The Art and Science of Bats Bat Facts

This fact sheet attempts to dispel the fears and answer some of the questions most often asked of the National Museum of Natural History by presenting some general facts about the biology and natural history of these shy, nocturnal creatures.

What are they? 

Bats are mammals belonging to the order Chiroptera, a name of Greek origin meaning "hand-wing," which accurately describes the animal's most unusual anatomical feature. The order is divided into two suborders, the Megachiroptera, consisting of a single family, the flying foxes and their Old World fruit and flower eating relatives, and the Microchiroptera, composed of the rest of the bat families, some 17 in all. These families are further classified into about 180 genera and over 900 species; only rodents have a greater number of species. Even though the names imply otherwise, not all Megabats are larger than Microbats. Although it is true that certain species of flying foxes have wingspans of up to 5 feet, one member of the Megachiroptera, the flower-feeding Macroglosus, has a wingspan of only 10 inches. However, some of the Microchiroptera are very tiny; the smallest is probably the Philippine bamboo bat, Tylonycteris pachypus, its forearm measuring only 22mm. and weighing only 0.05 ounce. The largest Microbat is the tropical American false vampire, Vampyrumspectrum, with a wingspan of up to 40 inches.

Where are they?

Because bats are small, secretive, feed at night, and are unfamiliar to most people, they are sometimes regarded as rare. On the contrary, they are found throughout the world except for certain oceanic islands, the Arctic and Antarctic. Though most species inhabit the tropical and semitropical areas of the world, they are still common in the United States and are most numerous in the Southwest. Some bats prefer to roost in barns, attics, caves or abandoned mines, those shelters providing safety from predators, protection from fluctuations in weather, and seclusion for rearing the young. Other species select hollow trees or rock crevices as their daytime resting site, while certain ones are known to roost in exposed locations, clinging to tree trunks or hanging upside down from tree branches. To survive in colder regions, bats either hibernate in shelters with high humidity and temperatures above freezing, or migrate to warmer areas where food is available.

When did they appear? 

Evidence for bat-like flying mammals appears as far back as the Eocene Epoch, some 50 million years ago; however, the fossil record tracing bat evolution is scanty. Based on similarities of bones and teeth, most authorities agree the bat's ancestors were probably insect eating placental mammals, possibly living in trees, and likely the same group that gave rise to shrews and moles. Bats are not rodents and are not even closely related to that group of mammals.

How do they fly? 

Bats are unique. Even though they share the characteristics of all mammals - hair, regulated body temperature, the ability to bear their young alive and nurse them - bats are the only mammals to truly fly. The commonly named "flying!' squirrels and "flying" lemurs actually glide or parachute by means of a furred membrane, but only bats have the structural adaptations that allow for full powered flight.

The structure of the wing membrane, the arrangement of the bones supporting it, and the positioning of the muscles provide the bat with the lightness and maneuverability necessary for catching insects, hovering above flowers, or quickly avoiding obstacles. Extending from the sides of the body and incorporating the hands, legs, and tail are the two thin layers of skin containing blood vessels, nerves, and tendons that essentially make up the wing membrane.

The bones of the arm and four of the fingers are light, slender, and lengthened to support, spread, and manipulate the membrane. The thumb, usually with a sharp claw, is not attached but remains free for clinging to various surfaces. Unlike other mammals, the hind limbs are positioned so that the knees, when bent, point backward while the bottom of the feet face forward. The toes have claws which help in gripping and in hanging head down, the normal bat resting position. A long spur, the calcar, extends from one of the ankle bones and helps spread the interfemoral membrane, that part of the wing membrane between the tail and the hind legs. This membrane can form a basket or pouch to help catch and hold insects captured in flight.

Most of the flight muscles controlling the wing beat are attached to the shoulder blades, unlike birds, whose muscles are braced against the rib cage. Birds usually have a prominent breast keel where heavy muscles are attached; in bats, only one muscle is attached to the breastbone, the ribs are flattened, and there is some fusing of the vertebral joints, all adaptations that make the frame light and give the bat its incredible agility.

What do they eat? 

Although various bat species eat different kinds of food, the vast majority consume a variety of insects such as moths, beetles, gnats, and crickets. In this respect, the bat's nocturnal behavior provides certain advantages. Enormous numbers of insects fly at night, and with the exception of spiders, there are few competitors for such food. Other than the occasional owl or snake, there are also few predators that can capture or pursue a bat in the dark. Night brings cool temperatures which help dissipate the heat generated by the muscular activity of flight. Because the bat has a thin wing membrane, flying during the heat of the day could be hazardous causing excessive absorption of heat and resulting in dehydration and possible heat prostration. Nocturnality offers protection from the heat and helps the bat maintain its body temperature and moisture.

"Blind as a bat" is a common saying yet one that is false. All bats can see, even though vision may be less important than other senses. To locate and catch prey, insectivorous bats use an acoustic orientation called echolocation. They emit a series of supersonic cries through the mouth or nose and detect flying insects by the echoes reflected back. Those species that produce sound through their noses usually have a flap of skin called a nose leaf above the nostrils. This may help to direct the sound, but its precise function is not known. The man credited with discovering that bats emit sounds in the ultrasonic range and navigate in response to echoes is Donald R. Griffin. Since his first findings were announced in 1940, numerous studies have been made in the field of echolocation, and it is the one area of bat life that has been investigated in any depth.

In addition to the ultrasonic sounds used in echolocation, bats also emit other sounds possibly to communicate or to indicate emotion. Purrs, clicks, and buzzing often precede mating of some species; some of these sounds may be ultrasonic. It has been observed that certain North American insectivorous bats vibrate when at rest and content. This vibration does not occur when they are asleep. Also, recognition of mothers and babies involves both audible and ultrasonic sound. The bat's ear is extremely mobile and sensitive to sound. The tragus, a lobe projecting in front of the ear opening, may have a sensory function; however, its exact use is not known.

According to analyses of stomach contents and the insect remains found near bat roosts, species vary in the insects they prefer. For example, the cave myotis, M. velifer, is known to enjoy a diversified diet of beetles, leafhoppers, flies, and moths while the Malaysian free-tailed bat exhibits greater selectivity and seems to prefer to catch ants engaged in nuptial flight. Small insects are often caught directly in the mouth, but larger ones are more frequently captured by the wing membrane, transferred and readjusted in the basket formed by the hind legs and the interfemoral membrane, then pulled to the mouth and eaten.

Not all bats seize their prey on the wing. Some specialize in catching land insects like grasshoppers or roaches that are found on the ground, rocks, or trees. In this instance, less energy is required to catch a few large insects than hundreds of tiny moths or gnats. Other bats use their feet to snatch insects from the surface of ponds and lakes. With the exception of three flower-eating species that migrate from Mexico, the bats in the United States are insectivorous.

Those species that eat pollen, nectar, and in some instances, petals from flowers, are found mainly in the tropics and semitropics. Pollen, like insects, is highly nutritious. The flower-eaters are usually small and have long pointed heads, long tongues, and the ability to hover and fly slowly. Fruit and sometimes insects also supplement their diet.

Most of the Old World fruit and flower eaters rely on vision and sense of smell to locate food rather than sonar. Unlike birds, bats are color blind and, therefore, not dependent on color for locating and choosing their food. Many prefer dark green or brown fruits with musty or sour odors instead of the more colorful varieties. These fruits usually hang from vines or grow from the trunk of a plant unobstructed by heavy tropical foliage.

A few species of bats are carnivorous, eating small frogs, lizards, and birds. Other species are known for their fish-eating habits and are usually specialized by having huge hind feet and claws. The best known of the fish-eating bats is the fishing bulldog bat, Noctilio leporinus, of Central and South America. How this bat actually finds its food is not completely understood, but many authorities believe that sonar is used to detect ripples on the water made by the fish. It catches its prey with large hooked claws, spears the fish with its canine teeth, gathers it up in the wing membrane, bites through the skull and eats it, all in a matter of seconds.

Can bats swim?

Although there is little scientific data on the subject, observations by naturalists in the field seem to support the fact that some bats swim in stressful situations but that it is not normally part of their ordinary behavior patterns. For example, flying foxes, often island inhabitants, may have to fly long distances to obtain food. A forced landing or a foray over water to collect fruit which has dropped and floated there may necessitate an unexpected swim. Photographs of the flying fox, Pteropus giganteus, show the animal actually swimming, using its wings and feet to reach land rather than floating or paddling.

Vampire Bats

Many questions received by the Museum concern the vampire bat. Because the true vampire bat of Central and South America feeds on blood, a popular misconception has been to link it to the human vampire legend. The Eastern European tale of a vampire, a corpse that came back to life and sucked blood from the neck of its human victim, dates back to the Middle Ages. In fact, there are no vampire bats native to Europe or Asia; they were not even known to exist before the 1500's when explorers visited the New World and observed their unusual eating habits. Indeed, the bat was named for the legend rather than the legend originating with the bat.

The vampire bat feeds mainly on the blood of cattle, horses, and wild mammals such as deer and peccaries. It seldom bites humans. When feeding it chooses to land near a hairless area of thin skin -- perhaps the hoof or shoulder of the animal -- hop or walk cautiously to a likely spot, lick it, then make a small incision and lap the animal's blood. The harm from such bites arises not from the amount of blood lost, which is relatively small, but rather from the exposure of the livestock to secondary infections, parasites, and the transmission of viral-borne diseases such as rabies.

Rabies occurs naturally in many wild animals. Actually, a higher incidence of the disease is found in skunks and foxes than in bats. In the United States the rate of occurrence is so small, barely a fraction of a percent, that there is very little danger to humans. Though experiments have shown that theoretically rabies is a virus that can be air-borne or transmitted through contact with infected bat urine, there is not one documented case of a human contracting the disease this way. Bat bites of humans are uncommon and rabies in humans resulting from such bites is extremely rare. But, bats can harbor the rabies virus and, therefore, should be handled with caution.

The greatest incidence of rabies occurs in the large vampire bat populations found in South America. Here the danger is not so much to the human population but rather to the livestock exposed to bat bites. In South America a major economic problem existed when large numbers of cattle were lost to rabies and infection. An effort to control and eradicate the vampire bat was initiated in 1968 by the Agency for International Development in cooperation with the Mexican government. The U.S. Fish and Wildlife Service was assigned the task of studying its habits to obtain some clues for its control. Using several different methods of nighttime surveillance, observation of the bat's grooming behavior provided the answer. Bats are extremely clean, sometimes grooming, licking, and scratching themselves for hours. The vampire bat, in particular, grooms both itself and its neighbors intensively. A paste-like anticoagulant poison that causes internal bleeding in the bat was smeared on likely areas, fetlocks or shoulders, of cattle that had been bitten. Vampire bats usually return to their victims for more food, so when they attack again, they come in contact with the poison. Back at the roost in the process of grooming, they ingest and spread the poison with the subsequent death of a large percentage of the colony. The treatment is specific for the vampire bat; no other species is affected, and the anticoagulant does not appear to hurt the livestock even when injected directly into the stomach. At the present time, the poison is being used successfully in Mexico, Central and South America.

What is the bat's life style? 

For humans, daytime brings work and play. For bats it is the time to rest in some secluded shelter, hang head down, groom, sleep, and perhaps even quarrel. When night approaches, bat activity increases; they fly about inside their cave then finally emerge to forage for food and drink water. After feeding for an hour or two they may rest again, then have a second feeding before daybreak.

In many species, the males and females roost separately except when mating. In migratory species mating occurs in the fall and winter. The female stores the sperm until spring when ovulation and fertilization occur. In May or June the females congregate in large colonies and give birth. Most species bear only a single young per litter, but others may have two, three, or even four. The female hangs head up as the young is born, feet first. She catches and holds the new born in the pouch formed by the interfemoral membrane. The baby bat, already large and well developed, crawls to the mother's nipples, attaches itself and feeds. In the evening when the mother forages for food, she may, for the first few days, carry the young with her. Later the baby remains behind, clinging to the wall or roof of the cave or shelter. The mother may return several times during the night to feed her young.

How long do they live? 

Among young bats, mortality is high. Some fall from the roof of their shelter and are not able to climb back; some succumb to disease or parasites. However, they do grow rapidly and are able to fly in about three weeks. Once the hazards of youth are over, bats enjoy a relatively old age, some reaching the age of 20 and in one documented case of a brown bat, to 30. Certainly they attain a longer life span than other mammals of similar size such as rodents that live for only one or two years.

How do they affect us?

The Smithsonian frequently is asked how to get rid of a bat or bats that invade a house or building. If a lone bat flies into your home, simply open all the doors and windows and allow it to find its own way out. If this is not successful, take a large towel or blanket, throw it over the animal, gather it up and put it outside. The bat will extricate itself from the cloth without any additional handling.

Eradicating large numbers of bats from buildings or barns is much more difficult since once they choose a place to roost, they continue to return to that site. The best approach is to take preventive measures. Screen various parts of the roof so that there are no crevices or openings where a bat can hide, or repair the roof or building to eliminate entrance holes. Sprays are not advised and can be much more dangerous than the presence of the bats.

In the United States bats are beneficial members of the animal community. They help to control night-flying insects, in some instances pollinate flowers, and scatter the seeds of plants. Their guano is rich in nitrogen and has been sold as fertilizer. Bats are valuable subjects for study. Investigations in the field of echolocation try to determine how bats assess their environment and how this information is coordinated and acted upon. What we learn about bats may help us understand similar physiological processes in man. In time, as we learn more of the bat's biology and habits, perhaps we can also adopt new attitudes that allow us to accept this soft furry creature as a natural part of our surroundings and have a greater appreciation for the role it plays in contributing to a healthy ecosystem.

Prepared by the Department of Systematic Biology, Vertebrate Zoology,
National Museum of Natural History, in cooperation with Public Inquiry Services,
Smithsonian Institution

November 1980