Inbiology,dactyly is the arrangement ofdigits (fingers andtoes) on thehands, feet, or sometimeswings of atetrapodanimal. The term is derived from theGreek wordδακτυλος (dáktylos) meaning "finger."
Sometimes the suffix "-dactylia" is used. The derived adjectives end with "-dactyl" or "-dactylous."
Pentadactyly (fromGreekπέντεpénte "five") is the condition of having five digits on each limb. It is traditionally believed that all livingtetrapods are descended from an ancestor with a pentadactyl limb, although many species have now lost or transformed some or all of their digits by the process ofevolution. However, this viewpoint was challenged byStephen Jay Gould in his 1991 essay "Eight (or Fewer) Little Piggies," where he pointed outpolydactyly in early tetrapods and described the specializations of digit reduction.[1] Despite the individual variations listed below, the relationship is to the original five-digit model.
In reptiles, the limbs are pentadactylous.
Dogs have tetradactylous paws but thedewclaw makes them pentadactyls. Cats also have dewclaws on their front limbs but not their hind limbs, making them both pentadactyls and tetradactyls.
Tetradactyly (from Greekτετραtetra 'four') is the condition of having four digits on a limb, as in manybirds,amphibians, andtheropoddinosaurs.
Tridactyly (from Greekτρίαtría 'three') is the condition of having three digits on a limb, as in therhinoceros and ancestors of the horse such asProtohippus andHipparion. These all belong to thePerissodactyla. Some birds also have three toes, includingemus,bustards, andquail.
Didactyly (from Greekδι-di- 'two') or bidactyly is the condition of having two digits on each limb, as in theHypertragulidae andtwo-toed sloth,Choloepus didactylus. In humans this name is used for an abnormality in which the middle digits are missing, leaving only the thumb and fifth finger, or big and little toes. Cloven-hoofed mammals (such as deer, sheep and cattle –Artiodactyla) have only two digits, as doostriches.
Monodactyly (from Greekμόνοςmonos- 'one') is the condition of having a single digit on a limb, as in modernhorses and otherequidae (though one study suggests that thefrog might be composed of remnants of digits II and IV, rendering horses as not truly monodactyl[2]) as well assthenurine kangaroos. Functional monodactyly, where the weight is supported on only one of multiple toes, can also occur, as in thetheropoddinosaurVespersaurus. The pterosaurNyctosaurus retained only the wing finger on the forelimb, rendering it also partially monodactyl.[3]
Among humans, the term "five-fingered hand" is sometimes used to mean the abnormality of having five fingers, none of which is a thumb.[citation needed]
Syndactyly (from Greekσυν-syn 'together') is a condition where two or more digits are fused together. It occurs normally in somemammals, such as thesiamang and mostdiprotodontidmarsupials such askangaroos. It occurs as an unusual condition in humans.
Polydactyly (from Greekπολυ-poly- 'many') is when a limb has more than the usual number of digits. This can be:
Oligodactyly (from Greekὀλιγο-oligo- 'few') is having too few digits when not caused by anamputation. It is sometimes incorrectly calledhypodactyly or confused withaphalangia, the absence of thephalanx bone on one or (commonly) more digits. Whenall the digits on a hand or foot are absent, it is referred to asadactyly.[4]
Ectrodactyly, also known assplit-hand malformation, is the congenital absence of one or more central digits of the hands and feet. Consequently, it is a form of oligodactyly. News anchorBree Walker is probably the best-known person with this condition, which affects about one in 91,000 people.[citation needed] It is conspicuously more common in theVadoma inZimbabwe.
Clinodactyly is a medical term describing the curvature of a digit (a finger or toe) in the plane of the palm, most commonly the fifth finger (the "little finger") towards the adjacent fourth finger (the "ring finger"). It is a fairly common isolated anomaly which often goes unnoticed, but also occurs in combination with other abnormalities in certain genetic syndromes, such asDown syndrome,Turner syndrome andCornelia de Lange syndrome.
Anisodactyly is the most common arrangement of digits in birds, with three toes forward and one back. This is common insongbirds and otherperching birds, as well as hunting birds such aseagles,hawks, andfalcons. This arrangement of digits helps with perching and/or climbing and clinging.This occurs inPasseriformes,Columbiformes,Falconiformes,Accipitriformes,Galliformes and a majority of other birds.
Syndactyly, as it occurs in birds, is like anisodactyly, except that the third and fourth toes (the outer and middle forward-pointing toes), or three toes, are fused together almost to their claws, as in thebelted kingfisher (Megaceryle alcyon).[5] This is often found inPicocoraciae, thoughrollers,ground rollers, andPiciformes (who are zygodactyl) are exceptions.[6]: 37
Zygodactyly (from Greekζυγος, even-numbered) is an arrangement of digits in birds and chameleons, with two toes facing forward (digits 2 and 3) and two back (digits 1 and 4). This arrangement is most common inarboreal species, particularly those that climbtree trunks or clamber through foliage. Zygodactyly occurs in theparrots,woodpeckers (includingflickers),cuckoos (includingroadrunners), and someowls. Zygodactyl tracks have been found dating to 120–110 million years ago (earlyCretaceous), 50 million years before the first identified zygodactyl fossils. AllPsittaciformes,Cuculiformes, the majority ofPiciformes and theosprey are zygodactyl.[7]
Heterodactyly is like zygodactyly, except that digits 3 and 4 point forward and digits 1 and 2 point back. This is found only introgons,[8] though theenantiornitheanDalingheornis might also have had this arrangement.[9]
Pamprodactyly is an arrangement in which all four toes point forward, outer toes (toe 1 and sometimes 4) often if not regularly reversible. It is a characteristic ofswifts (Apodidae) andmousebirds (Coliiformes).[6]: 37–38
The feet ofchameleons are organized into bundles of a group of two and a group of three digits which oppose one another to grasp branches in a pincer-like arrangement. This condition has been called zygodactyly or didactyly, but the specific arrangement in chameleons does not fit either definition. The feet of the front limbs in chameleons, for instance, are organized into a medial bundle of digits 1, 2 and 3, and a lateral bundle of digits 4 and 5, while the feet of the hind limbs are organized into a medial bundle of digits 1 and 2, and a lateral bundle of digits 3, 4 and 5.[10] On the other hand, zygodactyly involves digits 1 and 4 opposing digits 2 and 3, which is an arrangement that chameleons do not exhibit in either front or hind limbs.
In manysecondarily aquatic vertebrates, the non-bony tissues of the forelimbs and/or hindlimbs are fused into a singleflipper. Some remnant of each digit generally remains under the soft tissue of the flipper, though digit reduction gradually occurs such as in baleen whales (mysticeti).[11] Marine mammals evolving flippers represents a classic example ofconvergent evolution, and by some analyses,parallel evolution.[12]
Full webbing of the digits in themanus and/orpes is present in a number of aquatic tetrapods. Such animals includemarine mammals (cetaceans,sirenians, andpinnipeds),marine reptiles (modernsea turtles and extinctichthyosaurs,mosasaurs,plesiosaurs,metriorhynchids), and flightless aquatic birds such aspenguins.[13] Hyperphalangy, or an increase in the number ofphalanges beyond ancestral mammal and reptile conditions, is present in modern cetaceans and extinct marine reptiles.[14]
Schizodactyly is aprimate term for grasping and clinging with the second and third digit, instead of the thumb and second digit.