The platypus is the only living mammal known to produce venom in its hind-leg spurs.
Male platypuses have sharp ankle spurs connected to venom glands. The venom is delivered through the spur during disputes, especially in the breeding season, and can cause severe pain and swelling in humans. Females are born with rudimentary spurs that usually do not develop into functional venom-delivery structures.
The platypus is also remarkable for combining several unusual traits: it lays eggs, has a broad bill, webbed feet, and detects weak electrical signals from prey underwater. It is one of only five living monotreme species, the other four being echidnas. The platypus is native to eastern Australia and Tasmania, where it feeds on aquatic invertebrates in streams and rivers.