If you've ever seen a litter of kittens, you may know that many of them barely look related. With coats ranging from black to white, tortoiseshell to tabby, and even long-haired to short-haired, littermates can look quite different from each other.
So why do kittens often not resemble their close relations, whereas most human siblings look similar to each other? The answer comes down to the complicated nature of cat coat genetics and a phenomenon in cat ovulation.
The genetics of cat coat color
The Cat Fanciers' Association recognizes dozens of coat colors and patterns, including everything from basic solid colors to more distinctive patterns, like the Bengal's rosetted tabby coat or the Abyssinian's ticked fur. All of those variations stem from a complex system of interacting genes.
"There are a number of different genes involved in cat color and pattern, and it's particularly complicated because some genes overwrite other genes," Jonathan Losos, a professor of biology at Washington University in St. Louis and author of "The Cat's Meow: How Cats Evolved from the Savanna to Your Sofa" (Viking, 2023), told Live Science.
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Different genes control solid coat colors, spotting, patterns and hair length. According to Losos, these genes come in a "hierarchy" — for example, one gene mutation for white fur, called dominant white, will override any other colors. Other genes control patterns such as tabby fur, and these work in combination with genes for color to produce a coat. Some cat coat colors are also sex-linked, meaning the gene is contained on the X chromosome, so the range of colors and patterns for male and female cats is different. In general, only female cats have calico or tortoiseshell patterns.
If all of these gene variations are hard to keep track of, that's the point. Even a few different combinations of a handful of genes can have drastically different outcomes for coat color, pattern and length, and that's one big reason cat littermates can look so different even when they have most of the same genes. But there's another factor that makes the genetics of cat litters even more complicated.
Multiple paternity
Female cats are induced ovulators, which means their reproductive system doesn't release eggs until they've already mated with a male. This increases the chance of successful fertilization, but it also comes with a catch: The female can release multiple eggs during this time, so if she mates with another tomcat in the next few days, she can become pregnant by multiple males.
That means kittens in the same litter can have more than one father, so some siblings are only 25% genetically related to each other, leaving a lot more room for variation in their appearance. This phenomenon is called heteropaternal superfecundation, and it's actually not that rare in the animal kingdom; dogs, sheep and cows can also have multiple births with more than one father. In extremely rare cases, heteropaternal superfecundation has even been documented in humans.
"I don't think it's unusual that cats do this," Losos said.
Scientists don't know exactly why heteropaternal superfecundation is so common in cats, but there are theories.
For one, induced ovulation could be a more resourceful way to go about mating, and heteropaternal superfecundation is a side effect of induced ovulation. "It might be an evolutionary adaptation that allows the mating process to be more efficient," Bruce Kornreich, director of the Cornell Feline Health Center, told Live Science. He explained that releasing eggs only after mating prevents eggs from being "wasted" so that the female cat's reproductive system doesn't use energy on unneeded eggs.
Multiple paternity within a litter could even have its own advantages, Losos noted. When littermates have different fathers, it increases the genetic diversity of cat families, which is generally good for survival.
Heteropaternal superfecundation is much more likely to occur in urban areas, where the concentration of cats is high and tomcat territories are likely to overlap. In fact, one 1999 study found that 70% to 83% of urban cat litters had more than one father, whereas only zero to 22% of rural litters had more than one father, making diverse litters the norm rather than the exception.
