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

If you get lost in the luminous orange peepers of housecats or the baby blues of white tigers, thank the granddaddy of all felines—an ocelotlike creature that lived more than 30 million years ago. A new study finds that this distant ancestor of lions, tigers, and housecats sported brown and gray eyes, the latter of which allowed its descendants to evolve a veritable rainbow of iris colors.

“I love this paper,” says Juan Negro, an evolutionary biologist at the Doñana Biological Station who has spent decades studying animal coloration. “Eye coloration in cats is something that, surprisingly, hasn’t been approached by scientists before,” he says, perhaps because it’s so challenging to study in such elusive animals. “I really want to thank these guys for daring to deal with these things.”

The work began as a class project for Harvard University graduate student and evolutionary biologist Julius Tabin. He initially thought to look at patterns in eye color in people, but turned to wild animals when he noticed the topic was so understudied.

With the help of Katherine Chiasson, his romantic partner who at the time was a graduate student at Johns Hopkins University, Tabin collected images of cats from iNaturalist, a nature-focused social network and crowdsourced species identification system, and Google Images. After filtering for various quality aspects, such as a clear, unshaded view of at least one of the animal’s eyes, the duo narrowed down the collection to 279 irises from 52 cat species and subspecies. Tabin then employed algorithms to quantitatively identify and sort the animals’ iris colors into distinct groups. About 80% of the cat species and subspecies examined had more than one eye color.

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.

a cougar, snow leopard, and white tiger
Crowdsourced images revealed that most cat species, including cougars, snow leopards, and tigers (left to right), have more than one possible eye color. left to right: Evgeny555/iStock; Gypsy Picture Show/iStock; Voraorn/iStock

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.”