Have you ever looked longingly into your cat’s eyes and wondered “why are you pupils terrifying little slits, you nighttime killer of beasts?!”
You are reading this right now because the light from your screen is passing through your cornea, being bent in just the right way that it passes through your circular pupil into your eye hitting your retina so your brain can receive the image and, well, you can see!
In all that, the pupil plays the key role as the adjustable opening that controls the intensity of light allowed to enter your eye, just like the aperture on a camera. In humans, our pupils dilate to allow more light in and contract to allow less light in.
But not all eyes in the animal kingdom are created equal, nor are all pupils. When we think about different eyes, we have to remember that eyes didn’t develop on land. Nothing did!
The first eyes of the first living things were under water where light moves different and moisture is built into the environment, as different species developed in and out of water, pupil shapes have gotten weird, or at least, weird when we’re used to looking at our own circular pupils. Obviously they’re totally normal and natural!
The one most of us are familiar with is the vertical slit pupil, which, interestingly, has developed in a variety of species independently including canines, felines, some snakes, geckos, crocodiles, and some birds. That’s a pretty wide variety of animals, but what they all have in common is that they are nocturnal predators who don’t stand too high off the ground.
One benefit is that slit pupils can contract and dilate more than circular ones.
Think of a cat who is awake and running around in full daylight and in the middle of the night.
Cats can see far better than we can in those two extreme light environments.
Some more recent research suggests that slit pupils also help predatory animals see color in different light conditions, leaving a larger amount of the retina open to receive wavelengths of light that translate to different colors.
A vertical pupil also affects the depth of the image that the eye can see as well as an increased horizontal movement in sharp focus, something predators like cats need more than humans.
Vertical pupils are also less noticeable than round ones when a predator is staring you down.
But there’s another pupil shape that also developed independently among a number of species and that’s the horizontal slit.
Even-toed ungulates as well as all equines, mongooses, some frogs and toads, and octopi have horizontal pupils. That’s a pretty interesting mix that might not seem to have a lot of obvious commonalities to explain their shared pupil shape, but they are all predators with eyes on the side of their head and with a lateral pupil, they all get an extremely wide field of view, sometimes near a full 360 degrees. So those, vertical and horizontal slits, are the major ones.
There are some even stranger pupils out there, particularly lurking in the water or in species that have to see both underwater and in the air.
Some species of gecko have pinhole pupils, pupils that, when fully contracted, become just four tiny dots vertically in the eye that can help gauge distance.
Dolphins have pupils that look like the letter U when fully contracted.
The cuttlefish has a super neat looking W-shaped pupil that is actually a break in the retina.
They can look forwards and backwards at the same time and see in polarized light, cutting out the glare that can make it impossible for humans to see!
In the scheme of eye-things, our eyes are sort of the most basic, and they serve us well!
But there’s an intense variety of pupils out there that speaks to the amazing variety of species on our planet.