Dactyly
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In biology, dactyly is the arrangement of digits (fingers and toes) on the hands, feet, or sometimes wings of an animal. It comes from the Greek word daktulos, meaning "finger".
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Zygodactyly
Zygodactyl refers to birds which have "yoked" feet, that is with two toes facing forward and two back. 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.
Syndactyly
The more normal arrangement in birds, with three toes forward and one back is called syndactyly. This is common on songbirds and other perching birds, as well as hunting birds like eagles, hawks, and falcons.
In humans, syndactyly is when two digits are fused together. While syndactyly is considered normal in birds and in some mammals, such as the siamang, it is very unusual in humans.
Ectrodactyly
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 90,000 people.
Hyperdactyly
An excess of digits is called hyperdactyly, such as in the extremely rare case that a person has six fingers or toes on a single hand or foot.
Hypodactyly
A lack of digits not caused by an amputation is called hypodactyly.