This tabby cat sitting outside minding his own business with a nice collar and bell to help protect the birds has very interesting eyes because the pupils are triangular-shaped due to a congenital condition called coloboma. This is particularly noticeable in domestic cats because we know that they have pupils which vary from a circle when wide open to a narrow slit when closed down to block the light. Just to be clear, the pupil is the opening in the eye which allows light through onto the retina. It is surrounded by the iris which is made up of muscle which adjusts the size of the pupil. The iris is normally coloured with pigment granules.
This tabby cat has a congenital condition call coloboma which describes missing tissue in part of the eye. It can cause sensitivity to light and/or vision loss. Coloboma can affect one or both eyes.
In this instance part of the iris is missing leaving a triangular aperture rather than the usual slit when a cat is outdoors in daylight. This will result in the retina receiving more light than normal. I would expect that this cat is sensitive to strong daylight as he is unable to “stop-down” the pupil to the point where light is more effectively blocked out.
2 reasons why domestic cats have slitted eyes (there are actually 3!)
Let’s remember that the retina has a reflective layer behind it (tapetum lucidum) to make it more sensitive to light and therefore the iris does need to be stopped down very tightly to protect the retina. This cat is unable to do that.
This irregular shaped iris may also affect his/her depth control vision because the slit-like pupil of the domestic cat helps to manage depth perception and therefore improves predation. There is a third aspect of the slit-like iris in cats and that is that the eyelid passes down over it in very bright light to further protect the eye. This aspect of light control in the domestic cat will be less effective in this individual.
However, I wouldn’t expect that coloboma in this tabby cat affects him dramatically. Hardly at all, actually. In fact, I would expect the owner to see no difference in his behaviour compared to a normal domestic cat.
Coloboma affects humans as well. They can receive treatment such as glasses, surgery, low vision aids or an eye patch or eyedrops. Surgery in people is carried out to make their pupils look rounder. It seems that it is carried out mainly for cosmetic reasons. They can wear glasses to correct for “refractive errors” which allows them to see more clearly. And sometimes people with colobomas can also wear coloured contact lenses to make their pupils look rounder.
None of the above apply to domestic cats! People just accept it although I think that it might be useful to keep your cat inside in very bright sunlight if the coloboma resulted in a triangular-shaped opening as we see the photograph.
Below are some more articles on cat eyes.
