Bats

Adapted from Funk & Wagnall's Encyclopedia, found in Infopedia. Funk & Wagnall's Encyclopedia © 1993, 1994 Funk & Wagnall's, Inc., Licensed to Future Vision Media. Infopedia © 1994 Future Vision Media, Inc.

The only mammal capable of sustained flight. Bats make up a unique group, the order Chiroptera (Gr. cheir, "hand"; pteron "wing"). It is divided into 2 suborders, Megachiroptera ("larger bats" or "megabats") and Microchiroptera ("smaller bats" or "microbats"), and 17 families. One of these families includes all megabats (more than 150 species); the other 16 are microbats. In all, 850 to 900 species of bats exist, far more than in any other mammalian order except Rodentia, and bats exceed even rodents in total abundance. Distribution.Bats occur worldwide, except in the Arctic Zone and certain remote oceanic islands. All megabats and some families of microbats are confined to the eastern hemisphere; other microbats are confined to the western hemisphere, and a few families are worldwide in distribution. Bats are believed to have originated in a warm climate, probably in the early Eocene geologic epoch (the oldest known fossil bat, Icaronycteris index, is about 60 million years old), and the center of their distribution and abundance remains the tropical and subtropical areas. Members of only four families, all of them microbats, live in temperate regions, and the number of resident species diminishes poleward. Members of only two families reach subarctic regions, and then only in the summer. Size Range. The megabats include the largest bats, the giant "flying foxes" of Africa, India, and Australo-Malaysia. They are so called because of their long, doglike muzzles. The biggest, Pteropus vampyrusof Java, achieves a wingspan of 1.4 m (nearly 5 ft) and a body length of 42 cm (16.7 in). Many species of megabats are smaller than the largest microbats, however, the distinction between the suborders being based on tooth form and major skeletal differences rather than size alone. The smallest microbat, Pipistrellus nanus of Central Africa, is only 4 cm (1.5 in) long, has a wingspan of only 12.5 cm (about 5 in), and weighs less than 4 g (0.14 oz), making it one of the tiniest of mammals. The differences between microbats and megabats are sufficient, in fact, to suggest that they are of different evolutionary origin, with the megabats appearing much more recently than the microbats. Flight and Echolocation. Among living vertebrates, true flight is unique to bats and birds. Unlike most birds, however, bats are able to fly at relatively low speeds with extreme maneuverability. The wing is a thin, fleshy membrane supported near its leading edge by the greatly elongated bones of the forelimb and second finger, and toward the tip and rear by the even more attenuated third, fourth, and fifth fingers. It is attached along the midline of the trunk and outward-directed legs, and in various species it extends between legs and tail. Only the first finger, or thumb, is free, and in most bats it alone is clawed, together with the toes. This structure enables bats to vary dramatically the convexity of the wings and thus their aerodynamic lift.

All microbats navigate and most insectivorous species also target their prey by echolocation. This is the pulsed emission of high-frequency sounds that are reflected back as echos to a bat's ears from surrounding surfaces, indicating the position, relative distance, and even the character of objects in its environment. In this sense microbats "see" acoustically. This is the basis for their ability to navigate in total darkness. The physical properties of the emitted sounds vary in characteristic ways among species. The sound pulses are generated in the larynx, and in different species are emitted either from the mouth or nostrils.

By contrast, megabats use vision rather than acoustics for orientation. Only one genus (Rousettus of Africa, the Middle East, and Asia) has evolved an echolocation mechanism, involving the emission of audible "clicks," and it is used only when the bats fly in darkness. The eyes of megabats are also relatively larger than those of microbats. No bat is blind, however, and even echolocating microbats may use gross visual landmarks for homing during flight. Behavior. With a few exceptions, such as Saccopteryx of the western hemisphere and the African Lavia, all microbats are nocturnal. During the day they may rest in a variety of roosting places, such as caves, crevices, hollow trees, foliage, hiding places beneath rocks or bark, and in buildings. They may even roost in exposed situations; certain larger megabats hang upside down in enormous aggregations from tree branches, like so many gourds. Nocturnality gives bats many advantages, such as greatly reduced competition for insects and other food items, substantial freedom from attack, and protection from overheating and dehydration, to which bats are especially liable because of their enormous skin area relative to their size.

A few species of bats live solitarily, such as Lasiurus cinereus of North America, but most are gregarious. Aggregation during the day may vary from small groups consisting of a single male and a dozen or more females to enormous assemblages numbering many thousands or even millions of individuals (for example, Tadarida brasiliensis of the southwestern U.S.). Aggregations of members of specific species may show seasonal variation and sexual segregation in varying combinations. Mixed-species associations of a casual sort are common among bats using protected shelters such as caves.

Certain species of Temperate Zone bats are migratory to some degree, and movements of nearly 1600 km (about 1000 mi) between summer and winter quarters have been recorded in Tadarida brasiliensis. Others may fly only a few or up to 40 km (about 25 mi) or more daily between roosting and feeding sites, but the majority forage within more restricted home ranges.

Most bats are insectivorous and are able to hawk their prey on the wing or to seek out stationary insects on foliage or other surfaces. Most megabats, and many species of leaf-nosed bats of tropical America (so named for the remarkable folds of skin projecting upward from the nose), are fruit eaters. Still others in both groups consume flower parts or extract the nectar from flowering plants by means of greatly elongated tongues, aiding cross-pollination of the plants in the process. Some of the larger leaf-nosed bats as well as members of the Old World family Megadermatidae are carnivorous or omnivorous; they attack small amphibians, lizards, birds, mice, and even other bats, in addition to consuming insects and fruit. Closely related to the leaf-nosed bats are the three genera of true vampires of the American Tropics (Desmodus, Diphylla, and Diaemus), which subsist entirely on blood freshly drawn from small wounds inflicted on mostly warm-blooded prey such as fowl, cattle, horses, swine, and occasionally human beings. At least three species of bats supplement their diets with small fish, which are caught as the flying bats drag their enlarged feet and claws just beneath the water surface.Life Cycle. The gestation periods of bats are relatively long, ranging from about 44 days to 8 months in various species. Few produce more than a single offspring each year, and the young tend to mature slowly. The reproductive cycles of most species follow general mammalian patterns, but exceptions exist. Most interesting are Temperate Zone species such as Myotis, Rhinolophus, and Eptesicus,which hibernate during the winter months. Copulation occurs prior to hibernation, and sperm are retained within the females throughout the hibernation period. Fertilization occurs when the ovum is released from the ovary after the bats arouse in the spring. This is called delayed fertilization. A variant of this pattern is observed in the European genus Miniopterus, in which copulation, ovulation, and early development of the embryo all occur in normal sequence immediately prior to hibernation, but the embryo experiences developmental arrest before uterine attachment. It remains free and undeveloped until after the torpid mother resumes normal metabolism in spring. This is called delayed implantation.

The abundance of bats despite their low individual reproductive performances is attributable not only to the survival value of their habits but also to their remarkable longevity. Some larger species of megabats (Pteropus) and the smaller vampire bat (Desmodus) have survived in zoos for 20 years. Among various species of microbats banded and released in the wild, many have been recaptured after years of freedom. The record is a specimen of Myotis lucifugus recovered 31 years after it was initially marked and released in New England. Ecology. Insect-eating bats can be considered generally helpful to humans, and many bat species play an important role in plant pollination and seed dispersal. Because of their size and numbers, the larger fruit bats of the Old World can be an economic menace when they invade fruit orchards, but perhaps the greatest adverse effect of bats is the transmission of disease, especially rabies, to domesticated animals. This is a particular problem in the American Tropics, where local outbreaks of rabies among cattle, stemming from the bites of infected vampire bats, have decimated whole herds. Bats other than vampires can also contract and transmit rabies; most species of bats inhabiting the U.S. and Canada have been reported at one time and place or another to be infected. The prevalence of infection, however, has been low, outbreaks seldom, and unprovoked attacks on humans rare.

Many bats have low tolerance for certain insecticides such as DDT, which they can concentrate through the food chain. Some populations in the U.S. may have been adversely affected, inasmuch as three species on the list of endangered species have suffered from the agricultural use of pesticides. W.A.Wi.