The platypus is the Australian mammal with a venomous spur on its hind leg, used mainly by males during fights with rivals. The spur, called a calcar, is connected to venom-producing crural glands on each hind limb.
Only adult male platypuses retain functional venom spurs. Females develop small spur buds early in life, but these disappear and their venom glands do not become functional. Male venom production increases during the breeding season, supporting the idea that the spur evolved chiefly for competition over territory and mates.
Platypus venom is extremely painful to humans but is not normally fatal. The animal is also unusual for several other reasons: it lays eggs, detects prey underwater using electroreception, and has a bill covered with sensitive receptors. A common mix-up is the echidna, another egg-laying monotreme; echidnas have spurs but lack the platypus's functional venom system.