A study has helped researchers determine what led some, including goats, to have horizontal-shaped pupils and others, like rattlesnakes and domestic cats, to have vertical slits. Survival edge is the best answer scientists have linked to it.

The study, conducted on 214 species and published Fridayin the journal Science Advances, found that certain predators are able to hunt because of vertical pupils and circular pupils. On the other hand, horizontal pupils help other species to detect predators from a long distance.

Letterbox pupils distinguish prey species such as sheep and goats, while vertical slits are true in case of cats, crocodiles and other ambush predators.

The researchers said that evolution was responsible for the both cases to be different. Horizontal slits helped prey animals to get a panoramic view of the surroundings to spot any imminent danger. The horizontal pupil enhances vision in the horizontal plane and reduces dazzle from above.

“If an animal becomes nocturnal, they're likely to develop a vertical pupil, if they become diurnal, they have a round pupil. It's not just random.The changes are associated with their ecological niche”, said William Sprague, a postdoctoral researcher at UC Berkeley and a coauthor on the paper.

Sprague said that they were left surprised by the fact that an association existed between slit pupils andpredators that were close to the ground. While vertical slit pupils were present in ambush predators, active hunters that chased down their prey were more likely to have round pupils.