In the New Scientist of 12 May, 2001, it was asserted by Chris Hayes that “five is the magic number” of digits. This was based on the assumption that the genetic mutation that produces the extra toes is also responsible for deformities and that possession of more than five digits is a “counter-survival trait”. His assertion was based on observation of mice with deformities caused by genetic mutations. However, what is true for mice is not necessarily true for other animals and there are many healthy cats with six or more digits. Fossil evidence shows that early amphibians also had 7 or 8 toes.
WHAT IS POLYDACTYLY?
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 known by various names – “mitten cats”, “thumb cats”, “six-finger cats” and “Hemingway cats”. The latter is because of writer Ernest Hemingway who made his home on the small island of Key West, Florida. He shared the island with nearly 50 cats, including a 6-toed polydactyl given to him by a ship captain; the cats bred and the polydactyl trait became common, hence polydactyls are often known as “Hemingway Cats”. Hemingway’s colony of cats was free-breeding with the local cat population and the ratio of polydactyl cats to normal-toes cats was about 50/50. Another story suggests that the cat given to Hemingway was a female double-pawed cat and that the polydactyl cats on the island came from 19th Century ships’ cats. The high rate of polydactyl cats in Boston, USA has also led to the nickname “Boston Thumb Cats”. The nickname “double-pawed” cats is a misnomer since there is a specific double paw condition.
Polydactyly itself is also known as hyperdactyly or supernumary digits and occurs in many animals (including humans) as a spontaneous mutation or hereditary trait. The mode of inheritance – dominant or recessive – varies between species. Instances and illustrations of human polydactyly (for comparison purposes) are given at the end of
AN ALL-AMERICAN TRAIT OR AN AMERICAN CONCEIT?
Some American cat lovers, cat publications and breeders (notably early advertising literature about PixieBobs) have claimed that polydactyly is unique to America. In early PixieBob promotional literature “Bigfoot is in the Building!” Brenda Weatherby and Carol Ann Brewer wrote that polydactyly first appeared in Boston in the early 17th century and that polydactyl cats are only found in North America. In actuality, polydactyl cats may have arrived in Boston from England. Polydactyly is emphatically not unique to North America. Any suggestions that polydactyl cats are not found outside of America are factually incorrect and perhaps an American conceit. The trait is found in Britain, some parts of mainland Europe and in Asia. Polydactyly is common enough in some areas of Britain for it to be almost unremarkable and I see several polydactyls per year at the Chelmsford Cats Protection rescue shelter as well as having owned a polydactyl. I found a superb bobtailed polydactyl cat in rural Malaysia (close to Kuantan) where “six-finger cats” are considered lucky. Being an unneutered tomcat, the trait would be perpetuated.
An article from Cornell University, Cat Watch (1998), cited studies into polydactyl cats from the 1940´s through to the 1970´s. The study indicated that the trait may have occurred in cats taken to Boston by English Puritans during the 1600s and speculated that the mutation developed in cats already in the Boston area rather than in cats in
England. The progeny of these cats may have travelled on trading ships from Boston to Yarmouth, Massachusetts and Halifax, Nova Scotia, two areas which also have a high incidence of polydactyly. Charles Darwin wrote of polydactyl cats in his book “Variation of Animals and Plants Under Domestication”. He wrote: “I have heard of several families of six-toed cats, in one of which the peculiarity had been transmitted for at least three generations.”
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 mainland (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.
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, c
ompared 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.
Bjørn B Svingen, owner of a polydactyl cat, provided the following information on polydactyl cats in Norway and their associaiton 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 Trøndelag in mid Norway.” As a result, they are also known there as “Skipskatt” (ship-cat).
In a survey and detailed account of cats he found 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.
THE GENE(S) FOR POLYDACTYLY
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. 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, and possibly only, 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 was a variable expressed dominant gene with no reason to suspect it was lethal when homozygous: “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 did not find evidence of split foot or radial hypoplasia 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” (a book that had as much about mice and rabbits than cats!) 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.
Jude also described another form of polydactyly, the type we now call “mitten cats” but which he called “posterior reduplication”, in his 1955 book: 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 description was published by him in 1930. This deformity is mentioned here mainly to show how information of a helpful nature can be given by fanciers. In this instance it came from Mrs. A. Winsor of Hull, a well-known English Abyssinian breeder. Before the war, says Mrs. Winsor, “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.’”
It is interesting that Jude differentiated between the 2 types of polydactyly in 1955; differentiating between the two forms has recently become a concern for cat breeders due to the occurrence of Twisty Cats.
According to the late Roy Robinson in his book “Genetics for Cat Breeders”, polydactyly has been officially (scientifically) recorded as early as 1868, though it had been observed earlier and seen frequently since. The distinguishing feature is the presence of extra toes, most noticeably on the front feet. Robinson explains that 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.
Karen Kohl’s orange tabby kitten (Rufus, pictured) has no thumbs on his front paws and apparently normal front dew claws (described as 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).
In August 2002, Dominic emailed about the appearance of “thumbs” on the back feet of a polydactyl cat. Both of his polydactyl kittens appeared to have rear thumbs with claws (which were not fully retractile, probably due to the kittens’ young age). The photo above is one of Zelda Annabelle’s hind feet. On closer examination, the extra hind toe is not really a thumb. To be a thumb it needs to have not only a terminal pad (the “fingertip”) but also a palm pad (like the palm of a person’s hand or the ball of a person’s thumb). A feline “thumb” sticks out at a different angle to the other toes, like the thumb of a mitten. It can be wiggled independently of the rest of the foot. Zelda Annabelle’s additional hind toes are typical of polydactyl back feet – they do not have the palm-pads and they follow the curve of the foot. Because the hind paws are constructed differently to the fore paws, there are rarely true “thumbs” on the back feet (only one or two reported cases of opposable hind thumbs, but no photographic evidence).
In reviewing the various reports of polydactyly, Robinson noted that many indicate a dominant gene, but that not all cases need be due to the same mutant gene. It is possible for the exact same gene to have arisen by mutation in different localities at different times which could account for the similar heredity. Some gene loci (areas of chromosome) are more prone to mutation than are others. However, Robinson cautioned his reader that other cases of polydactyly might prove to be inherited differently.
During the 1990s, other researchers reported forms of polydactyly which they believed to be recessive. One researcher into feline curiosities suggested two different dominant forms and one recessive form all of which had subtly different effects on the structure of the paw. This was based on the sudden appearance of polydactyl cats in a population of normal-toed cats. This could only have occurred through a gene mutation or through recessive genes. Apparently the evidence among a number of random-breeding cats suggested a recessive gene for polydactyly.
Dan Williams contacted me in July 2006 about a localised population of cats with about 5-10% polydactyls in Syracuse, Central New York. The “7-toed cats” that are consistent in conformation: 4 regular toes, with an extra toe just outside the innermost digit, but the foot sits flat, including the innermost digit. The 7th toe comes from counting the vestigal toe (dew claw) on the outside, up from the actual paw. There is a digit-less claw between the opposable “thumb” and the wrist. The paw-pads resemble a human hand-print with no separate pad for the dew-claw toe. An analogy is a human with a tiny duplicate unjointed thumb (with nail) growing from the first joint of the normal thumb and which moves with the thumb. Dan thought the gene might be recessive as it only appeared in 5-10% of these random-breeding cats. He also thought it was a mainly male trait. If verified, this would be different from Danforth’s classical polydactyly which is a non-sex-linked dominant trait.
Since polydactyly is seen more commonly in cats compared to other mammalian species, what is it about the cat genome (or kitten developmental processes) that makes polydactyly this common? It could simply be the location of certain gene(s) on the chromosome(s). Because of the way chromosomes are duplicated and shared out in cell division when eggs and sperm are made, some areas of chromosomes are more prone to mutation than others. These are known as mutational hot-spots. During cell division, chromosomes duplicate and the chromosome pairs are physically joined together; these are pulled apart and during separation genes can cross over from one copy to the other. Genes adjacent to the join might be affected by the separation process, resulting in small changes. This hot-spot effect could account for the spontaneous appearance of unrelated polydactyl cats in widely separated areas.
Some polydactyls (mitten cats) have double or triple dewclaws because the genes seem to give the instruction “add another digit to what is already present” rather than saying “produce 5 toes instead of 4″ or “produce 6 toes instead of 4″. Back in the 1960s, a cat lover in an English village reported a high incidence of polydactyl cats, mostly fathered by a local tomcat. Each generation of cats had more toes than the mother – as if the genes simply said “add another toe”. Presumably there was either a limit to how many toes could be added or the tomcat left the area as the phenomenon was not reported again.
There is a similar report from the USA. Each successive generation of a colony of barn cats had more toes than the previous one. Eventually this led to crippling. The colony was severely inbred with each generation being fathered by the same polydactyl tomcat. When he disappeared, a non-polydactyl cat took his place and no further crippled kittens occurred. I also received a first hand account of crippling in later generations of polydactyl barn cats (relating to the writer’s grandmother’s farm). This may be the same case. Although the latter report of inbred polydactyl farm cats claimed up to 12 toes per paw, this extreme number is unlikely; 8 or 9 is generally the upper limit. The gene is variable in expression so in these colonies, paws range from mitten-paws (with a “thumb”) through to double paws.
So far, veterinary literature has not confirmed the “add another toe” gene. Even in the homozygous state (which would occur in inbred colonies), the most common form of polydactyly does not appear to be detrimental to health. However, there is another gene, which resembles polydactyly, which causes severe crippling. Radial hypoplasia (RH) is discussed later in this article.
Polydactyly is probably an incomplete dominant. With a normal dominant trait, a cat either has the trait or doesn’t have it. With an incomplete dominant there are different “levels” of the trait depending on whether the cat is homozygous or heterozygous for the trait. A homozygous cat shows the trait more fully than a heterozygous cat. This may have been the explanation for the apparent “add another toe” form of the gene in the English village cats. Observation suggests that the incomplete dominant is expressed only 40%-50% of the time when inherited – this concurs nicely with the 50/50 split of polydactyl/non-polydactyl in Hemingway’s colony.
The suggested recessive form may also be due to incomplete dominance. The apparently “normal footed” cats would have been polydactyls with barely discernible extra toes, but whose offspring had more prominent extra toes – giving the appearance of either spontaneous mutation or recessive genes.
Because several different genotypes (genetic make-up) produced a similar or identical phenotype (physical appearance) it could be difficult to identify which cat carried which mutation. As it turns out, there are several forms of polydactyly due to several genes which produce a similar-looking trait and in the 2000s, the need to identify and isolate the various forms became crucial due to fears of a link with radial hypoplasia (Twisty Cat mutation). There was evidence for a second dominant form of polydactyly which resulted in a different form and structure to the familiar “mitten cat” form and which affects not only the paw, but the whole leg.
The form of polydactyly most commonly seen in cats is a simple autosomal (i.e. not linked to gender) dominant trait which does not affect the cat adversely and is not associated with other abnormalities. Despite suggestions of “natural snowshoes” there is no real evidence that polydactyly has any significant natural selective advantage or disadvantage. If it was disadvantageous, polydactyl cats would quickly have died out. It is simply an endearing anomaly.
It has been said that if the parent has extra toes, the kittens inherit extra toes in the same configuration as the polydactyl parent. So, if the parent has double dewclaws, the kittens have double dew claws and if the parent has an extra toe, so will the kittens. This may not be 100% true because there are undoubtedly numerous gene mutations causing polydactyly. This may be true in polydactyl breeding programs where cats are carefully matched, but in the random-breeding population, the incomplete dominance of polydactyly means the configuration is variable – and it also depends on what genes the kitten inherits from the other parent!
It is reported that in Maine Coons (where there is work to restore the polydactyl form of the breed), non-polydactyl kittens born to a polydactyl parent appear to have heavier boning in the legs and chest than kittens born to two non-polydactyl parents. In random-breeding cats, polydactyls often appear robust, but this may be due to the overall impression caused by big feet.
Copyright & Credit:
Sarah Hartwell – MESSYBEAST.COM
Photo copyright and courtesy: Sarah Hartwell