A polydactyl cat is a cat with a congenital physical anomaly, with more than usual number of toes on one or more of its paws as a result of a cat body type genetic mutation. In animals including humans, polydactyly (or polydactylism, also known as hyperdactyly) is the anatomical abnormality of having more than the usual number of digits on the hands or feet.
Normal cats have five toes on each front paw and four toes on each hind paw. Polydactyl cats may have as many as seven digits on front and/or hind paws, and various combinations of anywhere from four to seven are common, although each of the front and rear paws are typically the same. Polydactyly is most commonly found on the front paws only, with polydactyly of all four paws being less common. It is rare for a cat to have polydactyl hind paws only.
The true polydactyly - commonly called mitten foot, mitten cat or thumb cat condition - is a congenital abnormality, genetically inherited as an autosomal dominant trait of the Pd gene with incomplete penetrance. This type of polydactyly is not life-threatening and usually not even debilitating to a cat. Some polydactyl kittens initially have more difficulty in learning to walk than normal animals. However in some cases polydactyly appears to improve the dexterity of the animal. For example, a common variation with six toes on the front paws, with two opposing digits on each (comparable in use to human thumbs), enables the cat to learn and perform feats of manual dexterity generally not observed in non-polydactyl cats, such as opening latches or catching objects with a single paw.
Feline radial hypoplasia (see squitten) is a mimic of polydactyly and is considered a severe condition. Radial hypoplasia may cause the formation of extra jointed toes, but it is not a result of the Pd gene normally associated with polydactyls. It thus does not cause the "mitten cat" or "thumb cat" condition where the extra toes occur separated from the normal ones just like a dewclaw, usually associated with an additional pad which makes them look like an underdeveloped foot sticking out near the base of the normal toes.
Rather, radial hypoplasia-related extra toes are immediately adjacent to the normal ones, giving the cat overly large, flat feet - colloquially known as "patty feet" or "hamburger feet". Though this looks less serious than true polydactyly (as the feet appear "normal" apart from having one or two extra toes), breeding such cats will eventually result in severely crippled offspring. Cats used in polydactyl breeding programs can be screened by x-ray for indicators of radial hypoplasia, and cats suspected to have radial hypoplasia should not be used for breeding.
Nobel Prize-winning author Ernest Hemingway was one of the more famous lovers of polydactyl cats, after being first given a six-toed cat by a ship's captain. Upon Hemingway's death in 1961, his former home in Key West, Florida, became a museum and a home for his cats, and it currently houses approximately fifty descendants of his cats (about half of which are polydactyl). Because of his love for these animals, Hemingway cat, or simply Hemingway, is a slang term which has come to describe polydactyls (Michael Palin describes one such cat named "Bill Clinton" he found at a former residence of Hemingway's in Key West during the filming of his "Hemingway Adventure" special).
Other nicknames include "boxers" or "boxing cats", "mitten cats", "thumb cats", "six-finger cats", "Boston Thumb Cats", "Cardi-cats" and "double-pawed cats" (a misnomer since there is a specific double paw condition, although they may be interrelated).
American Polydactyl Cats are also being bred as a specific cat breed, with specific physical and behavioral characteristics in addition to extra digits. A particular strain native to Ithaca, New York, is known as the "Ithacats". The American Polydactyl is not to be confused with the pedigree Maine Coon polydactyl. The polydactyl form of the Maine Coon is being reinstated by some breeders.
Polydactyly has also been observed in big cats.
