Out of all of the diverse, creative forms that body parts can take throughout the animal kingdom, eyes might be the weirdest. Maybe that’s our human bias — we do a lot of communicating via eye contact, after all. But whatever the reason, there’s something particularly bizarre about staring into the alien eye slits of a housecat or the W-shaped pupils of a cuttlefish. Here’s the explanation behind a few of the weirder pupils you can find in nature.
Niche: Ambush predators, active by day and night
Examples: Cats, foxes, crocodiles, snakes, geckos
A vertically oriented pupil is a precision instrument, capable of razor-sharp focus in many light conditions. One reason is their ability to open very wide or close to tiny slits, depending on how much light they need to let in. A cat’s pupils can expand by a whopping 135 times, for example; a gecko’s expand by more than 300-fold. (Compare that to the paltry 15-fold expansion of our own pupils!)
When vertical pupils are at their widest, these animals judge distance by which parts of the image are blurred. You can compare it to the portrait setting on a smartphone — that in-focus foreground and blurred background mimics what happens when a professional photographer uses a wide camera aperture, and clearly shows where the subject is in relation to the background.
When vertical pupils are narrowed, they rely on the slight differences between the images received by the left and right eyes, a process known as stereopsis. (You use this method too; it’s why you lose depth perception when you cover one eye).
The coolest part? That orzo-shaped pupil, with its narrow points and wide center, can achieve both of these feats at once.
Niche: Grazing prey animals
Examples: goats, sheep, horses, moose, white-tailed deer
If vertical slits are portrait mode on your smartphone camera, horizontal slits are panorama mode. An animal with horizontal pupils also tends to have its eyes on each side of its head, so these pupils can give it a panoramic view that lets in more light from its front, back, and sides. That helps them both detect an oncoming predator and keep tabs on it while they flee.
“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,” UC Berkeley optometry professor Martin Banks said in a statement about his research on the topic. “They have to see well enough out of the corner of their eye to run quickly and jump over things.”
But while vertical pupils stay vertical whether a predator is looking up or staring at the ground, pupils that are horizontal while a grazing animal is upright wouldn’t stay that way while their heads are down. Never fear! These animals adapt: their eyes swivel clockwise to remain aligned with the ground while they graze.
Niche: Water-dwelling animals that spend a lot of time motionless
Examples: skates, rays, flatfishes, catfish, some whales
Underwater animals have a different set of visual challenges than us landlubbers since light bends differently in water than it does in air. It only makes sense, then, that you’d see some completely foreign pupil shapes under the sea. Crescent shapes are a perfect example. One superpower they have is a wide visual field — they can take in more information than a circular pupil with the same surface area. In 1991, ophthalmology researchers at the University of California Davis set out to discover their other superpowers by putting a crescent-shaped filter on a camera lens.
What they saw was pretty incredible: Instead of keeping the subject focused and making everything else blurry, a crescent-shaped pupil turns points of light that are in front of the subject into U shapes (or crescents facing up) and points of light that are behind the subject into n shapes (or crescents facing down). The closer the object, the larger the U shape, and vice versa. For a flounder hanging out on the seafloor watching schools of fish swim by, the ability to know exactly how far away each one is would definitely come in handy.
More Than Two Pupils
Niche: Typically, animals that aren’t strictly active in the daytime or nighttime
Examples: Cats, llamas, horses, geckos, skates, rays, some sharks
You may have noticed that some of these are animals we’ve already mentioned above. That’s no mistake: in very bright light, many animals will constrict their pupils in a way that creates multiple openings. Goats, for example, have horizontal pupils that will constrict in a dumbbell shape with openings at each end. When vertical pupils are narrowed to slits, they’ll still let light in at the top and bottom; when crescent-shaped pupils narrow, they form a teardrop opening at each side. Still other animals have even more openings: skates and horses have pupils with “fringe” that create multiple openings in bright light, and some geckos have vertical pupils with a wiggly shape that constricts to form four smaller pupils.
Those same UC Davis researchers found that when you place a multi-pupil filter on a camera lens, an in-focus subject forms a crisp, single image, while everything out of focus forms multiple images. They also noted that multiple pupils achieve something similar to a crescent pupil by letting in less light, boosting image resolution, and allowing for a larger visual field than a single round pupil.
Just Plain Alien Pupils
Examples: Octopuses, squids, cuttlefish
Cephalopods are a breed all their own. They’re the smartest invertebrates on Earth, they can communicate by changing the patterns on their skin, and they can even edit their own DNA. (No wonder some people think they’re aliens.) Their pupils are very on-brand: You can find cephalopod pupils in the shape of Ws, crescents, dumbbells, and whatever the heck this is. But here’s the mystery: That thing about changing the patterns on their skin to communicate? They do that despite the fact their eyes don’t have cells for color vision. It just doesn’t make sense that they’d create this skin-based light show just for the color-sensing predators who can see them.
In 2016, researchers from Harvard and Berkeley found that cephalopods can see color — they just do it in a completely different way than we’ve seen before. (Typical.) If you’ve ever had your pupils dilated at the eye doctor, you know two things happen: you become sensitive to light (so you have to wear those dorky sunglasses) and objects are blurry, often with a weird colorful fringe around them. That fringe comes from the fact that a wide pupil spreads light out like a prism in a process known as chromatic aberration — and it’s generally a bad thing. But these researchers think cephalopods use chromatic aberration to their advantage. They might be able to spread out these colors and bring certain wavelengths into focus by changing not only the shape and position of their pupils but also the depth of their eyeball and the distance between their lens and their retina.
One quirk of this process, say the researchers, is that it takes a lot more brain power than the alternative. One more reason why cephalopods are the alien supergeniuses of the animal kingdom.