A group of warm-blooded animals with a bony skeleton, mammals include mice and other RODENTS, PRIMATES, such as monkeys and humans, and animals as various as hippos, deer, and cats. The 4,500 or so species include elephants, the largest creatures on land, and whales, the largest of all animals on Earth.
In almost all mammals, the babies develop inside the mother before they are born. This process is called GESTATION. Once born, baby mammals suckle, or feed, on their mother’s milk. Most mammals have hair, and all land mammals have four limbs. However, in whales, the rear limbs have disappeared.
Mammals have become very successful because of the wide range of foods they eat. Meat-eaters include cats, hyenas, and dogs. Shrews and hedgehogs eat insects. Plant-eaters include hoofed animals such as horses and deer, and also rabbits and rodents. Some mammals are omnivores, eating both plants and meat.
All mammals reproduce sexually—sperm from the male fertilizes the female’s egg. In some mammal species, males establish breeding territories, where they put on displays for the females, showing that they are healthy and strong. In others, the males fight for the right to mate. Many male hoofed mammals have horns or antlers, which they crash or lock together in tests of strength.
Mammals maintain a constant body temperature, which lets them stay active in any weather. Maintaining body temperature takes up a lot of energy, so mammals need large quantities of food. To help reduce the amount of food they must find, mammals in cold environments have thick fur or fatty blubber to retain body heat. Some go into HIBERNATION to survive winter.
|Mammals make up the vertebrate class Mammalia. There are about 4,500 species split into three main groups.|
|The first group—the monotremes—is also the smallest, with three species.|
|The second group contains the marsupials, with around 240 species in all.|
|The third group contains all placental mammals and is split into 17 orders. Rodents make up the largest order.|
Gestation is the time young mammals spend growing in their mother’s womb. Most mammals develop in this way, so the mother gives birth to fully formed young. Many MARSUPIAL babies, such as kangaroos, complete their development in their mother’s pouch.
In most mammals, the fertilized egg implants itself in the mother’s womb. There it develops into an embryo, which is nourished by the placenta. Marsupials have no placenta and give birth to tiny, helpless young. MONOTREMES, such as the platypus, lay eggs.
Many mammals, such as bats, bears, and dormice, survive winter in cool and polar lands by entering a deep sleep called hibernation. This strategy helps to conserve energy that would otherwise be lost in the struggle to keep warm and find scarce food.
Heartbeat, breathing, and other body processes slow down, and the animal’s temperature drops so that it feels cold to the touch. When the weather warms again in spring, these processes are reversed, and the mammal wakes up to resume active life.
These mostly tree-living mammals are divided into two groups. Prosimians, or primitive primates, include lemurs, lorises, and tarsiers. Anthropoids, or higher primates, include marmosets, apes, monkeys, and humans. Primates range in size from mouse lemurs weighing 3 1/2 oz (100 g) to gorillas, which are 2,000 times heavier.
Primates are intelligent mammals. As well as hairy bodies, most have long arms and opposable thumbs and big toes, which enable them to grasp branches. Primates’ eyes face forward, giving them binocular vision, which helps them judge distances as they swing through the trees. Their main senses are sight and touch; hearing and scent are less important.
By living in groups, primates can defend large feeding territories and are more likely to spot predators than they would be on their own. Group living also helps with raising young. Primate babies take a long time to grow up—three to five years in apes such as chimpanzees. Having other adults around helps the mothers and gives the babies added protection.
With around 1,800 species, rodents make up the largest group of mammals. The smallest rodents weigh just a few ounces. The largest, South America’s capybara, is the size of a large dog. All rodents have chisel-like incisor teeth at the front of their jaws to gnaw food.
Rodents can survive almost anywhere except the sea. Marmots and lemmings inhabit snowy mountains and Arctic wastes, while jerboas and gerbils live in deserts. Rats and mice have colonized our towns and cities. Different rodents are adapted for climbing, swimming, burrowing, or gliding through the air.
The group of marsupials includes kangaroos, wallabies, opossums, gliders, and wombats. All marsupials are born early and complete their development in their mother’s pouch or clinging to her fur.
Most marsupials live in Australia and surrounding islands, but some are found in South America, and one, the Virginia opossum, lives in North America. Marsupials multiplied and evolved into all sorts of species in Australia because there were no placental mammals there to compete with them.
Many marsupials are plant-eaters. Kangaroos and wombats feed mostly on grasses, while koalas eat leaves. Some gliders feed on nectar from flowers. Tasmanian devils are solitary and nocturnal, preying on rabbits, chickens, and other small animals. Virginia opossums are omnivorous, eating fruit, eggs, insects, and other small creatures.
The small group of egg-laying mammals contains just three species—the duck-billed platypus and two types of echidnas. Monotremes are found only in Australia and on the island of New Guinea. These secretive, burrowing creatures are rarely seen.
Monotremes eat invertebrates, which they search for at night. Echidnas, also known as spiny anteaters, feed on termites and other insects. They slurp them up with their long, sticky tongues. Platypuses hunt under water, searching out worms, crustaceans, and insects with their soft, sensitive beaks.
Platypuses and echidnas lay between one and three soft-shelled eggs. Female echidnas incubate their eggs in pouches on their abdomens. The platypus curls around her eggs in her burrow. When the eggs hatch, after about ten days, the babies feed on milk seeping from patches on the mother’s abdomen. The young become independent after four or five months.