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A dewclaw is a digit
vestigial Vestigiality is the retention, during the process of evolution, of genetically determined structures or attributes that have lost some or all of the ancestral function in a given species. Assessment of the vestigiality must generally rely on co ...
in some animals – on the
foot The foot ( : feet) is an anatomical structure found in many vertebrates. It is the terminal portion of a limb which bears weight and allows locomotion. In many animals with feet, the foot is a separate organ at the terminal part of the leg mad ...
of many
mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur ...
s,
bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...
s, and
reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates ( lizards and snakes) and rhynchocephalia ...
s (including some extinct orders, like certain
theropod Theropoda (; ), whose members are known as theropods, is a dinosaur clade that is characterized by hollow bones and three toes and claws on each limb. Theropods are generally classed as a group of saurischian dinosaurs. They were ancestrally c ...
s). It commonly grows higher on the leg than the rest of the foot, such that in
digitigrade In terrestrial vertebrates, digitigrade () locomotion is walking or running on the toes (from the Latin ''digitus'', 'finger', and ''gradior'', 'walk'). A digitigrade animal is one that stands or walks with its toes (metatarsals) touching the groun ...
or unguligrade species it does not make contact with the ground when the animal is standing. The name refers to the dewclaw's alleged tendency to brush dew away from the grass.Danziger, D., & McCrum, M. (2008). ''The Thingummy: A book about those everyday objects you just can't name''. London: Doubleday. On dogs and cats the dewclaws are on the inside of the front legs, similarly to a human's
thumb The thumb is the first digit of the hand, next to the index finger. When a person is standing in the medical anatomical position (where the palm is facing to the front), the thumb is the outermost digit. The Medical Latin English noun for thumb ...
, which shares evolutionary homology. Although many animals have dewclaws, other similar species do not, such as
horse The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 million yea ...
s,
giraffe The giraffe is a large African hoofed mammal belonging to the genus ''Giraffa''. It is the tallest living terrestrial animal and the largest ruminant on Earth. Traditionally, giraffes were thought to be one species, '' Giraffa camelopardal ...
s and the
African wild dog The African wild dog (''Lycaon pictus''), also called the painted dog or Cape hunting dog, is a wild canine which is a native species to sub-Saharan Africa. It is the largest wild canine in Africa, and the only extant member of the genus '' Lyca ...
.


Dogs

Dogs almost always have dewclaws on the inside of the front legs and occasionally also on the hind legs. Unlike front dewclaws, rear dewclaws tend to have little bone or muscle structure in most breeds. It is normal, although not biologically necessary, that certain breeds will have more than one dewclaw on the same paw. At least one of these dewclaws will be poorly connected to the leg, and in this case it is often surgically removed. When a dog has extra dewclaws in addition to the usual one on each front leg, the dog is said to be double dewclawed. For certain dog breeds, a dewclaw is considered a necessity, e.g. a Beauceron for sheep herding and for navigating snowy terrain. As such, there is some debate about whether a dewclaw helps dogs to gain traction when they run because in some dogs, the dewclaw makes contact when they are running and the nail on the dewclaw often wears down in the same way that the nails on their other toes do, from contact with the ground. In many dogs, the dewclaws never contact the ground. In this case, the dewclaw's nail never wears away, and it is often trimmed to maintain it at a safe length. The dewclaws are not dead appendages. They can be used to lightly grip bones and other items that dogs hold with the paws. In some dogs, these claws may not appear to be connected to the leg at all except by a flap of skin; in such dogs, the claws do not have a use for gripping as the claw can easily fold or turn.


Rear dewclaws

Canids have four claws on the rear feet, although some domestic dog breeds or individuals have an additional claw, or more rarely two, as is the case with the beauceron. A more technical term for these additional digits on the rear legs is ''hind-limb-specific preaxial
polydactyly Polydactyly or polydactylism (), also known as hyperdactyly, is an anomaly in humans and animals resulting in supernumerary fingers and/or toes. Polydactyly is the opposite of oligodactyly (fewer fingers or toes). Signs and symptoms In hum ...
''. Several genetic mechanisms can cause rear dewclaws; they involve the LMBR1 gene and related parts of the genome. Rear dewclaws often have no phalanx bones and are attached by skin only.


Dewclaw removal

There is some debate as to whether dewclaws should be surgically removed. The argument for removal states that dewclaws are a weak digit, barely attached to the leg, and thus they can rip partway off or easily catch on something and break, which can be painful and prone to infection. Dewclaw removal is most easily performed when the dog is young, around 2–5 days of age. It can also be performed on older dogs if necessary though the surgery may be more difficult then. The surgery is fairly straightforward and may be done with local anesthetics if the digit is not well connected to the leg. Many dogs cannot resist licking the surgery site in the weeks following the procedure, so an Elizabethan collar or bitterant may be used to curtail this behavior, thus preventing infection. Some pups are commonly sold by breeders "dew-clawed", that is with the dewclaws removed (as by a
veterinarian A veterinarian (vet), also known as a veterinary surgeon or veterinary physician, is a medical professional who practices veterinary medicine. They manage a wide range of health conditions and injuries in non-human animals. Along with this, vet ...
) for perceived health and safety reasons. A few breed standards, such as that for Portuguese Water Dogs, also call for it.


Dewclaws and locomotion

Based on stop-action photographs, veterinarian M. Christine Zink of Johns Hopkins University believes that the entire front foot, including the dewclaws, contacts the ground while running. During running, the dewclaw digs into the ground preventing twisting or torque on the rest of the leg. Several tendons connect the front dewclaw to muscles in the lower leg, further demonstrating the front dewclaws' functionality. There are indications that dogs without dewclaws have more foot injuries and are more prone to arthritis. Zink recommends "for working dogs it is best for the dewclaws not to be amputated. If the dewclaw does suffer a traumatic injury, the problem can be dealt with at that time, including amputation if needed."


Cats

Members of the cat family – including domestic cats and wild cats like the
lion The lion (''Panthera leo'') is a large cat of the genus '' Panthera'' native to Africa and India. It has a muscular, broad-chested body; short, rounded head; round ears; and a hairy tuft at the end of its tail. It is sexually dimorphic; adu ...
– have dewclaws. Generally a dewclaw grows on the inside of each front leg but not on either hind leg. The dewclaw on cats is not vestigial. Wild felids use the dewclaw in hunting, where it provides an additional claw with which to catch and hold prey.


Hoofed animals

Hoofed animals walk on the tips of special toes, the
hoof The hoof (plural: hooves) is the tip of a toe of an ungulate mammal, which is covered and strengthened with a thick and horny keratin covering. Artiodactyls are even-toed ungulates, species whose feet have an even number of digits, yet the rum ...
s.
Cloven-hoof A cloven hoof, cleft hoof, divided hoof or split hoof is a hoof split into two toes. This is found on members of the mammalian order Artiodactyla. Examples of mammals that possess this type of hoof are cattle, deer, pigs, antelopes, gazelles ...
ed animals walk on a central pair of hoofs, but many also have an outer pair of dewclaws on each foot. These are somewhat farther up the leg than the main hoofs, and similar in structure to them. In some species (such as
cattle Cattle (''Bos taurus'') are large, domesticated, cloven-hooved, herbivores. They are a prominent modern member of the subfamily Bovinae and the most widespread species of the genus '' Bos''. Adult females are referred to as cows and adult ...
) the dewclaws are much smaller than the hoofs and never touch the ground. In others (such as pigs and many
deer Deer or true deer are hoofed ruminant mammals forming the family Cervidae. The two main groups of deer are the Cervinae, including the muntjac, the elk (wapiti), the red deer, and the fallow deer; and the Capreolinae, including the re ...
), they are only a little smaller than the hoofs, and may reach the ground in soft conditions or when jumping. Some hoofed animals (such as
giraffe The giraffe is a large African hoofed mammal belonging to the genus ''Giraffa''. It is the tallest living terrestrial animal and the largest ruminant on Earth. Traditionally, giraffes were thought to be one species, '' Giraffa camelopardal ...
s and modern
horse The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 million yea ...
s) have no dewclaws. Video evidence suggests some animals use dewclaws in grooming or scratching themselves or to have better grasp during mating.


References


External links

{{commons category, Dewclaws
Another look at Dewclaws
Vertebrate anatomy Dog anatomy Limbs (anatomy) Felidae anatomy