Mammal

I

Introduction

Mammal, animal that raises its young on milk. Most mammals are covered with hair or fur, and most have specialized teeth that help them to cut or chew their food. Compared to other vertebrates (animals with backbones), mammals have highly developed nervous systems, and 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 human beings—a species that now dominates life on earth.

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.

Mammals evolved from a group of reptiles called therapsids. The earliest true mammals, appearing over 200 million years ago, were only 5 cm (2 in) long and probably spent at least some of their lives in trees. These tiny mammals resembled shrews with four roughly equal short legs and sharp claws. Since that time mammals have evolved an extraordinary variety of body shapes and sizes. Of the approximately 4600 species of mammals alive today, most live on land, and most still move about on all four legs. But modern mammals also include animals that hop on two legs, ones that live permanently in water, and ones that can fly. These swimming and flying species include the world's largest mammal—the blue whale, which can grow over 30 m (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).

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.

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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 –68° C (-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 6100 m (20,000 ft), while sperm whales can dive to depths of at least 2100 m (7000 ft), holding their breath for over an hour. 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 20,000 km (12,500 mi).

II

Types of Mammals

In biological classification, mammals form one of the six major classes of vertebrate animals. Mammals themselves are divided into three different groups, or subclasses, based on distinctive underlying features.

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.

The second subclass of mammals contains the 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 have evolved into 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.

The third subclass of mammals, called placentals, 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.

Biologists classify placental mammals into about 19 groups called orders (the exact number varies in different classification systems). 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. Another major group of mammals, with about 1000 species, contains the bats. Insect-eating bats are generally small animals, but some fruit-eating species have a wingspan of over 1.5 m (5 ft).

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. Mixed diets are also common in a different group of mammals—the primates. Primates include animals such as lemurs, monkeys, apes, and humans, and most of the 230 species live in trees.

The world's large plant-eating mammals are divided into two major groups. One group, called the artiodactyls, contains animals such as hogs, deer, cattle, and antelope, which have hoofed feet with an even number of toes. The other, a much smaller group called the perissodactylas, includes horses, tapirs, and rhinoceroses, which have an odd number of toes.

Some mammals have adapted to 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 and dugongs 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 adapted as fast, open-ocean predators. Still, like all other mammals, aquatic mammals would drown if they could not reach the surface to breathe.

III

Mammal Habitats

On land, mammals live in many different habitats, and at a wide range of altitudes. Many mammals dig burrows as refuges or as places to raise their young, but some have developed a largely subterranean lifestyle, feeding on small animals or plant roots beneath the soil's surface. These animals, including moles and mole-rats, 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 and mole-rats 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 has evolved to break 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. Forest mammals also include climbing carnivores such as martens, and species that spend most or all of their lives on the ground. In temperate regions (areas with cold winters and warm summers) the largest of these ground-based forest dwellers are bears, deer, and wild pigs, but in tropical regions they also include elephants, okapis, and tapirs.

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 adaptations, 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 have successfully 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 adaptation allows them to run effortlessly at altitudes of up to 4900 m (16,000 ft).

Some mammals, such as otters and river dolphins, have adapted to life 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.

IV

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 (see Brain: Evolution of the Brain). 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 7.25 kg (about 16 lb), which is about 0.02 percent of its body weight. By comparison, an average human brain weighs only about 1.4 kg (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 macaques 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. This kind of learning played a key part in human evolution, and has since become one of the outstanding features of our species.