Claw

A claw sheath from a cat

All carnivorans have claws, which vary considerably in length and shape. Claws grow out of the third phalanges of the paws and are made of keratin. Many predatory mammals have protractile claws that can partially hide inside the animal's paw, especially the cat family, Felidae, almost all of whose members have fully protractible claws. Outside of the cat family, retractable claws are found only in certain species of the Viverridae (and the extinct Nimravidae).[2] A claw that is retractable is protected from wear and tear.

Most cats and dogs also have a dewclaw on the inside of the front paws. It is much less functional than the other claws but does help the cats to grasp prey. Because the dew claw does not touch the ground, it receives less wear and tends to be sharper and longer.

A nail is homologous to a claw but is flatter and has a curved edge instead of a point. A nail that is big enough to bear weight is called a "hoof". (Nevertheless, one side of the cloven-hoof of artiodactyl ungulates may also be called a claw).

Every so often, the growth of claws stops and restarts, as does hair. In a hair, this results in the hair falling out and being replaced by a new one. In claws, this results in an abscission layer, and the old segment breaks off. This process takes several months for human thumbnails. Cats are often seen working old unguis layers off on wood or on boards made for the purpose. Ungulates' hooves wear or self-trim by ground contact. Domesticated equids (horses, donkeys and mules) usually need regular trimming by a farrier, as a consequence of reduced activity on hard ground.

Primate nails consist of the unguis alone, as the subunguis has disappeared. With the evolution of grasping hands and feet, claws are no longer necessary for locomotion, and instead most digits exhibit nails. However, claw-like nails are found in small-bodied callitrichids on all digits except the hallux or big toe. A laterally flattened grooming claw, used for grooming, can be found on the second toe in living strepsirrhines, and the second and third in tarsiers. Aye-ayes have functional claws on all other digits except the hallux, including a grooming claw on the second toe.[3] Less commonly known, a grooming claw is also found on the second pedal digit of night monkeys (Aotus), titis (Callicebus), and possibly other New World monkeys.[4]

At just under a meter, the claws of Therizinosaurus are among the largest recorded.

Using its claws for anchoring, a green lizard basks.

Most reptiles have well-developed claws. Most lizards have toes ending in stout claws.[5] In snakes, feet and claws are absent, but in many boids such as Boa constrictor, remnants of highly reduced hind-limbs emerge with a single claw as "spurs" on each side of the anal opening.

Lizard claws are used as aids in climbing, and in holding down prey in carnivorous species.

A talon is the claw of a bird of prey, its primary hunting tool.[6] The talons are very important; without them, most birds of prey would not be able to catch their food. Some birds also use claws for defensive purposes. Cassowaries use claws on their inner toe (digit II) for defence and have been known to disembowel people.[7][8] All birds, however, have claws, which are used as general holdfasts and protection for the tip of the digits.

The hoatzin and turaco are unique among extant birds in having functional claws on the thumb and index finger (digits I and II) on the forelimbs as chicks, allowing them to climb trees until the adult plumage with flight feathers develop.[9][10] However, several birds have a claw- or nail-like structure hidden under the feathers at the end of the hand digits, notably ostriches, emus, ducks, geese and kiwis.[11]

The only amphibians to bear claws are the African clawed frogs. Claws evolved separately in the amphibian and amniote (reptiliomorph) line.[12]