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  Cats evolved to be nocturnal hunters, and they can see well in very dim light. Because their eyes are so sensitive to light, cats need precise control over the amount of light reaching their eyes. 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; this ability is particularly important in bright sunlight. Vertical slits also have an 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.