Huge variety of eye colors in today’s cats may trace back to distant ancestor’s unusual peepers

Tabin mapped these colors onto a tree of their evolutionary relatedness. That allowed him to use statistics to infer which eye colors were likely present in the cats that gave rise to all living felids, a method known as ancestral state reconstruction. Those first cats likely had brown and gray eyes , the pair reported recently on the preprint server bioRxiv.

It’s “awesome” that the team found a way to survey iris colors for so many felid species, says Rosalyn Price-Waldman, a Princeton University graduate student and evolutionary biologist who studies coloration and color vision in birds. Traditionally, studies of animal coloration rely on museum specimens or preserved tissues, she says. But irises don’t keep well, making it hard to get good data. “It’s already a huge deal to be able to say, ‘These species have this number of eye colors.’”

The brown eyes aren’t remarkable, Tabin says, because that eye color is present in the closest relatives of the felid family like hyenas and civets. “But it was surprising that [as soon as] gray shows up,” he says, “the eye colors explode.” That could be because gray is an intermediate color—an interruption in the production of the melanin pigments that darken the iris. The interruption may have granted cats the flexibility to more easily evolve different balances of pigments and therefore different colors. Indeed, the analysis indicated that all the other iris colors seen in cat species today, including yellow, blue, hazel, and green, emerged independently in more than one lineage. Hazel and green eyes appear to have evolved a dozen different times.

Tabin suspects that these particular colors may have been so popular because cats preferred them in their mates, a bit like how humans tend to seek out partners with striking eyes .

Price-Waldman isn’t so sure. She points out that even for humans, the evidence for the idea is thin, and as long as a color isn’t notably detrimental to the animals, it could evolve at random.

Shu-Jin Luo, an evolutionary geneticist and wildcat expert at Peking University, similarly wonders whether eye color has less evolutionary meaning for cats, perhaps because most species are active at night.

She also points out that coat and skin color often correlate with eye color in animals—because they both rely on the production of melanin pigments—so it could be that eye color is a “side effect” of selection on coat color. Still, she says the information and insights in the paper are valuable to researchers like her and will form the foundation of research into the evolutionary questions raised by the findings. “It will be interesting to see where it goes.”