Polydactyl cats, also known as “polydacts” or “Hemingway cats”, are felines that have extra toes on one or more paws. While the average cat has 18 toes (five on each front paw and four on each hind paw), polydactyl cats can have as many as nine toes on their front or hind paws.
Polydactylism is caused by a dominant genetic mutation, meaning a cat only needs one copy of the mutated gene to exhibit the trait. Throughout history, polydactyl cats have been prized in some parts of the world, often being associated with good luck.
One question that arises regarding polydactyl cats is whether the mutation that causes extra toes is associated with inbreeding. Specifically, some have proposed that concentrated populations of polydactyl cats, such as Ernest Hemingway’s famous cat colony in Key West, may have high rates of polydactylism due to extensive inbreeding over generations.
In this article, we will examine the evidence around whether inbreeding does indeed lead to higher rates of polydactylism in cats. We will look at specific polydactyl populations, genetics, and alternative explanations for why certain groups of cats have more extra toes than others.
Background on Polydactyl Cats
Polydactylism, also known as hyperdactyly, is a genetic condition that causes animals to be born with extra toes on one or more paws. It is relatively common in cats, occurring in approximately 1 in 500 domestic cat births (https://en.wikipedia.org/wiki/Polydactyl_cat). Cats normally have a total of 18 toes, with 5 toes on each front paw and 4 toes on each hind paw. Polydactyl cats, however, can have as many as 9 toes on their front paws and 8 toes on their hind paws.
Polydactylism results from a dominant genetic mutation and is hereditary. Cats that possess the polydactyl gene have a 50% chance of passing it along to their offspring, even if they do not express the trait themselves. Interestingly, polydactylism occurs more frequently in male cats than females, for reasons that are not fully understood.
While polydactylism was once considered an undesirable trait, many cat owners and breeders now selectively breed polydactyl cats due to their unique appearance. Polydactyl cats are able to grasp and climb in ways that non-polydactyl cats cannot.
Genetics of Polydactylism
Polydactylism in cats is caused by a dominant genetic mutation, referred to as the Pd gene. This mutation causes extra toes to form during embryonic development. Cats with the dominant Pd gene will exhibit polydactylism, having extra toes on one or more paws, while cats without the mutation will have the normal number of toes.
The Pd genetic mutation is autosomal dominant, meaning a cat only needs one copy of the gene from either parent to exhibit polydactylism. The gene is highly penetrant, meaning nearly 100% of cats with the mutation will have extra toes. Even if a cat inherits only one copy of the Pd gene, it will likely be polydactyl.
Scientists have identified that the Pd mutation occurs in the gene called “Sonic Hedgehog.” This gene plays an important role in patterning the limb during embryonic development 1. When mutated, it causes extra toes to form. All polydactyl cats share this same genetic mutation originating from a single ancestor cat.
Inbreeding and Genetic Mutations
When closely related cats mate, such as siblings or parents and offspring, this is considered inbreeding. Inbreeding increases the chances that two cats will have similar DNA, including carrying the same recessive genes. According to Wikipedia, “Inbreeding results in homozygosity, which can increase the chances of offspring being affected by recessive traits. In extreme cases, this usually leads to at least temporarily decreased biological fitness of a population.”
Essentially, inbreeding can bring together rare, recessive mutations that may not be expressed when cats mate outside of family lines. But when closely related cats mate, they are more likely to both carry the same recessive genes, leading to a higher chance of those traits being expressed in offspring. As the Sciencedirect overview on inbreeding states, “Inbreeding can lead to higher frequencies of genetic defects.”
So while inbreeding itself does not cause new mutations, it increases the likelihood that recessive mutations shared by family members will be expressed phenotypically in their offspring. This helps explain why rare traits like polydactylism are more common in intensely inbred cat populations.
Sources:
https://en.wikipedia.org/wiki/Inbreeding
https://www.sciencedirect.com/topics/immunology-and-microbiology/inbreeding
The Boston Cat Population
Boston has a high population of polydactyl cats, with an estimated 25-40% of cats having extra toes. Some people have speculated that this is due to inbreeding among the Boston population of cats (Source). However, there are alternative explanations for the high rate of polydactyly in Boston cats besides inbreeding.
One idea is that polydactylism provided an evolutionary advantage for surviving the harsh New England winters, allowing cats with extra toes better stability and traction on snow and ice. The trait became concentrated in the Boston area over time through natural selection. But without definitive genetic evidence, the theory that Boston’s polydactylism frequency stems from sustained inbreeding remains unproven.
Ernest Hemingway’s Polydactyl Cats
The famous author Ernest Hemingway was gifted a six-toed cat named Snow White while living in Key West, Florida in the 1930s. Snow White and other polydactyl cats lived on Hemingway’s property and over the years they inbred and produced litter after litter of kittens with extra toes. When Hemingway died in 1961, his house and the polydactyl cat colony was left to his friend Toby Bruce who continued caring for the cats. Today, there are around 50-60 polydactyl cats living on the grounds of the Ernest Hemingway Home and Museum in Key West, all believed to be direct descendants of Hemingway’s original polydactyl cat Snow White.
The tight-knit colony has experienced many generations of inbreeding as the polydactyl cats bred amongst themselves within the confined area of Hemingway’s estate. In the 1990s, some outside cats were introduced to diversify the gene pool, but the majority of the population still descends from that original line of inbred polydactyl cats Hemingway cared for in the mid-1900s (Source).
Other Polydactyl Cat Populations
In addition to the famous polydactyl cat colonies started by Ernest Hemingway, there are other notable feral cat populations with high rates of polydactylism. One example is the cat population in Sitka, Alaska. According to an article on KTOO, around 40-50% of the feral cats in Sitka have extra toes, which is a significantly higher rate than the general cat population https://www.ktoo.org/2022/06/30/sitka-cats-have-many-toes/. The article cites the island’s limited gene pool and inbreeding as the likely cause.
Another feral cat colony with a high rate of polydactyl cats is found on the Isle of Man. According to Wikipedia, around 30% of the feral cats on the Isle of Man are polydactyl, compared to just 6-7% in the overall cat population https://en.wikipedia.org/wiki/Polydactyl_cat. Once again, the isolated island environment and inbreeding are believed to contribute to the increased incidence of polydactyly.
These examples demonstrate that in certain isolated feral cat colonies, likely due to limited genetic diversity and inbreeding, rates of polydactylism can be significantly elevated compared to the general cat population.
The Evidence for Inbreeding Causing Polydactylism
Several studies have examined the link between inbreeding and polydactylism in cats. One key piece of evidence is a 2020 study published in the Journal of Feline Medicine and Surgery that analyzed the genetics of polydactyl cats in Boston (https://journals.sagepub.com/doi/full/10.1177/1098612X20911448). The study found that the polydactyl trait in Boston cats was associated with a single spontaneous dominant mutation that became prevalent through inbreeding in the local cat population. This provides strong evidence that inbreeding can lead to a higher frequency of polydactylism.
However, other studies have found different genetic causes of polydactylism in other cat populations. For example, research on polydactyl cats in the Pacific Northwest traced the trait to a different spontaneous mutation (https://www.mdpi.com/2073-4425/5/3/211/htm). So while inbreeding may play a role in some polydactyl cat populations, it does not appear to be the universal cause.
Overall, the evidence suggests that inbreeding can contribute to polydactylism in cats, but likely acts in conjunction with spontaneous mutations arising and spreading within isolated cat colonies. More research is needed to fully understand the complex genetics behind this interesting trait.
Alternative Explanations
While inbreeding may increase the chances of polydactylism in some cat populations, there are other potential causes as well. Spontaneous genetic mutations can arise randomly and lead to polydactylism, even without inbreeding. As noted in an overview on polydactyly from Healthline, “The syndromes are thought to be caused by genetic mutations that affect the signaling pathways during an embryo’s development.” https://www.healthline.com/health/polydactyly These spontaneous mutations can happen in any cat population.
Additionally, polygenes – multiple genes working together – may play a role according to research: “The inheritance patterns observed indicate polydactyly is a complex trait with a major gene(s) along with modifier genes contributing to varied phenotypes.” https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9771235/ The interactions of multiple genes, rather than a single recessive gene, could lead to higher rates of polydactyly in some cat groups.
Environmental factors may also contribute in currently unknown ways. Overall, while inbreeding is one hypothesis for higher polydactyl rates in certain cat populations, spontaneous mutations, polygenes, and environmental factors provide alternative explanations worth considering.
Conclusion
In summary, there is some evidence that inbreeding can lead to an increased rate of polydactylism in cat populations, but it is not the sole or definitive cause. The genetics behind polydactylism are complex, involving multiple genes. While inbreeding can increase the expression of recessive traits, cats not from inbred populations can also be born with extra toes. The high rates of polydactyl cats in certain populations, like Ernest Hemingway’s cats in Key West, are likely due to founder effects and isolation, rather than inbreeding alone. So while inbreeding may play a contributing role, it is overly simplistic to say it directly causes polydactylism in cats. The true causal factors involve a complicated interplay of genetics, selective breeding, and closed populations. But the end result is generations of adorable polydactyl kitties!
