Research links pupil shape to whether the animal is a hunter or the hunted

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Pupil are either round, vertical slits or horizontal slits and according to a new research, this particular shape is based on the animal’s position in the ecological web i.e. whether it is a hunter or the hunted.

Researchers at University of California, Berkley, US and Durham University, UK present a hypothesis as to why pupils are shaped and oriented the way they are. The findings are published in the journal Science Advances.

The study, based on an analysis of 214 species of terrestrial animals, shows that a creature’s ecological niche is a strong predictor of pupil shape. Researchers found that species with pupils that are horizontal slits are more likely to be plant-eating animals and most likely the prey, while on the other hand species with pupils that are vertical slits are most likely to be predators that prey on the plant-eating animals. Those with circular pupils, like humans, are “active foragers” or animals that chase down their prey.

The study, led by vision scientist Martin Banks, a UC Berkeley professor of optometry, is based on upon a previous study by the late Gordon Walls, a UC Berkeley professor of optometry who revealed through his study that slit-shaped pupils allow for different musculature and a greater range in the amount of light entering the eye. This means that vertical slits of domestic cats and geckos undergo a 135- and 300-fold change in area between constricted and dilated states, while humans’ circular pupils undergo a mere 15-fold change.

For their latest study, Banks and study co-author Gordon Love, a professor of physics at Durham University, used computer models to study the effects of different pupil shapes. Their findings provide an insight into the role that pupils play in an animal’s eating habits.

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 researchers said.

Banks explains that the one of the key requirements of plant-eating animals is to ensure that they are able to detect an approaching predator. In most cases the predator is likely to come from the ground and so a panoramic view of the ground will provide a better view of the place they are in with minimal blindspots.

“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”, Banks adds.

When the animals lower their head to graze, their eyes rotate to maintain the pupils at an horizontal alignment. Love found this same pattern when observing sheep and horses at nearby farms. Grazing animals’ eyes can rotate by 50 degrees or more in each eye, a range 10 times greater than human eyes.

On the other hand ambush predators’ requirement is to precisely gauge the distance between them and their prey so as to accurately pounce on their prey. Researchers identified three cues generally used to gauge distance: stereopsis, or binocular disparity; 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.

However, they ruled out motion parallax as one of the factors as it needs head movement that could reveal the predator’s position and from what we known predators keep their head very still while they are ambushing their prey.

Binocular disparity and blur, work together with vertically elongated pupils and front-facing eyes, the researchers 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, the researchers said.

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,” said William Sprague, a postdoctoral researcher in Banks’ lab. “So domestic cats have vertical slits, but bigger cats, like tigers and lions, don’t. Their pupils are round, like humans and dogs.”

Among the 65 frontal-eyed, ambush predators in this study, 44 had vertical pupils, and 36 of them had shoulder heights that were less than 42 centimeters (16.5 inches). Vertical pupils appear to maximize the ability of small animals to judge distances of prey.

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

“We are learning all the time just how remarkable the eye and vision are,” said Love. “This work is another piece in the jigsaw puzzle of understanding how eyes work.”

The authors noted that this research focused on terrestrial species. They expect to examine associations of aquatic, aerial and arboreal life on eye position and pupil shape in future studies.