The most well-known feature of monotremes is their method of reproduction. They are the only living mammals in which females lay eggs instead of giving live birth. The length of time the egg remains within the mother is short, only twelve to twenty days. While the egg is still within the mother's oviduct (the tube leading from the ovaries to the cloaca), the tissues of the oviduct secrete a shell onto the egg, as happens in birds and egg-laying reptiles. The monotreme eggshell is soft and leathery, and porous enough to soak up nutrients secreted into the oviduct from the mother's circulatory system.
The embryo begins its development before the egg is laid. When the mother lays her egg, the embryo has already developed to about the same degree as a newborn marsupial. The eggshell is leathery, like a reptile's, spherical, and small, 0.5 to 0.6 inches (13 to 15 millimeters) in diameter, or the size of a grape. After about ten days of the egg's incubation, the young hatches by tearing at the shell by means of a temporary egg tooth on its snout. When the youngster is fully hatched, it nestles close to the mother and feeds on her milk. The young are weaned at four to six months of age.
Female echidnas and platypus may lay up to three eggs at a time, but one is normal, and monotreme females usually bear and raise only one young per year. Females do all the raising of the young. Except during the mating season, individual platypus and echidnas of both sexes lead solitary lives.
A platypus mother incubates her eggs by curling her tail and holding the eggs between the tail and her warm underbelly. She incubates and nurses her young in a "birth chamber" burrow, which she digs and lines with moist leaves and water plants to maintain humidity. Echidna mothers form simple, temporary pouches by constricting special long muscles of their underbellies, and in which they incubate the eggs and later carry the developing young.
The monotremes are unique in yet another way. They are the only mammals to carry a sensory system that detects electricity, along with their usual senses of sight, hearing, etc. The platypus bill contains tiny electroreceptors, specialized sensory nerve endings arranged in rows along the length of the bill, on the upper and lower surfaces. These detect electricity from the muscular systems of underwater animals that the platypus hunts, and even from the electricity created by water as it flows over rocks on the bottom of the lake or river. The electroreceptors are located together with mechanoreceptors that detect underwater turbulence. Together, the two senses allow the platypus to put together a three-dimensional "picture" of its underwater hunting territory.
The bills of echidnas also have electroreceptors, though much fewer than in platypus. Biologists have confirmed the platypus's use of the electrosense, while this has not been found working in echidnas. Most likely the echidnas are gradually losing the electrosense while platypus have developed it into one of nature's most complex sensory systems.