day, a behavior called roosting, inside curled leaves or in other sheltered spots within vegetation. At night, when they become active, the bats are often seen flying above open spaces, or over or near wetlands, rivers and streams, and lakes and ponds. During winter months, the bats typically hibernate. In colder climates, the bats overwinter in caves or other places with relatively stable temperatures. In warmer climates, they may simply choose a spot beneath a loose piece of bark or in the hollow of a tree.


The diet for most of the vespertilionid bats consists of insects, and many species eat their body weight in insects each night. A few species eat other things, including spiders, scorpions, fish, and lizards.


Like other bats, the vespertilionid bats use sound waves to find their way through their habitat and to find food. They make high-pitched sounds, ones that we cannot hear, and then listen as the waves bounce off of objects and return to them as echoes. Using this method of "seeing" with sound, they can fly quickly between tree limbs and around objects, while also finding and identifying prey insects. It is common for a vespertilionid bat to notice a moth or other flying insect while both the bat and insect are in flight, then swoop in and capture the insect in midair. Using echolocation (eck-oh-loh-KAY-shun), they can also spot insects on plants and pick them off of leaves. Echolocation is particularly useful in these animals that rest during the day and look for food in the dark of night. A few species become active around sunset, sometimes even a little earlier, but most wait until the skies darken before they leave their roost and begin looking for food. Because they are such excellent and swift fliers, the vespertilionid bats avoid most predators. Occasionally an owl is able to catch one at night, but their biggest threat of predation (hunting by animals that eat them for food) comes from larger land animals that stumble upon a roost while the bats are resting.

Bats have a fairly set schedule with certain activities occurring during specific seasons. Mating occurs in the fall in most species. Some bats don't engage in any courtship rituals, but for the most part, scientists know little about these behaviors in most bats. In the fall, bats that live in cooler climates begin to disappear, probably to start migrating to warmer climates for the winter. Cool- and warm-climate bats typically participate in hibernation, although some warm-weather bats remain active all year. Some vespertilionid bats hibernate alone, and others hibernate together in large groups, often numbering a hundred or more. If the temperature rises sufficiently in the winter, the bats may awaken and fly about in search of food. When spring arrives, males typically strike out on their own, but females usually form colonies in roosts, which may be in caves or other hideaways, and share the duties associated with raising young, which are born in late spring to early summer. (A few warm-weather species may be able to have young at other times of the year.) Most mothers have one or two young, called pups, a year. A few species may have up to four pups at a time. The pups begin flying in about a month and then start hunting for insects on their own. Some remain with the colony for their first year, but others leave earlier.

Bat behavior is a field with many unanswered questions. Although scientists know a good deal about the behavior of a few species, they know little about most of the vespertilionid bats.


Humans frequently don't recognize the benefits of bats. Vespertilionid bats eat many insects, including mosquitoes, crop-damaging beetles, and other pest species. Just five bats can eat 15,000 or more insects in a single night. Besides their benefit in keeping insect populations in check, bats have become a part of the folklore of many cultures. Much of the folklore, including that portrayed in horror books and movies, describes bats as evil creatures bent on sucking blood. Vespertilionid bats engage in no such activity, and rarely even fly close to a human.


When people hear about a fall migration, they usually think of birds that fly south for the winter. Other animals, including bats, migrate, too. Some bats may fly several hundred miles (kilometers) to escape the winter cold. The noctule, a medium-sized bat from Europe and Asia, makes migrations of 400 miles (670 kilometers) or more each year—quite a feat for an animal that is only about 3 inches (7.6 centimeters) long in body length and weighs about an ounce (28 grams).


The Red List of the World Conservation Union (IUCN) contains two Extinct, died out, species; seven Critically Endangered, facing an extremely high risk of extinction in the wild; twenty Endangered, facing a very high risk of extinction in the wild; fifty-two Vulnerable, facing a high risk of extinction; and seventy-three Near Threatened, not currently threatened, but could become so. Those categories total 154 bats, more than half of all vespertilionid species. The U.S. Fish and Wildlife Service lists thirteen vespertilionid bats as endangered. For many of the species, habitat destruction and pesticide use are major reasons for their declines. Both organized and grassroots efforts are now under way to protect many bat populations. These include the preservation of roosting and hibernation sites.


Physical characteristics: Unlike many of the dark-furred, small-eyed vespertilionid bats, the pallid bat is yellowish with larger eyes. It also has large ears. Adult size ranges from 3.6 to 5.5 inches (9.2 to 14 centimeters) in body length and 0.5 to 1 ounce (13 to 29 grams) in weight. Its tail is a little more than a third of its body length.

Geographic range: The pallid bat lives in western North America from southern Canada to northern Mexico, also western Cuba.

Habitat: Their daytime roosts are in cracks and crevices of rocky outcroppings and in caves, usually near a water source. At night, they typically roost nearby in tree hollows, under bridges, or in some other hiding place.

Diet: These bats eat insects, scorpions, and other invertebrates (animals without backbones); possibly small lizards and mammals, such as mice.

Behavior and reproduction: They leave their daytime roosts after sunset, then begin looking for insects by flying between about 1 and 7 feet (30 centimeters to 2.1 meters) above the ground. They mate in fall to early winter, and females give birth to one or two pups in late spring to early summer. The young stay with their mothers in maternity roosts, and begin flying about a month and a half later.

Pallid bats and people: This species visits plants, probably in search of insects. In so doing, it picks up and delivers pollen, which helps fertilize plants.

Conservation status: The pallid bat is not threatened. ∎


Physical characteristics: This large-eared bat ranges from 1.8 to 2.4 inches (4.5 to 6.0 centimeters) in body length with a tail nearly as long, and 0.2 to 0.4 ounces (6 to 12 grams) in weight. Its back fur is black with white tips, and its belly fur is lighter.

Geographic range: The western barbastelle lives in central and northern Europe.

Habitat: These bats prefer upland forests, usually near water.

Diet: This species eats mainly flying insects, which they catch in midair. They will also swoop down to plants and pluck insects from their leaves.

Behavior and reproduction: This bat becomes active before sunset when it emerges from its daytime roosts in trees, caves, and other secluded spots. It is more solitary than many other vespertilionid bats, with many individuals spending the summer alone. Females will sometimes form small maternity colonies. Hibernation begins in late fall. Many questions remain about this rather rare bat's behavior.

Western barbastelles and people: Like other insect-eating bats, the western barbastelle rids its habitat of many insects that humans might consider pests.

Conservation status: The IUCN Red List lists this bat as Vulnerable. ∎


Physical characteristics: Similar in appearance to the big brown bat, this species is a bit smaller. Its body length averages 3.1 to 3.7 inches (20 to 27 centimeters) with a tail a little less than half that size. It weighs 0.2 to 0.5 ounces (6 to 14 grams). The little brown bat is light to dark brown above with a lighter belly.

Geographic range: This bat lives in Canada, the United States, and Mexico.

Habitat: When they aren't flying in search of food, they do their summertime resting in tree hollows, underneath bark, or in barns, attics, and other such structures. During winter, they typically hibernate in caves.

Diet: Their diet consists mainly of flying insects.

Behavior and reproduction: They mate in the late summer to early fall. The females typically have just one pup in late spring or during the first half of summer. The young grow quickly, but don't mate until at least the following year.

Little brown bats and people: Like many other insect-eating bats, the little brown bat helps to control pest insect populations.

Conservation status: This bat is not threatened. ∎


Physical characteristics: Unlike other vespertilionid bats, bentwing bats have a long third finger that they can bend beneath their wing when they aren't flying. The common bentwing bat has a thick gray, yellow, or brown fur coat. It ranges from 2.0 to 3.1 inches (5.1 to 7.8 centimeters) in body length and weighs 0.3 to 0.6 ounces (8 to 16 grams). Its tail is about as long as its body.

Geographic range: This bat lives in Madagascar, southern and northwestern Africa, southern Europe, southern Asia, eastern and northern Australia, and New Guinea.

Habitat: They tend to prefer woodlands and fields that are near caves or other roosting sites.

Diet: Adult bats will eat up to a third of their body weight in flying insects every night.

Behavior and reproduction: They mate in the fall, and females typically give birth to one pup each summer. The females form large maternity roosts where they raise their young together. A roost can contain several thousand pups. The pups are old enough to mate and have their own families in about a year. Predators for common bentwing bats include owls that may occasionally catch the bats in the air, as well as snakes and cats that may find a roost.

Common bentwing bats and people: Like most other bats, the insect diet of this species helps to keep pests in check.

Conservation status: The IUCN Red List considers this species Near Threatened, likely due to predation and disturbance to maternal roosts. ∎



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Kurta, A. Mammals of the Great Lakes Region. Ann Arbor: The University of Michigan Press, 1995.

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Web sites:

IUCN 2003. 2003 IUCN Red List of Threatened Species. (accessed July 5, 2004).

Endangered Species Program, U.S. Fish & Wildlife Service. (accessed on July 5, 2004).