A domestic
cat's retractable claw in protracted position
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.
[1]
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.
Arthropods
Main article:
Chela (organ)
The correct term for an arthropod's "claw" is a
chela (plural
chelae). Legs bearing a chela are called
chelipeds. Chelae are also called
pincers.
Tetrapods
At just under a meter, the claws of
Therizinosaurus are among the largest recorded
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.
Amphibians
The only
amphibians to bear claws are the
African clawed frogs. Claws appear to have evolved separately in the amphibian and
amniote line.
[2]
Reptiles
Lizards, turtles and crocodylians
Most lizards have toes ending in stout claws. The claws form from the last scale on the toe.
[3] 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.
Birds
A
talon is the claw of a
bird of prey, its primary hunting tool.
[4]
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. 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.
[5]
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.
[6]
Mammals
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.
Many
predatory mammals have
protractile claws that can partially hide inside the animal's
paw, especially the
Felidae, where almost all of its members have fully protractible claws.
Primates
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.
[7]
See also
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