Bat

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 about 1,000 living bat species, accounting for approximately one-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.

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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 Earth. Bats also produce a nitrogen-rich fertilizer known as guano.

II

Range and Habitat of Bats

Bats are found in all habitable regions of 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 Characteristics of Bats

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.

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