A spur is a bone covered in horn that grows out of an animal's body. While claws or nails grow from the toes, spurs grow on other parts of the foot. These parts are usually connected with joints. Spurs are mostly found on the hindfeet, but some birds have spurs at the edge of their wings.
Not many mammals have spurs. The male platypus has spurs on the heels of its hind feet. The spurs are hollow and connected to a venom gland. This helps the platypus kick painfully. Echidnas have spurs as well, but they are not venomous. Fossils of some early mammals are found to have spurs also.
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