New research suggests pupil shape could indicate whether an animal is a predator or prey.

Researchers looked at 14 species of land animals and noticed those with vertical slit-shaped pupils tended to be prey for animals that are active both in the day and night, the University of California at Berkeley reported. Animals with horizontally elongated pupils were found to be extremely likely be plant-eating prey species with eyes situated on the sides of their head. The findings have implications for why pupils are shaped the way they are.

"For species that are active both night and day, like domestic cats, slit pupils provide the dynamic range needed to help them see in dim light yet not get blinded by the midday sun," vision scientist Martin Banks, a UC Berkeley professor of optometry. "However, this hypothesis does not explain why slits are either vertical or horizontal. Why don't we see diagonal slits? This study is the first attempt to explain why orientation matters."

The researchers used computer models to study the effects of differently shaped pupils. They found horizontal pupils enlarged the range of view, aligning with the ground to get more light from the front, back, and sides. The also cut down on the amount of dazzling light taken in from the sun, allowing the animals to see the ground better.

"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," Banks said. "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."

When these animals put their heads down to graze they rotate their eyes to maintain the pupil's alignment with the ground.  Ambush predators with vertical-slit pupils are believed to have pupil shapes that allow them to identify how far away their prey is.

"A surprising thing we noticed from this study is that the slit pupils were linked to predators that were close to the ground," said William Sprague, a postdoctoral researcher in Banks' lab. "So domestic cats have vertical slits, but bigger cats, like tigers and lions, don't. Their pupils are round, like humans and dogs."

The findings could help researchers gain more insight into how complex organs like the eye work.

"We are learning all the time just how remarkable the eye and vision are," said study co-author Gordon Love. "This work is another piece in the jigsaw puzzle of understanding how eyes work."

The findings were published in a recent edition of the journal Science Advances.