Claws, nails, and hooves

Claws, nails, and hooves are very often special growths of the outer skin (epidermis) of warm-blooded vertebrate animals. Nails are considered claws whenever they are thicker, longer, harder, and sharper than those of humans. Nails and claws contain large amounts of keratin, the hard substance present in the skin and nails of human fingers and toes. The skin below a nail or claw, from which it grows, is called the matrix. If a nail, claw, or hoof part is torn off, it grows again if the matrix has not been overly damaged. Nails and claws occur on the digits of forelimbs and hind limbs of vertebrates.

Nails and claws are made of dead skin cells, very rich in the fibrous substance keratin. Each nail or claw consists of a root, concealed within a fold of skin at its base; a body, the exposed part attached to the skin surface; and an anterior edge overlapping the end of the digit to which it is attached. In clawed animals such as badgers, moles, rodents, wolves, and cats this edge can become quite long. The matrix, the skin below the root and body of a nail or claw, is thick and covered with highly vascular papillae. The matrix color is sometimes seen through transparent horny tissue, in humans and in some other species with relatively thin nail bodies or claws. A nail or claw grows forward by a combination of continual growth of new cells both at the root and under its body.

The body of the nail or claw in the predatory and burrowing animals is much thicker than in humans. Unlike human nails, the claws of burrowing animals do not have to be trimmed; they are worn down by use. The claws of cats, from house cats to lions and tigers, are retractable. This allows these animals to walk easily without wearing their claws down and to unsheathe them for capturing and killing prey. Predatory animals, especially cats with retractable claws, must hone them down occasionally. Cats use scratching posts—whether fallen tree limbs found in the wild or specially made carpet-covered posts provided by hopeful pet owners—for this purpose, as well as to sharpen their claws.

Ungulate Hooves

A hoof is a thick, hard growth at the front of the foot of a horse or other mammal in the order Ungulata (the hoofed mammals). The ungulates include elephants, pigs, horses, and many horned animals, such as deer and cattle. A hoof, really a “hoof-nail,” is also made of keratin, and it arises from the outer skin layer, or epidermis. A hoof differs physically from a nail or claw in that it is blunt and encases a toe or foot. Hooves are thought to have developed from animal toes, and ungulates are divided into those with even and odd numbers of toes. For example, a horse hoof developed from one toe. It almost completely circles the bottom of the foot. Zebras and donkeys are also animals whose hooves are single toes, not divided (or cleft). Even-toed ungulates have two or four toes. Usually, the two middle toes are developed into a cleft (divided) hoof. This vertebrate group includes deer, antelope, sheep, goats, cattle, pigs, and hippopotamuses. Hooves adhere to the earth well and give animals a firm footing for pulling, running, walking on slippery or icy ground, and climbing mountains.

Lizard and Bird Claws

Lizards, reptiles in the order Squamata, which also includes snakes and crocodiles, are the largest extant group of reptiles, with over twelve thousand species. All have dry, scaly skin, clawed feet, and external ear openings. In many cases, lizard claws are used for traction on slick surfaces or in climbing trees. Large carnivorous and omnivorous species also use them to pick or tear apart food. Research indicates that their claws have evolved to fit the lizard’s environment. Lizards that climb often have short, curved claws, while those that live underground have long, thin claws.

Most land birds have clawed feet, which help them to perch in trees. The claws are thought to have derived partly from the evolution of birds from ancient lizards. The most extensive claws seen in the birds are the talons of birds of prey (the raptors). Raptors include the night-hunting owls and the day-hunting hawks, eagles, falcons, and vultures. All are meat eaters, although the meat devoured by the small species is insects. All the raptors have powerful bills, and all but vultures have grasping toes tipped with large, curved, sharp talons. It is believed that the weak talons of vultures are part of the reason why they eat carrion, not live prey. That is, they cannot strike live prey like the other raptors or carry it off in strong claws.

Invertebrate Claws

Among invertebrates, insects, scorpions, and crustaceans are examples of organisms that possess claws. Examples are praying mantises and beetles, among the insects; scorpions, among the arthropods; and lobsters and crabs, among the crustaceans. In all cases, the claws are restricted to the first two legs of these invertebrates. Also, unlike those of vertebrates, the claws are not keratin-containing epidermis outgrowths. They are portions of the chitinous invertebrate exoskeletons and, therefore, part of their shells.

Crustacean claws are huge compared to animal claws. Most often, both claws are of similar size and used to capture prey. However, some crustaceans, such as hermit and fiddler crabs, have claws where one is much larger than the other. In those cases, the larger of the claws is usually used in protection or mating.

Lobsters have two huge, body-length claws (also called pincers or chelae) used to grasp prey and to dig burrows in which they live. In “true lobster” species, the claws are especially huge and dexterous. One claw is heavier than the other and has blunt teeth to crush prey. The other is somewhat smaller and has sharp teeth to tear up prey. Not all lobsters have the heavy claw on the same sidethey may be right-clawed, left-clawed, or ambidextrous. During the day, a lobster stays in its burrow awaiting prey. At night, it comes out to seek food, catching it and tearing it into eating-sized chunks with its claws. Their feet and claws have receptors that allow them to smell their food. Lobsters grow by shedding their shells (molting). When they do this, their claws are soft. They cannot use them well for hunting until the new shells harden. At those times, they mostly eat carrion.

Crabs, related to lobsters, are also clawed decapods. Like other cephalopods, the crab body is sheathed in tough chitin. A crab has five pairs of walking legs and two front legs that hold claws for feeding and defense. Male hermit and fiddler crabs have one claw that is huge compared to the other. The oversized claw in hermit crabs is used defensively to close off the mollusk shell it uses as its abode to protect its soft, vulnerable body. Fiddler crabs also have one oversized huge claw; they use it in mating and combat. Some research indicates that, depending on the species, these crustaceans can snap their claws closed at 100 kilometers (62 miles) per hour.

Claws, nails, and hooves are very important to animals. They are weapons used for protection and to secure food, appendages that impress potential mates, and locomotor appendages that assure safe, optimized movement around habitats. It is interesting that in organisms as far apart, evolutionarily, as lobsters and ungulates, claws and hooves are made of similar, tough, insoluble materials, chitin, and keratin, respectively.

Principal Terms

Chitin: A transparent, horny substance of invertebrate exoskeletons

Crustaceans: Lobsters, shrimps, crabs, and barnacles

Epidermis: The outer, protective layer of the skins of vertebrates

Keratin: A tough fibrous substance found in hair, claws, nails, and hooves

Matrix: The skin beneath a nail, involved in its growth

Ungulate: A hoofed mammal

Vascular Papilla: A protuberance having a blood supply

Bibliography

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