Your cat’s eyes share the same standard architecture as all other mammals, including humans. Even so, they also have some adaptations that allow them to see much better than you in low light. An inheritance received from its wild ancestor, the North African cat (Felis silvestris lybica), a specialized hunter who manages in all light conditions, including at night.
Why don’t cats like to get wet?
The first thing that catches your eye: your cat’s eyes are quite large compared to your body size and your face. In fact, cat eyes are very similar in size to our human eyes: about 22 millimeters (mm) in diameter, while humans are 25 mm (on average).
Not only that: though cats see colors (except red), and they do not perceive the world in black and white, as is sometimes said, their eyes receive the world with less detail, and colored with less intense tones.
Your cat sees better than you at night
The pupils of cats (that is, the opening of the iris or pigmented membrane that gives the characteristic color of the eyes) has the ability to open in an amazing way: about three times the maximum size that your fully dilated pupils acquire.
When fully open, along with the generous size of its eyes, your cat’s pupils allow an enormous amount of light to enter the retinas; similar to that received by species with properly nocturnal habits, such as bats or badgers. That is, your cat’s dilated pupils allow five times more light to enter than typically diurnal species, such as humans, receive.
This adaptation allows cats to see at night or in very low light levels, such as those that exist at dawn or dusk; in conditions in which we hardly see. And this helps felines like the North African cat hunt when their prey (such as mice) are most active.
But why are cats’ pupils vertical?
It is not the only adaptation that allows your cat to see at night or in low light. In addition, feline retinas contain a large number of rods, cells that are very sensitive to light. Therefore, a very open pupil in full sun or daytime would overstimulate and damage the retina of your furry comrade. [Aprende cómo ven los perros y gatos el mundo; y por qué sus ojos son tan excepcionales.]
Thus, the pupils of cats not only grow strikingly: they also have an amazing ability to contract and shrink to a very small size: a vertical slit just 1 mm wide (fully contracted), something they achieve thanks to pressure exerted by the iris. This important adaptation protects your cat’s sensitive retina; and it also controls or modulates the amount of light it receives.
There’s more: your cat’s vertical pupil allows the poles to stay “on” and working in full sun. Otherwise, during the day, they would simply turn off (to avoid damage), so your cat would stop seeing. Almost nothing.
Your cat also winks to protect himself from the light
Not only that: cats also wink or partially close their eyelids in bright conditions. Again, they do this to keep out too much light and protect your sensitive retinas. In this way, they help the pupils to reduce (or modulate) the amount of light that hits their retinas.
Cats and vertical pupils: detect colors separately
But there is still another reason that explains why cats have vertical pupils, instead of rounded, like yours. This allows the lenses of their eyes, that is, the transparent ocular structure that all mammals have to help us focus, depending on the lighting, to be multifocal.
Your cat’s eye lens (unlike yours) is multifocal; that is, the center focuses a different color (or a different range of the visible spectrum) than the extremes. And a vertical pupil allows all areas of the lens, from the periphery to the central areas, to be used at the same time and for more than one color to be in focus. Something that you cannot achieve, no matter how much you wink your eyes.
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