Power of pupils is in their shape

Blurry vision sounds like a reason to visit an eye doctor. But visual fuzziness might actually help some animals catch dinner. Out-of-focus areas created by vertically elongated pupils help predators triangulate the distance to objects, scientists propose August 7 in Science Advances. Prey animals may gain different visual advantages from pupil shapes that provide panoramic views. 

Cats, foxes and many other predators that ambush prey have vertical pupils. Through these narrow slits, vertical objects appear sharp over great distances, the scientists report. Horizontal shapes are clear over a more limited distance, quickly going out of focus as an object moves farther away. This rapidly blurring vision should make it easy to detect even subtle changes in distance, the researchers say. That makes blur a good estimate of distance, says study author Martin Banks, a vision scientist at the University of California, Berkeley. A stalking predator might rely upon an object’s fuzziness to judge its location.

The benefits of this mix of visual cues make good sense, says Michael Land, a neurobiologist at the University of Sussex in Brighton, England. A predator that must pounce on its dinner needs to be able to accurately judge distances, he says.

Many herbivores, like horses and deer, have horizontal, rectangular pupils, rather than vertical slits. The authors don’t think these pupils help with depth perception. But rectangular pupils probably have their own advantages, the authors report, including better panoramic vision and shielding of potentially blinding overhead light. These benefits could help grazing prey spot – and flee from – an approaching slit-eyed hunter.

These visual benefits could explain why predators and prey evolved their pupil shapes, Banks’ team says. But vision scientist Ronald Kröger of Lund University in Sweden warns against assuming that an animal’s habits caused the evolution of a certain pupil shape. Counterexamples exist of predators without slit pupils and herbivores with them, he says. Additionally, many predators and prey animals, including most birds – which were excluded from the study’s analysis – have circular pupils.

But evolution is complex, and the new hypotheses about the advantages of pupil shape only address one aspect of the evolution of vision, Banks says. “There are multiple forces that push the eye to evolve in multiple ways.”