Scientists from the universities of Berkeley in California and Durham in Britain say they have discovered the reason that different animals have different-shaped pupils.
When the domestic cat narrows its eyes to slits, it does so vertically. Sheep, deer and horses however have eyes with horizontally elongated pupils.
According to the study, vertical suggests predator, whereas horizontal suggests prey.
The researchers of the study, published in the journal Science Advances, said they looked at the eyes of 214 terrestrial vertebrates. The idea was to predict a relationship between an animal's place in the ecological pecking order and the shape formed by the pupil in its eye.
They found small predators who ambush their prey tend have pupils that narrow vertically which helps to increase focus and depth perception. Prey animals tend to have pupils that narrow horizontally, creating a panoramic view which helps to detect and evade an approaching predator.
They also discovered that when a grazing or prey animal bends down to eat, their eyes rotate by as much as 50 degrees in order the keep them parallel with the ground.
Martin Banks, professor of optometry at Berkeley, said: "The first key visual requirement for these animals is to detect approaching predators, which usually come from the ground, so they need to see panoramically on the ground with minimal blind spots.
"The second critical requirement is that once they do detect a predator, they need to see where they are running. They have to see well enough out of the corner of their eye to run quickly and jump over things."
But how did the researchers explain how lions and tigers have round pupils? Humans, of course, also have round pupils.
They explained this discrepancy by noting that humans along with lions and tigers walk tall which means less visual cue compensation is necessary to find food.