The classic description of a mammal is an animal that gives birth to fully formed young that are initially nourished on mother’s milk. But there are two groups of mammals that defy this definition: the monotremes and the marsupials.
The monotremes (subclass Prototheria) lay eggs, and the marsupials (subclass Metatheria) give birth to embryonic young that continue their development within the protection of the mother’s abdominal pouch. The six species of monotremes and approximately 300 species of marsupials together make up only about 4 per cent of all mammal species.
The monotremes are the platypus and the echidnas, or spiny anteaters. The most striking difference between monotremes and other mammals is that monotremes lay eggs covered by a leathery shell. However, like other mammals, monotremes feed their hatched young on milk from mammary glands.
Monotremes are curiously reptilian in other ways as well. Just as in reptiles, the monotreme excretory and genital ducts have a common opening known as the cloaca. It is this feature that gives monotremes their name, which means “one hole.” The structures of the bones in the lower jaw and middle ear are similar to those of other mammals, but the gir die of bones that supports the forelimbs is reptilian in form. In addition, the brain and circulatory systems of monotremes are mammalian but have some reptilian features. This peculiar mixture of characteristics indicates that monotremes separated from the therian evolutionary lineage before many of the therian characteristics evolved.
There are two families of monotreme: Or-nithorhynchidae (bird-noses), which contains a single species, the platypus, or duckbill, and Tachyglossidae, which is made up of two species of echidnas, also called spiny anteaters. The platypus, which is found only in Australia, is a streamlined animal with a flat snout like a duck’s bill. It lives in burrows that it digs in riverbanks or inherits from other animals. It hunts for food underwater, depending largely on the tactile sense organs on the soft edge of its bill to find its prey, which includes small animals such as larvae, earthworms, and crustaceans.
The platypus has webbed feet and nostrils on the tip of its bill through which it breathes while it floats on the surface of the water. When it dives, a protective skin fold closes over its ears and eyes, and it can stay submerged for up to five minutes. Male platypuses have a venom spur on each hind leg, as do male spiny anteaters.
Spiny anteaters, or echidnas, occur in the sandy and rocky regions of Australia and New Guinea. They have a protective coat of short, sharp spines, rather like a hedgehog. These animals have no teeth but feed on termites, ants, and other insects by picking them up with their long, sticky tongue. The female spiny anteater temporarily develops a pouch on her abdomen during breeding season, into which she transfers her eggs after laying. The young hatch about a week later and feed from the milk ducts, which open into the pouch. They stay in the pouch for several weeks until their spines have developed.
Marsupials are now represented by only about 300 species, one in North America, 90 in South America, and 210 in Australasia. However, the fossil records of the Eocene age, which began about 58 million years ago, indicate that marsupials were once widely distributed and even more common than eutherians. One reason for the marsupial decline may be that their mode of reproduction is poorly adapted to unpredictable food supplies.
The most distinguishing feature of the marsupials is the pouch, or marsupium, which contains the mammary glands. Although there are many species in which the marsupium is not present, it is this feature that gives marsupials their name. The embryos of some marsupials are supplied with nutrients from the wall of the uterus, whereas in others there is no placental connection between the uterus and the embryo. In all species of marsupials, the embryo remains in the uterus for a very short period and develops quickly.
At birth, the embryo is tiny. At most it may be 1.2 inches (3 centimeters) long and many are no longer than a grain of rice. Its forelimbs and nervous center are well developed, but the hind limbs are mere buds. It crawls, unaided, from its mother’s birth canal to her pouch, where it latches onto a nipple that injects milk into the young animal’s mouth. A baby kangaroo, or joey, remains in its mother’s pouch for about six months, and will return to feed or seek refuge until it is about eight months old.
The kangaroos, the largest and perhaps the best known marsupials, have forward-opening pouches, but the form of pouch varies among species. For example, wombats and phalangers, including the koala, have pouches that open to the rear. The arrangement is an advantage for the young of digging animals, although it means the mother cannot clean the pouch.
The marsupial reproductive system differs from that of eutherians in various other ways. For example, the females have a double uterus and double vagina, and in many species the males have a forked penis with the testes in front of it.
Kinds of marsupials
The marsupials are an extremely diverse group, and they include tree-dwelling, fruiteating, grazing, burrowing, insect-eating, and meat-eating types. There are six families of marsupials. The opossums (Didelphidae) and the rat opossums (Caenolestidae) are found only in the Americas. Most didelphids eat any plant and animal food. Caenolestids live in western South America and eat insects and other invertebrates. Carnivorous marsupials (Dasyuridae) include insect-eating species often called marsupial mice as well as the native cat and the fierce Tasmanian devil. The insecteating bandicoots (Peramelidae) use their long, pointed noses to root in the soil for insects. Phalangers (Phalangeridae) eat mainly fruits, flowers, and nectar and nest in holes in trees. Kangaroos and wallabies (Macropodi-dae) feed mainly on grasses.