The question of why some cats appear to have extra digits, often described as “thumbs,” directs us to the genetic variation known as polydactyly. This term originates from the Greek words poly (many) and dactylos (digits), accurately describing the condition of having more than the typical number of toes. The extra digit is a congenital physical anomaly, not a true opposable thumb like a human’s, capable of flexing to touch other fingers. Polydactyly is a mostly harmless genetic trait that offers a distinct look to the paws of these unique felines.
Anatomy: What Polydactyly Looks Like
A standard cat typically possesses 18 toes in total: five digits on each front paw and four on each back paw. The fifth digit on the front paw is the dewclaw, which is set higher and does not make contact with the ground. Polydactyl cats, by contrast, can have anywhere from six to eight digits on a single paw, sometimes totaling up to 28 toes across all four limbs.
The most common presentation of this trait is preaxial polydactyly, where the extra digits appear on the inner, or “thumb,” side of the paw. This placement is what gives the paw its distinctive “mitten paw” or “snowshoe” appearance. The extra digits can range from a small, rudimentary nub to a fully formed extra toe complete with its own claw and pad.
While the extra digits are often located on the front paws, polydactyly can affect the hind paws or even all four paws, though the latter is less common. The presence of these additional toes increases the overall surface area of the paw, making it appear noticeably larger than a non-polydactyl cat’s paw. Importantly, this variation is not associated with any pain or typical health issues for the cat.
The Genetic Mechanism Behind Extra Digits
The presence of extra digits is not random but is the result of a specific, naturally occurring genetic mutation. Polydactyly is inherited as an autosomal dominant trait, meaning a kitten only needs to inherit the mutated gene from one parent to exhibit the trait. If a polydactyl cat breeds with a non-polydactyl cat, their offspring have an approximately 50% chance of inheriting the trait.
The genetic change occurs in a non-coding regulatory element called the Zone of Polarizing Activity Regulatory Sequence, or ZRS. This sequence acts as a distant switch that controls the expression of the Sonic hedgehog (SHH) gene during embryonic development. The SHH gene is fundamental to the proper patterning and formation of digits in the developing limb.
The mutation in the ZRS causes the SHH gene to be expressed incorrectly in an area of the limb bud where it is not normally active. This “ectopic expression” provides the cellular signals necessary to spur the growth of extra tissue, resulting in the formation of additional digits. This genetic mechanism is similar to that observed in other mammals, including humans and mice, when they develop polydactyly.
How Polydactyl Cats Use Their Unique Paws
The larger, wider paws resulting from polydactyly are often believed to provide functional advantages to the cats that possess them. The increased surface area of the foot can act like a natural “snowshoe,” which historically was thought to aid the mobility of cats, such as the Maine Coon, in snowy environments. The extra digits may also offer improved stability and better balance, particularly when navigating uneven surfaces or climbing.
Historically, polydactyl cats were favored by sailors and were common aboard ships, leading to their prevalence in ports along the East Coast of North America and in the UK. Sailors considered these cats good luck charms, believing their unique paws made them superior mousers and climbers in the unsteady environment of a ship. The extra digits, especially in the “mitten paw” configuration, can potentially enhance a cat’s ability to grasp or hook objects, though this ability varies widely among individual cats.
The tradition of these multi-toed felines continues today, notably in Key West, Florida, where the descendants of author Ernest Hemingway’s cat still reside, earning them the nickname “Hemingway Cats.” While the extra toes are not a crucial evolutionary adaptation, they are a harmless quirk of feline genetics. The most significant practical consideration for owners is ensuring the extra claws are trimmed regularly, as they may not wear down naturally and can become ingrown.