New Guinea is differentiated from its drier, flatter, and less fertile southern counterpart, Australia, by its much higher rainfall and its active volcanic geology, with its highest point, Puncak Jaya, reaching an elevation of 4,884 m (16,023 ft). Yet the two land masses share a similar animal fauna, with marsupials, including wallabies and possums, and the egg-laying monotreme, the echidna. Other than bats and some two dozen indigenous rodent genera, there are no pre-human indigenous placental mammals. Pigs, several additional species of rats, and the ancestor of the New Guinea singing dog were introduced with human colonization.