zygodactyly

Zygodactyly is a form of digit arrangement in which two digits point forward and two point backward. The term comes from the Greek zygos, meaning “yoked,” and daktylos, meaning “finger.” In birds, zygodactyly describes a foot with two forward-facing toes and two backward-facing toes, contrasting with other arrangements such as anisodactyly (three forward, one back) and pamprodactyly (all four toes forward).

In avian anatomy, zygodactyly is most notably found in parrots (Psittaciformes) and woodpeckers (Piciformes), where the toe configuration aids in grasping branches, climbing, and manipulating objects. The two forward-facing toes and two backward-facing toes provide a strong grip on vertical surfaces and help stabilize the body during movement and feeding. While the basic pattern is characteristic of these groups, there can be some variation in toe orientation among species.

Outside of birds, zygodactyly is not a common natural limb arrangement in humans or most other non-avian mammals. In medical or anatomical discussions, the term may appear when describing rare congenital variations or comparative anatomy, but it is not a standard human condition.

See also: anisodactyly, pamprodactyly.