VESPERTILIONID BATS – Vespertilionidae



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, cropdamaging 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.


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.