In cooler parts of their range, such as Tasmania, females may mate within a few hours of arousal from hibernation. Before mating, the male smells the female, paying particular attention to the cloaca. This process can take a few hours, and the female can reject the suitor by rolling herself into a ball. After prodding and sniffing her back, the male is often observed to roll the female onto her side and then assume a similar position himself so the two animals are abdomen to abdomen, having dug a small crater in which to lie. They can lie with heads facing one another, or head to rear. If more than one male is in the vicinity, fighting over the female may occur. Each side of the bilaterally symmetrical, rosette-like, four-headed penis (similar to that of reptiles and in length) is used alternately, with the other half being shut down between ejaculations. Sperm bundles of around 100 each appear to confer increased sperm motility, which may provide the potential for sperm competition between males. This process takes between a half and three hours. Each mating results in the production of a single egg, and females are known to mate only once during the breeding season; each mating is successful. Fertilisation occurs in the oviduct. Gestation takes between 21 and 28 days after copulation, during which time the female constructs a nursery burrow. Following the gestation period, a single, rubbery-skinned egg between in diameter and in weight is laid from her cloaca directly into a small, backward-facing pouch that has developed on her abdomen. The egg is ovoid, leathery, soft, and cream-coloured. Between laying and hatching, some females continue to forage for food, while others dig burrows and rest there until hatching. Ten days after it is laid, the egg hatches within the pouch. The embryo develops an egg tooth during incubation, which it uses to tear open the egg; the tooth disappears soon after hatching. Hatchlings are about long and weigh between . After hatching, young echidnas are known as "puggles". Although newborns are still semitranslucent and still surrounded by the remains of the egg yolk, and the eyes are still barely developed, they already have well-defined front limbs and digits that allow them to climb on their mothers' bodies. Hatchlings attach themselves to their mothers' milk areolae, specialised patches on the skin that secrete milk—monotremes lack nipples—through about 100–150 pores.