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If your beautiful feline has two different eyes - a yellow and a blue, perhaps, then you’ve got yourself an odd-eyed kitten! These captivating little creatures are carriers of a feline form of a condition known as complete heterochromia. Read on and find out more!

The anatomy of cat’s eyes

Cat’s eyes are extraordinary. Let’s take a peek to see for ourselves and find some answers in science! Cats’ eyes are significantly large compared to the size of their head. This is a key feature of all nocturnal animals. The actual eyeballs are placed in the bony cavities called the orbits. The white part of the eye is sclera and it’s covered with a thin membraneconjunctiva. Conjunctiva covers the inside of the eyelids. In front of the eye is a clear dome cornea. Cornea protects the eye and lets the light in. The round, colored area of the eye is the iris.

The light enters the eye through the pupils (the black area at the center of the eyes). The pupils are very sensitive to light changes and they adjust to them accordingly. In the high abundance of light, the cat’s pupils appear elliptical, while in darker conditions they are round, almost filling the entire iris area. The smaller the pupil, the less light comes in, and vice versa. Domestic cats are featured with vertical slit pupils. However, there are some big cats, such as Siberian tigers, which have circular pupils, similar to us. It is accepted that the feature of vertical eyes is an adaptation to a nocturnal lifestyle, as this pupil shape excludes light more effectively. Vertical pupils also change the size much faster than the round pupils.

Behind the iris sits the lens. Lining the back of the eye is the light-sensitive tissue retina. Light focuses on the retina through cornea, pupil and lens. It is the retina that will convert the light rays into nerve impulses and send them to the brain. The brain interprets these impulses and forms an image (Gelatt, Baker, & Eugster, 2018).

The eye colors

The iris in domestic felines’ may vary in pigmentation from blue and green tones, to yellow and brown. These colors are not discrete, but rather a continuum between tones. Sometimes, eye colors are linked to coat color or breed. For example, Siamese cats always have blue eyes. Generally speaking, pedigreed cats tend to have more vivid and distinct eye pigmentation due to selective breeding as breeds tend to favor the trait.

How do eyes get their color? The eye pigment is produced by the pigment-producing cells called melanocytes present in the iris. The exact color and its intensity will depend on the number and activity of melanocytes in the iris. If there are no melanocytes, the eye will be blue. A low abundance of melanocytes results in green coloration, and a high abundance of orange. If the melanocytes are less active, the intensity of the given color will be lighter and vice versa. Therefore, eyes with a high number of melanocytes (orange) can range from amber (less active melanocytes) to strong copper (active melanocytes).

The activity of melanocytes is genetically determined. Multiple genes in different chromosome locations are in control of the iris pigmentation (Bishko, 2011).

Odd eyes

Odd eyes, or heterochromia iridium, can be genetic, congenital or acquired. This is a condition most commonly seen among white-colored felines but may appear in all other cats who carry the white spotting gene too.

Beautiful odd-eyed British shorthair supermeowdel Starina

The cause for odd eyes

Odd eyes can be inherited from parents (genetic) or acquired due to certain illnesses, injuries, and medications. Acquired odd eyes cannot be passed on to the kittens. However, most commonly, odd eyes are congenital (associated with development defects).

How do congenial eyes form? During the development, stem cells are migrating to a different part of the body where they differentiate into specialized types of cells. Some of these stem cells migrate through the embryo and stop in the eye where they become melanocytes. In certain conditions, melanocytes are prevented from reaching one of the eyes. This results in one eye having melanocytes which will give it its pigmentation (green, amber, brown), while the other eye will remain blue due to the lack of melanin.

Beautiful odd-eyed sisters Iriss and Abyss (Saint-Petersburg, Russia)

This usually happens in solid white or white spotted cats (bicolor and tuxedo). The dominant white gene (the gene that makes the cats completely white) and the white-spotting gene (the gene that makes the cats white-spotted) sometimes interfere with the migration of melanocytes into one of the eyes. This condition doesn’t occur very often in cats that lack these two genes. In different color cats, odd eyes are often a result of differently developed eyes in the embryo. The odd congenital eyes are hereditary and may be passed on to the kittens (Bishko, 2011) (Wenger, 2009).

Odd eyes and deafness

About 10-20% of all cats are, unfortunately, either born deaf or develop deafness over time. And completely white cats with one or both eyes blue are at a higher risk from genetically determined deafness. About 40% of odd-eyed white cats are deaf, and 65-85% of white cats with both blue eyes are deaf. This is because the white gene can occasionally cause the degradation of the cochlea, the part of the inner ear involved in hearing (Starbuck & Thomas, 2004).

Did you know?

Cats have a visual field of view of 200°, compared to human’s of 180°.
Cats don’t need to blink regularly like us. This is considered an advantage when hunting. They do “squint,” however, usually as a way of communication with other cats and humans.
They have a third eyelid! The nictitating membrane, or the third eyelid, is a thin membrane that closes from the side of the eyes. You can usually spot it when your feline is sleepy or sick. It’s also briefly visible when the eyelid opens.
Cats aren’t color blind. Cats can recognize red, blue and yellow lights, as well as between red and green tones.
Cats do not see in complete darkness. But they do see well in a very low light. The reasons they see better in darkness than us, mere humans, are because 1) they have more rods (they can, therefore, detect more light than us) and 2) they have a tissue in the back of the eyes called tapetum lucidum. This tissue reflects the light within the eye and it’s also what makes their eyes shine in the dark.
Odd eyes are a national treasure in Turkey. The Turkish folklore suggests that “the eyes must be as green as the lake and as blue as the sky”, and since 1817, Turkish Angora (a pure white cat with blue and amber odd eyes) is being bred through a breeding program aiming to preserve and protect this trait.

Cats are captivating and wonderfully distinct from all other animals in so many different ways. Wouldn’t we all just love to see what the world looks like to our little friends? But today, we will have to satisfy with just a little science behind the secretive ways of these pawesome creatures!