Have you ever wondered why the pupil of a cat's eyes becomes a vertical slit when it contracts, while most other mammals, including humans, all have eyes that contract to a tiny circle?
There is a good answer for the difference.
Cats evolved to be nocturnal hunters, As a result, they can see well in very dim light.
Because their eyes are so sensitive to light, cats need very precise control over the amount of light reaching their eyes, this ability is particularly important in bright sunlight.
Being able to reduce the pupils to slits rather than tiny circles gives the cat greater and more accurate control of how much light enters their eyes.
Vertical slits also have another advantage over horizontal slits.
Because the cat's eyelids close at right angles to the vertical pupil, the cat can reduce the amount of light even further by bringing its eyelids closer and closer together. This combination of the vertical slits of the pupils and the horizontal slits of the eyelids, allows the cat to make the most delicate adjustments of the light reaching its eye compared to any other animal.
The pupils of the lion are an interesting comparison.
The lion hunts by day, not by night as the cat does.
The lion does not have the same sensitivity to light as the cat. And the lion's eyes contract like ours do - to tiny circles, not vertical slits.
So, next time your favorite kitty gives you that mystical look through slitted eyes . . . you will know why.