Bats
Bats
Sensory systems and echolocation
Reproduction and social organization
Ecological and economic importance
Bats are one of the most diverse and widely distributed groups of mammals on Earth, second only to rodents in the number of species. More than 1,000 species of bats have been described. They occur in most terrestrial biomes, except for the high Arctic and all of Antarctica. Bats are the only truly flying mammals, and are distinct from the flying lemurs and flying squirrels, which actually glide. Bats make up the order Chiroptera, named from the Greek words cheiro (hand) and pteron (wing); this is an appropriate name, since the wing is formed by modified bones of the hand.
The bat order Chiroptera is divided into two suborders. The Megachiroptera is composed of a single family that is restricted to the tropics, and includes the largest species of bats, such as fruit bats and flying foxes. The megachiropterans are characterized by large eyes, simple ears, and a doglike face. The Microchiroptera are made up of 17 families, and feature small eyes, complex ears, and an ability to find their prey and navigate by echolocation. Certain differences between the two suborders in flight and sensory capabilities have led some biologists to propose that they evolved from separate ancestral lineages, and that the megachiropteran bats are more closely related to primates. This idea is highly controversial, however, and other morphological data and DNA evidence support the hypothesis that all of the bats evolved from a common ancestor. Most bat scientists believe that bats evolved from tree-dwelling, shrewlike ancestors that scampered along branches and fed on insects. The proto-bat likely had long fingers supporting webs of skin attached to the body, which it used to glide while in pursuit of its insect prey.
Basic body plan
The bodies of bats are well designed for flight. However, they achieve flight differently from birds. Like birds, the bones of bats are light-weight and delicate. However, bats have a short neck compared
to birds, and they lack a deeply keeled sternum, or breastbone, where the flight muscles attach in birds. Instead, three shallow pairs of muscles on the breast power the downstroke of the wing during bat flight, while the upstroke is provided by three pairs of muscles on the back. Because they do not have a well-developed breastbone, bats have a flat profile through the chest, and so they can squeeze through small openings and roost in narrow crevices.
The wing structure of birds and bats is also different. The skin and feathers of the wings birds are supported mainly by their second and third “fingers.” In comparison, the wings of bats are formed by thin, elastic skin extending from the sides of the body to the tips of all four elongated “finger” bones. Their much-reduced thumb remains free of the wing membrane and is used to manipulate food, and as a hook when the bat climbs and clings to surfaces or vegetation. The wing membranes are also supported by the hind legs, and in species with a tail, it is entirely or partially enclosed by wing membrane stretching between the hind legs. The hind legs of bats are unique among mammals in being rotated 180°, so that the knees point backward, allowing the leg to flex in a reverse fashion. This is believed to assist in steering during flight, and in taking off from the characteristic head-down roosting position of bats.
The Megachiroptera have a doglike face. However, the face of the Microchiroptera is often striking and weird looking, with fleshy embellishments that form complicated dimples, wrinkles, and horseshoe- or leaf-like structures. Some species have tubular nostrils. Biologists have suggested that these facial embellishments function in the projection of sounds produced for echolocation, like megaphones or acoustic lenses. While Megachiroptera generally have simple ears, there is huge variation in the size, shape, and elaboration of ears among microchiropteran bats. Depending on the species, their ears may feature special folds and ridges that are thought to play a role in sound perception. For instance, many of these bats possess a large tragus, a fleshy projection on the bottom front edge of the ear opening, and believed to aid an echolocating bat in determining the horizontal position of a target.
Like many birds, bats pass the food they have eaten fairly quickly through their digestive tract, so as to reduce the amount of time they must carry the extra weight of undigested food. Total output time is as little as 20 minutes in some smaller bat species, which is similar to birds of the same size.
Diet
The dietary diversity of bats is unmatched among living mammals. Most bats living in temperate areas, about 265 species, eat insects. Fruit bats, restricted to tropical areas, eat fruit and leaves, which they chew, swallowing the juice and spitting out the pulp. The long-tongued fruit bats (Macroglossus species) specialize in a diet of pollen and nectar, which they acquire using their elongated snout and unusually long tongue (up to one-third their body length). The fisherman bats (Noctilio species) of Central and South America catch small fish, while the frog-eating bat (Trachops cirrhosus ) uses the calls of frogs to locate this prey, and can distinguish between the calls of poisonous and edible frogs. The large slit-faced bat (Nycteris grandis ) of Africa eats small birds and even other bats, which are caught on the wing.
The infamous vampire bats (three genera in the subfamily Desmodontinae) dine on the blood of other mammals, such as domestic livestock, by making a shallow cut with their incisors and lapping the blood that flows from the wound; their saliva contains an anticoagulant that keeps the blood from clotting. These bats are quite agile on the ground, typically landing beside their sleeping victim, and crawling gingerly onto them to feed. Vampire bats are dietary specialists, but most other species consume several food items, varying their consumption to get enough protein and other nutrients.
Sensory systems and echolocation
Contrary to popular myth, bats are not blind. In fact, the large eyes of many species suggest that they have well-developed vision. Like most mammals, they have keen senses of taste and smell, the latter being useful in locating food items, and in identifying roost sites and other bats, including family members. Bats also have excellent hearing. Many species use a wide range of vocalizations to communicate with one another. Some species hunt for food by listening to the sounds of their prey moving about.
The most remarkable sensory adaptation of bats is their capacity for echolocation. This sensory ability allows bats to maneuver in total darkness, using echoes of their ultrasonic calls to detect objects in their vicinity. Efforts to understand how bats can fly in complete darkness date back to the late eighteenth century, when the Italian scientist Lazarro Spallanzani (1729–1799) conducted experiments that included denying bats the use of their senses of smell, touch, and vision. He observed that bats lost their way only if their head was covered by a small sack, and concluded that bats must have a sixth sense, not shared with humans.
A Swiss scientist named Charles Jurine reported in 1794 that if a bat’s ears are blocked, it cannot maneuver. Spallanzani heard this report, and taking the experiment a step further, showed that bats with brass tubes inserted into their ears can only navigate when the tubes are open. He then concluded that bats must somehow “see” with their ears. How this could occur was not explained until the 1930s, when the echolocation pioneer Donald Griffen (then an undergraduate at Harvard University) detected ultrasonic signals produced by bats in the lab, using a microphone capable of picking up sounds above 20 kHzultrasound. In a series of experiments, Griffen showed that bats used the echoes of their calls to locate obstacles, and he coined the term “echolocation” to describe this sensory ability.
It is now known that some other animals are also able to echolocate, including whales, dolphins, shrews, and some birds such as cave swiftlets. It is also known that some bats, including all but one of the flying foxes, are not able to echolocate. (The only megachiropteran genus that can echolocate is Rosettus, whose sounds are produced by tongue clicks.) Evidence indicates that bats can echolocate using reflected sound as effectively as we see with our eyes, using reflected light. Bats do this by sensing the time elapsed between the production of the sound (by means of their larynx) and the return of its echo, thus gauging the distance of objects near them. Of course, they do not perform these computations in a conscious way, any more than a person willfully determines the frequency of incoming light waves to perceive an object as blue or green. Rather, the bat brain carries out the necessary functions in a split second, providing them with a continuously updated “picture” of their surroundings.
What bats “see” in this way might best be imagined as something like what a human visitor might see in a darkened disco, where a strobe light is flickering to illuminate dancers and other objects in a pulse-like fashion; every time the strobe light flashes, the observer gets a brief update on the position of nearby things. A faster pulse rate in the strobe means that more information about these objects can be conveyed to the observer. The same is true, more or less, for bats, which vary their calling rate depending on what they are doing. The rate for a bat on a routine cruise is 5-10 calls per second; as it locates and closes in on a flying insect the call rate increases, and it finally accelerates briefly to more than 200 per second in a terminal “feeding buzz” that pinpoints the location of the food item. However, such a high rate of calling is only suitable for near targets; if an object is too far away, the outgoing signal gets mixed up with those returning from other distant objects. In addition, echolocation takes considerable energy; the intensity of the calls of some bats, if they were of a frequency audible to humans, would make them as loud as the beeping alarm of a home smoke detector. Expending the energy required for the feeding buzz does not pay off during an ordinary commute, and so the calling rate at this time is relatively low.
The brain of an echolocating bat carries out some astonishing perceptual feats, and solves problems similar to those facing engineers during the early development of radar technology. For example, there is the problem of signal attenuation: sounds progressively degrade as they get farther away from their source. Pulses must be loud enough to survive degradation, but a loud sound can overwhelm the sensitive receiver structures needed to pick up the return signal. Through their evolution, bats solved this problem in a similar way as the early radar engineers: they are equipped with a send/receive switch that momentarily disconnects the receiver function just as the loud outgoing pulse is produced; the receiver is then reconnected in time to receive the echo. The switching is accomplished by muscles that attach to the bones of the inner ear; when the muscles contract, these bones do not transmit sound well. When the muscles relax, the ear returns to its normal sensitivity. Further signal attenuation happens within the brain itself, as neurons responsible for sound perception block the transmission of messages to higher regions of the brain at the moment that a bat vocalizes. In addition, special echo-detector cells in the brain respond more intensely to the second of two separate sound pulses, which is an excellent way of picking up the echo of a vocalization.
There is also the problem of sorting out echoes returning from near, medium, and distant objects; how can a bat tell the distance between itself and its next meal? They accomplish this by means of frequency modulation, so the sorting can be done by differences in pitch (frequency). If a bat utters a downward-sweeping whistle call, an echo returning from a more distant object will be older and thus higher in pitch compared with echoes from closer objects. The bat thus has a standard for comparison: lower pitch means close by, higher pitch means farther away. In addition, bats can measure the speed of a moving target by means of the Doppler effect; this is the phenomenon responsible for the change in pitch of an ambulance siren as it moves toward and then passes a stationary observer. To do this, the bat’s brain compares the pitch of an echo with that of the original call; they can do this reliably even in the midst of hundreds of echolocating colleagues engaged in a midnight feeding frenzy. All of this is accomplished automatically and instantaneously, with no more conscious effort than a person might exert while watching images on television.
Of course, the echolocation system does not guarantee a perfect success rate while hunting. Some flying insects, mice, and other potential prey can detect the echolocation signals of bats and then take evasive action.
Bats also use their senses of sight and smell to find food. Their other senses are also important in recognizing other bats, including their offspring, and perhaps also in identifying roost sites.
Roosting
Most bats rest during the day and disperse around dusk to feed. Bats typically spend more than half of their lives in their roost environment, which may be in a cave, mine, crevice in rocks, cavity in a tree, in dense foliage (sometimes rolled up into a tent), or in a human structure. Many bats roost communally, often for brooding of young or for hibernation; such colonies range in size from a few individuals to several millions in a large cave. Females of some colonial bat species will share food and nursing of the young during the breeding season. For a hibernating bat in temperate areas, a communal living arrangement offers a relatively stable microhabitat in which their body temperature may drop to within a few degrees of the ambient temperature, thus permitting the conservation of critical reserves of body fat.
Reproduction and social organization
Most bats have a breeding season, which is in the spring for species living in a temperate climate. Bats may have one to three litters in a season, depending on the species and on environmental conditions such as the availability of food and roost sites. Females generally have one offspring at a time; this is probably a result of the mother’s need to fly to feed while pregnant. Female bats nurse their youngster until it has grown nearly to adult size; this is because a young bat cannot forage on its own until its wings have assumed adult dimensions.
Female bats use a variety of strategies to control the timing of pregnancy and the birth of young, so as to make delivery coincide with maximum food ability and other ecological factors. Females of some species have delayed fertilization, in which sperm are stored in the reproductive tract for several months after mating; in many such cases, mating occurs in the fall, but fertilization does not occur until the following spring. Other species exhibit delayed implantation, in which the egg is fertilized after mating, but remains free in the reproductive tract until external conditions become favorable for giving birth and caring for the offspring. In yet another strategy, fertilization and implantation both occur but development of the fetus is delayed until favorable conditions prevail. All of these adaptations result in the pup being born during a time of high local production of fruit or insects.
Bats exhibit every kind of breeding system that has been described for mammals. Some species are monogamous, with a particular male and female mating only with each other. Other species are polygynous, which means that one male may mate with several females. In these species, males may fight for control of preferred female roosting sites, or over aggregations of females, called harems. In hammer-headed fruit bats (Hypsignathus monstrosus ) of Africa, the males assemble into leks, which are aggregations of displaying males at traditional sites that females visit for the purpose of selecting a mate. The males display by vigorous wing flapping, erecting patches of hair, and loud vocalizations. Still other bats have a promiscuous mating system, in which both males and females mate with more than one other individual.
While a mother and her young are the basic social unit, some species elaborate on this theme. For example, the social organization of the common vampire bat (Desmodus rotundus ) is based on roosts shared by several females and their young. Roost-mates are often close relatives who groom each other, share wound sites on their prey, and regurgitate blood for consumption by their colleague.
Ecological and economic importance
To many people, bats conjure up images of vampires, evil spirits, or creepy castles. Many people also feel that bats are dirty, dangerous, ugly creatures that can get tangled in their hair. These misguided images are not, however, promoted in all societies. In some cultures bats are symbols of long life, good luck, and fertility. It is true that some species of bats can carry rabies and histoplasmosis, both of which are potentially dangerous diseases of humans and other animals. However, bats also provide crucial ecological and economic services that many people overlook.
Bat guano (excrement) collected from roosts has been used for centuries as a source of saltpeter for making gunpowder and fertilizer. Gunpowder made in this way was used in the United States during the War of 1812 and the Civil War. During World War II, U.S. military commanders considered using bats to carry small bombs into enemy territory; however, they abandoned “Project X-Ray” when one of their own buildings was gutted by a fire caused by a stray bat-ferried bomb.
Some bat species are important in the pollination and seed dispersal of plants of economic importance, such as the durian fruit of Southeast Asia. Bats are also dispersers of the seed of certain plants that colonize disturbed areas, and therefore play an important role in the revegetation of denuded places. Bats also consume vast quantities of insects, including mosquitoes and species that are agricultural pests.
Unfortunately, some bat species have recently become extinct, and many others are endangered. The geographic ranges of many other species have been drastically reduced. These ecological damages are often caused by the loss of roosting sites, deforestation, insect control, and environmental contamination with toxic pesticides, all of which are associated with human activity. The ecological consequences of the continuing decline of bat species are unknown. However, even our limited understanding of these animals suggests that the outcome will not be favorable to natural ecosystems or to the needs of humans.
Resources
BOOKS
Fenton, M.B. The Bat: Wings in the Night Sky. Richmond Hill, ON: Firefly Books, 1998.
Kunz, T.H., and P.A. Racey, eds. Bat Biology and Conservation. Washington, DC: Smithsonian Institution Press, 1998.
Nowak, Ronald M. Walker’s Mammals of the World. 6th ed. Baltimore: Johns Hopkins University Press, 1999.
Tuttle, Merlin. America’s Neighborhood Bats. 2nd rev. ed. Austin: University of Texas Press, 2005.
Tuttle, Merlin, and Don E. Wilson. Bats in Question: The Smithsonian Answer Book. Washington, DC: Smithsonian Institution Press, 1997.
PERIODICALS
Friederici, Peter. “Graveyard Shift.” Audubon 108 (September–October 2006): 48–53.
Milius, S. “Babbling Bats: Do Pups Talk Baby Talk as Human Infants Do?” Science News 170 (July 29, 2006): 67–68.
Norlander, Britt. “Twilight Zone: Follow a Scientist as She Searches the Skies to Investigate One of Nighttime’s Fliers—Bats.” Science World 62 (March 27, 2006): 8–12.
Susan Andrew