Why do the eyes of some animals, including goats, have horizontal-shaped pupils, while others, such as rattlesnakes and domestic cats, have vertical slits?
It’s a question that has longed intrigued researchers, and a study of 214 species published Friday suggests the answer may be strongly linked to giving animals a survival edge: vertical pupils and circular pupils help certain predators hunt, while horizontal pupils help other species spot predators from afar.
Not all vision scientists accept the researchers’ hypothesis, however, citing examples of animals that do not fit cleanly into these classifications.
The research, which was conducted by a team of scientists from the University of California, Berkeley, and Durham University in Britain, was published in the journal Science Advances.
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“People had been saying that the horizontal pupil helps expand the horizontal view of the ground, they just hadn’t shown that,” said Martin S. Banks, a visual scientist from Berkeley and lead author on the paper. “Our contribution was to build a model and show that that happened.”
But there was an obvious caveat to this conclusion: What would happen if the sheep bent its head to the ground to eat? Logic would suggest that the horizontal pupil would become perpendicular to the ground.
But the researchers made a surprising discovery while taking pictures of goats at a petting zoo — the eyes actually rotate as much as 50 degrees when the head turns downward, keeping the pupils parallel to the ground. Dr. Banks assumed that other scientists had noted this ability, but he found no mention of it after doing an extensive search of the scientific literature.
The researchers then studied horses, antelopes and other grazing animals, and found that they could rotate their eyes as well.
Dr. Banks and his team also used the computer model to identify advantages of vertical slit eyes. They found that vertical pupils help an ambush predator better estimate the distance to its prey by sharpening depth perception and its focus on a target.
One asterisk on this explanation is that large predators like tigers and lions that ambush prey have circular eyes, not vertical slits. The authors reason that because these animals are taller, their eyes do not have to compensate as much for those same visual cues.
Fascinating stuff. And does seem obvious when one thinks about it, just like so many prey-animals having their eyes on the side of their heads, and predators forward-facing. Obvious...after someone's pointed it out to us. And, as for primates ? Optimised for an arboreal environment ?
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