rivers and the riparian zone for both a food supply of prey species, and banks where it can dig resting and nesting burrows. It may have a range of up to , with a male's home range overlapping those of three or four females. The platypus is an excellent swimmer and spends much of its time in the water foraging for food. It has a very characteristic swimming style and no external ears. Uniquely among mammals, it propels itself when swimming by an alternate rowing motion of the front feet; although all four feet of the platypus are webbed, the hind feet (which are held against the body) do not assist in propulsion, but are used for steering in combination with the tail. The species is endothermic, maintaining its body temperature at about 32°C (90°F), lower than most mammals, even while foraging for hours in water below 5°C (41°F). Dives normally last around 30 seconds, but can last longer, although few exceed the estimated aerobic limit of 40 seconds. Recovery at the surface between dives commonly takes from 10 to 20 seconds. When not in the water, the platypus retires to a short, straight resting burrow of oval cross-section, nearly always in the riverbank not far above water level, and often hidden under a protective tangle of roots. The average sleep time of a platypus is said to be as long as 14 hours per day, possibly because it eats crustaceans, which provide a high level of calories. Diet The platypus is a carnivore: it feeds on annelid worms, insect larvae, freshwater shrimp, and freshwater yabby (crayfish) that it digs out of the riverbed with its snout or catches while swimming. It uses cheek-pouches to carry prey to the surface, where it is eaten. The platypus needs to eat about 20% of its own weight each day, which requires it to spend an average of 12 hours daily looking for food. Reproduction When the platypus was first encountered by European naturalists, they were divided over whether the female lays eggs. This was finally confirmed by William Hay Caldwell's team in 1884. The species exhibits a single breeding season; mating occurs between June and October, with some local variation taking place between different populations across its range. Historical observation, mark-and-recapture studies, and preliminary investigations of population genetics indicate the possibility of both resident and transient members of populations, and suggest a