According to a study of 214 species of terrestrial animals in the journal Science Advances, an animal’s ecological niche is a strong predictor of pupil shape.

Species with pupils that are vertical slits are more likely to be predators that are active both day and night.

In contrast, those with horizontally elongated pupils are very likely to be herbivorous species with eyes on the sides of their heads.

Circular pupils were linked to active foragers, or animals that chase down their prey.

To explain why horizontally elongated pupils, with few exceptions, corresponded to grazing prey animals such as sheep, deer and horses, the study’s authors – Prof Martin Banks from the University of California, Berkeley, and co-authors – turned to computer models to study the effects of different pupil shapes.

The scientists found that the horizontal pupils expanded the effective field of view. When stretched horizontally, the pupils are aligned with the ground, getting more light in from the front, back and sides.

The orientation also helps limit the amount of dazzling light from the sun above 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. 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,” Prof Banks explained.

But what happens to this orientation when the animal lowers its head to graze? If the pupil follows the pitch of the head, they would become more vertical.

“To check this out, I spent hours at the Oakland Zoo, often surrounded by school kids on field trips, to observe the different animals. Sure enough, when goats, antelope and other grazing prey animals put their head down to eat, their eyes rotated to maintain the pupils’ horizontal alignment with the ground,” Prof Banks said.

“Grazing animals’ eyes can rotate by 50 degrees or more in each eye, a range 10 times greater than human eyes.”

For ambush predators with vertical-slit pupils, the scientists noted the importance of accurately gauging the distance animals would need to pounce on their prey.

They identified three cues generally used to gauge distance: stereopsis; motion parallax, in which closer objects move farther and faster across our field of vision; and blur, in which objects at different distances are out of focus.

They ruled out motion parallax as a factor since using that cue would require head movement that could reveal the predator’s position.

“The remaining two cues, binocular disparity and blur, work together with vertically elongated pupils and front-facing eyes,” the scientists said.

“Binocular vision works better at judging differences when contours are vertical and objects are at a distance, while blur comes into play for horizontal contours and near-field targets. Vertical-slit pupils maximize both cues.”

Vertical pupils are not equally distributed among ambush predators, however.

“A surprising thing we noticed from this study is that the slit pupils were linked to predators that were close to the ground. So domestic cats have vertical slits, but bigger cats, like tigers and lions, don’t. Their pupils are round, like humans and dogs,” said study co-author Dr William Sprague from the University of California, Berkeley.

Among the 65 frontal-eyed predators in the study, 44 had vertical pupils, and 82 percent of them had shoulder heights that were less than 16.5 inches (42 cm).

Vertical pupils appear to maximize the ability of small animals to judge distances of prey.

The team explained this by calculating that depth-of-field cues based upon blur are more effective for estimating distances for short animals than tall ones.

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Martin S. Banks et al. 2015. Why do animal eyes have pupils of different shapes? Science Advances, vol. 1, no. 7, e1500391; doi: 10.1126/sciadv.1500391