Polydactylism (Extra Toes)

As a veterinarian who specializes in cats, I frequently come across cats and kittens in need of a good home. Fortunately, my hospital is located in a fairly residential part of Manhattan, and our clients, friends, and neighbors have adopted over 100 of our kitties in the past three years. For years, I resisted (with difficulty) taking one home myself, for fear of upsetting Crispy, my high-maintenance diva cat. Recently, though, I succumbed to the charms of “Mittens”, a 14 week-old calico. What mesmerized me about Mittens wasn’t simply her cute face, her winning personality, and her adorable meow. I confess, I was bowled over by her freaky feet!

Mittens, you see, is a polydactyl cat. Polydactyly (from the Greek: poly = many, daktulos = fingers) is a common trait among cats. It is a naturally occurring genetic variation that occurs in many animals as well as in humans. Although the trait had been observed earlier, the first official scientific recording of polydactyly was in 1868.

The lore behind polydactyl cats is intriguing. It is believed that English Puritans may have taken polydactyl cats on their ships to Boston during the mid-1600s, although it is also possible that the mutation developed in cats already residing in the Boston area. The offspring of these cats are believed to have then traveled on trading ships from Boston to Yarmouth, Massachusetts and Halifax, Nova Scotia, which might explain why these areas have a higher than normal incidence of polydactyly. In Norway, polydactyl cats are known as “ship’s cats” because the extra toes supposedly give them better balance on ships in stormy weather. In fact, polydactyl cats were considered to be lucky by sailors. Sailors also considered them to be much better mousers. Their presence on ships as mousers and lucky mascots suggests that they would have indeed reached America with early British settlers, explaining the increased incidence in the northeastern United States. It has been said that there are less polydactyl cats in Europe because many of these cats were destroyed due to witchcraft superstitions.

Polydactyl cats are occasionally referred to as “mitten cats” (explaining my cat’s corny name), “thumb cats”, and “Hemingway cats”, the latter name referring to the writer Ernest Hemingway, who made his home on the small island of Key West, Florida. Hemingway shared the island with almost 50 cats, including a six-toed polydactyl named Snowball (or possibly Princess) given to him by a ship captain and drinking buddy named Stanley Dexter. For the next 100 years, unrestrained breeding between this cat’s descendents and the local cats (alas, they weren’t as keen on spaying and neutering as we are today) led to a high percentage (almost 50%!) of polydactyls in the local population. Hemingway isn’t the only famous person who’s linked to polydactyl cats. President Theodore Roosevelt had a polydactyl cat named Slippers who was one of the first feline residents of the White House.

Historically, the original unregistered Maine Coon cats had a high incidence of polydactylism – around 40%! It has been written that the extra toes evolved as a “snowshoe foot” to help Maine Coons walk in the snow, and local folk tales claimed that these cats used their big mitts to catch live fish right out of the streams, taking them home to feed their owners! These stories are charming, however, there is no evidence that polydactylism confers any natural selective advantage to affected cats. Breed standards required a normal foot configuration, and did not allow polydactyly in Maine Coons, and so the trait was deliberately bred out of this breed. In the Netherlands and Belgium, there is currently a move to restore the polydactyl form of the breed.

Although polydactylism is alluring, breeding cats deliberately for polydactylism is controversial. Some cat enthusiasts fear that unscrupulous breeders would try to produce cats with excessive and disabling numbers of toes on each paw. Fortunately, polydactyl genetics doesn’t work this way; you can only fit so many toes on a cat’s foot. Even so, a good compromise would be to write breed standards to define the maximum number of toes allowed, to discourage such attempts.

Normally, a cat has 18 digits. The front paw has five toes – four toes and one dewclaw (the small toe on the medial side of the foot that doesn’t touch the ground). Most polydactyl cats have one or two extra toes on each foot, with the extra toes appearing on the thumb side of the foot. The normal rear paw has four toes.

The gene for polydactylism can give rise to either extra toes or extra dewclaws. Each toe has its own “terminal pad” (the fingertip pad) and often an extension of the palmar pad (the larger pad on the front foot) or plantar pad (the larger pad on the rear foot). It is possible for cats to even have different numbers of toes on each of its front feet! Most cases of polydactylism affect the front feet only. The hind feet are less often affected. When they are, it is usually in addition to having polydactyl front feet. It is quite rare to find a cat with polydactyl rear paws and normal front paws. When polydactylism does occur on the hind paws, it tends to cause extra toes rather than a dewclaw. My mutant Mittens, however, has a dewclaw on each of her rear feet.

There is a lot of variation regarding the number of extra toes and how well-formed they are. The most common form of polydactylism results in cats with well-formed extra toes. Others have an enlargement of the inside digit to a degree that it looks like a “thumb”. This is conventional “thumb cat” polydactyly. And then there’s my freaky little Mittens, who has the other form (some would say the “bad” form) of polydactyly. While most expressions of polydactylism are harmless, there is a gene which can cause a wide range of defects ranging from extra toes (no big deal) to missing or abnormal bones (a potentially disabling deformity). Examples of this would be the “twisty” mutation, a genetic defect that causes hypoplasia (underdevelopment) or agenesis (absence) or the radius, a major bone of the forearm. One striking result of this gene is the “triphalangeal pollex” – a thumb with an extra joint (a “three-boned” thumb), making it look more like an extra finger than the usual little dewclaw. Cats with these three-boned thumbs, like my Mittens, carry the gene that could put their offspring at risk of producing kittens with underdevelopment or absence of forearm deformities. Of course, we don’t need to worry about Mittens. I spayed her myself at 6 months of age. Oh, to complicate matters further, Mittens has a tiny little toe between her normal index finger and her three-boned thumb.

Genetically, polydactylism is a simple autosomal (not related to gender) dominant trait. Cats with extra toes have the dominant gene, Pd. A cat needs only one copy of this gene from either parent to have the trait. If one parent has it, 40 – 50% of the kittens will have it too. Although it has been said that if the parent has extra toes, the kittens inherit extra toes in the same configuration the extra-toed parent, this isn’t 100% true, because polydactylism genes show “incomplete dominance”. In other words, the genes inherited from the normal parent do figure into things, and the configuration of the kittens’ toes can vary. Because many polydactyl cats carry the gene for normal toes, the trait is never “fixed”. In other words, even breeding two polydactyls doesn’t guarantee all the kittens will be polydactyl. Inbreeding would increase the percentage of polydactyl offspring, but there will always be a few normal-toed kittens in the litter, because of that recessive gene.

Polydactylism doesn’t affect cats adversely. It offers them no advantages, nor does it yield any disadvantages. (If it did, polydactyl cats would have likely died out fairly quickly.) It is simply an enchanting quirk. It is an anomaly – a deviation from the norm – rather than a deformity. While people often worry about cats catching the extra toes on furnishings, this is rarely a problem. The toenails associated with the extra toes tend to be normal nails, although occasionally, the extra toe is incompletely formed, and the nail bed is deformed, leading to claw problems like ingrown or overgrown claws. Like all kitty toenails, the extra ones require regular trimming.

In case you were wondering, the most toes ever found on a cat is 32 – eight on each paw – was reported in October 1974. The current verified record holder is “Tiger”, a 27-toed cat residing in Alberta, Canada. Tiger has seven toes on each front foot, seven on her left hind foot, but only six on her right hind. Mittens, with 22 toes, has a long way to go.