Cheetahs have “a conspicuous black tear line [which] runs from the inner corner of the eye to the mouth” – Wild Cats of the World. The classic often repeated reason why this line exists is to help reduce the glare of the sun. In other words reflections from the cat’s fur into the eyes are reduced thereby making the cheetah a more efficient hunter.
I don’t actually subscribe to this reason. There is nothing in the books which state that. This argument comes from the Internet and there’s lots of stuff on the Internet which isn’t true as we know.
There is a more likely reason why these black lines exist. It is probably simply camouflage just like the rest of the cat’s coat. The reason why the evenly spaced solid black dots cover the cat’s coat is camouflage. Incidentally, an early researcher of the cheetah, Varaday, counted the number of spots and came up with a total of 1,967.
You sometimes see a vestige of this line from the corner of the eye going towards the mouth in domestic tabby cats. I don’t think that it is located properly or is large enough to have any meaningful impact on reflections into the eye from the fur. Therefore I conclude that it cannot exist in order to reduce reflections from the sun.
I suspect that this theory about reducing glare comes from America where ‘eye black’ is applied under the eyes to reduce the sun’s glare amongst American footballers and baseballers. It is also used to reduce glare from stadium floodlights. It seems that amateur experts have extrapolated this human behaviour to find meaning for the camouflaged coat of the cheetah. I will defer to wiser people than me if they can come up with a better reason provided it is well researched.