“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,” lead author Martin Banks of UC Berkeley said in a statement. “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.”
Circular pupils, like our own (or like a big cat's) tend to belong to predators. Side-slanted eyes like a goat's belong to grazing prey.
Using computer models, the researchers confirmed that sideways eyes produce a much wider field of vision than eyes like our own.
The shape of their pupils also allows them to take in more light. Meanwhile, they aren't absorbing as much light from above their heads, which keeps the sun from bleaching out their view of the grass around them. This extensive peripheral vision lets them watch out for predators, and helps them see the entire terrain around them when they must plot an escape.
During his research, Banks confirmed that grazing animals rotate their eyes when they bow their heads down to graze, effectively keeping their eye slits nearly parallel to the ground at all times — no matter the position of their heads. They can rotate more than 50 degrees per eye, the researchers say, which is 10 times of the human eye. That way the grazers can maintain a preys'-eye-view of the world even when chowing down.
The researchers also examined why some predators — like snakes and small cats — have vertical slits in their eyes, while other predators have round pupils. They found that proximity to the ground was the big determining factor.
Vertical pupils make hunting by night a breeze, as they can expand much more than a round pupil, giving the animal more light to work with. The researchers believe they're also great for improving depth perception. Based on their calculations, the researchers think that the depth perception gained from vertical pupils diminishes as animals get farther and farther away from the ground. That could be why taller animals tend to have round pupils instead.
The researchers say their next focus will be on the pupil shapes of animals that don't live on land. That should get a bit more complicated, since at least one side-eyed sea creature we can think of — the octopus — is both predator and prey.