A new study, published in the journal Science Advances has revealed that the pupil shapes of animals can distinguish between a predator or a prey in the animal kingdom. Vertical and circular pupils help predators to hunt, and horizontal pupils provide a panoramic visual to spot danger.
About 214 species were analyzed in a study by the University of California and Durham University, indicating that there are strong evolutionary reasons for the optical designs. Ambush-predator species like cats and crocodiles are likely to have vertical slits, and plant-eating species such as sheep and goats tend to have horizontal and elongated pupils. Animals that chase down their prey instead of ambushing them called “active foragers” have circular pupils.
“A surprising thing that we noticed from this study is that the slit pupils were linked to predators that were close to the ground,” said Dr William Sprague, a researcher on the Berkeley team,. “So domestic cats have vertical slits, but bigger cats, like tigers and lions, don’t. Their pupils are round, like humans and dogs.”
Researchers used computer models based on sheep’s eye to support the theories on why different pupil shapes benefit animals. A horizontal pupil allows more light to be captured from the left and right, allowing grazing animals to detect predators approaching from different directions.
For instance, if a goat bent down its head to eat, the horizontal pupil would become perpendicular to the ground. The amazing discovery was made while pictures of goat were taken at a petting zoo — the eyes rotate up to 60 degrees when the head is turned downward.
However, ambush-predators like tigers and lions have circular pupils instead of vertical slits. The authors say that their tall, features compensate the visual cues. Several critics of the study say that there are many species to undermine the study. For instance, the chinchilla has vertical pupils, but eats grass. The research builds on the earlier work of late Gordon Walls, professor of optometry, who published The Vertebrate Eye and Its Adaptive Radiation in 1942.
Martin Banks, a vision scientist and UC Berkeley professor of optometry, said in a statement:
“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. 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.”