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Predator vs Prey: Eye Pupil Shapes Tell The Story [VIDEO]

Aug 10, 2015 12:57 PM EDT
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If you recall the "animal-eye" contact lenses that trended some years ago with their alarming, vertically angled pupils, you may not be surprised to hear that certain predators in the animal world are the ones with pupils as sharp as a church steeple. But more importantly, University of California, Berkeley and Durham University (U.K.) scientists have research that shows why that is, which they recently published in the journal Science Advances. Pupil shape could likely reveal whether one is a hunter or the hunted, they said in the research. 

The scientists analyzed 214 species of land animals, finding that species with vertical slits are usually ambush predators that are active both day and night.
"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, with UC Berkeley, in this statement. "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."

However, predators that are low to the ground and need a certain type of sight for that reason, such as domestic cats and geckos, were found to have vertically slanted pupils -- whereas big cats such as tigers have round pupils, like humans and dogs, according to the release.

Sheep and other plant-eating prey species, in contrast, usually have horizontally elongated pupils, with eyes on the sides of their heads. The researchers studied computer models to determine that horizontal pupils--which stay horizontal to the ground even when horses or other grazers lean over--allow for an expanded field of view. In this way, the pupils gather more light from the front, back and sides--but decrease glare from the sun above. Grazing animals' eyes are far more complex than human eyes: They can rotate by 50 degrees or more in each eye, 10 times greater in range than human eyes, the researchers said in a release.

"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 in the release. "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."

The researchers also examined predators' ability to gauge distances and successfully pounce on prey. When looking at stereopsis, or binocular disparity; motion parallax, which means that closer objects move farther and faster across our vision field; and blur, in which objects at various distances can be out of focus, the scientists found that vertical-slit pupils maximize binocular vision and focusing blur, as the release noted.

Later, the researchers plan to look outside land species: at aquatic, aerial and arboreal life regarding eye position and pupil shape, the release said. Hold tight for more predators!

To see a video, click on the UC Berkeley release here. 

Follow Catherine on Twitter at @TreesWhales

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