Reproductive biology

Together with the echidnas, the platypus is distinguished as a monotreme, or egg-laying mammal. Males and females have a single physical opening (the cloaca) that is used both for reproduction and excretion. To help maintain a streamlined shape, the male's penis and testes are carried inside the body; mating occurs in the water. In the female platypus, the right ovary is small and nonfunctional. At the time of ovulation, the platypus egg is about 0.16 in (4 mm) in diameter. After being fertilized, the first of three shell layers is formed in the fallopian tube before the egg moves into the uterus. There, the egg is supplied with additional nutrients and two more shell layers are secreted, so the egg is about 0.6 in (15 mm) in diameter when it is laid. Though the time required for gestation has never been determined precisely, it is believed that it takes around three weeks.

Platypus eggs are produced in late winter and spring (August-November), with some evidence that breeding occurs later in southern populations as compared to those found in Queensland. The eggs are laid in a burrow typically measuring 10-20 ft (3-6 m) in length, though sometimes much longer. Throughout incubation and juvenile development, the female keeps the burrow's entry tunnel blocked by several plugs of soil. Besides discouraging access by predators such as snakes and Australian water rats (Hydromys chrysogaster), the plugs reduce the likelihood that juveniles drown in the event of a flood. A few days before laying her clutch of one to three eggs, a female drags a large quantity of wet leaves and other vegetation into the rounded burrow chamber to make a nest. It is believed that the female incubates the leathery-shelled eggs for about 10 days, clasping them between her curled-up tail and belly as she lies on her back or side. When they hatch, the young are less than 0.5 in (9 mm) long. Their emergence from the egg is assisted by the presence of a prominent bump (caruncle) at the tip of the snout, an inward-curving egg tooth, and forelimbs armed with tiny claws. When they hatch, the young are less than 0.8 in (20 mm) long.

Juveniles develop in the nursery burrow for about four months before entering the water for the first time. Through out this period, they are nourished solely on milk. The female does not have nipples. Instead, milk is secreted directly onto the mother's fur from two circular patches of skin located about halfway along her abdomen. An orphaned platypus will drink milk from a human hand by sucking up the liquid while sweeping its short bill rhythmically back and forth against the palm. In the wild, such sweeping movements may help to stimulate the flow of milk.

Both males and females are physically mature at the age of two years, though some females may delay having offspring until they are four years old or more. Courtship involves two individuals swimming alongside or circling each other, sometimes accompanied by nuzzling or rubbing. One animal may use its bill to grasp the tip of the other's tail and be towed or swim behind. Little is known about the platypus breeding system, apart from the fact that the animals do not appear to form long-term pair bonds. Instead, it is believed that males tend to move about widely during the breeding season, trying to mate with as many females as possible. By the same token, adult females appear to rear the young without any help from their mates.

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