Dewclaws
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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 an organ at the terminal part of the leg made up o ...
of many
mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...
s,
bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...
s, and
reptile Reptiles, as commonly defined, are a group of tetrapods with an ectothermic metabolism and Amniotic egg, amniotic development. Living traditional reptiles comprise four Order (biology), orders: Testudines, Crocodilia, Squamata, and Rhynchocepha ...
s (including some extinct orders, like certain
theropod Theropoda (; from ancient Greek , (''therion'') "wild beast"; , (''pous, podos'') "foot"">wiktionary:ποδός"> (''pous, podos'') "foot" is one of the three major groups (clades) of dinosaurs, alongside Ornithischia and Sauropodom ...
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 (phalanges) on the ground, and ...
or
unguligrade Ungulates ( ) are members of the diverse clade Euungulata ("true ungulates"), which primarily consists of large mammals with Hoof, hooves. Once part of the clade "Ungulata" along with the clade Paenungulata, "Ungulata" has since been determined ...
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 Dew is water in the form of droplets that appears on thin, exposed objects in the morning or evening due to condensation. As the exposed surface cools by thermal radiation, radiating its heat, atmospheric moisture condenses at a rate grea ...
away from 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 mi ...
s,
giraffe The giraffe is a large Fauna of Africa, African even-toed ungulate, hoofed mammal belonging to the genus ''Giraffa.'' It is the Largest mammals#Even-toed Ungulates (Artiodactyla), tallest living terrestrial animal and the largest ruminant on ...
s and the
African wild dog The African wild dog (''Lycaon pictus''), also called painted dog and Cape hunting dog, is a wild canine native to sub-Saharan Africa. It is the largest wild canine in Africa, and the only extant member of the genus '' Lycaon'', which is disti ...
.


Dogs

Dog The dog (''Canis familiaris'' or ''Canis lupus familiaris'') is a domesticated descendant of the gray wolf. Also called the domestic dog, it was selectively bred from a population of wolves during the Late Pleistocene by hunter-gatherers. ...
s 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. One of these dewclaws will most likely be poorly connected to the leg, and 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 The Beauceron () is a herding dog breed originating from the plains of Central France. The Beauceron is also known as Berger de Beauce (, sheepdog from Beauce) or Bas Rouge (, red-stockings). Description Appearance This breed stands in height ...
for sheep herding and for navigating snowy terrain. As such, there is some debate about whether a dewclaw helps dogs gain traction when they run because, in some dogs, the dewclaw makes contact when they are running and the
nail Nail or Nails may refer to: In biology * Nail (anatomy), toughened protective protein-keratin (known as alpha-keratin, also found in hair) at the end of an animal or human digit, such as fingernail * Nail (beak), a plate of hard horny tissue a ...
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

Canid Canidae (; from Latin, ''canis'', "dog") is a family (biology), biological family of caniform carnivorans, constituting a clade. A member of this family is also called a canid (). The family includes three subfamily, subfamilies: the Caninae, a ...
s 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 The Beauceron () is a herding dog breed originating from the plains of Central France. The Beauceron is also known as Berger de Beauce (, sheepdog from Beauce) or Bas Rouge (, red-stockings). Description Appearance This breed stands in height ...
. A more technical term for these additional digits on the rear legs is ''hind-limb-specific preaxial
polydactyly Polydactyly is a birth defect that results in extra fingers or toes. The hands are more commonly involved than the feet. Extra fingers may be painful, affect self-esteem, or result in clumsiness. It is associated with at least 39 genetic mut ...
''. Several genetic mechanisms can cause rear dewclaws; they involve the
LMBR1 Limb region 1 protein homolog is a protein that in humans is encoded by the ''LMBR1'' gene. This gene encodes a member of the LMBR1-like membrane protein family. Another member of this protein family has been shown to be a lipocalin transmembran ...
gene and related parts of the genome. Rear dewclaws often have no
phalanx bones The phalanges (: phalanx ) are digital bones in the hands and feet of most vertebrates. In primates, the thumbs and big toes have two phalanges while the other digits have three phalanges. The phalanges are classed as long bones. Struct ...
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 An Elizabethan collar, E collar, buster collar, pet ruff or pet cone (sometimes humorously called a treat funnel, lamp-shade, radar dish, dog-saver, collar cone, or cone of shame) is a protective medical device worn by an animal, usually a cat ...
or
bitterant A bitterant (or bittering agent) is a chemical that is added to a product to make it olfaction, smell or taste Taste#Bitterness, bitter. Bitterants are commonly used as aversive agents to discourage the inhalation or ingestion of toxic substances ...
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) or veterinary surgeon 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, veterinarians also play a role in animal r ...
) 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 Felidae, cat of the genus ''Panthera'', native to Sub-Saharan Africa and India. It has a muscular, broad-chested body (biology), body; a short, rounded head; round ears; and a dark, hairy tuft at the ...
– 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 Felidae ( ) is the family of mammals in the order Carnivora colloquially referred to as cats. A member of this family is also called a felid ( ). The 41 extant Felidae species exhibit the greatest diversity in fur patterns of all terrestria ...
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
hooves The hoof (: 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; the ruminants with ...
.
Cloven-hoof A cloven hoof, cleft hoof, divided hoof, or split hoof is a hoof split into two toes. Members of the mammalian order Artiodactyla that possess this type of hoof include cattle, deer, pigs, antelopes, gazelles, goats, and sheep. The two digits ...
ed animals walk on a central pair of hooves, but many also have an outer pair of dewclaws on each foot. These are somewhat farther up the leg than the main hooves, and similar in structure to them. In some species (such as
cattle Cattle (''Bos taurus'') are large, domesticated, bovid ungulates widely kept as livestock. They are prominent modern members of the subfamily Bovinae and the most widespread species of the genus '' Bos''. Mature female cattle are calle ...
) the dewclaws are much smaller than the hooves and never touch the ground. In others (such as
pig The pig (''Sus domesticus''), also called swine (: swine) or hog, is an omnivorous, domesticated, even-toed, hoofed mammal. It is named the domestic pig when distinguishing it from other members of the genus '' Sus''. Some authorities cons ...
s and many
deer A deer (: deer) or true deer is a hoofed ruminant ungulate of the family Cervidae (informally the deer family). Cervidae is divided into subfamilies Cervinae (which includes, among others, muntjac, elk (wapiti), red deer, and fallow deer) ...
), they are only a little smaller than the hooves, and may reach the ground in soft conditions or when jumping. Some hoofed animals (such as
giraffe The giraffe is a large Fauna of Africa, African even-toed ungulate, hoofed mammal belonging to the genus ''Giraffa.'' It is the Largest mammals#Even-toed Ungulates (Artiodactyla), tallest living terrestrial animal and the largest ruminant on ...
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 mi ...
s) have no dewclaws. Video evidence suggests some animals use dewclaws in grooming or scratching themselves or to have better grasp during mating.


References

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