Why do the pupils of animals have different shapes?

The pupils’ orientation is related to the animals’ lifestyle.

Goats, sheep, horses, cats and many other animals have pupils that vary from circular in the dark, to slits or small rectangles in bright light, to modulate the amount of light entering the eye.

In fact, a cat’s pupil can increase enormously in size.

This is typical of animals that are active both during the day and at night, and allows them better vision in both cases.

The direction of the pupil is no less important than its ability to dilate.

Animals with vertically pupils are usually predators that ambush preys, with their eyes on the front of their heads. Vertical pupils improve their ability to accurately judge distance without having to move their heads, which could reveal their presence to potential preys.

It has also been found that having vertical pupils is more advantageous for small animals, with eyes closer to the ground (at a height of less than about 40 cm), than for larger ones. In large carnivores that actively chase their prey, the pupils are instead circular. This explains why large felines and dogs, but also primates, have them this way.