The typical vertebrate limb features five digits, a pattern known as pentadactyly, which is widely observed across amphibians, reptiles, birds, and mammals. However, nature frequently presents variations to this standard arrangement, resulting in animals born with more than the usual number of fingers or toes. This condition, involving the presence of extra digits, is a relatively common congenital trait across the animal kingdom. The animal most often associated with having six toes is a familiar household companion.
Polydactyl Cats: The Most Common Answer
The animal most commonly known for having six toes is the domestic cat, a trait referred to as feline polydactyly. While a typical cat has five toes on each front paw and four on each back paw, these felines can display extra digits on any or all of their paws. The extra toes most frequently appear on the front paws, sometimes giving them the appearance of an enlarged thumb or a mitten. This morphology is why polydactyl cats are affectionately called “mitten paws.”
These cats gained fame through the author Ernest Hemingway, who was gifted a six-toed white cat named Snow White. Hemingway’s home in Key West, Florida, became a sanctuary for these multi-toed animals, and many of the roughly 60 cats living there today are descendants of that original feline. Due to this connection, polydactyl cats are often colloquially referred to as “Hemingway Cats.”
The condition is particularly common along the East Coast of North America and in the southwest of Great Britain. Historically, sailors favored them as ship cats, believing their extra grip made them superior climbers and hunters. While the maximum number of toes recorded on a single cat’s paw is twenty-eight, six or seven digits per paw are the most frequent variation.
The Genetics of Extra Digits
The scientific term for having extra digits is polydactyly, which literally translates from Greek as “many digits.” This morphological variation is the result of a single, genetic mutation. In many species, including cats, the trait is inherited in an autosomal dominant fashion, meaning only one copy of the mutated gene is needed for the offspring to exhibit the condition.
The mutation affects the developmental pathways that control limb formation during the embryonic stage. Specifically, the genes involved in the Sonic Hedgehog (SHH) signaling pathway, which determines the anterior-posterior axis of a developing limb, are implicated. A mutation in a regulatory region near the LMBR1 gene, known as the ZRS, can cause the SHH signal to be expressed improperly, leading to the formation of extra digits.
This genetic change is usually benign, causing no apparent health problems for the animal. The extra digits can range from a simple flap of soft tissue to a fully formed toe, complete with bone structure, a claw, and a pad. The variable appearance is due to the degree of gene expression and the exact timing of the mutation during development.
Other Species That Exhibit Polydactyly
While cats are the most recognized example, polydactyly is observed in a wide range of other animal species. In the canine world, certain ancient herding and working dog breeds have been intentionally bred to preserve this trait. The Norwegian Lundehund, a specialized puffin-hunting dog, is genetically required to have at least six functional toes on each foot to meet its breed standard.
The Great Pyrenees and certain other livestock guardian breeds also commonly exhibit extra dewclaws on their hind legs, a form of polydactyly considered beneficial for traversing rough or snowy terrain. The trait is also observed in livestock such as pigs and sheep, as well as in laboratory mice used for genetic research.
An interesting example is the mole, where the presence of six digits is not a mutation but the species’ norm. The mole’s extra “thumb,” or the os falciforme, is a unique bone that develops later than the other five digits and provides a larger surface area for digging. This is a case of hyperdactyly—an adaptation for function—rather than the occasional genetic variation seen in domestic animals.