Little is known about the mating systems of prototherians. They are solitary for most of the year, coming together only to mate. During the mating season, duck-billed platypuses are found in pairs, but despite these observations, platypuses are not likely to be monogamous because males do not associate with females post-copulation, nor do they provide any parental care. Female short-nosed spiny echidnas have been observed with several males at a time, which may reflect a polygyny or polyandry. Even less can be inferred about the mating systems of long-nosed spiny echidnas because so little is known about their basic behavior and biology.
Prototherians are seasonal breeders. Typically, the breeding season lasts 1 to 3 months between July and October. At least one species (duck-billed platypuses) perform somewhat elaborate courtship behaviors prior to copulation.
The eggs layed by monotremes are small (13 to 15 mm diameter) and covered by a leathery shell. The number of eggs laid is small, usually 1 to 3, and they are placed in the mother's pouch. They contain a large yolk, which is concentrated at one end of the egg, very much like the yolk of a bird's egg. Only the left ovary is functional in the platypus, but both produce eggs in the echidna. Like the eggs of birds, monotreme eggs are incubated and hatched outside the body of the mother. Incubation lasts about 12 days. The young, which are tiny and at a very early stage of development when they hatch, break out of the eggs using a "milk tooth. They are protected in a temporary pouch in echidnas but not platypuses. They are fed milk produced by mammary glands; the milk is secreted onto the skin within the pouch and sucked or lapped up by the babies. Weaning takes place when the young are 16 to 20 weeks old.
Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); oviparous
The parental investment of male prototherians appears to consist entirely of acquiring mates and fertilizing a female's eggs. All other investment and parental care is provided by females. Young are born in a highly altricial state and require considerable care and protection from their mothers. As mammals, females produce milk and nurse their young. Echidnas develop a brood pouch on their abdomen within which eggs and hatched young develop for nearly two months. Young are weaned by about three months of age. Platypuses do not have a brood pouch, and instead lay their eggs in deep, complex burrows on the banks of streams and ponds. Young develop within the burrow and are weaned after 3 months.
Parental Investment: altricial ; pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Female)