Bats: Chiroptera

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BATS: Chiroptera

PHYSICAL CHARACTERISTICS

Bats are the second largest group of mammals after rodents. Almost one out of every four mammalian species on the planet is a bat species. Living bats are categorized into two main groups, each with its own distinct features. The Megachiroptera (mega-keer-OP-ter-ah), or "large bats" group includes one family. The Microchiroptera (micro-keer-OP-ter-ah), or "small bats" group includes all the rest of the bats.

Bats are the only mammals that can fly. Chiroptera comes from the Greek roots cheiro (hand) and ptera (wing), named for the similarity of a bat wing to a hand. Bat wings are long arms, hands and extra-long finger bones that are covered with a double layer of thin skin called a membrane. The membrane is thin enough that light can shine through it. The membrane contains blood vessels, nerves, and muscles.

In some bats, a membrane extends between the legs and encloses the tail. Some bats have tails that extend past the membrane and others have no tails. In most bats, the thumbs are free from the membrane. These thumbs have claws and are often used for climbing up trees or other structures.

Bat membranes are tough and flexible, allowing bats to move their wings much like people move their fingers. Changing the shape of their wings allows bats to turn and maneuver quickly. Some bats can hover in the air while others glide. When it is cold, the bats fold their wings around themselves. When it is warm, bats flap their wings to cool themselves.

Bats range widely in size, yet the majority of bats weigh less than 1 ounce (25 grams). The largest bat is the Malayan flying fox, which can have a wingspan of 6 feet (1.8 meters) and weigh 3.3 pounds (1.5 kilograms). The smallest bats are the Kitti's hog-nosed bats, also called bumblebee bats, of Thailand, with a wingspan of 6 inches (15 centimeters) and a weight of about 0.07 ounces (2 grams), less than a penny.

Like other mammals, bats are warm-blooded and fur covers their body. Megachiroptera are characterized by large eyes, small ears, and dog-like snouts. Most Microchiroptera species are characterized by wide, extended ears and odd shaped noses. Bats have weak legs and do not walk long distances. Their feet are small with sharp claws on each toe. Bats use their claws to hold the weight of their body when they hang upside down, which is their normal resting position.

GEOGRAPHIC RANGE

Bats live on every continent on Earth except Antarctica and some remote islands. Most bats live in the tropics and species are most numerous around the equator.

HABITAT

Bats need a roost, a place to settle or rest, and a place to find food. The type of roost a bat chooses depends upon the type of bat. Bats can roost in hollow trees, cracks in trees, and under bridges. Many bats depend on caves in the cool winter months to survive, and others roost in caves all year long. Some bat species find their roosting site in abandoned mines. The dome shaped ceilings can hide and protect the bats from predators, the animals that hunt them.

A few species of bats make their roost from large leaves, such as palm and banana leaves. These small bats chew across the leaves so that the sides droop down in the form of a tent. Other bats can roost in flowers and animal dens.

Bats often return to the same site at the same time each year. In warm weather, big brown bats commonly roost in buildings and then shift to caves and abandoned mines during the colder months. The Pallas's mastiff bats are found roosting in buildings, hollow trees, rock crevices, caves, and bridges.

DIET

While the most famous bats are the vampire bats, known for eating blood, the majority of bats eat only insects. Microchiroptera are generally carnivores, meat-eaters, that feed on insects, such as moths, flying beetles, and mosquitoes. Bats can capture insects while flying by catching them in their mouths or scooping them into their tails or wing membranes. Some bats pick the insects off leaves or the ground. One gray bat may eat up to 3,000 insects in one night.

Some bats feed on larger prey, animals hunted or caught for food, such as fish, frogs, birds, mice and other bats. A fisheating bat will swoop down and grab fish with its claws. A bat that eats mice will swoop down, wrap the prey in its wings, bite it and then whisk it away to eat it.

The three species of vampire bats are the only bats that feed on blood, sucking up the blood of cattle, sheep, or other relatively large animals. The bats use their razor-sharp teeth to pierce the animal's skin, often while the animal is sleeping. The bats then lap up about 2 tablespoons (30 milliliters) for their meal.

Most megachiropteran species are herbivores, plant-eaters, eating fruit, seeds, leaves, nectar, and pollen. Whatever it eats, bats eat only the parts of their prey that they want to ingest. When a bat catches an insect, it will generally bite off and drop its wings and legs. When eating another bat or bird it will not ingest its wings. An Old World fruit bat will chew its fruit thoroughly, swallow the juices then spit out the remaining pulp.

Bats drink by flying close to the water and taking up the water while flying. With the exception of three species of nectar-feeding bats that live along the Mexican border of Arizona and Texas, bats in the United States and Canada eat insects.

BEHAVIOR AND REPRODUCTION

Bats as a group are crepuscular (kri-PUS-kyuh-lur), meaning they are active at dawn and dusk, or nocturnal, meaning they are active at night. When they are roosting, bats generally hang upside down by their claws. This allows them to simply let go of whatever they are hanging onto and start flying.

With their large ears and small eyes, microchiropteran bats depend upon a complex sound technique called echolocation (eck-oh-loh-KAY-shun) to help them find prey and move. While flying, these bats send out high-frequency sounds that bounce off of other objects. The bat listens for the bounced sound, and then determines the location, size, distance, and speed of the object—all within a split second. In most bats, the echolocation is at such a high pitch that it is beyond the human hearing range, though humans can hear the sounds of some bats. Researchers are still working to understand exactly how echolocation works. Megachiroptera generally depend upon their eyes to navigate, but some of these bats also use echolocation.

Like all mammals, bats are warm-blooded, meaning they maintain their body temperature. Bats roost in warm places during the cool months to conserve the energy it takes to keep warm. Unlike other mammals, bats can allow their body temperature to drop to the ambient temperature, or surrounding temperature, when they are not active. As their temperature drops, metabolism slows down.

During the winter, some bats will drop their body temperatures for months at a time and go into hibernation, meaning they go into a resting state in a safe place, typically without eating or passing wastes. A bat's body temperature can drop to as low as 35.6°F (2°C). These bats survive the winter by living off their storages of fat and making occasional food trips during warmer weather.

Other bat species follow an annual migration pattern, traveling to warmer climates in the cool months and cooler climates in the warm months.

Bats are generally social animals and gather together in roosts. Bats can roost in colonies of several hundred to tens of millions. The number of bats in a roost depends upon the type of bat. Pipistrelle maternity, or motherhood, roosts usually contain between fifty and two hundred bats. Brown long-eared bats usually live in colonies of twenty-five up to fifty bats. Mexican free-tailed bats are one of the more social bat species and found in huge populations throughout their range. In Bracken Cave, Texas, the population of Mexican free-tailed bats was estimated at twenty million bats!

Like all mammals, female bats give birth to live young and feed their newborns milk. Females often roost in large colonies, with many females giving birth in the same area. Bats usually give birth to only one young per year. During their first weeks of life newborn bats cling to their mothers while in flight. Only the mother cares for the young, and there is no lasting relationship between the mother and father.

Bats grow quickly; the young are often flying at four weeks. Young microchiropterans become independent at approximately six to eight weeks, megachiropterans at about four months old. At the age of two years bats are sexually mature. Bats live about twenty-five years, far longer than most mammals of a comparable size.

BATS AND PEOPLE

Popular folklore and myths have led to many people having a negative reaction to bats. Because most people do not typically see or interact with bats, many misunderstandings about these creatures remain. The Eastern European tale of a vampire, a corpse that came back to life and sucked blood from the neck of its human victim, dates back to the Middle Ages. After Bram Stoker's Dracula was published in 1897, the misconception of bats as dangerous and mysterious became more popular. Although there are only three species of vampire bats, all living in South and Central America, all bats still have a reputation for sucking blood.

Bats also have a reputation for carrying rabies, a viral disease that affects the nervous system and can be spread through bite of an affected animal. Yet less than one half of one percent of bats carries the rabies virus. And bats are rarely aggressive, usually attacking only if they are frightened.

Not all people consider bats a bad omen or scary. In China, bats are considered good luck symbols. Fabrics and dishes are often decorated with bat-shapes for good luck. Native Americans considered the bat a protector.

Bats are beneficial to people in many indirect and direct ways. They are one of the few predators of night-flying insects, some of which are pests to crops and people. People have long used the nitrogen-rich bat droppings, called guano (GWAN-oh), as a fertilizer.

A WHOLE LOT OF BATS

Bracken Cave in Texas is home to the world's largest bat colony. Each year, some twenty million Mexican free-tailed bats gather at this cave near San Antonio to give birth and rear their young. And each night they emerge to forage for food. The twenty million bats can eat more than two hundred tons of insects in a single summer night!

Bats also play an important role in plant pollination, the transfer of pollen, the reproductive spores, for fertilization. When nectar-eating bats move from flower the flower to eat, the bats pick up pollen on their fur and disperse it as they move. Bats are the most important pollinators among mammals in the rainforest. They pollinate many plants that humans eat, including bananas, figs, mangos, and peaches. Bats also are integral for seed dispersal, having led to the continued survival of over 1,000 species of trees. The fruit bat disperses seeds away from the parent tree by either swallowing them and leaving the seeds in their droppings, or carrying off the fruit to eat.

CONSERVATION STATUS

Bat populations are in decline in the United States and throughout the world. In the United States, out of forty-five bat species, six are federally endangered, facing an extremely high risk of extinction, dying out, or threatened, close to facing the risk of extinction. Twenty species are categorized as being of special concern by the U.S. Fish and Wildlife Service.

The World Conservation Union (IUCN) includes 521 bats on its Red List of Threatened Species. Twenty-nine are Critically Endangered, facing an extremely high risk of extinction; thirty-seven are Endangered, facing a very high risk of extinction; 173 are Vulnerable, facing a high risk of extinction. The rest on the list are not currently threatened, but could become so, or there is not enough information about the bats to know how threatened they are.

With few natural predators, the primary reason for declining bat populations is directly and indirectly related to humans. Pesticides on plants have reduced insect populations, the food supply for many bats. Occasionally, people hunt bats for food, but far more harmful to bats is the destruction of their natural areas and living spaces. Deforestation, the clearing of trees for agriculture or people, decreases their food supply and habitats.

FINDING DINNER

Bats have an amazing ability to find food. Fishing bats have echolocation so sophisticated that they can detect a minnow's fin as fine as a human hair, which sticks up above a pond's surface only 0.08 inches (2 millimeters); that's about the thickness of about twenty human hairs. African heart-nosed bats can hear the footsteps of a beetle walking on sand from a distance of more than six feet. And when the Central American fringe-lipped bat hears the mating calls of mud-puddle frogs, it switches to the frogs' lower frequency so that it can detect its exact location.

People have also killed colonies of bats out of fear or ignorance. In Central America, where vampire bats can be a problem for livestock, locals find bat caves and blow them up, killing entire colonies whether they are colonies of vampire bats or not. In the United States, destroying bat habitats such as mines have killed them and any that remain are left without protection.