Dactyly
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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. It comes from the Greek word δακτυλος = "finger".
Sometimes the ending "-dactylia" is used. The derived adjectives end with "-dactyl" or "-dactylous".
[edit] Pentadactyly
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?"
These numerical variation from the pentadactyl limb are observed in terrestrial vertebrates and their aerial and aquatic descendants:-
[edit] Tridactyly
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.
[edit] Didactyly
Didactyly or bidactyly 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.
[edit] Monodactyly
Monodactyly is the condition of having a single digit on a limb, as in modern horses. These belong to the Perissodactyla.
[edit] Syndactyly
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 an unusual condition in humans.
Syndactyly can be simple or complex. In simple syndactyly, adjacent fingers or toes are joined by soft tissue. In complex syndactyly, the bones of adjacent digits are fused. The kangaroo exhibits complex syndactyly.
Simple syndactyly can be full or partial, and is present at birth (congenital). In early human fetal development, webbing (syndactyly) of the toes and fingers is normal. At about 16 weeks of gestation, an enzyme dissolves the tissue between the fingers and toes, and the webbing disappears. In some fetuses, this process does not occur completely between all fingers or toes and some residual webbing remains. The exact cause is not known. In cases, this condition appears to be hereditary.
In the case of human feet, syndactyly does not affect the function of the foot or toes and does not interfere with walking or swimming or any other activities. Although webbing of the fingers usually does not affect the function of the hand, it can impair function of the fingers. Surgery may be performed to separate webbed fingers or toes. As with any surgery, there are risks of complications. This procedure involves general anesthesia. In addition to the incision between the toes, it is necessary to remove some skin from elsewhere on the body to graft into the newly exposed space between the toes. This can leave additional scars on the site from which the skin is removed.
In the case of webbed toes, surgical separation is a purely cosmetic operation with no medical benefits. It is unethical to perform such surgery on infants or children.
[edit] Polydactyly
Polydactyly (or hyperdactyly) is when a limb has more than five digits. This can be:-
- As a result of congenital abnormality in a normally pentadactyl animal. Polydactyly is very common among domestic cats.
- Normality in some 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. For more information, see polydactyly.
[edit] Hypodactyly
Hypodactyly is having too few digits when not caused by an amputation.
[edit] Ectrodactyly
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).
A fusing of almost all digits on all of the hands and feet is ectrodactyly. News anchor Bree Walker is probably the best-known person with this condition, which affects about one in 91,000 people. It is conspicuously more common in the Vadoma in Zimbabwe.
[edit] In birds
[edit] Anisodactyly
Anisodactyly is the commonest 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.
[edit] Syndactyly (birds)
Syndactyly 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, as in the Belted Kingfisher Ceryle alcyon. This is characteristic of Coraciiformes (Kingfishers, Bee-eaters, Rollers, and relatives).
[edit] Zygodactyly
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, including flickers, nuthatches, and parrots.
[edit] Heterodactyly
Heterodactyly is like zygodactyly, except that digits 3 and 4 point forward and digits 1 and 2 point back. This is only found in trogons.
