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Monotreme, a member of the mammalian order Monotremata, an order of primitive, egg-laying mammals that includes the platypus and two species of spiny anteaters. These are, in fact, the only monotremes still in existence and the only surviving members of the subclass Prototheria.
Although they resemble reptiles in that they lay eggs, the monotremes are true mammals. They possess such distinctively mammalian characteristics as mammary glands, hair, a large brain, and a complete diaphragm. They range in body length from about 30 to 80 centimetres (12 to 32 inches) and weigh from about 1 to 10 kilograms (2 to 22 pounds). The three living monotremes are the platypus (Ornithorhynchus anatinus), one of the most remarkable of all mammals with its ducklike "bill," webbed feet, and a flattened beaverlike tail, and the echidnas, or spiny anteaters ( Tachyglossus aculeatus and Zaglossus bruijni), which have sharp-pointed spines and tubelike noses. The platypus is found in eastern Australia; Tachyglossus, the short-nosed echidna, is found throughout Australia and in New Guinea, while Zaglossus, the long-nosed echidna, is found only in New Guinea.
The amphibious platypus digs winding burrows 5 to 10 metres (15 to 30 feet) long in the banks of the streams and lakes where it lives. It feeds mainly on insect larvae, small crayfish, tadpoles, and other aquatic animals. It has few natural enemies.
The echidnas are generally solitary. Tachyglossus is seldom encountered by humans, although it is one of the most widely distributed animals in Australia. It lives in forests, scrubland, and deserts. When threatened, these anteaters have a remarkable ability to dig vertically into the ground and cover themselves with dirt. They also can draw in their appendages and erect their spines for defense. Zaglossus lives in the cloud-covered humid forests of New Guinea, often at elevations up to 100 to 2,000 metres. Little is known of monotreme social behaviour.
One of the most remarkable features of the monotremes is that they lay eggs. In the breeding season the female platypus usually lays two leathery-shelled eggs. The period from mating to egg laying is probably around 12 to 14 days, and the incubation period is an additional 12 days. The mother has no teats; the young suck the milk through the mother's skin as it is exuded from the mammary glands. In the echidna the period between copulation and egg laying is at least 16 days; the incubation period is about 10 days. In the breeding season the female develops an abdominal pouch into which the egg is apparently laid directly. One egg is usually laid at a time. After hatching, the young echidna lives for a short period in the pouch, but it continues to suckle for some time even after it leaves the pouch. Like the platypus, the echidna has no teats, and the young get milk through the skin.
The monotremes have no living relatives. Little is known of their ancestry; the earliest fossil monotremes found in Australia are only about 2,000,000 years old and differ little from contemporary species. Most authorities believe that the order Monotremata originated from a line of mammal-like reptiles different from that which gave rise to the other mammals. Nonetheless monotremes may well represent features of anatomy and development that characterized the earliest mainstream mammals.
Monotremes
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