Dactyly
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In biology, dactyly is the arrangement of (fingers and toes) on the hands, feet, or sometimes wings of a tetrapod animal. It comes from the Greek word δακτυλος, meaning "finger".
Sometimes the ending "-dactylia" is used. The adjectival forms end with "-dactyl" or "-dactylous".
By number
Pentadactyly is the condition of having five digits on each limb. All land vertebrates are descended from an ancestor with a pentadactyl limb, although many groups of species have lost or transformed some or all of their digits. Despite the individual variations, listed below, the relationship to the original, five-digit 'model' can be traced. This phenomenon featured in the work of Charles Darwin: "What could be more curious than that the hand of man formed for grasping, that of a mole, for digging, the leg of a horse, the paddle of a porpoise and the wing of a bat, should all be constructed on the same pattern and should include similar bones and in the same relative positions?"The following numerical variation from the pentadactyl limb are observed in terrestrial vertebrates and their aerial and aquatic descendants:
Tetradactyly is the condition of having four digits on a limb, as in amphibians and many birds. The hind limbs of dogs and cats also have only four digits.
Tridactyly 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 belong to the 'Perissodactyla'. Some birds also have three toes.
Bidactyly or didactyly is the condition of having two digits on each limb, as in the 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. Cloven-hoofed mammals (such as deer, sheep and cattle - 'Artiodactyla') walk on two digits.
Monodactyly is the condition of having a single digit on a limb, as in modern horses. These belong to the 'Perissodactyla'.
Syndactyly is a condition where two or more digits are fused together. It occurs normally in some mammals, such as the siamang. It occurs as a rare abnormality in humans.
Polydactyly refers to a greater number of digits than five. This can be as a result of congenital abnormality in a normally pentadactyl animal or can refer to early tetrapod aquatic animals, such as Acanthostega gunnari (Jarvik 1952), which is 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
In birds
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 like eagles, hawks, and falcons.
Syndactyly in birds is like anisodactyly, except that the third and fourth toes (the outer and middle forward-pointing toes) are fused together, as in the Belted Kingfisher, Ceryle alcyon.
Zygodactyly (from Greek ζυγον, a yoke) is an arrangement of digits in birds, 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 woodpeckers and flickers, nuthatches, and parrots.
Heterodactyly is like zygodactyly, except that it is digits 3 and 4 that point forward and digits 1 and 2 that point back. This is only found in trogons.
Other terms
An excess of digits is called hyperdactyly or polydactyly, such as in the case that a person has six fingers or toes on a single hand or foot.A lack of digits not caused by an amputation is called hypodactyly.
Ectrodactyly is the congenital absence of all or part of one or more fingers or toes. This term is used for a range of conditions from aphalangia (in which the some of the phalanges or finger bones are missing), to adactyly (the absence of a digit).
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