Bats
are the 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 for some 50 million years. 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 between two major groups. The 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 Australia. The microbats are smaller, eat a variety of foods from small mammals to fish, and are much more widely distributed.
Nearly all bats are active at night or 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 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.
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. 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.
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 quantities of insects consumed by bats are astonishing: as many as 250 tons of insects may be devoured by a colony of Mexican free-tailed bats in a single night.
During
the daytime, bats. Nearly all bats rest and sleep hanging upside
down by their hind feet; 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 for fertilizer.
PHYSICAL DESCRIPTION
Size
Bats range in body size from as small as 1 inch long, to as large as
the 16 inches long Malayan flying fox, which has a wingspan of 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 about 0.07
ounces to more than 2.9 pounds.
Wings
Bat wings are supported internally by modified hand bones the name of
the bat order, Chiroptera, is Greek for "hand-wing." Each
wing is made of a double layer of skin 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. Flying bats have been clocked at speeds as fast as 60 mph and
observed at altitudes of 2 miles. Unlike most birds, bats are able to
fly at relatively low speeds with extreme maneuverability.
Tails
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 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.
Fur
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.
Faces
The faces of bats also vary considerably. Old World fruit bats have
long, narrow, fox-like muzzles, while common vampire bats and their
relatives possess short, pig-like snouts.
Ears
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.
Eyes
Some bats have large, conspicuous eyes, while others may have small
beady eyes. 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.
One species can perceive colors.
Teeth
Like most mammals, bats have two sets of teeth. Depending upon the
species, newborn bats are equipped with as many as 22 milk 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
scissor-like motion ideal for grinding the hard outer shells of
insects. Fruit-eating bats have molars with large flat surfaces for grinding.
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.
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.
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, stoneflies, and other insects making up the rest.
Some
bat species eat small fish, reptiles, amphibians, birds, and mammals.
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.
Flying foxes, such as the Southeast Asian species pictured here, constitute most of the fruit-eaters of the Old World. This adult male short-nosed fruit bat, steals coconut sap from a can. The foam on top of the sap is caused by fermentation. |
Torpor and Hibernation
Unlike most other warm-blooded animals, they maintain their body temperature only when active. While resting bats let their body temperature drop to the temperature of their surroundings. If the surroundings are cold, bats enter a sluggish state known as torpor. In the colder reaches of their ranges, many bats enter an extreme form of uninterrupted torpor, known as hibernation, which can last through the winter months. However, bats hibernate to a greater degree than the other animals. Whereas the body temperature of most hibernating mammals drops fewer than eighteen Fahrenheit degrees, the temperature of some hibernating bats can fall slightly below freezing. The coldest recorded temperature for a hibernating bat is 23° F for a red bat.
In temperate climates, bats that do not hibernate migrate winter roosts. For example, the Mexican free-tail bat migrates nearly 1000 miles between summer roosts in the United States and winter roosts in Mexico.
REPRODUCTION AND GROWTH
Little is known about the reproductive cycles 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 mating, birthing, and rearing activities occur within a period of days or weeks.
The gestational periods of bats are relatively long, ranging from 40 days to 8 months. Most bats give birth once a year and only have one offspring, some have twins, and the hairy-tailed bats have triplets. Some hibernating bats mate in the fall, and then hibernate throughout the winter months. The sperm remains dormant 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.
Newborns are enormous, often weighing in at 25 or 30 percent of their mother's weight.
In all bat species, only the females take care of their offspring. The mothers must feed throughout the rearing time. 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. (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. Any bats that survive to adulthood 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.
STATUS
Worldwide, bat populations are declining at a rapid rate, due in large part to the destruction of feeding and roosting habitats. Major population losses have been recorded on all continents, and several island-dwelling bats, 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.
Contributed By:
David George Gordon