A dewclaw is a digit – vestigial in some animals – on the foot of many mammals, birds, and reptiles. It commonly grows higher on the leg than the rest of the foot, such that in digitigrade or unguligrade species, it does not make contact with the ground when the animal is standing. The name refers to the dewclaw's alleged tendency to brush dew away from grass. On dogs and cats, the dewclaws are on the inside of the front legs, similarly to a human's thumb, which shares evolutionary homology. Although many animals have dewclaws, other similar species do not, such as horses, giraffes and the African wild dog.
Paw of a dog: A. Claw, B. Digital pads, C. Metacarpal pad, D. Dewclaw, E. Carpal pad
A dog's dewclaw does not make contact with the ground while the dog is standing. This older dog's dewclaw is rounded from use while running, but it has grown.
Some active dogs' dewclaws make more frequent contact with the ground while running, so they wear down naturally, as do their other claws.
Double dewclaws on rear leg of dog
Vestigiality is the retention, during the process of evolution, of genetically determined structures or attributes that have lost some or all of the ancestral function in a given species. Assessment of the vestigiality must generally rely on comparison with homologous features in related species. The emergence of vestigiality occurs by normal evolutionary processes, typically by loss of function of a feature that is no longer subject to positive selection pressures when it loses its value in a changing environment. The feature may be selected against more urgently when its function becomes definitively harmful, but if the lack of the feature provides no advantage, and its presence provides no disadvantage, the feature may not be phased out by natural selection and persist across species.
The Darwin-tubercle (left) is a vestigial form of the ear tip (right) in the mammalian ancestors of humans—here shown in a crab-eating macaque.
Vestigial hindlegs (spurs) in a boa constrictor
The blind mole rat (Spalax typhlus) has tiny eyes completely covered by a layer of skin.
The uniform of the German Feldgendarmerie during WW2, complete with gorget.