Mammals

Characteristics:

Mammal, animal that raises its young on milk.

Most mammals are covered with hair or fur.

Most have specialized teeth that help them to cut or chew their food.

Compared to other vertebrates, mammals have highly developed nervous systems.

They show an intelligence and resourcefulness that few other animals can match.

Mammals include some of the most familiar members of the animal kingdom, such as cats, dogs, elephants, and whales, and also humans.

Life spans:

The life spans of mammals vary as enormously as their sizes.

Most shrews survive for less than a year, exhausting themselves in a life of almost ceaseless activity.

By contrast, horses can live about 20 years, chimpanzees can live to be over 50, and elephants can survive into their 60s. Humans have the longest life span of any mammal, with a few individuals living over 110 years.

TYPES OF MAMMALS �

Subclasses of mammals:

Monotremata:

The monotremes make up by far the smallest subclass of mammals, with just three species, found in Australia, Tasmania, and New Guinea.

One of these is the duck-billed platypus, and the remaining two are the echidnas, or spiny anteaters.

The reproductive and excretory systems of monotremes share a single body opening, but a much more striking feature of these mammals is that they lay eggs, a characteristic unique from all other mammals.

The female duck-billed platypus normally lays two or three eggs and incubates them in a waterside burrow.

Echidnas usually lay a single egg, which the mother incubates in a pouch formed by two folds of skin on her abdomen.

When monotreme eggs hatch, the young feed on milk, lapping it up from a special milk patch on the mother's underside.

Subclass: Marsupials

These mammals give birth to live young, but the young are born while still in a very undeveloped state.

They complete their development inside a special pouch on the mother's abdomen, feeding on milk supplied by her nipples.

In some marsupials the pouch is little more than a narrow flap, and the growing young soon protrude outside it.

In others it is a spacious bag, and the young are completely tucked away.

There are about 250 species of marsupials, and they are found in a variety of habitats.

About two-thirds of them live in Australia, Tasmania, or New Guinea, where they exist in a wide variety of forms, including plant-eaters such as kangaroos, koalas, and wombats, and also animals such as bandicoots and quolls, which have sharp teeth and feed largely on insects and other invertebrates.

The remainder of the world's marsupials live in the Americas. They include about 70 different kinds of opossum, one of which--the Virginia opossum--is the only marsupial found in North America.

Subclass Placentals

The group includes about 4300 species, making it by far the largest of all three mammal groups.

Unlike young marsupials, young placental mammals spend a relatively long time developing inside their mother's body before birth.

Warm and protected within the mother's womb, the unborn young are nourished by a spongy organ called the placenta, which absorbs nutrients from the mother's blood and transfers them to the developing animal.

By the time a young placental mammal is born it is usually fully formed, although it may not yet have fur or functioning eyes or teeth.

Orders of mammals:

Are about 19 groups called orders (the exact number varies in different classification systems).

Rodentia: The largest group, with about 1500 species, contains the rodents, such as rats, mice, squirrels, and porcupines. Animals with sharp, chisel-like front teeth that grow throughout life, rodents use these teeth to gnaw into their food, and also to cut through any obstacles in their path.

Chiroptera: Order has about 1000 species, contains the bats. Insect-eating bats are generally small animals, but some fruit-eating species have a wingspan of over 5 ft.

Carnivora: Most large predatory land mammals belong to a group called the carnivores, which contains about 240 species. Some of these animals, such as lions and wolves, rarely eat anything apart from meat, but others, especially bears, have a more mixed diet.

Primates: Primates include animals such as lemurs, monkeys, apes, and humans, and most of the 230 species live in trees.

Ungulata: Contains animals such as pigs, deer, cattle, and antelope, which have hoofed feet with an even number of toes and another much smaller group which includes horses and rhinoceroses, which have an odd number of toes.

Cetacea: Some mammals are suitable for life in the water. The seals, including sea lions and walruses, can sleep and feed in the open ocean but must return to land in order to reproduce. Manatees are large, plant-eating mammals that spend their entire lives in the water. The whales, including the huge baleen whales and the dolphins, are well equipped as fast, open-ocean predators. Still, like all other mammals, aquatic mammals would drown if they could not reach the surface to breathe.

MAMMAL HABITATS �

Mammals have adapted to some of the most extreme habitats on earth.

They are warm-blooded, or endothermic, meaning that they maintain their body temperature within a narrow range despite changes in the environment.

Polar bears survive on Arctic ice, while Arctic foxes can sleep on open snow in temperatures as low as -90� F.

Camels and kangaroo rats live in deserts, and can tolerate blazing temperatures that would kill many animals from cooler habitats.

Mammals can also tolerate the thin air of the highest mountains as well as the crushing pressures of the ocean depths. Yaks, for example, forage for food on mountain slopes at altitudes of up to 20,000 ft, while sperm whales can dive to depths of at least 7000 ft, holding their breath for over an hour.

Many mammals dig burrows as refuges or as places to raise their young, but some have a largely subterranean lifestyle, feeding on small animals or plant roots beneath the soil's surface.

These animals, including moles, dig through the ground either with spadelike front paws or with their teeth, and they detect danger by being highly sensitive to vibrations transmitted through the soil.

Most moles build permanent tunnels, but in Australia an unrelated burrowing animal--the marsupial mole--simply shovels its way through the loose sand of its habitat, leaving the dirt to collapse behind it.

Above ground, grasslands are one of the most productive habitats for mammal life. The most successful mammals in this environment are ruminants--hoofed species such as buffaloes and antelope--which have a highly specialized digestive system that breaks down cellulose, a tough substance that forms the walls of plant cells.

In a landscape that offers few places to hide, many of these grazing mammals protect themselves by forming large herds, and use speed to escape their enemies.

In some grassland areas, such as the plains of East Africa, herds of grazers carry out yearly migrations, arriving at fresh grazing areas just after rain has triggered new growth.

Unlike grassland mammals, those that live in forests are well concealed, rarely band together for safety, and are mostly nocturnal, or active at night.

Arboreal species: Ones that spend most of their lives in trees, include sloths, most primates, squirrels and their relatives, and a number of marsupials including opossums and tree kangaroos.

Squirrels use their tails to balance as they scamper along branches, but some arboreal mammals have prehensile tails that can wrap around branches.

In many South American monkeys, these tails are so strong that they can support the animal's entire weight.

Ground based: In temperate regions (areas with cold winters and warm summers) the largest ground-based forest dwellers are bears, deer, and wild pigs, but in tropical regions they also include elephants.

In mountains, tundra, and deserts, mammals have to overcome hostile conditions if they are to survive. Rodents have successfully colonized all three of these habitats, because their small size enables them to avoid extreme conditions by hiding away underground or in burrows beneath the snow.

Larger mammals do not have this option. Instead, they cope with cold with long fur and a layer of insulating body fat.

Some mammals, such as ground squirrels, survive cold winters when food is scarce by entering a sleeplike dormant state called hibernation and awaken when food is more abundant.

Larger mammals survive desert heat with a number of features, including sweat glands that produce perspiration to cool the body.

To combat arid conditions, many desert mammals have a sophisticated kidney function that produces a concentrated urine, so that less water is removed from the body.

At high altitudes, mammals face the additional problem of shortage of oxygen. In the Andes mountains of South America, guanacos, llamas, and alpacas overcome this problem by having more oxygen-carrying red blood cells than most mammals, and by having a special form of hemoglobin, the oxygen-carrying pigment in blood, which binds oxygen at very low pressures.

This feature allows them to run effortlessly at altitudes of up to 16,000 ft.

Some mammals, such as otters and river dolphins, live in freshwater habitats, but the great majority of the world's aquatic mammals live in the ocean.

Seals remain close to coasts or to floating ice, but whales and dolphins are truly pelagic, meaning that they wander far out into open water.

Most of these marine mammals live in areas where food is abundant, but where water temperatures are low.

They survive the cold in two different ways.

Some, such as sea otters and fur seals have a double coat of fur, with extremely dense underfur hairs that are so closely packed that the skin never gets wet.

By contrast, whales and dolphins have very sparse hair, and keep warm with a thick layer of fat called blubber.

MAMMAL INTELLIGENCE �

Compared to other vertebrates, mammals are highly intelligent animals. They are quick to exploit opportunities, and to learn from past mistakes.

This ability makes them greatly adaptable, and gives them the best chances for survival under difficult conditions.

In mammals, as in other animals, intelligence is linked to the size and structure of the brain. The brains of mammals and other vertebrates are divided into three parts--the hindbrain, midbrain, and forebrain.

The hindbrain deals mainly with essential body processes, such as breathing, while the midbrain receives and coordinates sensory and motor impulses. The forebrain integrates and processes information, enabling an animal to make decisions and respond to the world around it.

In mammals, the forebrain is highly developed and it has a folded surface that enables it to contain millions of interconnected neurons, or nerve cells.

On its own, however, brain size does not directly relate to intelligence. Equally important is the size of the brain relative to the rest of the body.

An adult male sperm whale has a brain that weighs about 16 lb, which is about 0.02 percent of its body weight.

By comparison, an average human brain weighs only about 3 lb), but makes up about 2 percent of the body weight. This difference allows a much larger part of the human brain to be devoted to processing information.

Even in small mammals, learning plays an important part in daily life.

Rodents are remarkably good at finding ways of getting at food and they soon learn to avoid obstacles such as traps and poisoned bait.

Small hunters such as weasels learn to anticipate the reactions of their prey, so they can make a lethal strike. Skill comes with experience, so the more often a mammal carries out a particular task, the better it becomes at the task.

In mammals, behavior is also acquired by imitation. If one animal in a group stumbles on a new way of doing something, others may follow the example.

This kind of behavior can be seen in many animals, from dogs to dolphins, but it is particularly well developed in primates.

Learning by imitation has been observed in chimpanzees and also in some species of monkeys. In one famous case, scientists fed sweet potatoes to a group of Japanese monkeys living on a small offshore island. One animal discovered that the food could be cleaned by washing it in the sea, and after several years, all the adult macaques had adopted this cleaning routine.

A key feature of this kind of learned behavior is that it can be passed on from one generation to another, allowing succeeding generations to benefit from the experiences of earlier ones.

DIET OF MAMMALS �

Mammals use food to keep themselves warm as well as to power their bodies and grow.

As a result, they have to eat more frequently than cold-blooded animals, or ectotherms, which do not maintain a constant warm temperature.

This is particularly true of the smallest mammals because, despite having fur, their small body size means that they lose heat at a rapid rate.

To facilitate eating, specialized teeth help them to collect their food, and also to process it before it is swallowed.

Mammals' teeth occlude, which means that they fit together in a precise way when the mouth is closed, allowing them to nibble, gnaw, slice, or chew.

Big insect-eaters, such as anteaters, usually locate their food by sight or smell. They gather food in large quantities using their sticky tongues, which can sweep up thousands of ants in a matter of minutes.

The majority of bats also feed on insects, but they have a very different technique for catching their flying prey. Using a system called echolocation, a bat sends out bursts of high-pitched sound toward objects and interprets the returning echoes as images that guide a bat toward its prey so that it can hunt even in total darkness.

Dolphins and sperm whales use a similar system to locate food underwater.

True carnivores, which include dogs, cats, and their relatives, often spend a considerable time tracking down their prey.

For most of these flesh-eating animals, the senses of smell and hearing are at least as important as vision.

Carnivores normally hunt alone, but some species--notably wolves and lions--hunt in organized groups. By doing this they can tackle prey larger than themselves.

Carnivores have pointed front teeth, called canines, which help them hold their prey. Most also have specialized rear teeth, called carnassials, which work like shears to slice through their food, and to crush bones.

The most successful of the plant-eating mammals are the ruminants--hoofed mammals that include cattle, sheep, goats, and their relatives.

Ruminants have a highly specialized three- or four-part stomach that helps these animals digest the cellulose in plant cell walls, which no mammal can digest on its own.

After eating their food, they regurgitate it and chew it a second time, before swallowing it once again.

Microorganisms that reside in the stomach then break down the chewed-up mass, releasing nutrients that the mammal absorbs.

In water, few mammals apart from manatees live purely on plants. Instead, most aquatic mammals eat animal food, but they catch it in two quite different ways.

Pursuit hunters, such as otters, seals, and toothed whales, chase individual prey through the water, much like carnivores chase their prey on land.

However, the largest whales feed on much smaller animals, scooping them up in vast quantities. These whales do not have teeth, and instead strain their food with a screen of fibrous plates called baleen. Using this system of filter feeding, a blue whale can consume over 4 metric tons of food in a day.

REPRODUCTION �

Reproduction:

With the exception of three highly unusual mammals called monotremes, all mammals give birth to live young.

Some young mammals are completely helpless when they are born, while others are relatively well developed.

Despite these differences, all young mammals initially rely on their mothers for food, and stay with them until they are ready to fend for themselves.

This close link between mother and offspring produces strong family ties, and allows young mammals to learn by copying their parents' behavior.

Some mammals breed throughout the year, but most have distinct breeding seasons that allow birth to coincide with times when food is plentiful.

In some species--for example, baboons--changes in skin color show when a female is ready to mate.

However, in most mammals, males detect fertile females by sensing airborne chemicals called pheromones.

The males of some mammals, including cats and horses, sense these chemicals with a receptor on the roof of the mouth called the Jacobson's organ. As the males sniff, they curl their upper lip and seal their nostrils to draw air over the scent organ.

Young mammals are normally conceived immediately after mating. The gestation period--the time between conception and birth--varies from as little as 12 days in the case of some marsupials, to over 20 months in the African elephant.

A mammal's size and its gestation period are not always directly related. The tiny mouse lemur, for example, weighs about 2 oz, but has a gestation period of nearly 9 weeks.

In a few mammal species, including rodents, bats, and kangaroos, development can be brought to a temporary halt at a very early stage. This system, called delayed implantation or embryonic diapause, allows the time of birth to be finely adjusted.

In rodents, it helps to make sure that successive litters are spaced apart. In kangaroos, it allows the birth of developing young to be kept "on hold" until rain produces an abundance of food.

Mammals usually give birth on their own, but in some species--for example, dolphins and elephants--nonbreeding females sometimes gather around to help during the birthing process.

Many placental mammals lick the embryonic membranes off the newborn young, and often nudge them toward the mother's mammary glands for their first meal of milk.

By contrast, female marsupials seem to pay little attention to their young, leaving them to find their own way into the pouch unaided.

Milk produced in the mother's mammary glands provides newborn mammals with water and important nutrients. Mammals are born with a strong sucking instinct that helps them feed immediately.

In monotremes, milk simply oozes out onto a patch of skin, but in marsupials and placentals, milk is released by a set of nipples or teats.

Young placental mammals leave their mother's nipples between feedings, but newborn marsupials remain firmly fastened to their mother's nipples by their mouths and cannot be dislodged.

The period before weaning forms a crucial part of a young mammal's development. During this time, it plays with its siblings and learns social and survival skills from its parents.

During weaning, young carnivorous mammals start to share food their parents have caught, while young plant-eating mammals begin to feed for themselves.

Independence comes only when the adult teeth are fully formed, enabling the young animal to switch to an adult diet.

LOCOMOTION �

Locomotion:

Of the approximately 4600 species of mammals alive today, most live on land, and most move about on all four legs.

But modern mammals also include animals that hop on two legs (kangaroos), ones that live permanently in water (whales), and ones that can fly (bats).

These swimming and flying species include the world's largest mammal--the blue whale, which can grow over 100 ft long--and also the smallest, the Kitti's hog-nosed bat. Discovered in 1973 in the forests of Thailand, this tiny bat is about the size of a bumblebee, and weighs just 2 g (0.07 oz).

Most primates are good climbers. Primates, such as monkeys and apes have long legs, grasping toes, and flattened nails instead of claws. Instead of moving along the top of branches, they can also hang below them.

Some species of primates--particularly gibbons--use a method of movement called brachiation. Hooking their hands over branches, they swing through the treetops. At top speed, a brachiating gibbon can move through the forest canopy as fast as a human can run.

On the ground, most heavily built mammals, such as badgers and bears, walk on the soles on their feet. This gait works well to maintain a grip, but it is not good for rapid movement.

Hunting mammals, such as dogs and cats, have legs that are long and more slender. Instead of walking on their soles, these animals walk on the pads of their toes, with their heels staying high off the ground. This reduces friction and increases leverage, permitting much more rapid movement.

Hoofed animals have very long legs, and they stand on the very tips of their toes. The combination of small, hard hooves and a long stride enables them to run quickly for long periods of time.

A small but varied assortment of mammals, including kangaroos, don't run on four legs, and instead hop on two. Hopping works best in open habitats, and is a surprisingly energy-efficient way of moving about.

Every time a hopping mammal makes contact with the ground, tendons in its legs stretch like elastic. The energy from the stretched tendon helps to power the next jump.

Many forest mammals have flaps of skin that act like wings enabling them to glide from tree to tree. Example: flying squirrels.

Bats, however, are the only mammals capable of powered flight. A bat's wings are essentially front legs, with four extremely long fingers that work like the spokes of an umbrella, opening up to support the wing. The remaining finger, which is much shorter, has a hooked claw and is often used for grooming the fur.

In the place of legs, marine mammals have sturdy flippers. Seals have all four limbs and they use their hind legs to swim.

In whales, however, the hind limbs are absent with only tiny bones hidden away inside the body.

To power themselves, whales use tail flukes, horizontal flaps that are stiffened by tough fibers instead of bones. As a whale bends its backbone up and down, its flukes speed it through the water.

Migration:

Mammals also include some of the world's greatest animal migrants. During its yearly migration from Arctic waters to the coast of Mexico and back, a gray whale may travel 12,500 mi.

On land, most migratory mammals are plant-eating species. They include tropical animals such as wildebeests and zebras, which migrate to take advantage of rain, and also Arctic species such as the caribou, which migrates between tundra and coniferous forest. Some herds travel up to 30 mi a day, covering over 1000 mi in the course of a year.

SURVIVAL �

Like all other animals, mammals have to overcome a wide range of hazards in order to survive.

Some of the problems they face, such as extreme cold or heat, are constant and predictable features of life.

They deal with these by physical adaptations such as good insulation, and by special forms of behavior such as hibernation and migration.

Attacks by predators present quite different problems, because they are more random and therefore much more dangerous. For most mammals, the key to surviving them lies in keen senses and rapid responses.

In open ground, most plant-eating mammals use special danger signals that warn if an attack is imminent. Rabbits, for example, stamp their hind feet on the ground if they see or hear danger, while many grazing mammals, such as antelopes, give a barklike alarm call if they sense an unfamiliar scent in the air. These sounds bring feeding to an instant halt, as the members of a group ready themselves to run.

In some primates, more elaborate warning systems are present. African vervet monkeys have specific alarm calls that are used for three kinds of predators--eagles, leopards, and snakes.

The eagle call makes the monkeys run for dense cover, while

the leopard call sends them running up trees to the highest branches.

The snake call simply increases general wariness as the monkeys continue to feed.

In other species, specific features of the body serve as effective weapons.

For example, many mammals communicate by scent, and mark their territories with strong-smelling secretions that are produced by special glands.

In skunks, these secretions have taken on a completely different role, and are used as chemical weapons. The skunk's conspicuous black-and-white markings warn that it can be dangerous if attacked.

More widespread defense systems are based on armaments made of keratin--the same substance that forms hair--and also on structures made of bone.

In hairs, keratin is thin enough to be flexible, but when it is laid down in much thicker layers, the same substance can form tough scales.

Armadillos are covered with sharp-edged scales, and they roll up if threatened, being protected by interlocking plates of bone.

Keratin also forms horns, spines, and quills.

Horns grow around a stubby core of bone, which locks the horn firmly in place.

Unlike antlers found in deer, which are shed and regrown annually, horns keep growing throughout life.

In some large grazers, such as African and Asian buffalo, they can span over 5 ft from tip to tip, making them formidable weapons.

Spines and quills are only anchored in the skin, but they grow over large parts of the body. Animals such as hedgehogs use their spines as a passive form of defense, but porcupines use their quills in a much more active way, charging backwards to leave them embedded in an enemy's mouth or skin.

For other mammals, hunting weapons such as teeth, tusks, and claws double as emergency defenses in the event of an attack.

The teeth of a hippopotamus are large enough to slice through a crocodile menacing its young.

The tusks of a walrus, which can be over 2 ft long, can sometimes fend off attacking killer whales.

Cats' claws are normally retracted in the paws, which keeps them permanently sharp, making them particularly effective weapons for attack and defense.

Many small mammals, such as rodents, are vulnerable to attack from birds or other predators when they are feeding out in the open.

To defend themselves, these animals employ camouflage, in which they use their natural coloring to blend in with their surroundings.

For instance, voles forage for food in areas littered with dead leaves that are the same color as the voles' fur, making them virtually invisible from the watchful eyes of a bird flying above.

Certain predators, such as the tiger, employ camouflage to remain unseen until they are ready to pounce on their prey.

MAMMALS AND HUMANS �

Throughout human history, mammals have played an important part in providing food and useful materials.

At first, humans exploited them solely by hunting, but thousands of years ago, humans started to domesticate certain species, controlling the way that they lived and bred.

The earliest domesticated mammal was almost certainly the dog, which is descended from wolves. Domestication probably began when hunters adopted wolf cubs, and then used them as allies for scenting out game.

Cats are thought to have been domesticated about 4500 years ago, and are descended from the African wild cat, an animal revered by the ancient Egyptians and widely valued for dealing with rodent pests.

Human hunters also discovered that herding mammals could be controlled. At first, they simply drove animals into places where natural boundaries such as cliffs and shorelines made them easier to attack, but over time, subtler techniques developed.

People learned how to follow herds and manage them, first by traveling with them, and later by confining them in limited areas.

Controlled breeding then produced distinct breeds, each with its own combination of useful characteristics.

Horses, for example, were initially hunted for food, but later became valued as a means of transportation.

Cattle were bred for meat, milk, and hides, while goats and sheep were raised for food and also for their wool.

Among the few nonhoofed mammals to have been domesticated--aside from dogs and cats--are the European rabbit and South American guinea pig, both of which provide meat.

Domesticated mammals now supply a large proportion of the meat humans eat and the fibers used for making clothes, although some wild animals are still hunted for food and for their fur.

Until recently, mammals also supplied two other commodities--ivory and animal oils.

Ivory is the hard white substance that makes up the tusks of elephants, and some other large mammals such as walruses.

Long prized for carving and for use in luxury goods, its sale is now strictly controlled worldwide in an effort to protect elephant populations.

Valuable animal oil is found in marine mammals, particularly the sperm whale. At the beginning of the 20th century it was an important industrial raw material and a fuel for domestic lighting, but with the current worldwide moratorium on whaling, this form of oil is no longer marketed.

Many kinds of mammals are kept as pets, but some species are kept in captivity for other reasons.

ANIMAL EXPERIMENTATION

In scientific research, mice and rats act as living testing laboratories for new drugs, and for new methods of medical treatment.

Primates are also used extensively in medical research, because they are closely related to humans.

Most of this work is carried out on animals born and raised in captivity, but in spite of its value, it remains controversial.

In future years it is likely that tests involving computer modeling will replace many experiments currently carried out with living mammals.

Animal Experimentation, the use of animals under controlled laboratory conditions for a variety of scientific and medical purposes.

Scientists use laboratory animals to investigate biological processes in humans and animals; to study the causes of diseases; to test drugs, vaccines, and surgical techniques; and to evaluate the safety of chemicals used in pesticides, cosmetics, and other products.

In many cases, scientific and biomedical research can be performed on laboratory cultures consisting of specific cells or tissues, or on simple life forms such as bacteria.

Some research, however, is better conducted by studying the more complex responses of an animal. Many animals function essentially like humans, and thus provide the best models for experiments on fundamental processes such as breathing, eating and digesting food, and reproducing.

Some animals suffer from the same diseases as humans do, with immune systems that respond similarly to disease-causing agents such as viruses and bacteria.

Animals also carry a number of genes that are identical to human genes, with information for similar traits.

Given these similarities, scientists have been able to learn much about the human body by studying animals.

The practice of experimenting on animals goes back many centuries.

As early as the 2nd century AD, Roman physician Galen experimented on pigs and apes to demonstrate that veins carry blood, not air, as had been believed.

In the early 1600s English physician William Harvey studied diverse animals such as worms, insects, fish, and frogs, providing pioneering insights into the principle by which blood circulates in the body.

Use of animal experimentation increased significantly in the 1800s, reflecting the rapid development of science during that century.

Among other investigations, scientists used animals to explore the major body organs, the function of the nervous system, and the role of microorganisms in causing disease.

Animals continue to play an essential role in research--between 18 and 22 million animals are used yearly in the United States. About 90 percent of these animals are rats, mice, and birds.

Other experimental animals include primates, rabbits, pigs, hamsters, guinea pigs, dogs, and cats.

The total number of animals used in experiments in the United States is falling, however. According to the United States Department of Agriculture (USDA), the use of primates, rabbits, pigs, hamsters, and guinea pigs fell from 2.1 million in 1985 to approximately 1.4 million in 1995, a decline of roughly 33 percent.

Since the 1970s, the use of dogs and cats has declined by about 50 percent. Animal use has also declined in the United Kingdom and in Europe.

REGULATING ANIMAL EXPERIMENTATION �

At least in part, this decrease reflects society's changing views about animal rights. Animal experimentation has long drawn outcry and protest from people committed to the protection of animals.

In the 1970s the animal rights movement gained new momentum and visibility, particularly with the 1975 publication of the book Animal Liberation, by Australian philosopher Peter Singer, which called animal experimentation morally indefensible.

In many incidents during the 1970s and 1980s, protest took the form of late-night raids on research facilities. Animal rights activists broke into laboratories, freed animals, and destroyed data and equipment.

The total damage from raids between 1985 and 1990, according to the Association of American Medical Colleges, exceeded $3.5 million, and universities and research facilities spent millions more on increased security.

Media coverage of these raids--and in some cases, evidence of serious animal mistreatment exposed by activists in some cases--brought animal experimentation into wider public view.

Animal rights groups pressed for changes in the laws that govern the protection of lab animals.

The main federal law pertaining to animal research is the Laboratory Animal Welfare Act. This law was first passed in 1966, partly because of reports about sales to laboratories of stolen pets.

The act initially required that certain animals used for research--including dogs, cats, primates, rabbits, hamsters, and guinea pigs--be obtained via legal means and given humane care.

In 1970 the Animal Welfare Act was amended to require that laboratory animals receive painkilling drugs whenever appropriate; in 1985 the act was amended again, requiring that each research facility receiving federal money create a special committee to oversee animal research.

Furthermore, resources must be devoted to the physical and emotional well-being of lab animals. The 1985 amendments also require federally funded researchers to consider alternatives to animal use.

BENEFITS OF ANIMAL EXPERIMENTATION �

Proponents of animal experimentation point to hundreds of years of medical advances made possible by research on animals.

Treatments for heart disease provide just one example, including open-heart surgery, in which circulatory functions are temporarily controlled by a heart-lung machine; coronary bypass to improve blood flow to the heart muscle; and valve replacement of a defective heart valve.

Techniques and equipment for kidney dialysis were also developed through animal experimentation.

More than 30 drugs for treating cancer, as well as anticancer radiation therapies, were first tested on rats and mice.

Vaccines for diphtheria, measles, smallpox, and many other previously feared diseases were developed through animal research.

Organ transplants and microsurgery to reattach severed limbs were made possible by work on animals.

And not just humans, but dogs, cats, and other domestic and farm animals have benefited from such research, with the development of treatments for distemper, rabies, anthrax, and other diseases of animals

.

Modern biotechnology is providing still more opportunities for advances with the development of transgenic organisms, such as mice that are specially bred to carry selected human genes.

Transgenic organisms permit researchers to investigate genetic causes of cancer and other diseases.

In other promising work with spinal cord injury and paralysis, neurobiologists experimenting on rats and mice are investigating ways to repair nerve tissue and restore movement.

Animals are also being used to seek cures for today's most pressing diseases, including Alzheimer's disease and acquired immunodeficiency syndrome (AIDS).

REASONS FOR OPPOSITION �

Opponents argue that animal research is cruel, immoral, and unnecessary. "The question," wrote English philosopher Jeremy Bentham, discussing animals used in experiments, "is not can they reason, nor can they talk, but can they suffer?"

More recently, Peter Singer, in Animal Liberation, argues that all species that can feel pain and suffering--animal as well as human--deserve equal consideration.

Many people believe that animals are no less complex than humans in their capacity to feel emotions--and to suffer pain. In this view, the infliction of discomfort, pain, suffering, and death on laboratory animals, which are incapable of giving consent to experimental procedures, is purely wrong.

Opponents also dispute the scientific validity of results obtained from animals. Many observers question whether data obtained from animals can be reliably applied to humans. They argue that physiological differences between animals and humans make them unsuitable as experimental models.

Animal rights activists cite figures of the United States General Accounting Office (GAO) showing that 52 percent of the new drugs marketed between 1976 and 1985 caused adverse reactions that were not predicted by animal studies.

Opponents of animal experimentation also point out that experimental animals are under great stress, often confined in small cages or held in special equipment designed to restrict movement.

The stress created by confinement, and by repeated handling for experimental procedures, may significantly alter an animal's physiological functioning, rendering any experimental observations meaningless.

Researchers in labs worldwide currently experiment with a variety of alternatives, such as in vitro methods, which use cell and tissue cultures in place of whole animals. One such test-tube method, designed to replace rabbits in evaluating the skin-irritating properties of new chemicals, has already won approval from the United States government, which requires extensive testing before chemicals can be used in commercial products.

Powerful computer programs, designed to imitate biological functions and demonstrate how a living body reacts to toxic chemicals or disease pathogens, are yet another alternative.

Animal rights activists representing the extreme view support a complete ban on animal experimentation in favor of alternative methods.

Despite increased interest in and success with alternatives, however, many scientists believe that there is no substitute for the complex response of a whole animal. In their opinion, animal research is vital for continued biomedical progress.

The complexities of the animal experimentation debate are certain to remain a topic of discussion for years to come.

ENDANGERED MAMMALS �

In today's rapidly changing world, many wild mammals face an increasingly difficult struggle for survival. Data released in 1996 by the World Conservation Union indicates that of about 4600 species of mammal known to exist, over 1000 are classified as being at risk for extinction.

The number of mammal species becoming extinct has increased consistently over the last few hundred years.

The quagga, an African relative of the zebra, died out in 1883, following in the footsteps of the African bluebuck, which became extinct in 1800.

During the preceding century, Steller's sea cow, an inhabitant of the Bering Sea, died out in 1768. This relative of today's manatees and dugongs could weigh over 5 metric tons, making it the largest animal to have become extinct in modern history.

Some of these animals vanished as a result of hunting, others through competition from other species.

Several species of mammals have become extinct in the 20th century, including the Caribbean monk seal, which was last seen in 1952; at least four species of bats; and eight species of marsupials, including one which disappeared when the last captive animal died in 1936.

Today hunting still poses a danger to some species, particularly animals such as elephants, rhinoceroses, and tigers, which fetch high prices on the black market for their tusks, horns, and other body parts.

During the past 30 years alone, the population of black rhinos has dropped by over 95 percent, and the number of tigers has been reduced to about 5000. International efforts have helped to stem the trade in products from these animals, but without further intervention, their future as wild species remains in doubt.

For mammals as a whole, a more important threat is habitat change. Deforestation threatens a third of the world's primates, while urbanization, agriculture, and water pollution have had a serious effect on a wide range of mammals, from prairie dogs to aquatic species such as otters and river dolphins. In the drive to reverse this decline, some mammals have become the focus of preservation programs aimed at increasing stocks by habitat management, and also by captive breeding.

These programs have saved several species from almost certain extinction. Mammals rescued in this way include the giant panda, which has become a worldwide emblem of wildlife preservation, the white-throated wallaby, and also the North American black-footed ferret, which was once regarded as an agricultural pest. By the late 1970s, the black-footed ferret was believed to be extinct, but a small colony of survivors was discovered in 1981. Over 400 animals now live in captivity, and a number have been released into the wild.

After centuries of exploitation, some marine mammals have also been helped by recent protective legislation. The gray whale, for example, once seemed certain to become extinct, until hunting was banned by the International Whaling Convention in 1946. Since then, the numbers of gray whales in the Pacific Ocean have risen to about 20,000, which may be close to the population before hunting began.

The Cape fur seal, which breeds on the rocky coasts of southern Africa, has shown an even more dramatic increase, growing from a few thousand in the 1930s, when it was first given protection, to over 1 million today. In the United States, the Endangered Species Act of 1973 prohibits federal projects from destroying the habitat of any endangered species.

Information about specific mammals

Dolphins

Dolphin (aquatic mammal), fast-swimming animal related to whales and porpoises. Sleek and powerful swimmers found in all seas, dolphins are distinguished from porpoises by well-defined, beaklike snouts and conical teeth. The porpoise has a blunt snout, chisel-shaped teeth, and a stouter body.

There are at least 32 species of dolphins. Typical examples are the bottle-nosed dolphin, a popular performer in seaquariums, and the common dolphin, which inspired many Mediterranean folk legends. Both often appear in open waters, making their characteristic arched bounds, frequently before the bow waves of ships.

Several freshwater species inhabit river estuaries in Asia and South America. A small, graceful dolphin has been sighted more than 1250 mi up the Amazon River. This is the smallest dolphin and is less than 4 ft long.

The largest, the bottle-nosed dolphin, reaches a length of 10 ft.

The killer whale is considered a dolphin despite its much greater length of 30 ft.

Dolphins once were hunted commercially, especially for the small quantity of valuable oil extracted from parts of the head and used to lubricate delicate watch mechanisms. Cheaper oils have now been found from other sources, and dolphins are no longer hunted for this reason.

Many dolphins, however, become accidentally trapped and drowned in tuna nets; between 1959 and 1972 an estimated 4.8 million dolphins died in this way. Under pressure from animal rights activists and United States consumers, both domestic and international tuna canners have refused to accept shipments from fishing fleets that do not protect dolphins.

Concern has also been expressed about the treatment of dolphins on display in public aquariums and in "swim with the dolphins" programs. The Marine Mammal Protection Act of 1972, amended in 1988 and 1992, was passed to prevent exploitation of dolphins and related aquatic animals. The National Marine Fisheries Service is the principal regulatory agency.

BEHAVIOR �
In one day dolphins eat an amount of food, mostly fish and squid, equal to nearly one-third of their weight.

Dolphins are swift enough to easily outdistance their prey.

They seize their catches with jaws that have from 200 to 250 sharp teeth. Dolphins follow schools of fish in groups of varying size. Some species, such as the Pacific white-sided dolphin, make up aggregations estimated at tens of thousands of members. Less gregarious species, such as the bottle-nosed dolphin, join in groups that often contain only a few members.

Dolphins, like whales, breathe through a blowhole at the top of the head. As they travel they break surface about every two minutes to make a short, explosive exhalation, followed by a longer inhalation before submerging again.

The tail, like that of other aquatic mammals, strokes in an up-and-down motion, with the double flukes driving the animal forward; the flippers are used for stabilization. Dolphins are superbly streamlined and can sustain speeds of up to 19 mph, with bursts of more than 25 mph.

Their lungs, which are adapted to resist the physical problems created for many animals by rapid changes in pressure, enable them to dive to depths of more than 1000 ft.

Adults of the bottle-nosed dolphin--the best-studied species--come to sexual maturity after 5 to 12 years in females and 9 to 13 years in males. They mate in the spring; after a gestation period of 11 or 12 months, a single calf is born, tail first. Calves swim and breathe minutes after birth; they nurse for up to 18 months. They are able to keep up with the mother by remaining close and taking advantage of the aerodynamic effects of the mother's swimming.

VOCALIZATIONS AND INTELLIGENCE �

Dolphins almost constantly emit either clicking sounds or whistles. The clicks are short pulses of about 300 sounds per second, emitted from a mechanism located just below the blowhole. These clicks are used for the echolocation of objects and are resonated forward by the so-called oily melon, which is located above the forehead and acts as an acoustic lens. Echoes received at the area of the rear of the lower jaw are transmitted by a fat organ in the lower jaw to the middle ear.

This echolocation system, similar to that of a bat, enables the dolphin to navigate among its companions and larger objects and to detect fish, squid, and even small shrimp. The whistles are single-toned squeals that come from deeper in the larynx. They are used to communicate alarm, sexual excitement, and perhaps other emotional states.

Because of the ability of dolphins to learn and perform complex tasks in captivity, their continuous communications with one another, and their ability, through training, to approximate the sounds of a few human words, some investigators have suggested that the animals might be capable of learning a true language and communicating with humans.

Most authorities, however, agree that although the dolphin's problem-solving abilities put the animal on an intelligence level close to that of primates, no evidence exists that dolphin communications approach the complexity of a true language.

Some studies have attempted to find intelligent meaning in the sounds of dolphins and to teach dolphins a symbolic language. These studies indicate that dolphins may be slightly superior to dogs in their abilities to communicate with humans. Research on communication among wild spinner dolphins in their own social world indicates that individuals behave abnormally when removed from a well-knit dolphin group. These studies also suggest that dolphins may convey information by elaborate postures and body language. These studies are preliminary, however, and scientists are still far from accurately measuring, or even knowing how to measure, the intelligence of whales.

Scientific classification: Dolphins belong to the suborder Odontoceti of the order Cetacea.

Kangaroos

Kangaroo, common name for a group of mammals found in Australia and neighboring islands. Kangaroos are marsupials, a type of mammal that gives birth to undeveloped young. In kangaroos and many other marsupials, the young are carried and nurtured in a special pouch on the mother's body.

More than 50 different kinds of animals are grouped together in two kangaroo families. The large kangaroos include red kangaroos and gray kangaroos, and wallabies. The other family is made up of assorted smaller species, such as various rat-kangaroos.

The largest kangaroos are the gray kangaroo and red kangaroo, which can stand up to 6.5 ft tall and weigh up to 187 lb.

The smallest are the musky rat-kangaroos measuring about 12 in, not including the tail.

Kangaroos are found only in mainland Australia and New Guinea and on some of their offshore islands, such as Tasmania.

A few species have been introduced to other countries, such as New Zealand and Hawaii. Kangaroos inhabit every imaginable type of habitat throughout Australia and New Guinea.

Red kangaroos and gray kangaroos are found in grasslands, savannas, and open woodlands. Rock wallabies live on nearly vertical rock walls in the southern desert. Other types inhabit burrows in arid scrubland.

As their name implies, tree kangaroos are found high in the rain forest canopy, while rat-kangaroos scamper nimbly through the dense, wet understory below.

Several species of kangaroos are so adaptable that they are common inhabitants of public parkland, suburban gardens, and even golf courses.

PHYSICAL CHARACTERISTICS �
Most kangaroo species have large ears and relatively small heads. Their small front feet have five digits of unequal length, while the back feet typically have four digits.

One of the toes on each hind foot has a sharp claw used in defense.

Many species are drably colored to closely match their surroundings, helping them to hide from predators.

Males of the larger kangaroo species are noticeably bigger in size than the females, a trait known as sexual dimorphism.

A physical feature that distinguishes kangaroos from other marsupials is the structure of the hind legs and tail for hopping. In red kangaroos, for example, the hind legs are very large--roughly ten times the size of the small front limbs.

When moving slowly, kangaroos use all four legs. When they need to move fast, they rise up on their muscular hind legs and start hopping.

The leaps of some kangaroos are legendary. Red kangaroos moving at top speeds can cover 16 ft or more in a single bound. They are able to reach speeds of more than 31 mph for short periods.

The kangaroo's heavy, muscular tail is also important for locomotion. It serves as a counterbalance and a rudder during hopping, and provides support when the animal sits.

Studies have shown that hopping is energy efficient for kangaroos, partly because of springlike tendons in their legs. The energy required to start hopping is quite high, but very little extra energy is needed to hop faster and faster. In contrast, mammals that run on all fours require increasing amounts of energy as speed increases.

REPRODUCTION �
Like all marsupials, kangaroos lack a true placenta, the structure in a pregnant female's womb that provides nutrients for and removes waste from the growing embryo.

Instead of a fully developed placenta, kangaroo females form a type of yolk sac in the womb. The embryo absorbs nutrients from this yolk sac for four to five weeks and then emerges from the birth canal, still in an embryo-like form but with well-developed forelimbs.

The tiny animal, weighing about 0.04 oz, crawls along its mother's body and into an abdominal pouch. Once inside, the offspring attaches firmly to a nipple, suckling milk. It remains there for several months until it is more fully formed and can move about on its own.

The young kangaroo, sometimes called a joey, may remain with the mother, climbing into her pouch for nourishment or safety, until it is more than a year old.

Almost all kangaroos produce a single young per litter. Depending on environmental conditions and food resources, one litter is typically born each year.

Some kangaroo species living in dry regions have the ability to breed whenever food and water resources are favorable. This may result in normal reproduction or no litters for several years if conditions are poor.

The unpredictable nature of resources has led to another specialized reproductive feature in some kangaroos. As soon as the tiny offspring emerges from the womb, the female is able to mate again.

While the newborn suckles a nipple in the mother's pouch, a fertilized egg begins developing in her womb. The development of the fertilized egg stops abruptly when the egg becomes a simple, hollow ball of about 100 cells, called a blastocyst. The blastocyst remains in a state of suspended development in the female's womb while the offspring in the pouch continues to grow.

After the pouch offspring is more mature and is able to leave the mother's pouch (up to six or seven months later), the blastocyst resumes development and soon emerges from the womb and moves into the now empty pouch.

This reproductive feature, called embryonic diapause, enables kangaroos to care for up to three litters simultaneously during desirable conditions: an older, weaned offspring still under the mother's care, another maturing in the pouch, and a third in suspended development in the uterus.

Should adverse conditions cause the weaned young to die, other offspring are still available for rearing.

The life span of the larger kangaroos ranges from 12 to 18 years in the wild, but this is greatly influenced by climate and food supply. In captivity, large kangaroos are known to live as long as 28 years. Smaller species have a somewhat shorter life span.

DIET �

Most kangaroos are herbivores, or plant-eaters.

The larger species, such as the red kangaroos and gray kangaroos, are grazers, feeding on grasses and other vegetation in open forests and savannas.

Medium-sized, forest-dwelling species browse on leaves, shoots, and twigs.

The small forest inhabitants tend to be omnivores, eating both plants and small animals. Some of these small forest-dwellers specialize in eating fungi, tree gum residues, insects, worms, and even dead animals, or carrion.

Kangaroos are similar to hoofed mammals, or ungulates, such as the deer and antelope, in their ability to digest plant matter that is high in fiber and low in protein. Like ungulates, they have multi-chambered stomachs containing bacteria that can break down plant cell walls and release the nutritious cell contents.

SOCIAL ORGANIZATION

Kangaroo social life is poorly understood, as few species have been studied in detail.

Small species tend to be solitary and widely dispersed, with a male's territory encompassing those of several females. The animals come together briefly for mating, and mother and offspring remain together only until the young can live on their own.

In contrast, red kangaroos and gray kangaroos are highly social, forming large groups, called mobs, of ten or more males and females.

Males form a hierarchy in the mob based largely on age and size. The most dominant male, called a boomer, has exclusive access to the females for mating. He spends much of his time wandering in and out of the mob, checking the reproductive condition of the females and also intimidating other males who attempt to mate with them.

It may take a male ten years to achieve the status of a boomer, but he will remain in this position for a year at most. A younger, more vigorous male soon forces him out of his position and even out of the mob, and he soon dies.

Larger kangaroos, although ordinarily timid, are dangerous when threatened by rivals. In a form of ritualized wrestling, kangaroos support themselves on their hind legs and tail and pummel attackers with forepaws and kick with their powerful hind legs.

SURVIVAL �

Aboriginal hunters once relied on kangaroos for their meat and hides but caused minimal impact on kangaroo populations.

As Europeans settled the Australian continent, overhunting coupled with the introduction of livestock that ate the grasslands providing cover for some smaller species caused some kangaroo populations to decline.

Today some smaller, more specialized kangaroos are threatened by habitat destruction, which reduces the areas where kangaroo species can survive.

Predatory mammals introduced to Australia, such as the fox, domestic dog, and domestic cat, prey on small kangaroos.

And other introduced mammals, especially rabbits and livestock, compete with kangaroos for food resources.

The larger kangaroos are commonly hunted and poisoned by ranchers, who consider them pests that overgraze land.

Despite these pressures, the larger kangaroo species are still thriving. About ten are considered endangered, including several rat-kangaroos, two hare wallabies, and two nailtail wallabies.

Scientific classification: Kangaroos make up the two kangaroo families, Macropodidae and Potoroidae, in the order Marsupialia.

Whales

Whale, large mammal that lives its entire life in the water. Like other mammals, whales have large, highly developed brains and nurse their young with milk.

Among aquatic mammals, whales are the most completely adapted to life in the water. Whales resemble fish in many ways, but they are not fish. Fish are cold-blooded and breathe underwater using gills; whales, on the other hand, maintain a warm and constant body temperature of about 99�F and breathe air with lungs. Unlike fish, whales move their tails up and down instead of side to side when they swim.

Whales are enormous in size compared to other mammals. The blue whale is probably the largest animal that has ever lived, reaching a length of nearly 00 ft and a weight of 180 metric tons. Its heart is as big as a Volkswagen Beetle and its body is larger and heavier than that estimated for any known dinosaur. The blue whale makes the loudest sound (over 150 decibels) of any animal, and can be heard over thousands of miles across entire oceans.

Whales are found in all the world's oceans and even in a few rivers. One species lives only in the Amazon River and its larger tributaries.

Some better-known whales, including the blue, finback, humpback, and gray whale, range widely and migrate between the tropics in winter and subpolar waters in summer.

Humpbacks have separate populations in the Northern and Southern hemispheres and do not seem to cross the equator.

Sperm whales and killer whales, or orcas, are wanderers without specific migratory routes. An individual sperm whale might, in its lifetime, swim around the world.

TYPES OF WHALES �
There are at least 75 species of whale, each with its own unique characteristics. Whales range from black to white in color and from nearly 100 ft to less than 7 ft in size.

They may live in salt water or fresh water, and can be found as far north as the Arctic Circle, and as far south as Antarctica.

The most common way of classifying whales is according to their feeding strategy. One group of whales has teeth, and the other does not.

Toothed Whales �

The toothed whales have jaws lined with simple, pointed teeth. These whales actively hunt fish, squid, and other prey. Toothed whales include the great sperm whales, whose adults average (55 ft long and weigh nearly 50 metric tons.

One of the most unusual toothed species is the narwhal inhabiting Arctic waters. The male narwhal possesses a single ivory tusk that grows almost 8 ft long, straight forward from its head.

Baleen Whales �

Instead of having teeth, the baleen whales have mouths lined with giant, flexible combs of a material called baleen, or whalebone, which is used to filter small fish and crustaceans from the water.

Gray whales live only in the North Pacific and measure up to 50 ft in length. They are slow swimmers that stay near shallow waters where they can feed from the bottom.

Right whales are thick-bodied, slow-swimming plankton feeders that reach 60 ft in length. One right whale, the bowhead, lacks a dorsal fin, or back fin, enabling it to use its huge back to break through the thick Arctic ice for air.

ANATOMY OF WHALES �
The streamlined body of whales appears remarkably fishlike. The front limbs are paddle-shaped flippers and the broad horizontal tail flukes provide the main propulsive thrust.

The whale's skeleton closely resembles the skeletons of other mammals. For instance, the bones of the flippers resemble jointed limbs and digits and the neck has exactly seven vertebrae.

The whale's body is enveloped in a thick layer of fat called blubber that aids in buoyancy, serves as a source of stored energy, and helps preserve body heat. Blubber permits whales to stay warm even in near-freezing waters.

In warmer waters they are able to release heat like a radiator by controlling blood flow to the skin. A whale's skin is free of sweat glands and oil glands, is nearly devoid of hair, and feels much like smooth, wet rubber to the touch.

Whales, like other mammals, have lungs. Whales do not breathe through their mouths, but rather through a nostril, or blowhole, located on the top of the head.

The blowhole opens by a slight muscular contraction and closes automatically when the muscle relaxes. As a whale surfaces it exhales powerfully, creating a loud sound and a characteristic cloud of mist known as the spout. The spout is caused by condensation from the warm, moist air that is exhaled, not from seawater trapped in the blowhole, as was once believed.

Large tail flukes provide thrust for whales when they swim. The power comes from body muscles that flex the lower spine up and down in a wavelike motion. The forelimbs of whales are flippers used for steering as the animals swim. In some species, such as humpbacks, the flippers are large and powerful and may be used for fighting among males.

Whales are able to drink seawater and yet maintain the same low salt concentrations in their body fluids that other mammals do. For every liter of seawater a whale drinks, its highly efficient kidneys concentrate the salt into only two-thirds of a liter of urine for a gain of one-third liter of pure water. If a human were to drink a liter of seawater, it would require one and one-third liters of urine to remove the salt for a loss of one-third liter of water.

Whales possess interesting features for diving, some of which are shared with other aquatic mammals such as seals.

Whales can store very high oxygen concentrations in their blood and muscles.

During a dive, the heart rate slows and arteries constrict to many of the animal's organs, greatly reducing blood flow into those areas. This conserves oxygen and maintains the blood pressure to vital systems, especially the brain, sense organs, and to the heart itself.

During a dive, the lack of oxygen triggers the build up of carbon dioxide and lactic acid in the blood and muscles. Unlike most animals, whales are able to resist pain and fatigue caused by lactic acid accumulation in muscle tissue.

Baleen whales can hold their breath up to 50 minutes when diving, and of the toothed whales, the sperm whales can hold their breath up to 80 minutes.

Whales have an excellent sense of hearing. They perceive sound that is waterborne and, as a result, they are able to hear without the external ear structures called pinnae that land mammals use to gather airborne sounds.

Whale external ears are reduced to small slits, but this does not hinder them from perceiving a wide range of sounds, many of which are not audible to humans.

FEEDING HABITS �
Baleen whales:

Groups of several humpback whales have been observed cooperating to trap schools of small fish. The humpbacks manage to corral fish inside a cylindrical column of bubbles released through the whales' blowholes. Each whale then takes its turn swimming inside the bubble curtain and rising to the surface with its mouth open, trapping many of the fish in its baleen net.

Instead of feeding in open water, gray whales feed on the ocean bottom, primarily on amphipods, a type of small crustacean that lives in mud. Gray whales have the shortest, stiffest baleen fibers of all whales. They approach the ocean bottom sideways, sucking material containing food into the side of their mouth and forcing water and mud out through the coarse baleen on the other side.

Toothed whales:

Toothed whales catch larger prey than the baleen species. Using echolocation, they actively track and seize fish and squid.

Perhaps the most formidable predator to appear on earth since Tyrannosaurus rex, the killer whale is the only whale to routinely pursue warm-blooded prey, chiefly seals and smaller dolphins, although they also eat fish.

One classic study found parts of 13 dolphins and 14 seals in a dead killer whale's stomach.

Occasionally, a group of killer whales will attack and kill a large whale such as a humpback.

Feeding dives by sperm whales last as long as 80 minutes and extend to depths of 6,500 ft. Water at these depths is in complete darkness and these whales find their prey by echolocation.

Sperm whales specialize in feeding on large deep-sea squid. Giant squid, measuring up to 30 ft long including tentacles, sometimes fight back, leaving deep cuts and scratches on the sperm whales.

REPRODUCTION �
Whales mate after extensive courtship rituals involving various sounds, postures, ritualized swimming, and touching and caressing.

Gestation may be as long as 16 months in sperm whales. Whales produce a single calf.

Most large species of whales migrate to the tropics to give birth and to mate. Humpbacks nurse the calf for several months before the young whale begins to feed on its own, although intermittent nursing may continue for a year or more.

The largest whales can produce an estimated 160 gallons of milk per day. Whale milk contains 25 to 50 percent fat, compared to 3 to 5 percent in cow milk.

INTELLIGENCE AND BEHAVIOR �
Studies of whales in captivity have taught us much about their physiology as well as their complex social behavior. Their learning and problem-solving skills, together with their curiosity and often eager interactions with humans, have led some scientists to believe that whales possess a high level of intelligence.

The large brains of whales also suggest a high intelligence. The sperm whale's brain is considerably larger than an elephant's brain and six times the size of a human brain. However, relative to the rest of the body, a sperm whale's brain is only about 0.02 percent of its body weight, while a human brain is about 2 percent.

Reflecting their heightened sense of hearing, in those whales that rely on echolocation to perceive their surroundings, the areas of their brains that process sound information are far larger than the corresponding parts in humans. These animals tend to rely on hearing more than vision, which explains why the visual centers of their brains are smaller proportionally than those in humans.

Another feature that scientists correlate with intelligence is the degree of folding of the upper surface of the brain, the area known as the cerebral cortex. This folding increases the surface area and has been observed in more intelligent animals. Whale brains generally show as much or more folding of the cerebral cortex as is seen in humans.

Complex behavior may reveal more about whale intelligence than sheer brain size. Perhaps the most intriguing indications of whale intelligence came with the discovery in the 1970s of whale singing, most notably in humpbacks.

Singing is most common on the winter mating grounds, for example in the Caribbean and Hawaii. Singing humpbacks are males, more specifically, escort males that closely follow females and their year-old calves.

These females are receptive to mating, and singing may be a mating ritual for humpbacks. There is no solid evidence that the songs encode language in an intellectual sense. The whale songs may be simply longer versions of the mating songs also noted in birds and amphibians.

Humpback songs, which may last more than 20 minutes, consist of a series of phrases or sequences. All of the singing whales of a particular migrating group sing very nearly the same song. The songs change progressively from year to year, resulting in entirely new songs after four or five years.

Some whale species exist for the most part as solitary animals, whereas others live in family groups or in pods numbering hundreds of individuals.

Observations of toothed whales have revealed a strong tendency toward cooperation, even among different species. In whales held in captivity, females of one species will assist a mother of another species as she gives birth, and will support the newborn calf, helping it to reach the surface to breathe.

WHALING �
Humans have hunted whales for centuries. Initially, they only killed and used whales that had become stranded on land, but eventually people began using hand-held harpoons and small boats to pursue whales at sea.

Primitive subsistence whaling relied on coastal species such as the gray whale and the bowhead. Commercial whaling in the open sea developed into big business by the 1700s. The major whale product was the oil made from the blubber and used in oil lamps.

A thick liquid from the head of sperm whales, crystallizes into a soft wax when exposed to air and was used as an ointment and to make high-quality candles. It burned with an especially clear, smokeless flame and was the most valuable form of whale oil.

Baleen was also in demand for several decades. Its resilient flexibility made it useful in a variety of products including umbrella spokes, horsewhips, and springy petticoats.

During the sailing ship era, when whales were caught by hand harpoon from oar-driven boats, the easiest whales to kill were the slower swimmers, such as grays, humpbacks, sperm whales, and right whales.

Heavy whaling pressure nearly decimated several species of whale during the 1800s. By 1850 the American whalers made up 80 percent of the world fleet and harvested over 10,000 whales per year.

In 1849, however, a method was devised for distilling kerosene from petroleum. Just as many whales were nearing extinction, kerosene began to substitute for whale oil, and the whalers' markets gradually dried up for a brief time.

By the turn of the 20th century, the process of hydrogenation made it possible to process whale oil into soap and margarine. Whale meat was still desired for human and animal food. Finally, the invention of fast steam-powered ships and cannon-fired harpoons with exploding heads ushered in a new era of whaling. With these new technologies, the whale slaughter began again. Japan, Norway, and Russia were the primary whaling nations.

In 1910 only 176 blue whales were killed; by 1931 the harvest exploded to 30,000.

Blue whales became ever scarcer over the years until 1966, when only 70 blue whales were caught worldwide. That year the International Whaling Commission (IWC) banned hunting for blue whales in the Southern Hemisphere.

Other species such as the humpback, however, which had been considerably depleted in the Northern Hemisphere a century earlier, were now driven toward extinction. These magnificent animals were ground up on factory ships to make lubricants, soaps, cosmetic products, and animal feeds.

WHALE CONSERVATION �
Most large whales are now very scarce. Their reproductive rates are so low that even if completely protected from whaling, some species will take many decades, perhaps a century or more, to rebuild their populations to healthy numbers.

Several countries and conservation organizations around the world are seeking to protect whales. In 1946 whaling nations established the International Whaling Commission (IWC). Unfortunately, member countries cannot always agree on the type of protection needed for the scarcer species and on quotas for hunted species, and the IWC has no power to enforce its agreements.

Whale-hunting abuses by a few nations continue to be uncovered. Humpbacks, which are supposedly protected in all oceans, are still hunted today.

The decimation of huge numbers of large whales over the years has caused krill and other whale prey species populations to increase, especially in the Antarctic. Some marine ecologists think that this ecosystem change is responsible for recent increases in the numbers of penguins and crabeater seals, which also feed on krill.

In addition to whaling, whales are endangered by increasing pollution in the oceans. This is particularly true of species that live in coastal waters.

These animals build up large concentrations of toxic chemicals such as lead, mercury, and pesticides in their bodies from eating contaminated fish and other food.

More and more whales are becoming stranded on beaches and dying; it appears that some of these animals have had their sense of echolocation damaged by infectious disease or parasites.

Even healthy whales may now be unable to hear well in the sea. Underwater noise pollution is steadily increasing and may be drowning out the tremendous calls of the blue and fin whales. Whales could once heard across thousands of kilometers of ocean, but some researchers believe noise pollution is now interfering with this form of long-distance communication between animals.

Scientific classification: Whales belong to the order Cetacea.

Seals

Seals are large aquatic mammals distinguished by having all four legs fully adapted into flippers.

There are three families of seals: the true seals, the eared seals, and the walrus.

True seals lack external ears and have relatively short flippers that are nearly useless for walking on land.

The eared seals, including the sea lions and fur seals, sport tiny external ears and can lift their bodies off the ground with their flippers.

The stout-bodied walrus, with its wrinkled skin and unique tusks, is the only member of its family.

The limbs of seals are flippers. In addition to their flippered limbs, seals have streamlined and flexible bodies that make them excellent and efficient swimmers.

Sea lions are the fastest seals and can swim at top speeds of nearly 25 mph. These seals are so flexible that they can nearly touch their rear flippers with their nose when bending backward.

Seals vary in size. As examples, the male southern elephant seal reaches more than 8000 lb, and the male walrus, which grows more than 11.5 ft in length and reaches weights of over 3700 lb. The diminutive ringed seal adults average about 110 to 130 lb and are a little more than 3 ft in length.

Seals make and hear sounds underwater. Their whoops, screams, barks, moans, and wails are used in simple communication, such as mating calls and territorial defense.

Some species, including bearded seals, Weddell seals, and walruses, sing complex songs that may last more than a minute.

Inuit seal hunters listen for bearded seal songs by placing one ear against a kayak paddle handle while the blade is held underwater. It is still unknown if seals use reflected underwater sound waves, or echolocation, to navigate through the depths or to track prey.

PHYSICAL DESCRIPTION �
Although similar in appearance, it is easy to distinguish differences between eared seals (sea lions and fur seals), true seals, and walruses.

Sea lions and fur seals have tiny external ears while only the ear openings are visible in true seals and walruses.

On land, sea lions, fur seals, and walruses rotate their large flippers back and forth to function as legs, enabling them to walk clumsily on land or ice.

The short flippers of true seals do not turn under the body to act as legs. These seals move on land mainly by flexing their bodies, although on snow and ice they can also use their foreflippers as paddles to reach surprising speeds. The crabeater seal, for instance, can cruise at 16 mph on level Antarctic ice.

Seals have many features for life in the water. External ears are small or absent, and in many species, testicles and mammary glands are located in slits or pockets under the skin, features that streamline the seal body for more efficient swimming.

When seals submerge underwater their nostrils close automatically. The pupils of their eyes expand widely to capture light in near darkness. This ability is important for finding prey at night or in deep water.

Seals conserve oxygen for long periods of time, enabling them to stay submerged at great depths, much longer than humans can. As a seal starts to dive, its heart rate slows to about one-tenth of its heart rate at the water surface. At the same time, the arteries, which transport oxygen-carrying blood to most of the animal's body, constrict or squeeze shut so that only the sense organs and nervous system continue to receive a normal flow of blood.

Seal muscles also store oxygen, and the spleen, an organ that stores oxygen-rich blood, is exceptionally large in seals, serving as a kind of biological scuba tank.

The deepest-diving seals can descend hundreds of meters and stay underwater for one to two hours. During a dive, carbon dioxide builds up in the blood and the lack of oxygen causes lactic acid levels to rise in the muscles. Unlike most animals, seals are able to resist pain and fatigue caused by lactic acid accumulation.

But once seals return to the water surface, they need a recovery period to bring their body chemistry back to normal. Rapid blood circulation through very large veins leading to the lungs helps to rid the seal's body of carbon dioxide. The big-branched veins carrying blood out of the walrus's lower body are so large that a person could pull them over their legs like pants.

Keeping warm is important for seals since water quickly conducts heat away from their bodies. Adult seals produce a thick layer of fat, called blubber, under their skin, which is an excellent insulator against the cold.

Blubber is also used to store energy for times when food is scarce; seals can live off the stored fat in blubber for weeks to months.

While most newborn seals have little or no blubber, many seal species develop a fur coat during infancy that traps air next to the skin for an extra layer of insulation.

The beautiful white coat of the infant harp seal, born on the Arctic ice, may actually set up a small greenhouse effect, trapping the energy of sunlight as heat near the skin.

Many species shed this fur coat as they grow older, replacing it with blubber for insulation. Fur seals, however, keep a dense coat of fur throughout their lives, made up of about 800,000 hairs per sq in. By contrast, an entire human head contains only about 100,000 hairs.

RANGE AND HABITAT �
Most seals live in cold waters near the Arctic and Antarctic. Some true seals live under ice for much of the year, finding cracks between ice floes or scattered holes in order to breath.

Depending on the species, true seals use strong claws, teeth, or their head to break through new ice that freezes over openings.

A few kinds of true seals are found in warmer regions. In North American waters, harbor seals are found from northern Canada to Georgia on the Atlantic coast--although they are not common south of Massachusetts--and from Alaska to Baja, Mexico, on the Pacific coast.

Monk seals are true seals that love warm, clear waters. The Mediterranean monk seal lives

Walruses are found only in the northern hemisphere. Atlantic walruses range from Arctic Canadian waters eastward to northern Europe, including western Russia, while Pacific walruses live mainly in the Bering Sea and off Siberia.

Large herds of walruses typically follow the broken edge of the pack ice north and south with the seasons, although some older males do not migrate. Walrus migrations cover distances as great as 1850 mi.

DIET OF SEALS �
Most seals eat fish and sometimes squid.

The leopard seal, an Antarctic species, may have the most diverse diet of all, commonly hunting penguins and other seabirds, smaller seals, as well as fish, squid, krill (small shrimplike crustaceans), and other invertebrates, as well as feeding on carcasses of dead whales.

Leopard seals sometimes hunt humans, lunging onto ice floes to chase people who are on foot, and also threatening scuba divers underwater.

Sea lions and walruses may occasionally kill and eat other seals, although more commonly sea lions eat fish, and walruses dive to the bottom and dig for clams, worms, crustaceans, and other organisms from the mud.

Walruses have a large "mustache" of especially sensitive whiskers, or vibrissae, that help them detect their food on the dark sea floor. A big walrus can eat about 100 lb of shellfish in one day.

REPRODUCTION �
Most seals mate on land or ice and, in all species, females give birth out of the water. In most land-breeding seals, such as elephant seals and sea lions, dense temporary colonies called rookeries form in the breeding season.

In these species, males tend to grow larger than females, and a harem system prevails in which the strongest males mate with several females and guard them from other males. This harem system ensures that only the strongest males pass their genes to the next generation. A study of northern elephant seals found that only 9 percent of males succeed in mating.

Seals that breed on ice, by contrast, do not typically organize into large harems. These species, including ribbon seals, harp seals, and Weddell seals, form colonies that spread out over wide areas.

Most ice-breeding species are monogamous, and the males and females are nearly the same in size and appearance.

An exception is the walrus, which breeds on ice islands where large dominant males gather sizeable harems. Walruses use their tusks in threatening displays toward rival males and in courtship rituals with females. Males have large throat pouches that produce bell-like sounds to attract receptive females.

Most females breed every year and on average bear a single pup (or calf in the walrus) 12 months after mating.

The seal embryo does not begin development for 3 to 5 months after conception, thus ensuring that young are born when food is plentiful.

Newborn seals vary in size according to species--a newborn ringed seal weighs only 25 lb while a newborn walrus can weigh 140 lb.

Infant seals grow extremely fast and rapidly build up a layer of blubber that is generally lacking at birth. Females nurse young for a few days to two years, depending on the species. The hooded seal nurses for only three to five days and the pups nearly double their weight from 49 to 95 lb in this time.

At the other extreme are sea lions, fur seals, and walruses, which continue to nurse their young for one to two years.

Seal milk is extremely rich in fat and protein. True seals have the highest milk fat levels, averaging 40 to 50 percent, while sea lions and fur seals pack 10 to 14 percent protein into their milk.

Elephant seal milk tastes bland and waxy and physically resembles melted vanilla ice cream.

ENDANGERED SEALS �
Natural predators of seals include large sharks, especially the great white shark, the orca, or killer whale, and other seals such as the leopard seal. Polar bears kill seals on land and ice in the Arctic. There are even reports of eagle attacks on baby Caspian seals.

Perhaps the greatest menace to seals are humans, who have long hunted seals for food and seal skins, used for clothing and even housing and small boats.

Commercial sealing developed into a profitable business in Europe and colonial America by the 1700s. Like whales, seals were hunted for their blubber, which was converted to oil for fuel, lubrication, and tanning. Fur seals were especially valued for their luxuriant pelts. Today seals are still killed for their skins in some countries, such as Canada and Russia.

After only fifty years of commercial hunting in the 1800s, the northern elephant seal became nearly extinct. Hawaiian and Mediterranean monk seals live along warm shores and are easily approachable, making them particularly vulnerable to human hunters. Both monk seal species are currently endangered. A third monk seal, once abundant throughout the Caribbean Sea, became extinct by about 1950.

One of the first formal attempts to protect seal populations occurred in 1911, when the United States, Canada, Russia, and Japan agreed to ban open-sea hunting of seals.

After Mexico banned seal hunting in 1922, the northern elephant seal population began to grow. In 1910, only 100 seals were found on Isla de Guadalupe; today this species numbers around 150,000 and has recolonized much of its former range from northern Mexico to southern Alaska.

However, another species, the Guadalupe fur seal, hunted during the same period, has not recovered its former abundance along the California and Mexican coasts.

Some countries have enacted laws to protect seals and other marine mammals. Sadly, these laws came too late to save the Caribbean monk seal. Even though the hunting of seals is now much less intense than in the past, threats from pollution, especially oil spills, and the accumulation of marine debris such as lost or discarded fishing line and nets still cause many deaths among seals.

Scientific classification: Seals are classified in the suborder Pinnipedia of the large mammalian order, Carnivora.

Bats

Bats are the only mammal that can fly. Bats have hands and arms that serve as wings capable of sustained flight.

In all the time of their existence, 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.

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 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 associated bats with other, more 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.

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 the earth.

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

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 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 6 oz, must eat 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.

PHYSICAL DESCRIPTION �

Bats range in body size from as small as the 1-in long Kitti's hog-nosed bat (also called the bumblebee bat), to as large as the 16-in 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.

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 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 60 mph and observed at altitudes of 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 known 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. 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 certain bats 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.

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.

Echolocation �
While flying at night, bats use a combination of vision, smell, and hearing to find food, to navigate, and to avoid collisions.

Some bats 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.

Cave-dwelling bats 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, stone flies, and other insects making up the rest.

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, these bats are unintentional agents of seed dispersal and pollination, both of which help food plants to reproduce and to spread. This contributes to the health of the forest environment.

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.

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 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 may migrate considerable distances to winter roosts in warmer locales where food is more plentiful. For example, the Mexican free-tail bat migrates 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.

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 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, and wild and domestic dogs and cats.

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.

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.

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.