Claw

A claw is a curved and pointed appendage found at the end of a digit.

This structure is common in most amniotes, which include mammals, reptiles, and birds. While many animals use them for survival, some invertebrates like spiders or beetles possess similar hooked structures for gripping surfaces. In the world of crustaceans, animals like crabs and lobsters have pincers known formally as chelae. A true claw is composed of a hard protein called keratin. These tools are essential for various tasks, such as catching prey, digging, climbing, or self-defense.

In tetrapods, claws are built from two distinct layers of keratin. The outer layer is called the unguis, which is the hardest part of the structure. The fibers in the unguis are arranged in layers at an oblique angle or perpendicular to growth. Beneath this lies the subunguis, a softer and more flaky layer. The grain of the subunguis runs parallel to the direction of growth. The claw begins growing from the nail matrix at the base of the unguis. As the subunguis travels across the nail bed, it grows thicker. Because the unguis grows outward faster than the subunguis, the claw develops a natural curve. Furthermore, the thinner sides of the claw wear away more quickly than the thicker middle. This specific wearing pattern results in a sharp point.

Mammals exhibit a wide variety of claw shapes and lengths. Many predatory mammals, specifically those in the family Felidae, possess protractile claws. These claws can be partially or fully hidden inside the paw to protect them from wear and tear. Aside from cats, retractable claws are found in certain Viverridae species. Most cats and dogs also possess a dewclaw on the inside of their front paws. Although it is less functional than other claws, it helps with grasping prey. Because the dewclaw rarely touches the ground, it often remains longer and sharper.

Claw growth is not always a continuous process. Occasionally, the growth of a claw will stop and then restart. In humans, this happens to our thumbnails over several months. When this occurs, an abscission layer forms, and the old segment of the claw breaks off. This is similar to how hair falls out and is replaced by a new strand. Some animals, like cats, may work old unguis layers off by scratching against wood. In some large animals, like horses or donkeys, the claws or hooves wear down through contact with the ground. These domesticated animals often require a farrier to perform regular trimming.

Primates show a different evolutionary path regarding these structures. Most primates have nails instead of claws. A nail is homologous to a claw, but it is flatter and has a curved edge rather than a point. In primates, the subunguis has disappeared, leaving only the unguis. This change occurred as primates evolved hands and feet designed for grasping. However, some small primates called callitrichids still have claw-like nails on most digits. Certain species, such as aye-ayes, possess functional claws on most digits except the big toe. Some primates even have specialized grooming claws on specific toes.

Reptiles and birds also rely heavily on specialized claw structures. Most lizards have stout claws used for climbing or holding prey. Some snakes, like the Boa constrictor, have reduced hind-limbs that appear as spurs near the anal opening. Birds use claws in many ways, but birds of prey use them as primary hunting tools. These specialized claws are called talons.

Without talons, most birds of prey could not catch food. Some birds, like the cassowary, use claws on their inner toes for defense. Unique birds like the hoatzin have functional claws on their wing digits when they are chicks to help them climb trees.

Claws appear in unexpected places across the animal kingdom. The African clawed frog is the only amphibian known to bear claws. Interestingly, claws also evolved separately in the amphibian line from the amniote line. The hairy frog has claw analogues, though it must intentionally dislocate its finger tips to use them. This shows how different biological paths can lead to similar tools. Whether they are used for hunting, climbing, or grooming, claws remain one of nature's most effective specialized tools.