A claw is a curved, pointed appendage, found at the end of a toe or finger in most amniotes (mammals and reptiles, including birds). However, the word "claw" is also often used in reference to an invertebrate. Somewhat similar fine hooked structures are found in arthropods such as beetles and spiders, at the end of the leg or tarsus for gripping a surface as the creature walks. Crabs', lobsters' and scorpions' pincers, or more formally, their "chelae", are sometimes called claws.
A claw is made of hard protein called keratin. Claws are used to catch and hold prey in carnivorous mammals such as cats and dogs, but may also be used for such purposes as digging, climbing trees, self-defense etc., in those and other species.
Similar appendages that are flat and do not come to a sharp point are called nails instead. Claw-like projections that do not form at the end of digits, but spring from other parts of the foot are properly named spurs.
Claws of animals like tigers, lions, and bears were used in making items such as ornaments, pendants, and brooches. Tigers' and lions' claws are illegal to trade.
In tetrapods, claws are made of keratin and consist of two layers. The unguis is the harder external layer, which consists of keratin fibers arranged perpendicular to the direction of growth and in layers at an oblique angle. The subunguis is the softer, flaky underside layer whose grain is parallel to the direction of growth. The claw grows outward from the nail matrix at the base of the unguis and the subunguis grows thicker while travelling across the nail bed. The unguis grows outward faster than the subunguis to produce a curve and the thinner sides of the claw wear away faster than their thicker middle, producing a more or less sharp point. Tetrapods use their claws in many ways, commonly to grasp or kill prey, to dig and to climb and hang.
Lizards, turtles and crocodylians 
Most lizards have toes ending in stout claws. The claws form from the last scale on the toe. Most reptiles have well-developed claws. 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. 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 seriously injure people. All birds however have claws, which are used as general holdfasts and protection for the tip of the digits.
The hoatzin is unique among extant birds in having functional claws on the thumb and index finger (digit I and II) on the forelimbs as chicks, allowing them to climb trees until the adult plumage with flight feathers develop. However, several birds have a claw- or nail-like structure hidden under the feathers at the end of the hand digits, notably ducks, geese and kiwis.
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' (see also Horse hoof. However, 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 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, and in prosimians, which possess one or two laterally flattened toilet claws, used for grooming. These can be found on the second toe in lemurs and lorises, and the second and third in tarsiers. Aye-ayes have functional claws on all other digits except the hallux, including a toilet claw on the second toe.
See also 
- Rand, A.L. (1954). "On the Spurs on Birds' Wings". The Willson Bulletin 66 (2): 127–134. Retrieved 16 November 2012.
- Maddin, HC; Eckhart, L; Jaeger, K; Russell, AP; Ghannadan, M (2009). "The anatomy and development of the claws of Xenopus laevis (Lissamphibia: Anura) reveal alternate pathways of structural evolution in the integument of tetrapods". Journal of anatomy 214 (4): 607–19. doi:10.1111/j.1469-7580.2009.01052.x. PMC 2736125. PMID 19422431.
- Alibardi, L. (2008). "Microscopic analysis of lizard claw morphogenesis and hypothesis on its evolution". Acta Zoologica: Morphology and Evolution 89 (2): 169. doi:10.1111/j.1463-6395.2007.00312.x.
- Fowler, D.W., Freedman, E.A., & Scannella, J.B. (2009). "Predatory Functional Morphology in Raptors: Interdigital Variation in Talon Size Is Related to Prey Restraint and Immobilisation Technique". PLoS ONE 4 (11): e7999. doi:10.1371/journal.pone.0007999. PMC 2776979. PMID 19946365.
- Parker, W. K. (1891). "On the Morphology of a Reptilian Bird, Opisthocomus hoazin". Transactions of the Zoological Society of London 13 (2): 43–89. doi:10.1111/j.1096-3642.1891.tb00045.x.
- Sir Walter Lawry Buller (1888): A History of the Birds of New Zealand. London excerpt from Zealand Electronic Text Centre collection
- Soligo, C., Müller, A.E. (1999). "Nails and claws in primate evolution". Journal of Human Evolution 36 (1): 97–114. doi:10.1006/jhev.1998.0263. PMID 9924135.
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