How wild cats got their striking eye colors

Fans of Clementine, the cat whose rare eye color recently took TikTok by storm, may be intrigued to learn the results of a new study. 

The striking eye colors found in many wild cats, such as the piercing gold of cheetahs, the vivid blue of snow leopards, and the glowing green of leopards, can all be traced back to one single ancestor.

This feline progenitor, similar to an ocelot, roamed the Earth over 30 million years ago.

Variety of cat eye colors

Harvard University researchers suggest that this ancient cat population likely had both brown- and gray-eyed individuals, the latter of which set the stage for the wide variety of iris colors seen in cat species today.

Study lead author Julius Tabin is a student at the Griffin Graduate School of Arts and Sciences in Harvard’s Department of Organismic and Evolutionary Biology.

“When I started this study I asked, ‘What do we know about eye color?’ And the truth is, very little, as there are basically almost no phylogenetic evolutionary studies on eye color,” said Tabin. 

Studying eye color diversity in cats

Previous research has mostly focused on eye color distribution within a species or the genes involved in determining eye color in humans and domesticated animals.

However, studying eye color across wild animal populations is rare due to the challenges of preservation and a general lack of diversity – most animals have brown eyes.

While eye color in humans may be shaped by sexual selection, and in domestic animals by artificial selection, Tabin wanted to explore the reasons behind the eye color diversity in wild cats. 

Creative approach to mapping eye color

Lacking fossil evidence, Tabin and co-author Katherine Chiasson, a Ph.D. candidate at Johns Hopkins University, took a creative approach.

They analyzed digital images from online databases to categorize and study the eye colors of 52 species of wild cats.

Tabin and Chiasson used an algorithm to map these colors onto a phylogenetic tree of the Felidae family. “We found a lot of variability of color between species, but shockingly, we also found a lot of intraspecific variability,” said Tabin. 

“Most species have a singular eye color with no variation. So, it’s really surprising that once you get into the cats – lions, tigers, panthers, etc. — we see all these different eye colors. There are actually very few Felidae species that have only a singular eye color in their population.”

Eye color of early cats

With their findings mapped onto the family tree, the researchers set out to reconstruct the ancestral eye color of early cat species. 

They discovered that the pre-felid lineages, including the ancestor of modern cats and their closest relatives (linsangs), had only brown eyes.

However, after the linsangs diverged, felines with gray eyes emerged alongside those with brown eyes.

“It’s likely this happened due to a genetic mutation that drastically decreased the pigment in the eye,” Tabin explained. 

Melanin, the pigment responsible for color, can be either eumelanin (which is brown) or pheomelanin (which is yellow).

A decrease in eumelanin could lead to eyes that are neither fully brown nor gray but a mixture of both, which the researchers found in their analysis.

Evolutionary significance of gray eyes

The appearance of gray-eyed cats, according to the study, paved the way for the evolution of more colors, including greens, yellows, and blues.

These gray eyes acted as a transition between the original brown eyes and the development of new shades.

“Blue eyes require carefully balanced low levels of pigment and are likely recessive in felids. A wild population would probably not be able to maintain blue eyes in a population with only one blue-eyed individual among a sea of brown eyes,” Tabin said. 

“It’s probable that you would need something lighter than brown, but not as light as blue, to be the mediator. And that’s what you see: In every single cat species with blue eyes, they also have gray eyes.”

Many questions remain

The researchers also found that brown and yellow eyes rarely coexist within the same species. Additionally, they observed a surprising correlation between yellow eyes and round pupils, as well as a negative correlation between brown eyes and round pupils.

Despite these intriguing correlations, the researchers did not find any significant connections between eye color and factors such as activity patterns, habitat, or geographical region.

This leaves the question of why eye color evolved to be so varied in wild cats an open field for future study.

Not only did the team reconstruct general eye color types for each evolutionary ancestor, but they were also able to predict the exact color of these ancient felines’ eyes. 

“Being able to reconstruct color quantitatively is one of the paper’s greatest strengths, because it means we are the first animals to see the color of these eyes since these felids were alive millions of years ago,” Tabin said.

Increasing accessibility of science

For Chiasson, part of the excitement of the study came from using publicly available resources for their research. 

“The fact that rigorous studies like ours can be done by anyone with an internet connection and some curiosity is indicative of a field-wide revolution that is increasing the accessibility of science around the world.”

Future research directions 

Looking ahead, Tabin hopes this study will inspire further research into the evolutionary significance of gray eyes and the broader patterns of eye color evolution in natural populations. 

“I’m still riding high on the excitement of knowing that the felid ancestor had both brown and gray eyes, because that’s something I didn’t go in expecting or even thinking about,” he said.

This study opens new avenues for understanding how the mesmerizing eyes of wild cats came to be, adding yet another layer of mystery and fascination to these majestic creatures.

The study is published in the journal iScience.

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