Polydactyly, or extra digits, is a common trait among cats, particularly it seems, among Celtic cats and cats on part of America's Eastern coast and South West Britain. This distribution may well be linked. Except for Twisty Cats, polydactyly is not a product of bad breeding. It is simply a naturally occurring genetic variation and, as noted later on, polydactyly is found in fossil reptiles - meaning that five digits might be the abnormal form! Only one form of polydactyly is known to be harmful.

In a 1967 issue of Britain's "The Cat" magazine, Mary Collier of Axminster, Devon wrote Can any readers of THE CAT give me any information about 6-toed cats, sometimes called 'Boxers' or 'Boxing Cats'? I have recently acquired a very fine kitten of this type [...] What I particularly want to know is their district or origin, or country if outside the British Isles and the date they may first have been recognised." In February 1978, the Daily Mirror carried a series of letters on polydactyl cats. Jennifer Wellstead, of Penzance, Cornwall, had asked if any other readers had cats with 6 toes on each paw. A "Mrs I" of Kettering, Northants, replied that 6-toed cats were favoured as witches' familiars of witches. Mrs Farley, of Havant, Hampshire, said she had 6 cats, 3 of whom had 6 toes and 2 had 4 toes on each foot in place of the normal 5. She added that a recent litter had produced one kitten with 7 toes, 4 with 6 toes and 2 with the normal number of toes.

Polydactyly (six or seven toes) varies from the classic "mitten cat" through to cats which simply have more toes than normal, but no "thumb". A correspondent to the New Scientist noted that the innermost extra toes on the front paws are often opposable and some cats use them with quite startling proficiency to manipulate small objects with almost human dexterity. Some owners of polydactyl cats joke that their cats are more intelligent because of this and represent the next stage in feline evolution - the ability to open cartons and cans unaided.

Polydactyl cats are said to be virtually non-existent in Europe, because "unusual looking cats" were destroyed due to witchcraft superstitions, practically eliminating the trait (Kelly, Larson,1993). I do not know whether Britain was included in the generic term "Europe" or whether it meant continental Europe only. In Norway, polydactyl cats are known as "ship's cats" as the extra toes supposedly gave them better balance on ships in stormy weather; they are not uncommon and polydactyl kittens are sought after pets. Polydactyl random-bred cats have been reported in Sweden though other European cat lovers (locations not reported) had apparently never seen a polydactyl. They are common enough in Britain to be considered unremarkable. A polydactyl gene pool has become concentrated in Cardiganshire (Ceredigion), south west Wales; cats with extra toes became common there and known as "Cardi-cats". Six-toed cats are apparently more common in Cardigan (Aberteifi) that elsewhere in Britain; seven-toed cats are also sometimes found in the area. Cardigan/Aberteifi sits on the Teifi estuary above Cardigan Bay and was once one of the busiest shipping ports in Wales which probably explains the high density of polydactyl cats in the area. I regularly saw polydactyl cats at the Chelmsford Cats Protection rescue shelter as well as pwning a polydactyl.

Polydactyl cats were considered "lucky" by sailors. Sailors also believed polydactyl cats to be superior mousers and ratters. Employed as ships' ratters and lucky mascots, they would have reached America with early British settlers hence their greater frequency in Eastern states. A disproportionately high number of "lucky" polydactyl cats, compared to normal-toed cats, would have found their way there. This would lead to a greater proportion of polydactyls than usual for a random-breeding cat population. Back in Britain, with its large cat population of which the polydactyl formed only a small part, the trait remained less common (though there have been localised pockets of higher than average incidence). There is a higher incidence of polydactyly in South-West England, possibly associated with ports from which ships set sail for the New World.

Bjorn B Svingen, owner of a polydactyl cat, provided the following information on polydactyl cats in Norway and their association with ships: "I have heard people say that these genes came to Norway long ago. The story was that the genes were inherited from Spanish or Portuguese ship cats with this 'disorder'. These cats were supposedly common on ships trading on the Norwegian coastline. They have again become popular, at least in our area, and are plentiful in Trondelag in mid Norway." As a result, they are also known there as "Skipskatt" (ship-cat).

In his survey and detailed account of cats in Singapore in 1959, Searle had not noted any polydactyls. The only polydactyls I noted in the Malaysia/Singapore region were the Lake Chini cats. I also found a superb bobtailed polydactyl cat in rural Malaysia (near Kuantan) where "six-finger cats" are considered lucky. Being an unneutered tomcat, the trait would be perpetuated.

There are two forms of polydactyly described by embryologists. Pre-axial polydactyly refers to extra digits on the inside edge (thumb-side) of the paw. Post-axial polydactyly refers to extra digits on the outer side (little finger side) of the paw and is lesscommon. One of the earliest inheritance studies was by Poulton (probably 1880s/90s) which is mentioned by Thomas Hunt Morgan (Professor of Experimental Zoology, Columbia University) in Experimental Zoology (Publ. Macmillan & Co, London, 1910): Poulton has given some records of polydactyl cats that appear to be exphcable, so far as they go, along Mendelian Hnes. Three young were produced from a polydactyl female by an unknown father. They were all polydactyl. If polydactylism dominates over the normal condition, this result is simple dominance. One of these individuals (F1) produced three litters (by unknown fathers), in which four normal and six abnormal kittens appeared. If the father was normal, five normal and five polydactyl young would be expected. Thus:- (P + N) + (N + N) = (2NP + 2NN) [P being poly foot and N being normal foot]

Sis & Getty (1968) describe polydactyly as an monogenic autosomal dominant trait affecting the pre-axial part of the limb. This is the typical and harmless form of polydactyly though other forms exist. Both Danforth and Chapman & Zeiner proved in their research that normal feline polydactyly is a simple incomplete dominant gene with variable expression. According to geneticist and TICA Genetics Committee Chair Dr Solveig Pflueger "most polydactyl cats... have a form of pre-axial polydactyly with the extra digits(s) on the thumb side of the foot... " The Pd gene causes the entirely harmless form of polydactyl. The main studies on feline polydactyly are Danforth's studies Heredity of Polydactylism (1947) and Morphology of the Feet in Polydactyl Cats (1947) plus Chapman & Zeiner's study The Anatomy of Polydactylism in cats with Observations on Genetic Control (1961). The Danforth and Chapman & Zeiner studies observed examples of both mitten-foot and patty-foot polydactyly in their studies (some refer to the patty foot as "snowshoes" but I avoid that term as there is a cat breed called the Snowshoe). Danforth, and Chapman & Zeiner, stated that polydactyly was never observed in the hind feet except when it was also present in the front feet. A hind-foot polydactyl must have extra digits on the front feet, though sometimes these are not readily discernible and can only be found by x-ray.

The gene(s) for polydactyly specifically affects the tissue formed at the very end of the limb (apical cap) of a developing embryo. This is the area where the toes will form. Danforth studied the way normal pre-axial polydactyly developed in embryos based on data from 150 cats. He discerned evidence of polydactyly in the 20th day of gestation in the form of excess development on the edges of the limb buds that would form the forelimbs. If the apical cap is larger than normal, extra toes will develop from it. It is worth noting that physical damage to the apical cap might also trigger the development of extra toes. Branching of the apical cap will lead to complete double paws or, if it branches early enough, to doubled limbs.

The first major study into classical polydactyly (thumb cats) was conducted in 1947. For his study "Heredity of polydactly in the cat" (Journal of Heredity 38, 107, 1947) Danforth studied two female polydactyl cats that he housed at his laboratory. The cats came from locations 85 km from each other in California, but the possibility of them being related could not be ruled out. He mated these females with different males and recorded the dates of mating and the physical traits of the kittens. He collated additional information from friends with polydactyl cats. He noted that the effects of polydactyly could always be seen on the front paws and sometimes also on the hind paws. On the front paws, the first digit was at least enlarged in one front paw and might be doubled or tripled. If the hind paws were also affected, there was at least a rudimentary dew-claw and sometimes additional claws. Where there was a doubled first digit, it was sometimes incompletely formed with the additional digit between the first digit and the other claws. The position of the first digit was also changed a little from normal to resemble a thumb. The four images show different footprints: a normal-footed front paw and 3 expressions of polydactyly:

Danforth mated all possible combinations of his cats: Poly x Poly, Poly x Non-Poly, Non-Poly x Non-Poly. This confirmed that polydactyly was a dominant gene, because in every mating where at least one parent was polydactyly there were polydactyl offspring. In matings between two normal-footed cats there were never any polydactyl offspring. In 3 matings, all offspring were polydactyl and therefore one or both parents were probably homozygous for polydactyly.

In guinea pigs, one form of polydactyly is lethal when homozygous. Danforth's studies indicated that this was not the case in cats. In a mating Pp x Pp (heterozygous parents) on average 25 % of the young will be homozygous for polydactyly, 50 % heterozygous for polydactyly and 25 % normal-footed. If the gene was lethal, the 25% of homozygous offspring would die before birth and the litters therefore would be smaller than expected. Danforth found the average litters to be almost the same size (4.12 with offspring homozygous for polydactyly compared to 4.35 for litters where no offspring could be homozygous). This also affected the ratio of polydactyl and normal-footed offspring in a litter. If the gene was lethal when homozygous, there would be (on average) 2 poly kittens for every normal-footed kitten. Danforth's cats produced a ratio of 77 poly kittens to 22 normal-footed kittens.

From these studies, Danforth concluded that polydactyly (at least the form that he studied) was a variably expressed dominant gene: "these data lend no support to the assumption that polydactyly in the cat is lethal when homozygous" nor was it associated with the cat's gender "The trait is not related to sex, and no evidence is found that its gene is lethal" He found no evidence of split foot or radial hypoplasia (a.k.a. radial hemimelia) in his studies though his second study into feline polydactyly ("Morphology of the Feet in Polydactyl Cats", 1947) found that cats with 6 metatarsals (toes) tended to have fusion at the ulna which caused varying degrees of rotation of the joint of the radius.

In 1955, Albert C Jude, author of "Cat Genetics"  documented two forms of polydactyly. He wrote that polydactylism (extra digits), and oligodactylism (reduction of toe number) were sometimes reported by the fancy, but were the exception (not selected for) rather than the rule within cat breeding. This meant little data was available on feline polydactyly to permit proper scientific study. He noted that polydactyly involving only the preaxial side of the limb had been documented in cats by Danforth in 1947 and was dominant over the normal form. Most cases of polydactylism in cats observed by Jude had affected the front feet only. In his 1955 book Jude also described a form of polydactyly which he called "posterior reduplication": Another interesting deformity - only very occasionally seen in cats, but more frequently seen in some other animals - is known as "posterior reduplication." The condition was found in a stock of mice by Danforth in 1923, and a described by him in 1930. The information came from Mrs. A. Winsor of Hull, a well-known English Abyssinian breeder: "Before the war, I had two little black she-cats. One came into season, and a strange gray tom came to investigate. His feet were really amazing. His front legs were very thick and stout, big feet, with normal number of toes. On the inner side of each foot was another smaller foot. A sort of stalk grew from the ankle, as if the ankle bone had been split, and this ended in a complete foot which rested on the ground alongside the normal foot, and turned slightly inward, When sitting he had to advance one leg, as he could not possibly put all his four feet side by side, and when walking he sort of lifted one foot over the other. He mated my queen who was calling at the time and there were two black kittens whom we put to sleep, and two gray-striped, both females. One had just thumbs; the other had seven toes - four ordinary, and three extra where the thumb would be. There was no stalk, but these three toes had a separate pad; they were about the same length as the others, and her feet spread out like paws. She also had a sort of 'thumb' half-way up each hind foot, with a claw on the end. The other gray-striped female - the one that had just thumbs - we kept for eight years and then she died. I managed to get a granddaughter who is now seven years old. She has had countless kittens, and about half of every lifter have the 'Family Feet.' "

According to the late Roy Robinson in his book "Genetics for Cat Breeders", there is considerable variation in the number of extra toes and in how well-formed they are. The trait ranges from an enlargement of the inside digit into a "thumb" to the formation of three apparently well formed extra toes (i.e. 7 toes on the affected foot). A cat may even have different numbers of toes on each of its front feet.

The hind feet are rarely affected and are only ever affected if the front feet are also affected. I have received a report of a Maine Coon with hind foot polydactyly and apparently normal fore paws; it seems likely that it was genetically polydactyl for all four paws, but that the extra toes had not been visibly expressed in the fore paws for some reason. I also received the following report about a random-bred hind-foot polydactyl with normal front paws.

Rufus (Photo Karen Kohl)

Karen Kohl's orange tabby kitten (Rufus, pictured) has no thumbs on his front paws and apparently normal front dew claws ("a short splinter of a nail on the inside of his left front paw"), but has 6 toes on each of his hind paws. The inside toes of each hind paw are thumb-like and have both a toe pad and an extra palm pad. One of them is retractile like his other toes and one isn't. Rufus was born in Boston in June, 2003. He and two normal-toed brothers were rescued feral kittens and probably inbred. Rufus was the only healthy one of the three. They were taken to a local cat shelter which had seen one or two cats with more toes on the back paws than on the front so Rufus isn't the only one and this form of polydactyly me be present in the local feral population. One explanation is that the polydactyly of the front paw has been suppressed by other genes. An alternative is that Rufus and other cats in that area have a mutation which affects only the hind paws - a form of polydactyly previously only seen leopards. Boston has a high incidence of polydactyly; either due to mutations occurring there or due to "lucky" polydactylous ships' cats being taken there with early settlers.

The normal cat's front paw has 4 toes and one dewclaw (rudimentary toe or thumb which does not touch the ground) while the back paw has 4 toes. A polydactyl will usually have one or two extra toes on each foot. Most polydactyl cats have a form of pre-axial (i.e. situated in front of the axis of a limb) polydactyly with the extra toes appearing on the thumb side of the foot. The gene for Polydactyly can give rise to either extra toes or extra dewclaws. Each extra toe has its own terminal pad (fingertip) and normally an additional palmar pad and additional plantar pad. When extra toes occur on the hind paws, these are not generally dewclaw. Some owners report their cats have five toes on the hind paws, however by definition a dewclaw does not touch the ground. Possession of a hind dewclaw or extra digit is considered a throwback in cats, but is relatively common in dogs (a photo is shown below for comparison).