Eye Shapes Of The Animal World Hint At Differences In Our Lifestyles

Aug 7, 2015
Originally published on August 12, 2015 1:52 pm

Take a close look at a house cat's eyes and you'll see pupils that look like vertical slits. But a tiger has round pupils — like humans do. And the eyes of other animals, like goats and horses, have slits that are horizontal.

Scientists have now done the first comprehensive study of these three kinds of pupils. The shape of the animal's pupil, it turns out, is closely related to the animal's size and whether it's a predator or prey.

The pupil is the hole that lets light in, and it comes in lots of different shapes. "There are some weird ones out there," says Martin Banks, a vision scientist at the University of California, Berkeley.

Cuttlefish have pupils that look like the letter "W," and dolphins have pupils shaped like crescents. Some frogs have heart-shaped pupils, while geckos have pupils that look like pinholes arranged in a vertical line.

Needless to say, scientists want to know why all these different shapes evolved. "It's been an active point of debate for quite some time," says Banks, "because it's something you obviously observe. It's the first thing you see about an animal — where their eye is located and what the pupil shape is."

For their recent study, Banks and his colleagues decided to keep things simple. They looked at just land animals, and just three kinds of pupils. "We restricted ourselves to just pupils that are elongated or not," Banks explains. "So they're either vertical, horizontal or round."

The researchers gathered information on 214 species. They noted the pupil shape and the location of the eyes on the head, plus the animal's lifestyle. For example, was it predator or prey, and active during the day or night?

One of the researchers, Bill Sprague, also at the University of California, Berkeley, says some animals have such dark eyes, it's hard to even see the pupil.

"I remember one in particular was the hyena," says Sprague. "It actually has a vertical pupil but it's very difficult to judge unless you work with them."

When they pulled everything together, a clear pattern emerged. In the journal Science Advances, the scientists report that there's a strong link between the shape of an animal's pupil and its way of life.

"If you have a vertical slit, you're very likely to be an ambush predator," says Banks. That's the kind of animal who lies in wait and then leaps out to kill. He says these predators need to accurately judge the distance to their prey, and the vertical slit has optical features that make it ideal for that.

But that rule only holds if the animal is short, so its eyes aren't too high off the ground, Sprague says.

"So for example foxes, in the dog lineage, have vertical pupils, but wolves have round pupils," he says.

And while a small pet cat has vertical slits, Sprague says, "the larger predators, like lions and tigers, have round pupils."

In general, round pupils seem to be common in taller hunters that actively chase down their prey, says Banks.

Meanwhile, he says, if you're the kind of animal that gets hunted, "you're very likely to have a horizontal pupil" and to have your eyes on the side of your head. That makes sense, he says, because it gives prey animals a panoramic view, so they can best scan all directions for danger.

But then the scientists began to wonder. This trick would only work if the animal's pupils were parallel with the horizon. And creatures like horses and sheep are constantly pitching their heads down to graze. When the researchers went to watch the animals in action, they discovered something unexpected.

"When they pitch their head down, their eyes rotate in the head to maintain parallelism with the ground," says Banks. "And that's kind of remarkable, because the eyes have to spin in opposite directions in the head."

"I've spent a lot of time handling horses, and having them put their head down to eat, and up to look around, and so on, and I had never noticed this," says Jenny Read, a vision scientist at Newcastle University in the United Kingdom. "It's just an ordinary observation that anyone could make, and yet apparently it wasn't known to science."

Read wasn't on the research team, but she says its conclusions seem right to her. "I think they're the first people to come up with a convincing explanation," she says, "for why the orientation should be chosen differently depending on your ecological niche."

Now, all of this isn't just important to scientists. Novelists and movie-makers constantly have to imagine the pupil shape of fictional creatures like Lord Voldemort in Harry Potter, or the dinosaur Indominus Rex in Jurassic World.

Giving their eyes vertical slits may make them look nice and evil, but Read says "I think their paper suggests that's unrealistic, because both of those creatures are sufficiently high off the ground that they probably should have round pupils."

Copyright 2018 NPR. To see more, visit http://www.npr.org/.

RENEE MONTAGNE, HOST:

If you've ever stared deeply into the eyes of your pet or another animal - and haven't we all? - you're probably thinking more about their inner lives than their actual eyes. Scientists do think about the animal's eyes, in particular, the dark pupil at the center, which can come in all sorts of strange shapes - not just round, like those of us who are people. NPR's Nell Greenfieldboyce reports that scientists now think they understand the reasons behind the shape of some animal eyes.

NELL GREENFIELDBOYCE, BYLINE: The pupil is the hole that lets light into the eye. And it isn't always a little circle.

MARTIN BANKS: There's some weird ones out there.

GREENFIELDBOYCE: Martin Banks is a vision scientist at the University of California, Berkeley.

BANKS: There's some that are W-shaped and some that are shaped like crescents.

GREENFIELDBOYCE: Some frogs have heart-shaped pupils. Geckos have pupils that look like little pinholes arranged in a vertical line. Needless to say, scientists want to know why all these different shapes evolved.

BANKS: It's been an active point of debate for quite some time because it's something you obviously observe. And it's the first thing you see about an animal is where their eye is located and what the pupil shape is.

GREENFIELDBOYCE: But he says there's never been a comprehensive study. He and his colleagues decided to do one. To keep things simple, they looked at just land animals and just three kinds of pupils.

BANKS: We restricted ourselves to just pupils that are elongated or not. So they're either vertical, horizontal or round.

GREENFIELDBOYCE: The researchers gathered information on over 200 species, not just the pupil shape and the location of the eyes on the head but also the animal's lifestyle. Was it a predator or prey, active at night or during the day? One of the researchers, Bill Sprague, says some animals have such dark eyes it's hard to even see the pupil's shape.

BILL SPRAGUE: I remember one in particular was the hyena. It actually has a vertical pupil, but it's very difficult to judge unless you work with them, basically.

GREENFIELDBOYCE: When they pulled everything together, a clear pattern emerged. Banks says there's a strong link between the shape of an animal's pupil and its way of life.

BANKS: If you have a vertical slit, you're very likely to be an ambush predator.

GREENFIELDBOYCE: The kind of animal who lies in wait and then it leaps out to kill. These predators need to accurately judge the distance to their prey. And the vertical slit has optical features that make it ideal for that. But Sprague says that rule only holds if the animal is short, so its eyes aren't too high off the ground.

SPRAGUE: So for example, foxes, in the dog lineage, have vertical pupils. But wolves have round pupils.

GREENFIELDBOYCE: Your small pet cat has vertical slits.

SPRAGUE: But the larger predators, like lions and tigers, have round pupils.

GREENFIELDBOYCE: Round pupils seem to be common in taller hunters that actively chase down their pray. So much for the predators - what kind of eyes do you see in their prey? Banks says if you're the kind of animal that gets hunted...

BANKS: You're very likely to have a horizontal pupil.

GREENFIELDBOYCE: And to have your eyes on the side of your head. Banks says that makes sense because it gives animals a panoramic view so they can best scan all directions for danger. But here's the thing. It only works if their pupils are parallel with the horizon. And that made the scientists wonder because they knew animals like horses and sheep are constantly pitching their heads down to graze. So they went to watch the animals in action, and they discovered something unexpected.

BANKS: When they pitch their head down, the eyes rotate in the head to maintain parallelism with the ground. And that's kind of remarkable because the eyes have to spin in opposite directions in the head.

JENNY READ: So I've spent a lot of time handling horses and, you know, having them put their head down to eat and up to look around and so on. And I had never noticed this.

GREENFIELDBOYCE: Jenny Read is a vision scientist at Newcastle University in the United Kingdom.

READ: It's just an ordinary observation that anyone could make. And yet apparently, it wasn't known to science. I think it's cool to have something like that.

GREENFIELDBOYCE: Read wasn't on the research team. But she says its conclusions seem right to her.

READ: I think they're the first people to come up with a convincing explanation for why the orientation should be chosen differently depending on your ecological niche.

GREENFIELDBOYCE: She says this is not just important to scientists. Think about novelists and moviemakers who have to imagine the pupil shape of fictional creatures, like Voldemort in "Harry Potter" or the dinosaur Indominus Rex in "Jurassic World."

(SOUNDBITE OF FILM, "JURASSIC WORLD")

BD WONG: (As Dr. Henry Wu) She was designed to be bigger than the T Rex.

CHRIS PRATT: (As Owen Grady) What happened to the sibling?

BRYCE DALLAS HOWARD: (As Claire Dearing) She ate it.

GREENFIELDBOYCE: Giving their eyes vertical slits may make them look nice and evil. But Read says...

READ: I think their paper suggests (laughter) that that's unrealistic because both those creatures are sufficiently high off the ground that they probably should have round pupils.

GREENFIELDBOYCE: Hollywood, take note. The study appears in the latest issue of the journal Science Advances. Nell Greenfieldboyce, NPR News. Transcript provided by NPR, Copyright NPR.