Thomas H. Kunz

Bat Facts and Folklore
by Dr. Thomas H. Kunz, Boston University
Adapted from: Kunz, T.H. 1984. The American Biology Teacher, 46:394-399.

Myths, Legends, and Folklore
Diversity and Distribution
Characteristics
Food Habits
Roosting and Social Habits
Echolocation: Seeing with Ears
Daily and Nightly Activity
Migration
Hibernation
Conservation
Health Concerns and Precautions
Impact of Bats on Property
Prevention and Control

Myths, Legends, and Folklore

The news media, movies, television, and comic books often perpetuate myths, "old-wives-tales," folklore, legends, and fears about bats that a surprising number of people believe. Bats do not get into your hair, they are not flying mice, they don't come "out of hell," they are not blind, and only three species (not in the U.S. or Canada) make a diet of blood.

Myths about bats are found in many human cultures. The ancient Egyptians believed that bats could prevent or cure poor eyesight, toothache, fever, and baldness, and a bat hung over the doorway of a home was thought to prevent the entry of demons that carried these "diseases." Bat gods were important to many pre-Colombian civilizations in central America, and bats have been used in vodoo worship in parts of Africa as well as in many parts of the Caribbean even today. The association of bats with the legend of human vampires has an uncertain origin, but since the time of Cortez and his Conquistadors, peoples of western civilization have linked vampire bats with the legendary "human" vampires of Transylvania. The writings of William Shakespeare, Robert Louis Stevenson, and others have contributed to legends that cast a veil of fear on people, as they associate bats with graveyards, death, ghosts, and goblins.

To the Chinese, bats are regarded as symbols of happiness and good fortune (health, wealth, serenity, virtue, and long life). At one time Chinese mothers would sew small jade buttons in the shape of a bat on the caps of their babies, a custom believed to impart long life. Ancient and modern-day art objects, tapestries, Imperial robes, home furnishings and the like often include bats as part of the motif.

Diversity and Distribution

Bats belong to the mammalian Order Chiroptera, meaning "hand-wing." Some members of this group of flying mammals have existed (in their present form) for at least 50 million years. Approximately 1,116 different species are known world-wide, represented by approximately 20% of mammal species of the world. Bats are second only to rodents in number of species and they probably outnumber all other mammals in total numbers. Bats are distributed on every continent except Antarctica and they are known from many oceanic islands. The overwhelming number of species live in tropical regions. 44 species are known from the North American Continent, north of Mexico.

Characteristics

Adult bats found in North America range in weight from approximately two grams (0.07 ounces) with a wing span of about 10 centimeters (4 inches), to those weighing over one kilogram (2.2 pounds) and having a wing span of nearly 1.5 meters (6 feet). The skeletal features of bats are comparable to those of humans and most other mammals. Unlike most birds which have hollow bones, the bones of bats are typically small and delicate. The bones of the wings are lengthened to provide support for the wing membranes. A highly resilient double membrane stretches between the elongated fingers, attaches to the side of the body, and extends to the ankle.

Food Habits

In temperate North America, bats feed almost exclusively on insects. In the warm months of the year individuals may eat up to one-half of their body weight on a given night. If this level of consumption is extrapolated to a population of 50,000 bats (a conservative estimate for the number of bats living in a 100 square mile area in New England), this would amount to over 13 tons of insects eaten in one summer. Many bats are also important predators of insect pests, including mosquitoes, biting midges, beetles, and moths. Thus, in reality bats are really valuable controllers of insects.Vampire bats feed exclusively on blood by licking it from small cuts they make in the skin with razor sharp teeth. However, these small blood-eating bats are only found in the American tropics, ranging southward from central Mexico, down to Brazil, thus there is nothing to fear from vampire bats in the United States and Canada.

Roosting and Social Habits

Many bats are gregarious animals which may seek daytime shelter in a variety of man-made structures. Bats do not build nests; instead, when at rest most species cling to walls and ceilings of caves and to rafters of buildings using their hind feet. Their wings are folded next to the body. Bats that roost in small crevices commonly assume a horizontal posture. Most female bats give birth during a two to three week period in early summer when there is an abundance of food. American bats usually give birth to a single annual litter of one or two offspring that may weigh from 20 to 30% of their mother's weight. This is comparable to a 100 pound human female giving birth to a 20 to 30 pound baby. Female bats suckle their young with milk and weaning usually occurs at the age of four to six weeks. Most bats grow rapidly and reach 90% of adult size by the time they are weaned.

Some bats found in the tropics make their own roosts called "tents" from the large leaves found in the rainforest. The bats chew on the veins of the leaves so that the leaf collapses downward forming a tent-like structure.

Echolocation: Seeing with Ears

Although bats do have eyes, most cannot see very well. They compensate for this lack by having a biosonar system (called echolocation). Echolocation allows bats to navigate in total darkness and to detect and capture food on the wing. In order to perform this feat, many bats have evolved specialized facial structures and bizarre looking ears designed for sound production and hearing. When hunting or flying in the dark bats produce pulses of high frequency sounds and detect the echoes returning from moving or stationary objects. Humans usually cannot hear echolocation calls produced by bats since they are produced at frequencies above our hearing sensitivity (>20 kHz). Some sounds made in a social context made by roosting bats and sometimes by flying bats are audible to the human ear.

    Among the nearly 1,116 different species of bats, almost two-thirds of them navigate and capture their prey in the dark by means of echolocation--an active sonar that usually involves the transmission and reception of brief chirps or squeaks at increasingly short intervals. Most species of echolocating bats transmit these chirps at durations of 0.5 to 10 ms at relatively long intervals. Some chirps are frequency modulated (FM) in the ultrasonic range of 15-150 kHz (kilohertz). For example, the common big brown bat (Eptesicus fuscus) sweeps two frequency ranges simultaneously (approximately 50 to 22 kHz in the first harmonic and 100 to 44 kHz in the second. This and other species that produce pure FM signals use their sounds as broadband signals, deriving images by integrating echo information across many frequencies. Distance or range from the target is determined by assessing the delay of FM echoes. For example, in an echo with a known signal-to-noise ratio of 36 dB (decibel), the big brown bat can detect delay changes of about 40 ns (nano second--1/10,000 of a second!). The shape of the target can be determined with great accuracy.

    The integration time for echo processing by the big brown bat is about 350 ms (microseconds). Thus, if echo components are in the range 50 ms apart they will be smeared together into a single, spectrally complex echo with only one directionally recognizable delay. The interference spectrum of the overlapping echoes is what represents the echo time separation of the so-called "glints," and the bat is able to convert the echo spectrum back into the echo time separation.

    Other species such as Old-world leaf-nosed bats emit complex signals that include a relatively long constant-frequency (CF) signal followed or preceded by FM signals. For example, the horseshoe bat, Rhinolophus ferrumequinum, emits CF components as narrowband signals, with the individual frequency serving as a carrier for the frequency and amplitude modulations that the flutter of their insect prey imposes on echoes. This and other bats prefers fluttering targets which they can identify by registering the frequency of CF echoes with great accuracy. To do so, they must characterize "glints" within an echo from the head and wing tips. They use a kind of spectrum analyzer that zooms in to expand the display of the CF frequency region.

    The ears of bats can hear both the emitted sounds and their echoes. This is accomplished because the auditory system of echolocating bats has thousands of parallel channels--auditory receptors and associated neurons-- tuned to different frequencies across the ultrasonic range of sounds. The spacing of these frequencies defines a scale that the bats use to encode the FM sweeps as spectrograms. The echo and emission spectrograms are distinguished by the time that elapses between sound emission and an echo at each frequency in the FM sweep. The bat determines target range or distance from these spectrogram delays by storing the volley of discharges representing the emission and then comparing it with the pattern formed upon the reception of the echo.

    Remember that the bat's auditory system has a 350 ms integration time. If a moth reflects two echo components 50 ms apart, this will evoke one volley of neural discharges in the bats auditory spectrogram. However, such a spectrogram will show two peaks and a notch. The notch is caused by spectral interference and appears at regular frequency intervals, according to the different times of arrival of different glints. To the ear, these notches indicated brief dips in the amount of auditory receptor excitation occurring at specific frequencies. The net effect is a scalloped appearance of the echo spectrogram.

    Experimental studies of echolocation at Brown University and California Institute of Technology have progressed to the point of developing a model sonar receiver based on a bat that has a simple geometric structure ideal for VLSI (very large scale integrated) chips, capable of real-time operation. Possible applications of these chips are in sensing and communication, including locating and classifying radar and sonar targets, determining propagation times of seismic phenomena, and distinguishing multipath reverberation in complex acoustic environments.

Daily and Nightly Activity

Bats typically seek shelter in roosts during the daytime and are active on the wing at night departing their roosts shortly after sunset and returning before sunrise. The timing of nightly departure and return is closely synchronized with light levels and is associated with seasonal changes in day length. One or more feeding periods may occur on a given night, depending on food availability and the temperature of the night air. Feeding activity is often interrupted by periods of night roosting, where individuals seek temporary shelter alone or in small groups in open buildings, on the rafters of porches, carports, and breezeways. Moonlight may influence the nightly activity of some species.

Migration

In late summer and early autumn, some bats migrate to warmer climates where they spend the winter. Others make short distance migrations from their summer quarters to caves where they hibernate. Fat typically is deposited prior to migration and serves as one of the principal sources of energy during migratory flights. Although it is thought that migration occurs at night, there are various reports of migratory flocks being observed during day-light hours. During migration bats supplement their stored energy reserves with the intake of insects. Three species of bats known to reside in New England make long-distance annual migrations. These are the eastern red bat (Lasiurus borealis), the hoary bat (Lasiurus cinereus) and the silver-haired bat (Lasionycteris noctivagans). The little brown myotis (Myotis lucifugus), the long-eared bat (Myotis septentrionalis), the small-footed myotis (Myotis leibii), and the eastern pipistrelle (Pipistrellus subflavus) are known to migrate from their summer quarters in buildings and hollow trees to hibernation sites in caves and mines. The big brown bat (Eptesicus fuscus) generally does not migrate, but rather it may make short movements in spring and autumn between buildings and other structures where it alternately hibernates and forms maternity colonies in the warm months.

Hibernation

While some species of bats migrate to warmer climates in late summer and autumn, other bats seek shelter in places protected from freezing temperatures (sometimes in houses or barns) where they enter hibernation. Among the eight bats known from New Hampshire, five are known to hibernate. These are the little brown bat (Myotis lucifugus), the keen's bat (Myotis septentrionalis), the small-footed bat (Myotis leibii), the big brown bat (Eptesicus fuscus), and the eastern pipistrelle (Pipistrellus subflavus).

    During hibernation the body temperature of a bat decreases and the heart and breathing rates become greatly reduced. During hibernation the heart rate may be as little as 20 times per minute, in contrast to a rate of almost 600 beats per minute in an aroused, but resting state, and 1,300 beats per minute during flight. Bats survive hibernation by relying on fat reserves which are deposited in late summer and early fall. Bats may arouse periodically from the hibernating state during unseasonably warm periods during the winter. Arousals can also be caused by human disturbance resulting from exposure to light, sounds, and physical contact. One should avoid causing unnecessary arousals during the winter since bats cannot feed during this period and they depend on a limited supply of fat for survival.

    Brock Fenton from the University of Western Ontario found that the more often little brown bats are disturbed in the wild for weighing (up to three times) the more mass they lost. Each disturbance resulted in a mean weight loss of 0.25 g in bats weighing 1.1 and 8.4 g. This loss is equivalent to to 9.85 kJ energy expended per disturbance, if the mass loss represents fat.

    In laboratory experiments, Donald Thomas from the University of Sherbrooke found that Myotis lucifugus takes about 44 minutes to arouse from 5 cC, which amounts to 86.6 J.g of warming. Assuming that the body warms to 37 oC, the cost of warming is 2.71 J g-1 C-1. for a 6.58 g M. lucifugus, warming from one arousal requires 14.5 mg fat. If the homeothermic phase lasts 3 h, an arousal period would require 83.7 mg fat. Conversely, the cooling phase was estimated to be 67.2% of warming and costs 9.7 mg fat. Thus, a complete arousal cycle requires 107.9 mg of fat and more if it flies. If the hibernation period lasts 193 dys at the latitude of Ottawa, Canada, 15 arousals during the course of the winter would require 1618.5 mg of fat to cover arousal costs. Thomas determined that the cost of hibernation was 0.02 ml O₂ g^-1 h^-1, which represents t total expenditure of 308 mg fat during 193 days of hibernation. Thus the minimum fat requirement for hibernation would be 1925 mg for a 6.58 g M. lucifugus. This amount of fat amounts to 29.3% of the live mass at the beginning of hibernation.

    Numerous studies have shown that hibernating bats arouse spontaneously during the winter. These periods of arousal may be necessary for bats to eliminate accumulated metabolites, urine, or to adjust to different temperatures of their hibernaculum. If bats are disturbed during hibernation and causes additional arousals, this may mean that the bats will not have adequate fat reserves to sustain them through the hibernation period. Thus, visits to caves in which there are hibernating bats should be avoided to minimize the risk of causing unnecessary arousals and threatening the survival of the bats.

    Recent studies by John Speakman and his colleagues at the University of Aberdeen in Scotland, indicate that non-tactile disturbances under experimental conditions had a negligible effect on fat depletion and reduction in the potential duration of hibernation. By contrast, tactile disturbances had a much greater effect, although the precise consequences varied with temperature and the body mass of the bat. Speakman and his colleagues suggested that avoiding non-tactile disturbances may be unnecessary. However, they pointed out that their interpretation of responses to non-tactile stimuli may be an artifact of experimental conditions. They also noted that the duration of torpor in the wild might be a significant factor causing bats to arouse in natural hibernacula.

Conservation

In recent years, the numbers of bats have declined dramatically in some regions of the world. These declines have resulted from the destruction of native habitat (deforestation, cave flooding, vandalism, commercialization of caves, etc.), unintentional disturbance to bats caused during cave exploration, and use of pesticides. In some cases bats are destroyed as perceived pests. Because of their importance in controlling insects, dispersing seeds, and pollinating flowers, bats should be protected as important members of natural communities. For further information about bat conservation efforts, you should contact Bat Conservation International (Austin, TX).

Health Concerns and Precautions

Bats may attempt to bite if handled, and they should not be handled unless you are wearing heavy gloves. Rabies can be transmitted to humans by bites from bats and other carnivorous mammals. In temperate regions of North America the incidence of rabies in most colonial bat species is less than one-half of one percent. The incidence is somewhat higher (4 to 10%) in bats that are found on the ground or are unable to fly. If bats are discovered in this condition, one should not handle them with unprotected hands. If bitten by a bat, an effort should be made to capture it and have it tested for rabies. A wound from a bite should be washed thoroughly with soap and water and one should immediately contact a physician and the public health department. Post-exposure treatments to bat bites are recommended.

In some regions of the United States, people may develop histoplasmosis, a disease of the lungs which is caused by a fungus that grows in the moist bat droppings (guano). The pneumonia-like symptoms are caused by the inhaled spores which cause fungal infections of the lungs. This fungus occurs naturally in the soil in some parts of the U.S., and residents of these areas may develop a natural immunity to this fungus. The prolonged presence of humans in enclosed areas of buildings and caves where bats roost may increase the risk of contacting histoplasmosis in some parts of central and southeastern U.S. There are no reports of histoplasmosis from bat colonies in New England.

Impact of Bats on House and Property

Bats do not gnaw or chew on wood, metal, or plastic to gain entry or exit to and from buildings. Nor do they build nests as do birds. Their roosting places in buildings are often recognized by the presence of a whitish or darkened stain on rafters and the accumulation of guano (feces and urine) beneath the roost site. Because bats also defecate and urinate while in flight, fecal droppings and drops of urine may be become splattered on the outer and inner walls of a building near where the bats gain entry or exit. Fecal droppings and urine may also become splattered on items stored in an area where the bats roost. This problem can be remedied by covering the items with sheets of plastic for protection. A musty odor is often associated with the accumulation of guano produced by bats. If guano is removed at the end of each summer season (in autumn), the effect of the odor will be lessened. Guano may be safely added to a compost pile or used directly as fertilizer on the lawn or garden.

While roosting in buildings bats may emit audible clicking or squeaking sounds which may be annoying to some people and especially to dogs (which can hear high frequency sounds). Sometimes bats may be heard clamoring in the spaces between chimneys and walls and between walls, especially at night before departing to feed, and upon their return from feeding.

Prevention and Control

    Bats that seek shelter in houses often gain entry to a building through small crevices and cracks, where a chimney has pulled away from an outer wall, where the facial board has become loosened, from beneath shingles and roofing, through unscreened air vents and open windows, and occasionally through open doorways. The occasional single bat discovered in the room of a building during the warm months is more than likely to be a young individual that has lost its way. Sometimes bats may arouse from hibernation during an unseasonably warm period in winter; they may become disoriented and eventually find their way into the interior of a house.

    The most effective and environmentally sound method of preventing bats from roosting in houses is to close off the openings that are used for entry and exit. This can be accomplished by determining where they depart at dusk and then sealing the openings with metal, wood, or plastic foam insulation. This procedure should not be attempted during the maternity period in June and July, otherwise young bats will be trapped in the building. In late summer and autumn openings that were identified during summer at the peak of bat activity can be sealed after bats have left for the night. Because big brown bats often hibernate in buildings, it is not advisable to seal the openings during the winter. The use of toxic chemicals as repellents is not recommended, because these can have adverse effects on the homeowner, and they are less effective than physical exclusion.

    Bats sometimes take up residency in, or accidentally enter, houses and other man-made structures. If it becomes necessary to evict a colony of bats from a house, the only environmentally sound, effective, and permanent method is to close the openings that bats use for their exit and entry. Nuisance bats usually enter buildings through small openings along the edge of the roof, through crevices where masonry has pulled away from the clapboard, or through unscreened air vents. To determine where bats are entering or exiting a building, two or more people should position themselves outside the house shortly after sunset (in the warm months). Bats can best be observed by silhouetting them against a clear view of the twilight sky. It may take up to one hour for all bats to depart on a given evening. Usually they will not fly out on cold, rainy nights. If you see something flying into your chimney at dusk, you probably don't have bats. Instead, you most likely have swallows or chimney swifts.

    After you have located places that bats are using as exit and entry portals, and all the bats have departed to feed at night, the opening should be sealed so that the bats cannot reenter. It may be necessary to repeat this procedure several times until all openings have been closed. The best time to seal the openings is in the early spring or in autumn. One should avoid sealing openings from late May to mid-July since young bats too young to fly may be trapped inside. An effective method of excluding bats from some buildings is to install one-way exits. This can be accomplished by attaching a PVC tube that has a collapsible plastic extension on one end. Bats can exit through this device but they cannot reenter.

    Chemical repellents (e.g. moth balls, sulfur candles, ammonia) are generally ineffective for excluding bats from buildings. Bats may be temporarily evicted by using these repellents but they are likely to return after the chemical has dissipated. Moreover, pesticides (e.g. Rozal, chlorodane, lindane, etc.) often sicken bats before killing them, and seldom is the application effective. Often, when these poisons are used, bats become sick and fall to the ground, which increases the risk of exposing children and pets to sick bats. Application of chemicals to the interior of a house for controlling bats also may impose adverse health risks to the occupants, consequently this practice should not be used. Commercially available devices that produce sounds at ultrasonic frequencies (sonar type) are not effective in discouraging bats from roosting in buildings. Use of incandescent or fluorescent lights in roosting areas may reduce the number of bats present under some circumstances. The safest and soundest approach for eliminating bats from buildings is to close the openings that bats are using for exit and entry. However, this may be difficult to accomplish if the building is old and has numerous holes and crevices that bats can use.

    If one or two bats accidentally get into the living quarters of a house (a common occurrence in late summer when young bats are learning how to fly), the lights should be turned on, doors or windows should be opened, and usually the bats will leave on their own accord. If a bat is discovered in the living quarters of a house during the day, efforts to encourage it to leave will be most successful at night.