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.

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 pigs, 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.

V

 

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.

The smallest predatory mammals feed on animals such as earthworms and insects, and often find their prey by touch. Bigger insect-eaters, such as pangolins and anteaters, usually locate their food by sight or smell. They gather food in much bigger quantities using their sticky tongues, which can sweep up thousands of ants or termites 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 and dugongs 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.

VI

 

REPRODUCTION

Mammals produce relatively few young each time they breed, but they provide their young with the best chances of survival. In all mammals except monotremes, the young start their development inside their mothers, where they are safe from most of their enemies. The parents continue to protect their young after they are born, fending off predators and providing the offspring with food.

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 directly related. The tiny mouse lemur, for example, weighs about 57 g (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.

VII

 

LOCOMOTION

The distant ancestors of today's mammals had five-toed feet and small, roughly equal limbs. Many small mammals—particularly insectivores and rodents—still follow this pattern, and move about by running on all four limbs on the ground, or by climbing tree trunks and along branches. But in many other mammals, the shape of the limbs has completely changed, allowing very different methods of movement.

Most primates are good climbers. Some primitive primates climb like early mammals, using claws to maintain their grip, but advanced primates, such as monkeys and apes, climb in another way. These animals 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 adaptation reduces friction and increases leverage, permitting much more rapid movement. Hoofed animals take this trend even further. These 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 jerboas and kangaroos, have given up running 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 evolved flaps of skin that act like wings enabling them to glide from tree to tree. Bats, however, are the only mammals capable of powered flight. A bat's wings are highly modified 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 evolved sturdy flippers. Seals have all four limbs and they use their hind legs to swim. In whales, however, the hind limbs have disappeared, leaving only tiny vestiges of 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.

Given their speed and stamina, large mammals are able to exploit different habitats by migrating (see Animal Migration). In water, most toothed whales occupy the same range throughout the year, and it is unclear how far they migrate. Baleen whales, on the other hand, are well-known migrants. Most species give birth in tropical waters, and then migrate to their feeding grounds in the colder waters much closer to the poles.

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 48 km (30 mi) a day, covering over 1600 km (1000 mi) in the course of a year.

VIII

 

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 have evolved. 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, evolution has transformed specific features of the body into 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. Pangolins are covered with sharp-edged scales, and they roll up if threatened, leaving only their scales exposed. Many armadillos use a similar defensive technique, although instead of having scales, they are protected by interlocking plates made 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 1.5 m (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 echidnas and 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, while the tusks of a walrus, which can be over 61 cm (2 ft) long, can sometimes fend off attacking orcas, or 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.

IX

 

ORIGIN OF MAMMALS

Mammals evolved from reptiles called therapsids, and first appeared over 200 million years ago. The transition from reptile to mammal was a gradual one and involved a slow accumulation of mammalian features over a long period of time. One of these new mammalian features, found in all modern mammals, is a space behind the eye socket that allows extra room for muscles powering the jaws. Another is a hard bony palate that separates air moving through the nose from food moving through the mouth. Mammal-like reptiles also developed a new kind of jaw hinge, and increasingly complex teeth. The number of bones in the lower jaw progressively decreased from seven, until only one was left. Instead of being completely lost, the extra bones changed shape and took on a completely new role, helping to conduct sound waves from the eardrum to the inner ear. At some point in this long transition, fur, mammary glands, and a warm-blooded lifestyle also evolved.

One of the earliest mammals known, a species called Morganucodon, looked like a small weasel, and was only about 2.5 cm (about 1 in) long, while a related species, called Megazostrodon, was about the size of a shrew. These early mammals were almost certainly active at night and are thought to have lived on insects or small vertebrates.

By the Jurassic period, when dinosaurs dominated life on land, mammals had already been in existence for millions of years. During the long reign of the dinosaurs, several different groups of mammals evolved. Most of these groups have been named for the structure of their teeth. Morganucodon and Megazostrodon, for example, belonged to a group called the triconodonts—so called because their teeth had three cusps, or conical points, roughly in a line. Symmetrodonts had molars with three cusps arranged in a triangle, while multituberculates had rodent-like incisors, and large molars adapted for grinding up plant food. Teeth are made from hardy material that fossilizes well, and in many cases they provide scientists with the only clues that show how these early mammals lived.

When dinosaurs disappeared 65 million years ago, mammals found themselves in a world of unprecedented opportunities, without the ruling reptiles that had held them in check for so long. Some mammal groups, such as the multituberculates, eventually became extinct themselves, but others flourished and diversified as they filled the ecological roles that reptiles had left vacant. In particular, small placental animals developed a wide range of new diets, and an even bigger range of body forms. By about 30 million years ago, some of them had already taken to the seas, while on land their descendants included saber-toothed cats, the forerunners of today's pigs, and also giant browsers (leaf eaters) such as Indricotherium. Standing up to 5.5 m (18 ft) at the shoulder and weighing about 20 metric tons, this hornless relative of the rhinoceros was probably the largest mammal that has ever lived on land.

Despite their huge variation in size, these animals all shared the basic mammalian body plan that they had inherited from their distant ancestors. Most mammals still have four limbs, hair, and relatively large brains, yet their shared ancestry also explains similarities that are much less obvious. All mammals have four-chambered hearts, a muscular diaphragm separating their heart and lungs from their abdominal cavities, a lower jaw made of a single bone, and the same arrangement of tiny bones in the inner ears. Almost all mammals, including whales, mice, and giraffes, have seven vertebrae in their necks. The arms of humans, the flippers of seals, and the wings of bats all have the same number and arrangement of bones.

X

 

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 by about 14,000 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 cavy, or South American guinea pig, both of which provide meat, and the ferret. Descended from the European polecat, the ferret is sometimes kept as a pet, but was originally raised to drive rabbits from their burrows so they could be caught in hunters' nets.

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. 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 (see Animal Experimentation).

XI

 

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 (also known as IUCN) indicates that of about 4600 species of mammal known to exist, over 1000 are classified as being at risk for extinction (see Endangered Species).

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 the thylacine, 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.