by Kim Doner
When we look deep into animals’ eyes, we humans might wonder what they’re thinking — but have you ever wondered how another creature sees in the first place? Think about it — not just the different species we interact with, but all those various sizes and colors of eyeballs and the shapes and responsiveness of their pupils. We already know our minds differ; let’s look a bit at how our sight differs.
Remember Those Rods and Cones?
At the back of the eyeball is a layer of cells called the retina. It has two very specialized kinds of cells: rods and cones. Light passes through the cornea onto those cells. They send information through the optic nerve to the brain, and “Aha!” We see an image.
Rod cells process black-and-white light so we see at night and pick up on motion. Cone cells process color and detail, which give images depth and clarity. They’re best for brighter light, although we use both types of cells simultaneously.
We might recall painting color wheels in elementary art class; human eyes see across a visible light spectrum, but most mammals don’t. Studies show that dogs are dichromatic, meaning they don’t see oranges, reds, or purples; rather, they see blues and yellows, with shades of gray adding nuance to perception. That is because dogs have only two kinds of cone cells but more rod cells than humans, meaning dogs don’t see a stationary object very well … until it moves. Then the rod cells come into play and signal the brain: “Prey.”
Cats also see blues, yellows, and grays; however, their vertical pupils control light exposure and enhance depth perception, giving them great nighttime vision.
Both felines and canines have a special structure behind the retina called a tapetum lucidum. It’s a reflective layer of cells that enhances their vision in low lights, hence that demonic look in a family photo. (At least, that’s what science claims. I personally think certain pets are boogers anyway, no need for photography — but that’s just an opinion.)
Most other mammals pretty much see yellows and blues (humans are trichromatic; we see red tones via our third cone receptor), but other eyeball adaptations have occurred, depending on the survival needs of each species of animal.
Eyes in the Back of the Head?
Take goats, for instance; they have horizontal pupils that rotate when the goat tilts its head so that the pupils stay horizontal. They can see as much as 340 degrees around themselves, checking out the entire panorama for danger. And they also possess 63 degrees of binocular vision that expands their ability to jump and climb — thus their brains grasp what they need to know in leaping to a narrow ledge to land on it. Needless to say, that is quite helpful when a predator wants you for dinner, and I do not mean as a guest.
Vertical pupils aren’t limited to mammals, either; the leaf-tailed gecko has that shape, along with “pinholes” that allow all possible light in wherever or whenever they need it. Their eyes also have more photoreceptor cells than most other animals, giving them nearly supernatural night vision — just missing eyelids. Instead, a transparent membrane provides protection for the vision, and if it gets dirty, it’s simply cleaned with a swipe of the tongue.
Other members of the animal kingdom also have sight clarity far exceeding mere mortals. We have our three kinds of cones, but birds have tetrachromatic vision — which means a fourth cone type allows them to see ultraviolet light. It’s believed that these cones give visual and magnetic information to an avian brain, which makes sense. Think of migration. How do they home in on home twice a year?
And wouldn’t it be cool to have that gift? I’ve got friends who can’t find their fannies with both hands. It would be delightful for them to have that fourth cone so they could locate any meeting place with no problem!
Another plus to that fourth cone is how raptors track rodents. Many small furry critters mark their territory with urine, but it’s as if they drew a map right to their own location. All a hawk needs to do is follow the yellow brick road, and voila! Dinner!
What about Underwater Vision?
So we’ve briefly considered land and air, but what about eyeballs beneath the sea? The biggest eyes in the animal kingdom are owned by the colossal squid, who can see 2,000 meters (about 1.25 miles) beneath the ocean. And if it’s too dark, squids can focus their eyes and generate enough light from them to see prey.
The cephalopod family differs even more from us in eye development, although we all share common parts. Like goats, they have horizontal pupils, and when they’re relaxed, they can see about eight feet in any direction. When they’re excited, they squint like we do — which constricts a circular muscle around the eyes and moves the lens closer to the retina to get 20/20 vision. Some scientists are studying whether this shape also allows different wavelengths of light to come into focus when the animal wants and that the brain perhaps interprets color that way.
An octopus can rotate its eyes 80 degrees, and by using seven eye muscles, can dilate or constrict the pupils in a split second to see 360 degrees. Although they are fully color-blind (few animals actually are), the photoreceptor cells see polarized light from different directions. It’s as if they’re wearing sunglasses to stop the glare; it’s also quite helpful to spot the flash of nearby scales (again — dinner).
All this would certainly lend itself to a unique sci-fi story, wouldn’t it? Taking the realities of vision from a variety of animals and managing to implant those cells into a human … having an optic nerve generate the ability to send such images to the brain … having the mental ability to interpret the images….
Then have the newly adapted characters nod and say, “I see.” Because in every possible way, they certainly would.
