Reproduction and life history
The platypus is a monotreme, or egg-laying mammal. Males and females have a single physical opening (known as the cloaca) which is used both for reproduction and excretion.
Platypus have been observed mating in the wild in Victoria and New South Wales from early August to early November, with animals believed to breed a few weeks earlier in Queensland and a few weeks later in Tasmania. The animals do not appear to form lasting pair bonds: males probably court as many females as possible, and females rear their young without any assistance from their mates.
Based on observations made in captivity, a female becomes receptive to males for a period of 4-6 days. Afterwards, she digs or renovates a nesting burrow and then spends 2-5 days collecting vegetation from the water (leaves, grass, bark strips, etc.) to line the nest. It is believed that wet nesting material is required to help keep platypus eggs and newly hatched young from drying out.
A clutch of 1-3 whitish, leathery-shelled eggs (like those of lizards and snakes) is laid approximately 2-3 weeks after mating. The eggs are incubated underground for around 10 days, clasped between a female’s curled-up tail and belly as she lies on her back or side. The eggs are about 15 millimetres in diameter, and the young are correspondingly small when they hatch (about 9 millimetres in length). Their exit from the egg is assisted by a prominent bump (or caruncle) at the end of the snout, an inwardly curving egg tooth and tiny claws on the front feet.
After hatching, juveniles (there is no well-established special term for a baby platypus) develop in the nesting burrow for about 3-4 months before entering the water for the first time. Throughout this period, they are nourished only on milk. A female platypus does not have nipples. Instead, milk is secreted directly onto her belly fur from two round patches of skin.
Platypus milk is thick and rich, containing on average about 39% solids (as compared to 12% solids in cow milk). The average fat content of platypus milk (22%) is about six times greater than that of cow milk, while its protein content (8%) is more than double the average value for cow milk.
The newly emerged juveniles are fully furred, well co-ordinated and about 80% of their adult length. They apparently are not taught to swim or how to feed by their mother, but have to learn by them through trial and error.
Males and females both become mature at the age of two years. However, some females may not produce young until they are four years old or more, with a long-term study carried out by Dr Tom Grant along the Shoalhaven River in New South Wales indicating that less than half of females breed on average in a given year (range = 18-80% over 27 years).
The juvenile mortality rate generally appears to be high, with only a small proportion of young platypus surviving to adulthood. However, it is not uncommon for adults to live for a decade or more. The oldest known platypus (a female) survived to the age of at least 21 years in the wild.
Grant, T.R., Griffiths, M. and Temple-Smith, P.D. (2004). Breeding in a free-ranging population of platypuses, Ornithorhynchus anatinus, in the upper Shoalhaven River, New South Wales � a 27 year study. Proceedings of the Linnean Society of New South Wales 125: 227-234.
Griffiths, M., Green, B., Leckie, R.M.C., Messer, M. and Newgrain, K.W. (1984). Constituents of platypus and echidna milk, with particular reference to the fatty acid complement of the triglycerides. Australian Journal of Biological Sciences 37: 323-329.
Hawkins, M. and Battaglia, A. (2009). Breeding behaviour of the platypus (Ornithorhynchus anatinus) in captivity. Australian Journal of Zoology 57: 283-293.