Pupil Shape Can Show Who's Predator and Who's Prey

Is an animal a predator, or someone’s likely dinner? The shape of its pupils can answer that question, even for humans.

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."

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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.

Shown is the rattlesnake

Pupil shape strongly predicts both an animal's basic position on the food chain, and what time of day it is most active, a new study has found. The research, published in the journal Science Advances, discovered that species with vertical slit pupils are more likely to be ambush predators that are active during the day and night. Rattlesnakes, such as this species from Mexico, fit the pattern. A boa constrictor included in the study had vertical-shaped pupils as well. In all animals, light comes through the pupil before passing through the lens, which focuses the image before it goes on to the retina. The retina changes the light into nerve messages that are sent to the brain via the optic nerve.

Pupil Shape Can Show Who's Predator and Who's Prey

Another animal with vertical-shaped pupils is the common domestic cat. Lead author Martin Banks, of the University of California at Berkeley, explained: "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." Cats undergo an amazing 135- and 300-fold change in area between constricted and dilated pupil states. Humans, in contrast, undergo a measly 15-fold change.

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Giraffes exhibit classic prey pupils. They are horizontal. Banks and his team determined that horizontal pupils expand the animal's effective field of view. That is because when the pupils are stretched horizontally, they become aligned with the ground, permitting more light in from the front, back and sides. Additionally, such an orientation also helps to limit the amount of bright sunlight from above, permitting the individual to see the ground better. "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," Banks said. He added: "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."

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Both African and Asian elephants were included in the study. Each has circular-shaped pupils. Humans have this type of pupil as well. The researchers suspect that height and overall body size are linked to pupil shape. During the study, Banks and his colleagues discovered that only a small percentage of species with circular pupils were shorter than 16 inches tall. Most, like elephants and humans, were much taller. Nearly all birds have circular pupils too, though. Banks and his team explained: "A near and fore-shortened ground plane is not a prominent part of birds' visual environment," so having the other types of pupils would not offer any additional benefits.

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Turtles have circular pupils too. Other animals included in the study with this type of pupil include: Eastern chipmunks, black-tailed prairie dogs, wild boars, Bornean bearded pigs, Alabama red-bellied cooters, knob-scaled lizards, yellow-bellied racers, Western gopher snakes, and common garter snakes.

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There is little doubt that this Cuvier's dwarf caiman, with its big eyes and vertical pupils, is a major predator. A member of the alligator family, the caiman feasts on fish, mammals, shellfish, birds, and whatever else might have the misfortune of being in its presence.

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The eyes of deer, with their horizontal pupils, put them in the "hunted" category. Like giraffes, their eyes are optimized for panoramic vision and the ability to detect approaching predators. While conducting the study, the scientists wondered what happened to the pupils' orientation when such animals lowered their heads to graze. "To check this out, I spent hours at the Oakland Zoo, often surrounded by school kids on field trips, to observe the different animals," Banks said. "Sure enough, when goats, antelope and other grazing prey animals put their head down to eat, their eyes rotated to maintain the pupils' horizontal alignment with the ground."

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Both dogs and their grey wolf relatives have round pupils, just like humans. It's an intriguing characteristic that man's best friend shares with us, as opposed to cats, given the feline predator-associated slit pupils. Wolves are considered to be active predators. This simply means that they search or hunt for their prey, as opposed to being an ambush predator.

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Hares and rabbits tend to have circular pupils like us, but they are eight times more sensitive to light than human eyes. Rabbits and hares are an exception to the pattern of larger and taller animals possessing round pupils. Their bunny eyes tend to be off the scientific charts anyway, given that their eyes are so large, relative to head size. So many predators want to eat furry little hares and rabbits that these prey animals will blink as little as 12 times an hour. Their field of vision is almost 360 degrees.

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The eyes of actress Bette Davis were so distinctive that they inspired a hit song in 1981. Davis had round pupils like the rest of us, though. For this study, the researchers only investigated terrestrial animals. They next hope to examine aquatic species, as well as aerial and arboreal animals. "We are learning all the time just how remarkable the eye and vision are," co-author Gordon Love, of Durham University, said. "This work is another piece in the jigsaw puzzle of understanding how eyes work."