To explain why horizontally elongated pupils, with few exceptions(are there any explanations for the exceptions?), corresponded to grazing prey animals such as sheep, deer and horses, the researchers turned to computer models to study the effects of different pupil shapes.
They found that the horizontal pupils 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, the researchers said.