June is Adopt A Cat Month, and here at the Schwarzman Animal Medical Center, we’re celebrating our feline friends with an entire month of blogposts devoted just to cats. I’m hoping many kittens and cats will get “fur”ever homes this month.

Earlier this month, I wrote about red eyes in cats, a common condition affecting our feline companions. For my final blogpost of the month, I’m returning to eyes—but this time exploring the evolution of feline eye color and what it tells us about our feline companions.

The Science Behind Feline Eye Color

A study published in late 2024 used sophisticated image analysis on a large database of high-quality photographs of 52 different feline species to investigate eye color patterns in cats. The researchers made sure to select images of adult cats since kittens may have a different eye color than they will as adults—like my darling foster kitten, Snickers, with his striking blue eyes in the photo above.

After image analysis, the authors mapped their findings onto the evolutionary tree of these feline species to draw conclusions about the development of feline eye color. The authors also investigated the relationship between eye color and pupil shape. Fun fact: our housecats and most other felines have vertical pupils, but a few feline species—like the Pallas cat of central Asia, lions and tigers—have round pupils.

Five Distinct Eye Colors in Cats

The study identified five different eye colors across feline species. Grey was the most common eye color, appearing in 38 species, followed in descending order by brown, yellow, green and blue. No species had all five colors, and only five groups had four colors.

This finding reflects an important biological principle: feline species with brown-eyed members never have blue-eyed individuals, and vice versa. Brown and blue are at opposite ends of the pigmentation spectrum, so either a species has abundant pigmentation (brown) or little pigmentation (blue). This makes evolutionary sense and helps explain the distribution patterns we observe.

Analysis of the study data suggests ancient ancestral cats had brown and grey eyes. Green and blue eyes, the least common colors in modern species, must have appeared late in feline evolution, which explains why they are the least common eye colors. The emergence of green and blue eyes was independent of one another.

The Evolution of Domestic Cats Eyes

Over millennia, today’s house cats have evolved (or been selectively bred) to have green eyes and no longer have brown in their repertoire of eye colors. While green eyes can be found in any cat, they are particularly prized in purebred cats like the Egyptian mau, Russian blue and Havana brown.

Our companion cats typically have vertical slit pupils, a feature commonly found in “ambush” predators. If you’ve ever watched your favorite feline stalk a toy mouse, you’ve witnessed this predatory behavior firsthand! Vertical slit pupils have extra muscles to allow for greater constriction than round pupils, giving the predator a visual advantage over their prey.

Interestingly, round pupils evolved exclusively in cats with yellow eye color, suggesting some link between the two features that needs further investigation.

Feline Eye Color and Behavior

A second study looking at feline eye color investigated the link between breed, coat color, eye color and behavior. While the researchers found that coat color did influence certain behavioral traits, they determined that these connections were primarily driven by breed characteristics rather than coloration itself, since coat color patterns are often breed-specific.

The study also found no correlation between eye color and behavior—proving, at least in cats, that their eyes are not a window to their soul.