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Bats of the World
A Golden Guide from St. Martin's Press
By Gary L. Graham, Fiona A. Reid
St. Martin's PressCopyright © 2002 St. Martin's Press
All rights reserved.
BATS ARE IMPORTANT to our world. Over the last few decades, much has been learned about how bats help keep our environment healthy. Many species of bats, such as those known as flying foxes, are also surprisingly appealing and intelligent. Ironically, though, bats continue to be among the most misunderstood and feared of all our wildlife. This fear and ignorance have contributed to the almost total destruction of several bat species and threatened the existence of many others. Such losses can seriously harm ecosystems and reduce the quality of life for many living things, including humans. But with our help, bats can continue to function beneficially in nature.
BATS ARE MAMMALS, and like all other mammals, the females possess mammary glands, where milk is produced and fed to the young. Baby bats, called pups, are born alive and have to be taken care of for an extended period of time. The body of a bat (but not the wings) is covered by hair, as it is in most other mammals.
BATS FLY, which makes them unique among mammals. Although flying squirrels and flying lemurs have names that seem to indicate otherwise, these animals cannot fly. They do not have wings, and thus they can only glide. Many of the distinctive characteristics of bats are a result of their evolutionary specializations for flight.
Bats belong to the order Chiroptera, which means "hand-wing." Species in this order are divided into two suborders: Megachiroptera, which includes the various species of flying foxes, and Microchiroptera. Flying foxes have foxlike faces with large eyes. Most flying foxes do not use echolocation, a kind of natural sonar for locating prey and other objects. Megachiropteran bats are found only in the Old World (Europe, Asia, etc.). Microchiropteran bats do echolocate and are a much more diverse and widely distributed group.CHAPTER 2
FACT AND FICTION
BATS ARE INDEED BENEFICIAL TO US, although this is a notion that some people find difficult to accept. In many parts of the world, even today, bats are hated and feared, and they are often associated with evil and death. This is unfortunate.
Perhaps the reason for these attitudes is that most bats are small and are active mostly at night, when they are difficult to observe. People tend to fear what they do not fully understand. In contrast, where bats are large and can easily be seen during the day, they are seldom feared and are often even highly regarded.
For example, in Chinese folklore the bat has few rivals as a symbol of good luck and good fortune. In fact, the Chinese word for bat, fu, is also the Chinese word for good fortune. Five bats are frequently shown together to represent the five blessings — a long life, wealth, good health, love of virtue, and a natural death.
On the Pacific islands of Samoa, flying foxes are depicted as heroes in a folktale where they save a princess. Folktales from many native cultures and ancient societies, including those of Aesop, the famous Greek storyteller, depict conflict or competition between mammals and birds to explain the nocturnal nature of bats.
Many people believe that bats are blind. This is not true. All bats can see, and many have excellent vision.
Bats are often thought of as flying mice. In fact, the German word for bat is fledermaus, which means "flying mouse." Most people are surprised when they learn that bats are actually more closely related to primates, including humans, than they are to mice.
People often use the term "dingbat" to imply that someone is not very smart. Bats, however, are actually quite intelligent and under certain special circumstances can be trained.
Bats are often mistakenly depicted as dirty. However, they spend a great deal of time grooming themselves and keeping themselves clean.
Misinformed people think of bats as symbols of evil and associates of Dracula. But in reality, most bats are very gentle animals that will bite only if they are frightened or improperly handled.
As predators of nocturnal insects, pollinators of flowers, and disperses of tropical plants (by scattering their seeds), bats are crucial to many of the ecosystems upon which we depend. Bats are also very important to the economies of some developing countries. There, many commercially important trees may owe their very existence to bats.CHAPTER 3
ALMOST 1,000 SPECIES OF BATS are found around the world. This represents nearly a quarter of all known mammals. Rodents are the only mammals that have more species. Such impressive diversity is certainly matched by the tremendous variation in ecology and behavior displayed by bats. Bats eat everything from insects and fruits to nectar, fish, meat (small land vertebrates), and even blood.
Bats are often compared with birds because both of them can fly. However, there are about eight times more species of birds than there are of bats. This may be because bats are a younger group than birds and have had less time to evolve new species. Another reason may be anatomy. In bats the legs, which are part of the wings, cannot be used effectively during various food-gathering activities, such as swimming, diving, running, and digging. In birds the wings are separate from the legs. Thus, many species of birds that use their legs for food gathering have evolved.
BATS CAN BE FOUND in almost all habitats except extremely hot deserts and the cold polar regions. Species are more diverse in the warmer latitudes. For instance, only one Insect-eating species can be found in northern Canada, whereas approximately 150 species are found, along with several non-insect-eating bat species, in some tropical areas near the equator. A similar increase in the number of species and individuals can be observed as one travels down tropical mountainsides from the alpine tundra to lowland rainforest habitats. Bat diversity on islands also decreases as the distance to nearby continents increases. For instance, five species of bats are known to exist in Fiji and three in American Samoa. Only one is native to Hawaii.
In the United States the areas with the greatest diversity of bats are in the Southwest — for instance, in the Chiricahua Mountains of Arizona and Big Bend National Park in Texas. These regions have both diversified habitats and many months when food is abundant.CHAPTER 4
THE FOSSIL RECORD for bats is far from complete. This is partly because bats are small and have delicate bones that seldom become fossils. The bones are difficult to recognize even when they are fossilized. Thus, the origin and evolution of these mammals are poorly understood.
The oldest bat fossil, Icaronycteris index, became extinct about 60 million years ago in what is now North America. Early bats were insect eaters, as indicated by their teeth and by the fossilized insects found in the stomach of one ancient bat. Megachiropteran bats first appear in the fossil record about 35 million years ago.
The abilities to fly and to catch insects in the dark were important developments in bat evolution. Many scientists believe that bat ancestors were small, shrewlike mammals that chased flying insects among the leaves of trees and evolved limb membranes that enabled them to glide from branch to branch. The transition from a fixed limb, like that of a flying squirrel, to a movable wing was a critical step in the development of bat flight and evolution. It allowed bats to pursue prey above the trees.
Scientists also suspect the early ancestors of bats were able to echolocate, which enhanced the capture of insects at night. However, the exact origin of echolocation in bats is not known. The type of echolocation used by the microchiropteran bats is not found among flying foxes. This difference, among others, has led some scientists to question the supposedly close relationship between these two particular groups.
THE RELATIONSHIP BETWEEN FLYING FOXES AND OTHER BATS has been debated by scientists and other experts for over ten years, yet it remains basically unresolved. Some scientists think that flying foxes and primates are more closely related to each other than flying foxes are to other bats because they share some characteristics not found in microchiropteran bats. These characteristics include the nerve pathways associated with vision, part of the male reproductive system, and an inability to echolocate.
Such a view implies that the similarities, including flight, shared between the two bat groups evolved two times in mammals. Recently, however, a number of studies using new genetic and analytical technologies dispute this view.
Most, but not all, scientists who study the evolutionary relationships between animals now conclude that megachiropteran and microchiropteran bats are much more closely related to each other than either group is to primates. This means that all bats belong to the some order and that flight evolved only once.CHAPTER 5
TRUE SUSTAINED FLIGHT distinguishes bats from all other mammals and is a trait shared only with birds and insects. To those creatures that possess it, flight offers numerous advantages. It allows access to additional food resources, as well as to roosting and nesting sites that are unavailable to flightless animals. Flight makes long-distance migrations possible and allows animals to get past barriers that are difficult for nonflying animals. In addition, a flying bat or bird uses less than a quarter of the energy consumed by similarly sized land species moving over the same distance. Finally, flight is also an effective way to escape from most predators.
Flight has thus been one of the principal factors in the evolutionary success of bats and birds. The reason flight has not evolved among other vertebrates (animals with a backbone) may have to do with the many complex specializations required for flight.
THE BASIC REQUIREMENTS for flight include a method for maintaining the body above the ground (lift), a means of propulsion through the air (thrust), and a streamlined body to cut down on air resistance. The bat's wing, which provides both lift and thrust, is truly a masterpiece of evolutionary design. It incorporates the same basic arm and hand bones found in humans and most other mammals, except that in bats the hand and finger bones are very long and slender and there are fewer digits. One of the forearm bones, the ulna, is reduced in size.
Flight membranes are very thin sections of skin stretched between the arms, fingers, body, legs, and feet. Although rather delicate-looking, these membranes are even more resistant than rubber gloves to tearing by sharp objects.
The muscles that move the wing are located on the chest, back, and shoulder rather than on the wing. This allows the bat to fly with less expenditure of energy. Bat legs are used more for flight than for moving about on land. Hence, the pelvis and legs are reduced in size, which contributes to a slender body shape.
LIFT AND PROPULSION are achieved by the downstroke of the wing. Lift is caused by air moving faster over the top of the wing than under it. It can be increased by increasing the speed of air moving past the wing, by changing the curvature of the wing, and by adjusting the angle of the downstroke.
The membrane between the body and the fifth finger is the lift-generating part of the wing. Forward thrust is produced by the membrane between the second and the fifth fingers.
Wingbeats are similar to the arm movements of a human swimming the breaststroke. Many species can take off from the ground, but most bats begin flight by taking off from a roosting site. Landing involves decreasing airspeed until the bat "stalls." Most bats grab hold of a branch or other object and then assume an upside-down position. Others perform a flip first and then grab hold.
Relatively short, broad wings, such as those belonging to many species in the family Phyllostomidae, permit highly maneuverable flight in a tight space. Species with long, narrow wings are usually swift flyers but maneuver poorly (for instance, many bats in the family Molossidae).
Most bats have a membrane between their legs. This tail, or interfemoral, membrane increases the overall lifting surface of the bat's wings and may also act as an air brake. It is even used by a number of species to trap insects during flight.
Birds have evolved somewhat different methods to accomplish flight. Perhaps the most obvious difference is the use of feathers rather than membranes. Bird wings are also characterized by the fusion of the bones and their reduced size.
In addition, birds have fewer flight muscles than bats. These muscles are attached to the keel, or ridge, on a bird's sternum. Bats do not have a keeled sternum.CHAPTER 6
ECHOLOCATION AND OTHER SENSES
ALL BATS CAN SEE, but their eyes are better adapted to seeing in the dark. For example, they see objects only in black and white. Microchiropteran bats use their eyes to monitor varying levels of light in the environment and to help them orient themselves in space.
Flying foxes use vision, rather than echolocation, for most of their activities. Consequently, their eyes are large compared with the typically small eyes of echolocating bats. In addition, the eyes of flying foxes possess a reflective layer of tissue under the retina that is typical of most nocturnal mammals. This improves vision by allowing more light to pass over the retina, where there are light-sensitive cells. This reflective layer is not found in other bats, probably as a consequence of their having evolved the ability to echolocate.
The sense of smell is well developed in most bats. It is used to locate and identify certain food items and to recognize roost mates and young. It is also important in the social behavior of some species.
ECHOLOCATION is the process used by some animals to identify, and measure the distance to, objects in the environment. Although this ability, which involves listening to echoes of sounds the echolocator itself produces, is associated mostly with bats, some birds, cetaceans, insectivores, rodents, and even people can echolocate. A person who shouts in a canyon and listens for the echo is using a simple form of echolocation to detect canyon walls.
Humans cannot normally hear bats echolocating. The low-frequency "squeaky" bat sounds that humans hear can also be used by some mother bats to communicate with their young or as warning or recognition calls.
Bats use echolocation to orient themselves in space and to determine the size, shape, texture, distance, speed, and direction of movement of prey or other food items. Some species can detect objects as thin as a human hair. All 800 or so species of microchiropteran bats are thought to use some form of echolocation.
SOUND VIBRATIONS form waves in the air, waves that have successive peaks and troughs. Sound frequencies, the number of these wave cycles that occur in a second, are expressed in kilohertz (kHz), one of which equals 1,000 cycles per second. The frequencies used by echolocating bats range from 4 to 210 kHz, but most species use signals between 20 and 80 kHz.
High-frequency, short-wavelength signals are better for identifying small objects than are low-frequency, long-wavelength sounds. Thus, the high-frequency echolocation calls of most bats are more useful for detecting the small insects that make up so much of their diet. Because the maximum frequency usually perceived by humans is around 20 kHz, frequencies above this number are referred to as ultrasonic.
ECHOLOCATION CALLS, or pulses, are produced in the larynx, or voice box, by forcing air past very thin vocal membranes that are unique to bats. Most species emit signals through the mouth, but some may use a complex nose structure, called a nose leaf, to direct signals from the nostrils. Members of the Old World genus Rousettus, the only megachiropteran bats that echolocate, use tongue clicks instead of vocal cords to produce sounds.
Usually, echoes are received by large funnel-shaped ears that face forward. Many species have a vertical flap inside each ear, called a tragus, that may help direct incoming echoes. The internal ear parts are similar in structure and function to those of other mammals, except that the eardrum is somewhat thinner and the cochlea is somewhat larger. The cochlea is specialized for frequencies in the ultrasonic range. The nerve impulses that are generated by these sounds are transported to the brain for processing.
THE BAT'S BRAIN processes information and forms images by comparing outgoing pulses with the echoes. The time between when the bat's call is sent and when the echo returns indicates how far away a particular object is. Echoes also convey information about the size of the target. The direction in which the target is moving is determined by the different arrival times of the sound at each ear and also by the intensity of the echoes as they reach the bat's ears.
Excerpted from Bats of the World by Gary L. Graham, Fiona A. Reid. Copyright © 2002 St. Martin's Press. Excerpted by permission of St. Martin's Press.
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