Short-beaked echidna

The female lays a single soft-shelled, leathery egg 22 days after mating, and deposits it directly into her pouch. Hatching takes place after 10 days; the young echidna then sucks milk from the pores of the two milk patches (monotremes have no nipples) and remains in the pouch for 45 to 55 days, at which time it starts to develop spines. The mother digs a nursery burrow and deposits the young, returning every five days to suckle it until it is weaned at seven months. Male echidnas have a four-headed penis. During mating, the heads on one side "shut down" and do not grow in size; the other two are used to release semen into the female's two-branched reproductive tract. Each time it has sex, it alternates heads in sets of two. When not in use, the penis is retracted inside a preputial sac in the cloaca. The male echidna's penis is 7 centimeters long when erect, and its shaft is covered with penile spines. During breeding season, males will form lines up to ten individuals long, the youngest echidna trailing last, that follow the female and attempt to mate.