If you have different pets, you may wonder why their eyes shapes are different from one another. For example, your cat has vertically elongated slits pupils and your dog – rounded pupils like humans.
By eye shapes, we are technically talking about pupils. So, why do animals have pupils of different shapes?
A study by a combined team of researchers from University of California – Berkeley and Durham University suggest that pupil shape is directly linked to an animal’s ecological niche. It can reveal whether an animal is predator or prey.
The researchers studied the pupils of 214 species of land animals. Their study revealed that animals with vertical slits pupils, like domestic cats and geckos, are more likely to be ambush predators that are active both day and night.
Species of animals with horizontally stretched pupils like goats and sheep are extremely likely to be herbivores. In other words – plant-eating prey species with eyes on the sides of their heads. Circular pupils, found in humans eyes, dogs and big cats like tigers and lions, were linked to active hunters. The good, far and wide vision help them to chase down their prey.
Martin Banks, who is the professor of optometry at UC Berkeley and the lead author of the study, presents a new theory that explains why pupils are shaped and tailored the way they are.
For ambush predators or species with slit pupils, like domestic cats and geckos, their pupils provide the active vision range needed to help them see in dim light, yet not be blinded by the midday sun.
Banks also noted the importance of vertical-slit pupils in accurately measuring the distance in which ambush predators would need to pounce on their prey. In predators, vertical-slit pupils help enhance binocular stereoscopic vision, motion parallax, and blur, in which objects at different distances are out of focus.
However, the cue, motion parallax would require head movement, which could reveal and the predator’s position. So the researchers ruled out motion parallax as one of the factors in which predators use to gauge location of the prey. But, the remaining two cues, binocular stereoscopic vision and blur, work together with vertically stretched pupils and front-facing eyes.
Binocular stereoscopic vision works best when the contours are vertical and the objects are at a distance. Blur works best at horizontal contours and near-field targets.
Researchers said vertical-slit pupils enhance both cues. However all ambush predators do not have vertical-slit pupils.
For plant-eating animals or species with horizontal stretched pupils like – sheep, deer, horses and goats, their pupils provide the wide effective field of view.
Their pupils stretch horizontally and stay aligned with the ground. This give them more light in from the front back and sides. The horizontal orientation of their pupils limits the amount of light coming from the sun. This helps them see the ground better.
For species with horizontal stretched pupils, the first key visual requirements is to detect the approaching predators. Most predators happen to come directly from the ground. The second key requirement is that once they detect the predator, they need to see where the predator is running.
Plant-eating prey animals have to see well enough out of the corner of their eye to run quickly and jump over things, said Banks. Even when these species lower their head to graze, their pupils stays horizontally aligned with the ground.
However, if their pupils follow the pitch of the head, instead of staying horizontally with the ground, they would become more and more vertical, which would revoke the hypothesis.
Therefore, there is no such alignment for pupils as diagonal alignment or diagonal pupils.
“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.”
“We are learning all the time just how remarkable the eye and vision are,” said co-author Gordon Love. “This work is another piece in the jigsaw puzzle of understanding how eyes work.”
The present study focuses only on land species. However, in the future, the researchers hope to expand further studies on eye shapes of aquatic, aerial and arboreal species.