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In biology, dactyly is the arrangement of digits (fingers and toes) on the hands, feet, or sometimes wings of a tetrapod animal. The term is derived from the Greek word δακτυλος (dáktylos) meaning "finger."

Sometimes the suffix "-dactylia" is used. The derived adjectives end with "-dactyl" or "-dactylous."

As a normal feature

Pentadactyly

Pentadactyly (from Greek πέντε pénte "five") is the condition of having five digits on each limb. It is traditionally believed that all living tetrapods 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 of evolution. However, this viewpoint was challenged by Stephen Jay Gould in his 1991 essay "Eight (or Fewer) Little Piggies," where he pointed out polydactyly 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 the dewclaw makes them pentadactyls. Cats also have dewclaws on their front limbs but not their hind limbs, making them both pentadactyls and tetradactyls.

Tetradactyly

Tetradactyly (from Greek τετρα tetra 'four') is the condition of having four digits on a limb, as in many birds, amphibians, and theropod dinosaurs.

Tridactyly

Tridactyly (from Greek τρία tría 'three') is the condition of having three digits on a limb, as in the rhinoceros and ancestors of the horse such as Protohippus and Hipparion. These all belong to the Perissodactyla. Some birds also have three toes, including emus, bustards, and quail.

Didactyly

Didactyly (from Greek δι- di- 'two') or bidactyly is the condition of having two digits on each limb, as in the Hypertragulidae and two-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 do ostriches.

Monodactyly

Monodactyly (from Greek μόνος monos- 'one') is the condition of having a single digit on a limb, as in modern horses and other equidae (though one study suggests that the frog might be composed of remnants of digits II and IV, rendering horses as not truly monodactyl^[2]) as well as sthenurine kangaroos. Functional monodactyly, where the weight is supported on only one of multiple toes, can also occur, as in the theropod dinosaur Vespersaurus. The pterosaur Nyctosaurus retained only the wing finger on the forelimb, rendering it also partially monodactyl.^[3]

As a congenital defect

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

Syndactyly (from Greek συν- syn 'together') is a condition where two or more digits are fused together. It occurs normally in some mammals, such as the siamang and most diprotodontid marsupials such as kangaroos. It occurs as an unusual condition in humans.

Polydactyly

Polydactyly (from Greek πολυ- poly- 'many') is when a limb has more than the usual number of digits. This can be:

As a result of congenital abnormality in a normally pentadactyl animal. Polydactyly is very common among domestic cats. For more information, see polydactyly.
Polydactyly in early tetrapod aquatic animals, such as in Acanthostega gunnari (Jarvik 1952), one of an increasing number of genera of stem-tetrapods known from the Upper Devonian, which are providing insights into the appearance of tetrapods and the origin of limbs with digits. It also occurs secondarily in some later tetrapods, such as ichthyosaurs. The use of a term normally reserved for congenital defects reflects that it was regarded as an anomaly at the time, as it was believed that all modern tetrapods have either five digits or ancestors that did.

Oligodactyly

Oligodactyly (from Greek ὀλιγο- oligo- 'few') is having too few digits when not caused by an amputation. It is sometimes incorrectly called hypodactyly or confused with aphalangia, the absence of the phalanx bone on one or (commonly) more digits. When all the digits on a hand or foot are absent, it is referred to as adactyly.^[4]

Ectrodactyly

Ectrodactyly, also known as split-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 anchor Bree 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 the Vadoma in Zimbabwe.

Tetradactyly
Tridactyly (Mikhail Tal)
Didactyly
Monodactyly

Clinodactyly

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 as Down syndrome, Turner syndrome and Cornelia de Lange syndrome.

In birds

Four types of bird feet
(right foot diagrams)

Anisodactyly

Anisodactyly is the most common arrangement of digits in birds, with three toes forward and one back. This is common in songbirds and other perching birds, as well as hunting birds such as eagles, hawks, and falcons. This arrangement of digits helps with perching and/or climbing and clinging. This occurs in Passeriformes, Columbiformes, Falconiformes, Accipitriformes, Galliformes and a majority of other birds.

Syndactyly

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 the belted kingfisher (Megaceryle alcyon).^[5] This is often found in Picocoraciae, though rollers, ground rollers, and Piciformes (who are zygodactyl) are exceptions.^[6]^: 37

Zygodactyly

Zygodactyly (from Greek ζυγος, 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 in arboreal species, particularly those that climb tree trunks or clamber through foliage. Zygodactyly occurs in the parrots, woodpeckers (including flickers), cuckoos (including roadrunners), and some owls. Zygodactyl tracks have been found dating to 120–110 million years ago (early Cretaceous), 50 million years before the first identified zygodactyl fossils. All Psittaciformes, Cuculiformes, the majority of Piciformes and the osprey are zygodactyl.^[7]

Heterodactyly

Heterodactyly is like zygodactyly, except that digits 3 and 4 point forward and digits 1 and 2 point back. This is found only in trogons,^[8] though the enantiornithean Dalingheornis might also have had this arrangement.^[9]

Pamprodactyly

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 of swifts (Apodidae) and mousebirds (Coliiformes).^[6]^: 37–38

Chameleons

The feet of chameleons 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.

Aquatic tetrapods

In many secondarily aquatic vertebrates, the non-bony tissues of the forelimbs and/or hindlimbs are fused into a single flipper. 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 of convergent evolution, and by some analyses, parallel evolution.^[12]

Full webbing of the digits in the manus and/or pes is present in a number of aquatic tetrapods. Such animals include marine mammals (cetaceans, sirenians, and pinnipeds), marine reptiles (modern sea turtles and extinct ichthyosaurs, mosasaurs, plesiosaurs, metriorhynchids), and flightless aquatic birds such as penguins.^[13] Hyperphalangy, or an increase in the number of phalanges beyond ancestral mammal and reptile conditions, is present in modern cetaceans and extinct marine reptiles.^[14]

Schizodactyly

Schizodactyly is a primate term for grasping and clinging with the second and third digit, instead of the thumb and second digit.

References

^ Stephen Jay Gould. "Stephen Jay Gould "Eight (or Fewer) Little Piggies" 1991". Archived from the original on 2011-08-05. Retrieved 2015-10-02.
^ Solounias, Nikos; Danowitz, Melinda; Stachtiaris, Elizabeth; Khurana, Abhilasha; Araim, Marwan; Sayegh, Marc; Natale, Jessica (2018). "The evolution and anatomy of the horse manus with an emphasis on digit reduction". Royal Society Open Science. 5 (1): 171782. doi:10.1098/rsos.171782. PMC 5792948. PMID 29410871.
^ Witton, Mark (2013). Pterosaurs: Natural History, Evolution, Anatomy. Princeton University Press. ISBN 978-0691150611.
^ József Zákány; Catherine Fromental-Ramain; Xavier Warot & Denis Duboule (1997). "Regulation of number and size of digits by posterior Hox genes: A dose-dependent mechanism with potential evolutionary implications". PNAS. 94 (25): 13695–13700. Bibcode:1997PNAS...9413695Z. doi:10.1073/pnas.94.25.13695. PMC 28368. PMID 9391088.
^ Dudley, Ron (14 February 2016). "Belted Kingfisher With A Fish (plus an interesting foot adaptation)". FeatheredPhotography. Retrieved 4 December 2022.
^ ^a ^b Nupen, Lisa (September–October 2016). "Fancy Footwork: The Dazzling Diversity of Avian Feet" (PDF). African Birdlife. Vol. 4, no. 6. BirdLife South Africa. pp. 34–38. ISSN 2305-042X. Retrieved 4 December 2022 – via FitzPatrick Institute of African Ornithology.
^ Lockley, Martin G.; Li, Rihui; Harris, Jerald D.; Matsukawa, Masaki; Liu, Mingwei (2007). "Earliest zygodactyl bird feet: evidence from Early Cretaceous roadrunner-like tracks" (PDF). Naturwissenschaften. 94 (8): 657–665. Bibcode:2007NW.....94..657L. doi:10.1007/s00114-007-0239-x. PMID 17387416. S2CID 15821251.
^ Botelho, João Francisco; Smith-Paredes, Daniel; Nuñez-Leon, Daniel; Soto-Acuña, Sergio; Vargas, Alexander O. (2014-08-07). "The developmental origin of zygodactyl feet and its possible loss in the evolution of Passeriformes". Proceedings of the Royal Society of London B: Biological Sciences. 281 (1788): 20140765. doi:10.1098/rspb.2014.0765. ISSN 0962-8452. PMC 4083792. PMID 24966313.
^ Zhang, Z.; Hou, L.; Hasegawa, Y.; O'Connor, J.; Martin, L.D.; Chiappe, L.M. (2006). "The first Mesozoic heterodactyl bird from China". Acta Geologica Sinica. 80 (5): 631–635.
^ Anderson, Christopher V. & Higham, Timothy E. (2014). "Chameleon anatomy". In Tolley, Krystal A. & Herrel, Anthony (eds.). The Biology of Chameleons. Berkeley: University of California Press. pp. 7–55. ISBN 9780520276055.
^ Cooper, Lisa Noelle; Berta, Annalisa; Dawson, Susan D.; Reidenberg, Joy S. (2007). "Evolution of hyperphalangy and digit reduction in the cetacean manus". Anatomical Record. 290 (6): 654–672. doi:10.1002/ar.20532. ISSN 1932-8486. PMID 17516431. S2CID 14586607.
^ Chikina, Maria; Robinson, Joseph D.; Clark, Nathan L. (2016-09-01). "Hundreds of Genes Experienced Convergent Shifts in Selective Pressure in Marine Mammals". Molecular Biology and Evolution. 33 (9): 2182–2192. doi:10.1093/molbev/msw112. ISSN 0737-4038. PMC 5854031. PMID 27329977.
^ Fish, F.E. (2004). "Structure and Mechanics of Nonpiscine Control Surfaces". IEEE Journal of Oceanic Engineering. 29 (3): 605–621. Bibcode:2004IJOE...29..605F. doi:10.1109/joe.2004.833213. ISSN 0364-9059. S2CID 28802495.
^ Fedak, Tim J; Hall, Brian K (2004). "Perspectives on hyperphalangy: patterns and processes". Journal of Anatomy. 204 (3): 151–163. doi:10.1111/j.0021-8782.2004.00278.x. ISSN 0021-8782. PMC 1571266. PMID 15032905.