Bat

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.

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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 non-nursery 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 island-dwelling 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 free-tailed bat family, Molossidae, and the Mexican free-tailed bat is classified as Tadarida brasiliensis.

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