The platypus is classified among egg-laying mammals - monotremes. Male platypuses have a poison apparatus on their hind legs. There are poison glands in the thighs and a hollow spur near the heel. The sting is not dangerous to humans, but is extremely painful and causes rapid swelling in the stung area. It may also cause increased temperature. For smaller mammals, like dogs, the sting can be fatal. The purpose of the platypus's poison system has not been satisfactorily explained. It may be for defending the burrows when young are present.