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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.
Updated 2/9/06 |
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