Eyes of the Kitten

My cat Romana has beautiful eyes. If I had to describe them as being a certain color, I’d call them yellow, but they’re actually multi-colored and the shades vary. Sometimes they’re greenish blue near the pupil, then gradually range to greenish yellow, and finally yellow at the outside edge of the iris. Other times, they’re completely yellow, but the center is vaguely greenish and the outer part is orangish. They’re very fascinating eyeballs. And they make me wonder, why don’t cats have the same eye colors as humans? Why is it possible for cats’ eyes to change colors so much more drastically than humans do? And why does Romana have multiple eye colors?

So, of course, I googled it.

Bo and his yellow eyes

It is common knowledge that cats’ eyesight works differently than humans’. They can see in the dark, they are more nearsighted than people are, and their perception of color is less precise than that of humans. (Contrary to common belief, it is not true that cats or dogs can’t see color, but it’s true that they cannot distinguish nearly as many shades of color as humans can, particularly on the red end of the spectrum.) All of these differences are due to the fact that the anatomy of a cat’s eye is slightly different than that of a human’s eye. The most visibly obvious differences are that a cat’s eye is larger in proportion to its head than a human’s eye is and that the cat’s pupil varies more in size. This second property is related to the fact that cats’ pupils are slit-shaped rather than round, and it is one of the main reasons that cats have good night-vision. The other reason is that cats have a reflective layer behind the retina, which is called the tapetum lucidum. The reflective property of the tapetum lucidum is the reason that cat’s eyes glow in the dark, and that the pupil sometimes appears to glow green. Many other types of animals, including dogs, also have this layer, but humans do not.

Heidegger and her yellow eyes (Although you can’t see it in this picture, they have more of a greenish tint than Bo’s do)

All of these facts were the things that showed up on my google searches, but it took a little while longer to find information about eye colors. I know that eye color is determined genetically and has to do with pigmentation, and it makes logical sense that different species have different genes, but I was looking for an answer that was a little more technical and specific than that. I wrote a few paragraphs earlier today, which I later deleted, which speculated about the shape of a cat’s eyeball. Based upon various diagrams I found, it does appear that cats’ eyes have a greater space between the cornea and the iris than people’s eyes do. This is interesting, and it verifies something that I had already guessed, based upon the fact that the surface of a cat’s eye appears transparent when you see the cat’s face in direct profile. But I was just randomly speculating when I thought that might affect the appearance of the iris. The reason that I deleted those paragraphs, and instead summarized that information in just a couple sentences, is that I realized that it was probably wrong when I finally found more information about the pigments that determine eye color.

This lovely kitten lived on my college campus. I think I took this picture in May my sophomore year.

There are two types of these pigments.  One, melanin, determines how dark the eye is.  People with a lot of melanin in their eyes have brown eyes, while people with less melanin have blue, green, gray, or hazel eyes. The other type of pigment is called lipochrome. Lipochrome is yellowish, and people with a lot of lipochrome in their eyes will have green eyes while people with very little lipochrome have brown, blue, or grayish eyes. The amounts of these two pigments are determined by two different types of genes, and their combination defines the shade of eye color. For example, hazel eyes have a bit more melanin and a little less lipochrome than green eyes. As a side note, violet eyes occur when there is absolutely no melanin in the iris, which means that light actually reflects off the blood vessels in the retina. The purple color is a combination of the colors of the blue iris and the red blood vessels, and it is extremely rare.  This reflective phenomenon is also the cause of red eyes in photographs.

Much less research has been done on the pigmentation of cat eyes, but it is my guess that cat eyes have the same pigments, just in different quantities. Based upon the eye colors that are common in humans and cats, it would seem that cats, in general, have less melanin and more lipochrome in their irises than humans do. This explains why human eye colors are most often blue or brown, and the most common cat eye colors are yellow and green.

A picture of an odd-eyed cat that I got on Google, since I don’t know any odd-eyed cats personally.

Odd-colored eyes (which are more common in cats than in humans) obviously occur when the eyes have different amounts of pigment. Multi-colored irises, (like those in my Romana’s eyes, or the eyes of the actor Baconstrip Cucumberpatch, who plays Sherlock and who was Khan in the new Star Trek movie) evidently are caused by an uneven distribution of pigment throughout the iris. Based upon my observations, it would seem that lipochrome is more likely to be uneven than melanin. Green and yellow eyes are more likely than blue or brown to be flecked or to consist of a spectrum of shades

Pictured: the aforementioned eyeballs of Buttermilk Colorswatch. You know, that actor who plays Sherlock and who was Kahn in the new Star Trek movie.

It is also worth noting that, strictly speaking, a person’s eye color doesn’t change according to mood or what they’re wearing. When the color of the iris appears to change, that is actually an effect of the lighting, which could be affected by the change of facial expression. The iris itself is a consistent color, except in the eyes of very young babies and eyes that have suffered some kind of physical trauma. However, cat’s eyes really do change colors drastically according to mood; I’ve seen it quite a lot. I’ve known many yellow-eyed cats whose eyes will definitely gain a greenish tint when they’re calm and relaxed. Romana’s eyes, which are more of a greenish color in the first place, sometimes get bluish when she’s very content and half-asleep. It’s undeniable that this happens, and I found a reasonable explanation for how it’s possible. The internet informs me that there is some pigmentation on the tapetum lucidum, that reflective layer in cats’ eyes that people don’t have. Cats tend to narrow their eyes when they are in a peaceful and happy mood, and I think it seems perfectly plausible that this could alter the position or angle of the tapetum lucidum, perhaps causing its color to show through the iris less clearly, resulting in a lower-lipochrome color.

Romana

According to this information, it would seem that the answer to my original question about Romana’s distinctive eyes is that she has a lot less melanin and a lot more lipochrome in her eyes than humans do, and that in her particular case, the lipochrome is concentrated more around the outer edge of her iris and less in the area of the pupil.