The distal ends of mammal digits possess keratinized sheaths or plates that are epidermal derivatives forming claws, nails, or hooves. Only the members of the whale and sirenia (seacows) families lack these structures. Claws are usually sharp, curved, and pointed. In many cases mammal claws are very similar to the claws found in other vertebrates. A claw consists of a dorsal plate called the unguis and a ventral plate called the subunguis. The unguis is curved both in length and width and encloses the subunguis, which is connected to the digital pad at the distal end of the digit. In addition to protection, claws assist predator species, such as lions and tigers, in holding their prey. They provide traction for some arboreal species (e.g., squirrels) when scampering on branches. Sloths have long curved claws that serve as hooks for hanging. Digging mammals, such as anteaters and moles, have long claws that help them dig.
Nails are modified claws found on the first digit of some arboreal mammals and on all the digits of some primates. Nails cover only the dorsal part of digits. The unguis (called a nail plate in human anatomy) is broad and flat, and the sub-unguis is vestigial. It has been suggested that nails evolved in primates to prevent rolling and provide flat support for the large pad of tactile sensory tissue found on the underside of the digit. Thus nails allow both increased tactile perception and enhanced manipulative abilities. The Callitrichidae (small monkeys found in South and Central America) have secondarily evolved claws, which are not true claws because they are derived from the laterally compressed nails of their ancestors. Nails and claws may be found on the same mammal (e.g., hyraxes).
Hooves are constructed of a prominent unguis that curves around the digit and encloses the subunguis. The well-developed unguis has lent its name to the group that have hooves: the ungulates (although this is not a true taxonomic group). For ungulates, the unguis is much harder than the subunguis and does not wear away as quickly, thus developing a sharp edge.
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