Why do animals have different pupil shapes?
It’s easy to think of pupils as simple holes that widen in the dark and narrow in the light. But if you look at the animal kingdom, you’ll see a variety of pupil shapes: vertical slits in cats and snakes, horizontal rectangles in goats and horses, and W-shaped crescents in cuttlefish, for example. THE shape of an animal’s pupil can reveal a lot about how this animal sees and what it needs to survive.
In a perfect optical system, the shape of the pupil should not matter much. “In an ideal world, it doesn’t matter to the student how optics is typically taught, because all the light should be arriving at a specific point anyway,” said Jenny Readvisual neuroscientist at Newcastle University in the UK
But real eyes are imperfect: light passing through the pupil creates diffraction and defocus that different pupil shapes resolve in different ways. “It actually turns out to be very complicated,” Read said.
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An important factor is depth of field, Marty Banksprofessor emeritus of optometry at the University of California, Berkeley, told Live Science. Her 2015 study in the journal Science Advances was the first to systematically explain why the orientation of an animal’s pupil is important for survival.
Depth of field is most easily illustrated by a camera lens, where the aperture of the camera acts as the pupil. A narrow aperture creates an image where things are in focus both far and close to the camera. A wide aperture brings an object into focus and blurs everything in the foreground and background. But when a pupil isn’t perfectly round, it can produce more variation in focus, and some animals’ eyes benefit from this.
Ambushes predators, such as cats And snakeshave forward-facing eyes that judge distance by comparing the slight differences between their two points of view – a process called stereopsis. Since the eyes are side by side, the differences appear most clearly along the vertical edges, which means those edges should be sharp.
“How do you make sure they’re sharp? Well, you [narrow] the pupil, and you increase the depth of field,” Banks said. “But they only need to do that for vertical contours. So it’s really clever to stop the pupil horizontally and leave it wide open vertically, because for other contours where stereopsis isn’t helpful, like the horizontal contour, they can now use blur to estimate the distance.
This is why the pupils of ambush predators are shaped like slits: narrowing the pupil horizontally facilitates stereopsis, while widening it vertically helps them estimate depth from blur.
However, it works best for predators close to the ground. Large predators, like lions and tigers, tend to have round pupils because they look at the ground at a steeper angle, reducing the benefit of blur for estimating distance.
Prey animals have different survival priorities, and the shape of their pupils reflects this.
“For prey, it’s more about field of view than image sharpness,” Banks said. “They need to be able to see the ground panoramic because most predators that approach them will be on the ground.”
In contrast, the eyes of prey animals like goats, sheep and horses tend to be positioned on the sides of their heads, and their pupils are shaped like a bar – wide horizontally and short vertically. This wide shape lets in more light from the horizontal plane in front and behind them to help them scan their surroundings, while the short vertical aperture accentuates the horizontal contours – basically the opposite of what predators’ eyes do.
But there’s a problem: Prey that lowers its head to graze would turn its horizontal pupils sideways and ruin its ability to scan the horizon. However, these animals have evolved a surprising solution, Banks discovered: When the animal’s head changes orientation, its eyes rotate in its sockets to compensate.
“They developed this ability to move their eyes in opposite directions on both sides of the head to keep the pupil parallel to the ground,” Banks said.
There are other animals with even stranger pupil shapes, especially in the sea. Cuttlefish, for example, have W-shaped pupils. Scientists still don’t have definitive answers to this question.
“Some people have argued that it makes them less visible to other animals,” Banks said. “I’m not sure I believe a W would be harder to see than a circle.”
Other theories suggest that these pupil shapes could help reduce light coming from above to minimize scattering and improve contrast. A first theory proposed that shape may help with color perception: cuttlefish have only one photopigment, which should mean they only see in black and white, despite their dazzling colors and talent for camouflage.
Banks’ study focused on land animals, and he recognized that many aquatic pupils remain unexplained. Read suspects the unknowns go even deeper. “It makes you think about what other capabilities animal eyes can have that we have no idea about,” she said.
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