Mammals are the group of animals to which humans belong. They are vertebrates (animals with a backbone) that give birth to live young and then feed them with milk. Most mammals have four limbs and a covering of hair or fur. Sea mammals, such as whales, have adapted to life in the water by losing their hair and hind limbs and forming their front limbs into flippers.
All mammals are warm-blooded, 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 the 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.
Mammals have large brains and complex sensory and circulatory systems. They rely on their sense of smell more than any other kind of animal. 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.
The most important feature which sets mammals apart from other animals is the way they care for their young after birth. 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.
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.
Most mammalian young, apart from the monotremes, are born live (not inside an egg). Small mammals can give birth to several young at one time, which grow and become independent quite quickly. The young of large mammals take longer to grow to full size, and need more care, so litter size (the number of young born at once) is smaller, often only one at a time.
All mammals except the marsupials and the monotremes are born fully formed, though some, such as rabbits, are hairless, blind and helpless. They can grow further while in the safety of their burrow or den.
In contrast, animals 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 run away with the rest of the herd.
Many 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 predators.
Like marsupials, monotremes give birth to under-developed young. However, monotremes young are born inside soft-shelled eggs. These hatch after a few days, and the young are suckled until they have developed fully.
There are two kinds of monotremes: the echidna and the platypus. Both are found only in Australia and surrounding islands. Echidnas have a small pouch that holds their young after hatching. Platypuses do not have a pouch, but instead leave their young in a special burrow.
Apart from a few types of shrew, monotremes are the only venomous mammals. Echidnas cannot use their venom but the platypus can kill a dog with poison ejected from a sharp spur on its leg.
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