Spur (zoology)

A spur is an outgrowth of bone covered in a sheath of horn found in various anatomical locations in some animals. Unlike claws or nails, which grow from the tip of the toes, spurs form from other parts of the foot, usually in connection with joints where the toes meet the foot or the foot meets the long bones. Spurs are most commonly found on the hindfeet, though some birds possess spurs at the leading edge of the wings.

A spur is much like a true horn; it is a bony core attached to the skeleton and has an outer horny layer. Like horns, the spur grows from the base outwards, so the tip is older than the base. Some spurs form as an outgrowth of an existing bone, though most are secondarily formed as dermal bone hinged to the skeleton through a semi-rigid joint. Spurs on the hind-feet do not appear to molt, but the wing spurs of birds are molted once a year along with the wing feathers.

Unlike claws, spurs are normally straight or only slightly curved, making them suited to striking or stabbing. In birds and mammals, their function appears to be for fighting, defense and territory marking, rather than for predation. In reptiles, spurs are usually only found in the males and are used as holdfasts or to stimulate the female during copulation.

The term spur is sometimes used to describe the pelvic spur, vestigal limbs found in primitive snakes, such as boas and pythons and in the striped legless lizard. The spurs primarily serve as holdfasts during mating. As these form at the terminal end of the limb, they may properly be claws rather than true spurs.

The Boinae (a sub-family of boas) possess pelvic or anal spurs on each side of their cloaca. These spurs have a high degree of mobility and can be moved by appropriate musculature from their normal position lying along and against the body, into a perpendicular plane to resemble miniature legs. The pelvic spurs are usually held in the "erect" position and used by the male (at least) to stimulate the female. During mating, the male has a tendency to dig his pelvic spurs into the female's body. Whilst crawling over the female the male's spurs will scratch the surface of the female snake often making a distinct scratching sound. On other occasions the male's spurs will move rapidly in and out, up and down, again to stimulate the female snake. This supports the theory that the pelvic spurs have a sexual role. In at least one species, the Madagascan Boa (Sanzinia madagascariensis), the spurs are also used in combat where they are erected perpendicular to the body and flexed vigorously against the scales of the opponent. It has been claimed that pelvic spurs are retained parts of the pelvic girdle and are small, vestigial hind legs. However, it has also been claimed the spurs are modified scales and do not appear to be linked to any bone structure.

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