Zygodactylous (pronounced zai-go-DAKT-uh-lus) is an anatomical term used in zoology. It refers to hands or feet with two or three toes pointing forward and two pointing backward. The term is most often used in ornithology, as a description of the feet of certain species of birds (see image at http://www.kidwings.com/bodyparts/feet/zygodactyl.htm), though there are also zygodactylous lizards, notably chameleons.
Usually it is the front outer toe or “hallux” (the morphological equivalent of the human thumb or “big toe”) that points backward, along with the fourth toe. The second and third point forward. Four-toed animals with toes pointed both forward and backward may also be called “heterodactylous,” http://www.kidwings.com/bodyparts/feet/heterodactyl.htm which is a similar, much less common arrangement.
Etymology of the word “zygodactylous”
Zygodactylous literally means “having paired toes.” The word is compounded from two Greek roots: “zygon” or “zugon” meaning yoke (implying the verb ‘to yoke,’ or to join into pairs), and “daktylos” or “daktulos,” meaning a finger. The first known use of the word ‘zygodactylous’ is in a work thought to date from about 1828.
Words related to “zygodactylous”:
zygodactyly or zygodactylism – the condition of being zygodactylous
zygodactyl – a bird or reptile that is zygodactylous (plural form: zygodactyls)
semizygodactylous – having a toe that is reversible, ie, that can point either forward or backward
Examples of zygodactylous animals
Zygodactyly is usually found in species that are arboreal (spending most or all of their time in trees). Zygodactylous animals include many parrots (family Psittacidae), toucans (family Raphastidae), chameleons (family Chamaeleonidae), woodpeckers (family Picidae), and cuckoos (family Cuculidae, which also includes roadrunners). Some varieties of owls are also zygodactylous, including the Great Horned Owl.
Based on fossilized footprints, scientists speculate that the first zygodactylous animals may have lived as long ago as the early Cretaceous period, between 110 and 120 million years ago.
Significance of zygodactyly
Having toes that point both forward and backward enable a bird or lizard to grip a branch or perch more effectively, because it can wrap its foot all the way around the branch. This gripping ability also helps a bird get a better hold on its prey. It also helps the animal balance better, because it creates a broader base.
Selected sources:
Michael Allaby. “zygodactylous.” A Dictionary of Zoology. 1999.
Encyclopedia.com. (November 9, 2010). http://www.encyclopedia.com/doc/1O8-zygodactylous.html.
Jody Hildreth. “Zygodactyl foot.” 2006. KidWings.com (November 9, 2010). http://www.kidwings.com/bodyparts/feet/zygodactyl.htm
Biology Online. (November 9, 2010). http://www.biologyonline.org/dictionary/Zygodactylous
YourDictionary.com. (November 9, 2010). http://www.yourdictionary.com/zygodactylous (includes audio pronunciation guide)
