How do animals find their food? Clues are as close as an animal's eyes, as Claire Maldarelli writes for Popular Science. Pupil shape provides differing advantages to those who crave the hunt and those who hide, scientists suggest in a study published August 7 in Science Advances.
Pupil shape varies across the animal kingdom, Maldarelli explains. It doesn’t take a rocket scientist to figure out that goats have horizontal pupils and domestic cats have vertical ones — but why do they vary in the first place?
Vertical slit pupils — like those sported by cats and geckos — might provide the optimal shape to dilate for use at night. But that hypothesis only explains half the story.
Researchers at UC Berkeley and Durham University in the U.K. compared pupil size across 214 species. Most animals (cats, foxes and snakes) with vertical pupils acted as ambush predators, sneaking up on their prey day and night. In contrast, animals with horizontal pupils tended to be grazing animals like horses and sheep — prey to predators with vertically-slitted eyes.
The team simulated how each of these eye types might see in the wild and learned more about what their advantages might be. Stretching pupils horizontally allows more light to enter the eye from the sides, so grazers can better spot attackers in the periphery. Even when horses and goats bend their heads down, their pupils rotate to stay parallel to the ground, researchers observed.
Predators, on the other hand, need a more acute sense of distance to better execute a pounce on their prey, writes Sarah Schwartz for Science News. Through vertical pupils, horizontal things show up blurry (or at least blurrier than vertical things). But blur can help animals estimate distance, and that helps the eye focus on the target as well — perfect for low predators that need to track prey that's also close to the ground.
(Lions and tigers flout the rule, though. Because of their higher vantage point, they're served better by round pupils, like those of humans and dogs.)
Though many factors can influence the evolution of an animal’s eye, it turns out there are plenty of lenses — and pupils — through which to view the world.