colour of their spines, which is similar to the vegetation of the dry Australian environment, to avoid detection. They have good hearing and tend to become stationary if sound is detected. It is likely that echidnas are keystone species in the ecosystem health in Australia, due to their contribution through bioturbation, the reworking of soils through their digging activity. This is based on the estimation that a single echidna will move up to 204 m3 of soil a year, that it is the most widespread of any terrestrial Australian species, is relatively common, and that other bioturbators have been heavily impacted by human settlement. In Australia, they are most common in forested areas with abundant, termite-filled, fallen logs. In agricultural areas, they are most likely to be found in uncleared scrub; they may be found in grassland, arid areas, and in the outer suburbs of the capital cities. Little is known about their distribution in New Guinea. They have been found in southern New Guinea between Merauke in the west and the Kelp Welsh River, east of Port Moresby, in the east, where they may be found in open woodland. Echidnas have the ability to swim, and have been seen cooling off near dams during high temperatures. They have also been seen crossing streams and swimming for brief periods in seas off Kangaroo Island. They swim with only the snout above water, using it as a snorkel. Reproduction The solitary short-beaked echidna looks for a mate between May and September; the precise timing of the mating season varies with geographic location. In the months before the mating season, the size of the male's testes increases by a factor of three or more before spermatogenesis occurs. Both males and females give off a strong, musky odour during the mating season, by turning their cloacas inside out and wiping them on the ground, secreting a glossy liquid believed to be an aphrodisiac. During courtship—observed for the first time in 1989—males locate and pursue females. Trains of up to 10 males, often with the youngest and smallest male at the end of the queue, may follow a single female in a courtship ritual that may last for up to four weeks; the duration of the courtship period varies with location. During this time, they forage for food together, and the train often changes composition, as some males leave and other join the pursuit.