Dewclaw

A dewclaw is a digit – vestigial in some animals – on the foot of many mammals, birds, and reptiles (including some extinct orders, like certain theropods). It commonly grows high on the leg such that in digitigrade species, when the animal is standing, it does not make contact with the ground. On dogs a dewclaw is on the inside of the leg, positioned analogously to a human thumb.

Dogs almost always have dewclaws on the inside of the front legs and occasionally also on the hind legs. Unlike front dewclaws, rear dewclaws tend to have little bone or muscle structure in most breeds. It is normal, although not biologically necessary, that certain breeds will have more than one dewclaw on the same paw. At least one of these dewclaws will be poorly connected to the leg, and in this case it is often surgically removed. When a dog has extra dewclaws in addition to the usual one on each front leg, the dog is said to be double dewclawed. There is some debate about whether a dewclaw helps dogs to gain traction when they run because in some dogs, the dewclaw makes contact when they are running and the nail on the dewclaw often wears down in the same way that the nails on their other toes do, from contact with the ground. In many dogs the dewclaws never contact the ground. In this case, the dewclaw's nail never wears away, and it is often trimmed to maintain it at a safe length.

The dewclaws are not dead appendages. They can be used to lightly grip bones and other items that dogs hold with the paws. In some dogs these claws may not appear to be connected to the leg at all except by a flap of skin; in such dogs the claws do not have a use for gripping as the claw can easily fold or turn.

Canids have four claws on the rear feet, although some domestic dog breeds or individuals have an additional claw. A more technical term for this fifth claw on the rear legs is hind-limb-specific preaxial polydactyly. Several genetic mechanisms can cause rear dewclaws; they involve the LMBR1 gene and related parts of the genome. Rear dewclaws often have no phalanx bones and are attached by skin only.

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Wikipedia