A young African wild dog feeds on its mother’s milkA young African wild dog feeds on its mother’s milk Mammals are warm-blooded vertebrates: animals with backbones. Their key characteristic, one possessed by no other kind of animal, is their ability to nurture their young with milk. Milk is produced by the mammary glands in the female, a term which gives its name to the Class to which all such animals—humans included—belong: Mammalia.

Greater white-toothed shrewGreater white-toothed shrew

A diverse group

There about 5500 different species of mammal. They range in size from tiny shrews, only a few centimetres long, to the 30-metre (100-foot) blue whale. Most mammals have four limbs and a covering of hair or fur. Sea mammals, such as whales and dolphins, have adapted to life in the water by losing their hair and forming their hind limbs into a tail.

All except five species of mammal give birth to live young. The monotremes—the platypus and four species of echidna—all lay eggs. The majority of mammals are placentals: they give birth to young that develop fully in the womb.
Platypus, a monotremePlatypus, a monotreme
Marsupials, a much smaller group than the placentals, give birth to their young that have not yet fully developed. The young then continue to develop while clinging to their mother’s body.

Polar bear and cubsPolar bear and cubs

Keeping warm

All mammals are warm-blooded or endothermic, which means that their bodies can control their own temperature. They obtain warmth from the process of burning fuel (food) inside the body, rather than from basking in the sun as cold-blooded reptiles do. The skin and hair of the body are very important both for keeping in warmth in cold conditions and letting it escape to cool off the body in hot conditions. Most mammals also produce sweat, which cools the skin as it evaporates.

A sambar deer scent markingA sambar deer scent marking

Big brains

Mammals have large brains and complex sensory and circulatory systems. Human eyes can see in detail and in a wide range of colours, but some other mammals have other senses that are more highly developed. With its superb hearing, a dog, for example, can detect sounds far higher in pitch than we can, and hear sounds much further away.

Some mammals rely on their sense of smell more than any other kinds of animal do. Scent is used to communicate between members of the same species, to mark the borders of a territory, to find food and to detect an approaching predator.

A kangaroo suckling her youngA kangaroo suckling her young

Raising their young

All female mammals have mammary glands which produce milk after their young are born. The young feed on this milk, which gives them all the nutrition they need, until they are able to survive and feed on their own. By not having to find their own food, they can put all the energy gained from feeding into growing larger and stronger.

Female baboons caring for their youngFemale baboons caring for their young

During the period of suckling (feeding on milk), and also while the young learn to feed for themselves, the mother will protect them as much as possible from predators. Some female mammals raise their young alone while others share the task with the father or other members of a group.

Young mammals

 A newborn fawn walking just two minutes after birth A newborn fawn walking just two minutes after birthA family of week-old baby rabbitsA family of week-old baby rabbitsAll mammals except the marsupials and the monotremes are born fully formed, though some, such as rabbits, are hairless, blind and helpless. They grow further while in the safety of their burrow or den. By contrast, mammals that live in open spaces, such as horses, cattle or deer, are able to walk and even run a few minutes after birth. If a predator appeared, they would need to be able to run away. Small mammals give birth to several young at the same time. They all grow and become independent quite quickly. The young of larger mammals take longer to grow to full size, and need more care, so litter size is smaller—often just one at a time.

Social groups

A vast herd of wildebeest, Serengeti, TanzaniaA vast herd of wildebeest, Serengeti, TanzaniaMany mammals live in social groups that can be made up of a few or many animals. Often the whole group will help to rear and protect the young. Carnivores such as lions or hyenas also hunt together, using their combined strength and skill to kill larger animals than they could tackle alone. Herbivores, such as deer or cattle, form large herds that give them some protection against such predators.

Hyenas, members of the Carnivore orderHyenas, members of the Carnivore order


Members of the Carnivore order include cats, dogsbears and hyenas as well as weasels, raccoons and mongooses. Most are meat-eaters, but others feed on insects and fruit and some eat only plants.

The teeth of a carnivore—a tigerThe teeth of a carnivore—a tiger
They are grouped together because they are all descended from mammals that had teeth shaped for cutting through flesh. Many of the meat-eating carnivores still have these teeth, but in others they have been adapted for grinding plant material.

Carnivores have keen senses, especially their sense of smell. Scent is used to mark their territory or to communicate with others. Many live in packs, hunting together and even helping to raise the young.


Springboks, close relatives of gazellesSpringboks, close relatives of gazelles


Ungulates are a group of mammals that have hooves instead of claws on their feet. This gives them speed to escape from danger. They are mostly quite large, with long faces and good senses of smell and sight. Ungulates that live in forests or that have to search for their food are often solitary, while those that live in open spaces and feed on abundant plants such as grasses tend to live in herds.

The teeth of a donkey, an ungulateThe teeth of a donkey, an ungulate
Most ungulates have a coat of coarse hairs rather than soft fur. They are all plant-eaters, taking food straight from the plant or the ground with their lips, teeth and tongue.

Ungulates have large, flat teeth for grinding down plant material, and specially adapted digestive systems to get as much nourishment as possible from their food. Because of the low nutritional value of the leaves and grasses they eat, many ungulates have to spend most of their time eating.

Odd-toed or even-toed?

The black rhinoceros has three toes on each foot.The black rhinoceros has three toes on each foot.Ungulates are divided into two groups: the odd-toed ungulates, called perissodactyls, which have one or three toes, and the even-toed ungulates, the artiodactyls, which have two or four toes. The odd-toed ungulates include horses and zebras, which have a single toe, and rhinoceroses and tapirs, which have three toes on each foot.

Pigs' feet, known as trotters, have four toes.Pigs' feet, known as trotters, have four toes. There are two main groups of even-toed ungulates. The first group is made up of the pigs, peccaries and hippopotamuses. These animals have short legs, large heads and four toes on each foot. The second group comprises all other even-toed ungulates, which have longer legs and two toes on each foot. They include cattle, antelope, sheep and goats, all members of the bovid family, as well as camels, giraffes and deer.


Digestion system of an antelope—a type of ruminantDigestion system of an antelope—a type of ruminant


Many ungulates are able to digest their food more efficiently than other animals. This means that, despite their large size, they can get the nourishment they need from their plant-eating diets. These animals are known as ruminants, and include the even-toed ungulates and some marsupials. 

Their stomachs are divided into compartments that break down tough food in stages. In some ruminants, such as cattle, the food is sent back to the mouth to be chewed again after the first stage of digestion. This is known as "chewing the cud".

A camel, a ruminant mammal, chewing the cudA camel, a ruminant mammal, chewing the cud

Black and white colobus monkeysBlack and white colobus monkeys


The group of mammals known as primates is made up of lower primates (prosimians), such as lemurs, lorises and galagos, and higher primates (simians), which include monkeys, apes and humans. Apart from humans, primates are found in all parts of the world except North America, Australasia and Antarctica.


Great galago, or bush babyGreat galago, or bush baby

Lower primates

The lower primates have quite pointed faces, forward-facing eyes, a good sense of smell and smaller brains than the higher primates. Most of the lower primates are tree-dwellers that leap or climb from branch to branch feeding on fruit, insects or leaves. They usually have long tails to help them balance on landing.


A monkey using its hands to grasp thingsA monkey using its hands to grasp thingsClick to play video

Higher primates

The higher primates, have flat faces, forward-facing eyes with good stereoscopic vision and larger brains than the lower primates.

Monkeys have well-developed fingers and toes with nails instead of claws, which they use to grasp things and pick up food. They feed on fruit, leaves, insects and other small creatures. Most monkeys live in trees, although some come down to feed or travel, and some, like baboons, live on the ground all the time.

A chimpanzee holding a simple toolA chimpanzee holding a simple tool All apes have long arms, no tails and can walk upright on two legs for a while. Gibbons are known as the lesser apes, while chimpanzees, orang-utans and gorillas are the great apes. The great apes make “nests” to sleep in at night. They have strong, nimble fingers for grasping or carrying out precise tasks. The closest relatives to humans, the great apes are able to use tools, solve problems and learn quickly. They live in structured societies and even wage war on each other’s societies much as humans do.

 Chris Jarvis

All except five species of mammals give birth to live young. The monotremes—four species of echidna, together with the platypus—all lay eggs.

Mammals are the only kinds of animals that have the three tiny middle ear bones, (the hammer, anvil and stirrup) and four chambers in their hearts.

About 70% of mammal species are either a rodent, a bat or a shrew.

No animals other than mammals have hair.

Mammals are the only animals that can sweat.

All but one mammal are endothermic (warm-blooded): they get heat energy from the food they eat. The single exception is the naked mole rat.

While there is no doubt that the 30-metre (100-foot) blue whale is the largest mammal, the record for smallest is between Kitti's hog-nosed, or bumblebee, bat and the Etruscan shrew. Both are about 3.5 cm (1.4 inches) long and weigh 2 g (0.07 oz).

The longest-living mammal known is the bowhead whale. In 2007, one was discovered to have a harpoon dating from 1880 embedded in its neck. It was therefore at least 130 when it died in 2010, but scientists think bowheads may live for up to 200 years.

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