Bat - biology.

Bat - biology. I INTRODUCTION Bat, only mammal that can fly. Bats have modified hands and arms that serve as wings capable of sustained flight. They have been flitting across the night skies since the Eocene Epoch, some 50 million years ago. In all this time, the physical features of these animals have changed very little. Fossil specimens from Europe and North America are nearly indistinguishable from living forms. There are nearly 1000 living bat species, accounting for almost a quarter of all mammal species. These species are divided among two major groups. The Megachiroptera, or megabats, are large animals, commonly known as Old World fruit bats. They are mainly fruit-eaters and are found only in tropical habitats of Africa, India, and Australasia. The Microchiroptera, or microbats, are smaller, eat a variety of foods from small mammals to fish, and are much more widely distributed. Nearly all bats are nocturnal (active at night) or crepuscular (active during the twilight of dawn and dusk). To get around in the dark, many nocturnal bats rely on a sophisticated form of sonar known as echolocation for navigating and finding prey. Many bats, especially the crepuscular ones, have exceptionally good eyesight designed for low levels of light. Only those bats that live on remote islands lacking birds of prey routinely fly during the day. Throughout history, people have considered bats to be dangerous and frightful creatures of the night. As such, bats have endured centuries of unjustified abuse. While most medieval superstitions about bats have perished, some misconceptions surrounding these animals persist today. Bats continue to appear as villains in motion pictures and children's cartoons. Despite studies that give evidence to the contrary, they are still frequently miscast as carriers of disease, such as rabies. On average, only one person per year dies from rabid bat bites in the United States, far fewer than die from dog bites or bee stings. However, studies published in 2005 suggested that fruit bats may carry the deadly Ebola virus in Africa and a horseshoe bat may be a host for the SARS virus in Asia. In recent decades, people have begun to appreciate bats for their beneficial roles. Bats are the most important natural enemies of night-flying insect pests. Bats are important pollinators of plants, pollinating at least 500 species, including bananas, mangoes, cashews, dates, figs, and agave, from which tequila is produced. Without these helpful night fliers, many bat-pollinated plants--and the many animals that depend on them for food and shelter--would disappear from the earth. Bats also produce a nitrogen-rich fertilizer known as guano. II RANGE AND HABITAT Bats are found in all habitable regions of the earth, from the cold, treeless plains of northern Canada to the rich tropical rain forests of Borneo. The majority of species occupy tropical habitats. Their long-range flying abilities have enabled some species to colonize remote islands in the Indian and Pacific Oceans, including Hawaii. Bats favor habitats that provide ample food and adequate roosts. The majority of bat species are insectivorous (insect eaters), and sizeable bat populations (often estimated in the millions) are usually found in tropical regions, where insect populations are abundant year-round. The quantities of insects consumed by bats in these regions are astonishing: as many as 250 tons of insects may be devoured by a colony of Mexican free-tailed bats in a single night. Bats that thrive on fruit are nearly as demanding. To meet its nightly juice-drinking needs, the dwarf epaulletted bat of tropical Africa, which weighs about 170 g (6 oz), must eat 500 g (18 oz) of food--approximately three times its body weight in fresh fruit. During the daytime, bats sleep in caves, crevices, tree cavities, and human-made structures. Some choose exposed sites on trunks, limbs, and branches of trees. Nearly all bats rest and sleep, or roost, hanging upside down by their hind feet. Bats do not need to expend energy to maintain this position--as a bat hangs, its own weight causes the foot tendons to automatically grasp, firmly holding the animal in place. Most species are colonial, roosting in dense clusters, often comprising several thousand individuals. Wherever large numbers of bats roost, huge piles of bat droppings, or guano, accumulates. In many countries, guano is collected and applied as a high-quality fertilizer for agricultural crops. In the past, the nitrogen and phosphorus in guano was extracted and used in the manufacture of explosives. III PHYSICAL DESCRIPTION Bats range in body size from as small as the 3-cm (1-in) long Kitti's hog-nosed bat (also called the bumblebee bat), to as large as the 41-cm (16-in) long Malayan flying fox, which has a wingspan of 1.7-m (5.6-ft). The Kitti's hog-nosed bat is the world's smallest mammal, even smaller than a shrew. Body weights of bats range from 2-g (about 0.07-oz) to more than 1.3-kg (2.9-lb). In most instances, the sexes of adult bats are easily distinguished. Adult males have a conspicuous penis, and adult females have an area around the nipples of their breasts where the hair has been worn away by nursing young. Many species also have subtle secondary characteristics--differences in size, vocalizations, and behaviors--that distinguish females from males. Bat wings are supported internally by modified hand bones--a fact reflected in the name of the bat order, Chiroptera, which is Greek for 'hand-wing.' Each wing is made of a double layer of skin (called the wing membrane or patagium) that stretches between the bat's elongated finger bones and attaches to the side of the body and the hind leg. The clawed thumb is free of the wing, and the bat uses the thumb to cling to tree bark or the walls and ceiling of its roost. Three pairs of flight muscles that attach to the bat's upper arms and chest produce the power for flight. When these strong muscles contract, they tug at the wing, producing a downstroke that lifts the bat skyward. The contractions of three other muscle pairs attached to the back cause the wing to raise in an upstroke, readying it for another downstroke. Flying bats have been clocked at speeds as fast as 100 km/h (60 mph) and observed at altitudes of 3 km (2 mi). Unlike most birds, bats are able to fly at relatively low speeds with extreme maneuverability, a feat achieved by the bat's extraordinary ability to vary the shape and curvature of their wings and thus dramatically vary their aerodynamic lift. The tails on bats are extremely variable. Many bats have no tail or an extremely short tail that serves no purpose. Other bats, such as the mouse-tailed bats, have a tail that is nearly as long as the bat's body. In some species, such as the Natterer's bat, the tail is used as a net to catch insects, while other species use the tail as a pouch to hold insects until they can be eaten. Bat fur is typically long, silky, and gray, but there are many exceptions--notably the pale white northern ghost bat of tropical America, the canary-hued yellow house bat of Africa, and the nearly black greater spear-nosed bat of Central America. The painted bat has scarlet fur and black and orange wings, which allows it to hide among flowers. There is even one species of hairless, or naked, bat. The primary function of bat fur is for warmth; however, some hairs (especially those on the face and other exposed areas) receive and transmit sensory impressions, much like the whiskers of cats or mice. The faces of bats also vary considerably. Old World fruit bats have long, narrow, foxlike or doglike muzzles, while common vampire bats and their relatives possess short, piglike snouts. The ears of some species, including the aptly named long-eared bats, are as long as the bat's entire body. On the other hand, the black-bearded tomb bat has rather small ears. The shapes of bat ears are extremely varied. The wide ears of the Australian false vampire bat meet above the head and are fused. Many bats are able to turn their ears in the direction of faint noises. Bat hearing and its use in echolocation are highly developed. Some bats have large, conspicuous eyes, while others may have small beady eyes. This variation suggests that vision plays different roles in the lives of various species. Despite the familiar expression, 'blind as a bat,' none of these animals are truly sightless. On the contrary, several bats can see better in dim light than do humans. However, only the Megachiroptera can perceive colors. Like most mammals, bats have two sets of teeth. Depending upon the species, newborn bats are equipped with as many as 22 milk, or deciduous, teeth. These teeth are soon replaced with 20 to 28 permanent teeth. Bats have widely differing teeth patterns specialized for the diverse diets among the species, and teeth are often used to help identify bat species. Flesh-eating, or carnivorous, bats, such as the false vampire bats, have sharp and strong canine teeth for shearing flesh, and strong molars for crushing bones. Insect-eating bats have molars with sharp-edged surfaces that slide past each other in a scissorlike motion ideal for grinding the hard outer shells of insects. Fruit-eating bats have molars with large flat surfaces for grinding. IV BAT BEHAVIOR Bats have been quite successful in diversifying to colonize many different environments. To thrive in their varied habitats, bats display a wide range of specialized behaviors, including echolocation, diverse diets, and hibernation. A Echolocation While flying at night, bats use a combination of vision, smell, and hearing to find food, to navigate, and to avoid collisions. The Microchiroptera tend to rely heavily on a form of sonar called echolocation. In echolocation, bats emit short pulses of high-frequency sounds that are usually well above the threshold of human hearing. The sound waves spread out in front of the bat, striking any objects in its flight path and bouncing back in the form of an echo. By interpreting the echoes, bats are able to discern the direction, distance, speed, and in some instances, the size of the objects around them. Such information is instrumental in avoiding mid-air collisions and in tracking winged insects and other live prey in the dark. Interestingly, most Megachiroptera do not use echolocation. The exceptions are the cave-dwelling Megachiroptera, who only use echolocation inside their caves. Once outside, they rely on sight. Research published in 2006 indicates that some bats can also sense magnetic fields, an ability used by some birds when they migrate. The bats may rely on magnetic directions along with other clues to find their way back to home roosts after flying long distances at night. B Diet More than 65 percent of bats eat insects. One of North America's most common bats, the little brown bat, can consume as many as 600 mosquitoes in an hour. Beetles account for more than a third of the diet of big brown bats, with flying ants, flies, crane flies, mayflies, stone flies, and other insects making up the rest. Some bat species, such as the greater false vampire bat, eat small fish, reptiles, amphibians, birds, and mammals--including other bats. These bats have exceptionally long hind feet, tipped with sharp claws that are well suited for nabbing prey on the fly. Other carnivorous bats are more specialized: the fish-eating bats (also known as bulldog bats) feed mostly on fish, and the fringe-lipped bat feeds mostly on frogs. Other bats feed on fruit and nectar. Because they are fairly sloppy foragers and leave droppings as they travel long distances, these bats are unintentional agents of pollination and seed dispersal, both of which help food plants to reproduce and to spread. Such bats contribute to the health of the forest environment. Some species are also highly specialized. The recently discovered tube-lipped nectar bat of Ecuador has proportionately the longest tongue of any mammal--three-and-a-half times the length of its body. The base of the bat's tongue is actually in its chest rather than at the back of its mouth. This unusual species is specially adapted to feed on an elongated bell-shaped flower that only the bat pollinates. Much attention has been focused on the eating behavior of the vampire bats. These South and Central American bats feed solely on blood. They are equipped with razor-sharp incisor teeth, with which they make small incisions into the flesh of birds or mammals, lapping the blood as it seeps from the open wound. To survive, each vampire bat requires about two tablespoons of blood per day. The saliva of vampire bats contains an anticoagulant to prevent the blood from clotting. This anticoagulant is twenty times stronger than any other known anticoagulant and is used to make the medical drug Draculin, prescribed for heart attack and stroke patients. C Torpor and Hibernation Bats are warm-blooded animals, but unlike most other warm-blooded animals, they maintain their body temperature only when active. During the day, while resting in their roosts, bats let their body temperature drop to the temperature of their surroundings. If the surroundings are cold, bats enter a sluggish state of suspended animation, known as torpor. During torpor, a bat's metabolism, or rate of biological activity, drops, enabling the bat to conserve energy. In the colder reaches of their ranges, many bats enter an extreme form of uninterrupted torpor, known as hibernation, that can last through the winter months. Hibernation permits bats, as well as other animals such as squirrels and mice, to conserve precious energy, allowing them to survive in the leanest of seasons when food is scarce. However, bats hibernate to a greater degree than the other animals. Whereas the body temperature of most hibernating mammals drops fewer than ten Celsius degrees (eighteen Fahrenheit degrees), the temperature of some hibernating bats can fall slightly below freezing. The coldest recorded temperature for a hibernating bat is -5° C (23° F) for a red bat. In temperate climates, bats that do not hibernate may migrate considerable distances to winter roosts in warmer locales where food is more plentiful. For example, the Mexican free-tail bat migrates nearly 1600 km (nearly 1000 mi) between summer roosts in the United States and winter roosts in Mexico. Magnetic materials in the brains of some species may help measure the earth's magnetic fields, providing subtle clues that enable migrating bats to find their way over great distances. V REPRODUCTION AND GROWTH Little is known about the reproductive cycles of bats because their nocturnal lifestyles and secretive natures have posed challenges to the study of bats in the wild. Similarly, the vast diversity of species has defeated most attempts to make generalized statements about the life histories of bats. Among species that have been extensively studied, many have annual cycles of sexual activity. The cycles of entire populations are synchronized so that nearly all mating, birthing, and rearing activities occur within a narrow time frame of days or weeks. Most bat species are promiscuous, meaning that each individual mates with several others. In many species, pregnant females migrate to special nursery roosts, where they are joined by hundreds of other pregnant females. These roosts are usually warmer than nonnursery roosts--a feature that may speed up the rate with which the baby bats develop inside and outside of the womb. The gestational periods of bats are relatively long, ranging from 40 days to 8 months. Most bats give birth once a year. Many have only one offspring, but some have twins, and the hairy-tailed bats have triplets. The reproductive cycles of hibernating bats are often interrupted. Some hibernating bats, such as the little brown bats, mate in the fall, then hibernate throughout the winter months. The sperm remains dormant in females until these bats arouse in early spring, at which time fertilization takes place. In other bats, such as the straw-colored fruit bat and the Mexican fruit bat, fertilization occurs immediately after mating, but then the fertilized egg stops developing for several months. Bats are born bottom-first--an arrangement that minimizes the chances of the wings getting tangled in the birth canal. Newborns are enormous, often weighing in at 25 or 30 percent of their mother's weight. For the first few days of its life, the baby remains upside-down, suckling at its mother's breast. Since most bats are born singly, it would disturb the mother's balance if her newborn hung on one side. To remedy this, the baby hangs at an angle across the chest, its mouth grasping one breast and its hind legs hooked under the opposite armpit. In all bat species, only the females take care of their offspring. The mothers must feed throughout the rearing period but cannot hunt efficiently while carrying their young, so young bats are often left on their own in the nursery roost for several hours each day. When the mother returns to the nursery roost, she must find her baby in the crowd of seemingly identical young. She is guided by a general memory of the area where she left her baby and by the baby's distinctive scent and shrill cries. A mother Mexican free-tailed bat can pick out her offspring from a crowd of as many as 3000 seemingly identical young per square meter (as many as 300 per square foot) of cave space. In 1994 scientists reported the capture of ten adult male Dayak fruit bats in Malaysia and discovered that these bats had breasts full of milk. It is not known if these males breast-feed, but if they do, they will not only be an exception to the rule that male bats do not care for their young, but they will be the only species of mammal where the male is known to breast-feed. Young bats grow rapidly. Some species learn to fly and forage in about 18 days. Others require considerably more parental care: the most dependent youngsters are common vampire bats, which continue to nurse for six to nine months after birth. As the bats mature, they must avoid an assortment of predators, including snakes, hawks, owls, weasels, raccoons, wild and domestic dogs and cats, and even a giant centipede in Venezuela that hunts bats in caves. Bats are eaten by humans in parts of Asia, Africa, Oceania, and South America. Like other animals, bats can succumb to various diseases, and fatalities from airborne accidents do occur. However, any bats that survive such trials can have exceptionally long lives--from roughly 10 to 20 years, depending on the species. The world's longest-lived mammal for its size, the little brown bat has a life span exceeding 32 years. VI STATUS Worldwide, bat populations are declining at a rapid rate, due in large part to the destruction of feeding and roosting habitats and the misuse of toxic pesticides. Human interactions with bats have also contributed to their decline. In many nations, bats are unjustifiably earmarked as nuisances or threats to public health and killed. Between 1963 and 1970, the world's largest bat colony--some 30 million Mexican free-tailed bats in Eagle Creek Cave in the Apache Sitgreaves National Forest in southeastern Arizona--was reduced to 30,000 individuals, a decline of 99.9 percent. Major population losses have been recorded on all continents, and several islanddwelling Megachiroptera, such as the little Mariana fruit bat of Guam, have recently become extinct. In the United States, nearly 40 percent of native bat species are currently protected under the federal Endangered Species Act or are official candidates for inclusion on the nation's endangered species list. Several other countries have adopted conservation strategies designed to protect already beleaguered local populations of bats. Additional programs to study and save all remaining bats are being sponsored by the World Conservation Union (also known as IUCN) and by Bat Conservation International, a nonprofit organization based in Austin, Texas. Scientific classification: Bats belong to the order Chiroptera. Megabats make up the suborder Megachiroptera. Microbats make up the suborder Microchiroptera. The vampire bats make up the vampire bat family, Desmodontidae, although some authorities include the vampire bats in the American leaf-nosed bat family, Phyllostomatidae. Little brown bats are members of the common bat family, Vespertilionidae, and their genus name is Myotis. The free-tailed bats make up the freetailed bat family, Molossidae, and the Mexican free-tailed bat is classified as Tadarida brasiliensis. Contributed By: David George Gordon Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.