Six Toes and a Lesson in Limb Development
What the Hemingway cats can teach about bones and joints
I should admit a bias before I start. I have two polydactyl cats of my own, adopted from shelters years apart, neither one labeled as anything unusual. Tisha has six toes on each front paw plus the beginnings of a seventh, and Dusya has six on each front paw too. In both cases I only discovered the extra toes at home, counting them on a lap one evening the way you do. So when I went to Key West and found a house full of cats with the same trait, it felt less like a tourist attraction than like running into relatives.
There are about sixty cats living on the grounds of the Hemingway house, and roughly half of them have extra toes. They trace back to a white cat named Snow White, given to Hemingway by a ship’s captain in the 1930s, and tourists come specifically to count their paws. I came for the same reason as everyone else, but I am a rheumatologist, and I cannot look at an extra digit without wondering what the rest of the limb is doing to accommodate it. The cats turned out to be a surprisingly good way into a serious subject: how limbs are patterned, and what happens at the joints when the pattern is altered.
A normal cat has eighteen toes, five on each front paw and four on each back. The Hemingway cats can have considerably more. The Guinness record is twenty-eight, held jointly by a Canadian cat named Jake and an American one named Paws. The extra toes usually appear on the inner, thumb side of the front paws, which is why these cats look like they are wearing mittens.
A switch flipped in the wrong place
The genetics connect directly to human medicine. During development, the position of each digit is governed by a signaling protein called Sonic hedgehog, which is produced in a small zone at the back edge of the limb bud, the side that will become the little-finger side. It diffuses across the bud, and the local concentration tells each future digit what to become. The gene is switched on and off by a distant regulatory sequence called the ZRS.
In the Hemingway cats, a single-letter change in that ZRS sequence causes Sonic hedgehog to switch on a second time, in the wrong place, at the front edge of the limb bud. The limb now has two signaling zones instead of one, and it builds extra digits on the thumb side to match. This is called preaxial polydactyly, preaxial meaning on the thumb side. The trait is autosomal dominant with incomplete penetrance, so one copy of the variant is enough to produce extra toes, but not every cat that carries it shows them to the same degree.
What makes this more than a feline curiosity is that the same regulatory sequence, mutated in the same small region, causes preaxial polydactyly in mice and in people. When researchers sequenced Hemingway’s cats, they found the changes sitting in the ZRS one nucleotide away from variants described in British cats, within the same conserved stretch implicated in human limb malformations. The sequence sits about eight hundred thousand base pairs from the gene it controls, and a few letters in it are enough to change the number of digits a mammal grows. The cats in that Key West garden are carrying a textbook example of long-range gene regulation, whether anyone photographing them knows it or not.
Where the musculoskeletal interest begins
For most polydactyl cats, the extra toes are harmless. The classic mitten-paw form does not compromise the animal, and these cats run, climb, and hunt like any other. There is an old story that the extra toes act as natural snowshoes or give better grip on a rolling ship deck, which is offered to explain why polydactyl cats cluster on the east coast of North America and the west coast of England, the old sailing routes. It is a charming idea with no solid evidence behind it, and I mention it mainly because every Key West tour guide will tell it to you.
What interests me clinically is what an extra digit does to the mechanics of the paw, and where the benign trait shades into something that is not benign at all. A few real problems do come with the extra toes.
The first is mundane but genuine. Extra toes often come with extra claws, and those claws can grow at odd angles, thicker than usual, in positions the cat cannot wear down or clean normally. An extra dewclaw tucked against the paw is prone to growing in a curl, catching on carpet and fabric, and in some cases turning into the paw and embedding in the pad, which is painful and invites infection. Sometimes an extra toe in a kitten fuses with the nail bed of its neighbor and has to be separated surgically. None of this is dramatic, but it is the kind of low-grade mechanical and inflammatory trouble that a clinician, or an attentive owner, has to stay ahead of with regular nail trimming.
The serious lookalike
The important distinction, and the reason a veterinarian reaches for an x-ray, is that not everything that produces extra toes is the harmless ZRS trait. There is a separate and much more damaging condition, radial hypoplasia, in which the radius, one of the two long bones of the forearm, fails to develop properly or is missing altogether. Cats with severe forms cannot use their front legs normally; they sit up and hop on their hind limbs, which has earned them the unfortunate nicknames twisty cats and kangaroo cats. In the most severe cases the forelimb is twisted and effectively non-functional.
The trap is that radial hypoplasia is often accompanied by extra toes, so on casual inspection it can look like ordinary polydactyly. The warning sign is a particular variant in which the cat’s thumb has three bones instead of the usual rudimentary dewclaw, resembling a human finger. This is called a triphalangeal pollex, a three-boned thumb, and breeders who know the field treat it as a reason to keep a cat out of breeding, because it travels with the more disabling forelimb defects. The two conditions can produce a similar-looking paw over a completely different skeleton, and you cannot tell them apart by looking at the toes. You have to image the forearm and see whether the radius is present and well formed.
It is worth being careful about the genetics, because the popular sources overstate the link. The everyday mitten-cat trait and the radial defect are caused by different genes. There is no good evidence that ordinary polydactyly causes radial hypoplasia; the concern is specifically with the three-boned-thumb form, which appears to track with the forelimb problem. Most polydactyl cats are simply carrying a harmless dominant trait and will never have an orthopedic problem because of it. Tisha and Dusya are firmly in that majority: ordinary extra toes, normal forelimbs, no trouble beyond the nail trimming that any six-toed cat earns.
The rheumatology, even if it is feline
There is a downstream joint story that makes all of this musculoskeletal rather than merely developmental. When a limb is built abnormally, the joints pay for it over time. Cats with radial hypoplasia commonly develop degenerative joint disease in the malformed leg, because the joint surfaces were never aligned to load correctly in the first place. It is the same principle I see in human clinics every week: abnormal mechanics, whatever their origin, concentrate stress where the joint was not built to take it, and the cartilage wears accordingly.
If you want a closer feline parallel to inflammatory rheumatology, it is not the polydactyl cats but their distant cousins the Scottish Folds, whose folded ears come from a dominant cartilage mutation. In cats carrying two copies of it, the same mutation produces a severe, progressive arthritis that fuses joints throughout the body, with a particular fondness for the spine, feet, and hind limbs. In effect it is a heritable ankylosing arthropathy, and the folded ear that makes the breed popular is the visible end of a gene that is also remodeling the skeleton.
I did not expect, walking into a writer’s garden full of cats, to spend the afternoon thinking about signaling gradients, the limits of inspection, and secondary arthritis. But that is what the Hemingway cats are good for, and the two asleep on my own couch. The extra toe is what everyone photographs. The more interesting part is underneath it: the developmental switch that built it, the rare and serious conditions that can hide behind a paw that looks just like Tisha’s, and the joints that record, over years, how well the limb was put together. The toes are never the whole story, which is roughly what I tell myself in front of every radiograph.
Olga Goodman, MD — RheumaView