UC Berkeley and Durham University optometry scientists have discovered the reasons for the horizontal pupil shape of some animals’ eyes — after analyzing  214 species of land animals, they have concluded that species with vertical pupils are more likely to be ambush predators that are active both day and night. Animals with horizontally elongated pupils are likely to be plant-eating prey species with eyes on the sides of their heads. And animals with circular pupils are either active foragers or animals that chase down their prey.

Researchers found that the horizontal pupils of grazing animals like horses, expanded the effective field of view. When stretched horizontally, the pupils are aligned with the ground, getting more light in from the front, back and sides. The orientation also helps limit the amount of dazzling light from the sun above so the animal can see the ground better.

When horses, sheep and other grazing prey animals put their head down to eat, their eyes rotated to maintain the pupils’ horizontal alignment with the ground.

Read more about the study here.