The shape of an animal's pupils reveals whether or not it is a hunter or the hunted, a new study has found.
The research helps explain why creatures throughout the animal kingdom have evolved different pupil shapes, from the vertical slits of rattlesnakes to the circular pupils of humans.
Each shape, according to the Science Advances study, is generally tied to a particularly evolved way of being in the world.
As lead author Martin Banks and his team wrote: "We found a striking correlation between pupil shape and ecological niche."
Banks, a professor of optometry at the University of California at Berkeley, and his team analyzed 214 species of land-dwelling animals for the study. While Banks spent hours observing animals at the Oakland Zoo in California, senior author Gordon Love, of Durham University, monitored sheep and horses in the United Kingdom.
When all of the data came together, the researchers and their colleagues determined that four basic pupil shapes corresponded to particular behaviors and other factors:
Vertical: Predators that are active both day and night, such as domestic cats, tend to have vertical pupils.
Vertically elongated: Animals with what the scientists called "subcircular eyes," such as lynxes, are usually ambush predators that capture their prey using stealth and strategy, as opposed to primarily relying upon speed and strength.
Horizontal: Grazing prey animals, such as sheep, deer, and horses, typically have eyes with this pupil shape.
Circular: Most predators that are active during the day, like humans, have evolved this type of pupil.
The predicted patterns come with a few exceptions, though.
"A surprising thing we noticed from this study is that the slit pupils were linked to predators that were close to the ground," co-author William Sprague, a postdoctoral researcher in Banks' lab, said. "So domestic cats have vertical slits, but bigger cats, like tigers and lions, don't. Their pupils are round, like [those of] humans and dogs."
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Sprague explained that for each pupil type the shape optimizes the animal's field of vision for its particular height and hunting needs. Vertical slits allow for better control of light and estimated distance, but humans are too far away from the ground to obtain these benefits. Therefore, the authors theorize that circular pupils can, in many cases, be related to an animal's height.
It's intriguing that the human pupil shape is so tied to daytime activity, though. That suggests our ancestors were not night owls, as are some people today. Instead, our early ancestors must have actively hunted, gathered, and done other things during the day and then snoozed the night away.