pentadactyl

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".

Pentadactyly

Pentadactyly (from Greek pente-="five" plus δακτυλος = "finger") is the condition of having five digits on each limb. It is 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."^[1] Despite the individual variations listed below, the relationship is to the original five-digit 'model'.

Tetradactyly

Tetradactyly (from Greek tetra-="four" plus δακτυλος = "finger") is the condition of having four digits on a limb, as in many amphibians, birds, and theropod dinosaurs. Some mammals also exhibit tetradactyly (for example pigs and the hind limbs of dogs and cats). Cartoon characters are commonly drawn with four digits on each hand/foot as it's clearer to see than five.

Tridactyly

Tridactyly (from Greek tri- = "three" plus δακτυλος = "finger") 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 di-="two" plus δακτυλος = "finger") 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 monos- = "one" plus δακτυλος = "finger") is the condition of having a single digit on a limb, as in modern horses. These belong to the Perissodactyla.

Syndactyly

Human foot with partial simple syndactyly.

Syndactyly (from Greek συν- = "together" plus δακτυλος = "finger") 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- = "many" plus δακτυλος = "finger") (or, in humans, hyperdactyly, from Greek hyper- = "too many" plus δακτυλος = "finger") 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 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. It also occurs secondarily in some later tetrapods, such as ichthyosaurs.

Hypodactyly

Hypodactyly (from Greek hypo- = "too few" plus δακτυλος = "finger") is having too few digits when not caused by an amputation.

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 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.

Schizodactyly

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

In birds

Types of bird feet

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 like eagles, hawks, and falcons.

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, as in the Belted Kingfisher Ceryle alcyon. This is characteristic of Coraciiformes (Kingfishers, Bee-eaters, Rollers, and relatives).

Zygodactyly

A Green-winged Macaw has raised its right foot to its beak

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 parrots, woodpeckers (including flickers), cuckoos (including roadrunners), and some owls. It is also seen in chameleons.

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.

Pamprodactyly

Pamprodactyly is an arrangement in which all four toes point forward. It is a characteristic of swifts (Apodidae).

References

External links

• Coates, Michael (2005-04-25). "Why do most species have five digits on their hands and feet?". Scientific American. http://www.scientificamerican.com/article.cfm?id=why-do-most-species-have. Retrieved 2009-07-05.