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.

Rodents represent nearly 40 percent of all mammal species. Over 1700 species of rodents, including (top from left) porcupines, beavers, chinchillas, pacas, (bottom from left) flying squirrels, mice, muskrats, and capybaras, can be found in nearly every terrestrial and arboreal habitat. The success of this group is due in part to its adaptability to new food sources and habitats and its relatively brief reproductive cycle.

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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 perissodactyls, 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.

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