Pupil Shape of Animals Linked to Being a Predator or Prey: Where Do Humans Fall?

First Posted: Aug 08, 2015 07:33 AM EDT

Why do animals have different pupil shapes? It all has to do with their ecological niche. Scientists have analyzed 214 species of land animals and have found that whether or not they're a predator is a strong predictor of pupil shape.

"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," said Martin Banks, one of the researchers, in a news release. "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 actually turned to computer models to study the effects of different pupil shapes. They found that horizontal pupils expanded the effective field of view. When stretched horizontally, pupils are aligned with the ground and get more light in from the front, back and sides. This also limits the amount of dazzling light from the sun so the animal can 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," said Banks. "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."

In contrast, ambush predators with vertical-slit pupils maximize binocular vision, which works better ad judging differences when contours are vertical and objects are at a distance, and blur, which comes into play for horizontal contours and near-field targets. Interestingly, though, slit pupils are linked to predators that are close to the ground. For example, cats have vertical slit pupils, but lions and humans have round pupils.

The scientists found that depth-of-field cues based on blur are more effective for estimating distances for short animals than tall animals, which explains the differences in pupils.

The findings are published in the journal Science Advances.

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TagsPupils, Animals, Animal Behavior, Predator, Prey