Ever wondered why some animals have vertical pupils while other have horizontal-shaped pupils? This question has perplexed scientists for decades. And now researchers have found that the shape and size of pupils can reveal whether an animal is a hunter or the hunted. Findings of the study were published in the latest issue of the journal Science Advances.

Vertical Pupils Make For A Better Predator, Research Shows

Plant-eating prey mostly have horizontal pupils

Animals with vertical slits or circular pupils are more like to be ambush predators that are active both day and night. Examples include domestic cats and rattlesnakes. On the other hand, horizontal pupils help animals spot predators from afar. Animals with horizontally elongated pupils are highly likely to be plant-eating prey with eyes on the sides of their heads, said Martin Banks of the University of California—Berkeley and lead author of the study.

Researchers used computer simulations to conduct the study. When the pupils are horizontal, they capture more light from the left and right of the eye, and less light from above and below. It helps grazing animals detect predators from different directions. But what happens if the animal bent its head to the ground to graze? In that case, the horizontal pupils should become perpendicular to the ground.

Big cats like lions and tigers lack vertical pupils

However, scientists found that when such an animal turns its head downwards, the eyes rotate up to 50 degrees to keep pupils parallel to the ground. Then researchers studied the advantages of vertical pupils. They discovered that vertical slit pupils help a predator sharpen its focus on the target and depth perception. But there is an exception, big predators like lions and tigers have circular pupils rather than vertical.

Scientists argued that since these animals are larger, they don’t have to compensate as much for these visual cues. They found that vertical pupils of geckos and domestic cats undergo between 135 and 300 fold change in areas between their dilated and constricted states. By comparison, the circular pupils in humans undergo just 15-fold change.