Eyes across all species serve for one purpose: sight. These globular organs sole function is to process light in order to create an image of the outside world. It is through vision that most animals are able to navigate the world and survive. Without it, survival may seem almost impossible unless other senses become more sensitive to bring in more information. Usually, this is not a problem as many species, including humans and cats, can function just fine with some little help.
Where are the Eyes?
Eye locations vary between predators and prey because its placement in the skull affects an animal’s ability to see.
Those who are hunted, like deer and mice, have their eyes on the side of their heads, allowing for a predominantly monocular field of view. In monocular vision, both eyes are used separately, with very little overlap of vision between the eyes. This allows prey species to have a wider field of view to monitor for predators at all times.
On the other hand, those who hunt have their eyes facing forward, allowing them to have a predominantly binocular field of view. With the eyes placed closer together, it provides a greater degree of visual overlap for focus and depth. Predators need these skills to be able to spot their prey and accurately judge distance to go for the kill.
Humans do not have the same comprehensive vision as that of the cat. Felines have about a 200° field of vision of which 140° is binocular overlap and a 30° peripheral vision on both sides compared to the 208° field of view for humans, with a 124° binocular overlap and 42° peripheral vision on both sides.
However, unlike humans, cats rely more on motion rather than focus to see and to hunt. Each eye views the world a little differently from the other due to its placement. And when these perceptions are merged into a final picture by the brain, cats can readily perceive even the slightest of movement. Because of the importance felines place on discerning motion, eyes who lose an eye compensate by making exaggerated bobbing movements of their heads. This allows the disabled cats to monitor the images of various objects they can see relative to one another.
Cats also have a highly specialized ability to make extremely rapid eye movements to keep tack of rapidly moving prey. Known as saccades, the eyes move in a series of jerks about a quarter of a second apart. To avoid image blurring, the cat’s brain can process each image separately and then merge them together to follow its target.
Overall, feline eyes function in the same way that human eyes function. They are also made up on the same structural and functional components.
The first outmost layer is the cornea, the transparent covering of the eyeball.
Next is the pupil, the circular membrane in the center of the eye that permits light to enter from the environment. Depending on the amount of light entering, the pupil will expand or contract. A cat’s pupil can get much larger than a human’s but it does so at the expense of a good depth of field. So while a cat can see in environments darker than what a human can, it cannot focus on objects as far of a distance as humans can.
When the cat’s pupil contracts, it doesn’t stay round like it is with humans. Instead, the pupil becomes a vertical slit. Both diurnal and nocturnal animals have slit pupils. Their advantage is that the slit pupils can cover a great range of sizes than a round pupil but because of its shape, the slit pupils create optical interference, making perfect focus difficult.
The pigmented membrane that surrounds the pupil is the iris. The membrane is responsible of contracting and expanding of the pupil to regulate the amount of incoming light. The shape of the iris is believed to assist in minimizing certain aberrations to the lens, and combined with a multifocal lens, improves the cat’s vision. Squinting also helps the cat to focus better.
Once light passes through the pupil, it reaches the lens, a transparent structure that can adjust its shape as needed to focus the light rays. A cat’s lens is much larger than the human lens, enabling the cat to gather more light. Again while this is advantageous in being resourceful, the len’s large size makes it difficult to change its shape as easily as a small human lens. As a result, cats have difficulty focusing on objects very close or very far from them.
Next is the retina, a sensitive membrane that lines the interior surface of the eyeball. It receives the focused light impulses that entered through the lens. Here, the eye focuses on the image being received by the light entering. The retina is made up of cells called rods and cones that react to the light. The rods are responsible for vision at low light levels. However, they do not discern color and have a low spatial acuity. Cones, on the other hand, are capable of color vision and are responsible for high spatial acuity. Depending on the type of cone, one can see different colors. Cats have a rod-rich retina and no cone-only area unlike humans who do have a cone-rich retina. Once the retina is able to focus the image, it sends it to the brain as visual information through the optic nerve, which is located at the back of the eyeball behind the retina.
The eerie glow that both cats and dogs have at night is caused by the tapetum lucidum, a layer of highly reflective cells behind the retina. It reflects back any light entering the eye, allowing the animal to make use of the limited light availability. Light that wasn’t absorbed by the retina during its first passage through the eye gets reflected again to be absorbed while it is on its out. Thanks to the tapetum lucidum, a cat’s sensitivity to light is thought to be about six times greater than that of a human’s, using twice as much available light.
As seen from above, a feline’s eye is structured similarly to a human’s eye. However, feline eyes have acquired a number of distinctive features that improves its chances for survival. Among these features is a third eyelid or “haw”. It is a thin, pale membrane positioned at the inner corner of the eye, between the lower eyelid and the eyeball. This extra eyelid helps help the surface of the eyeball moist, protects it from being scratched by the hairs, and can help shield it during a scrap with a rival cat or another animal.
For the most part, cats are mostly red-green colorblind. Humans have three types of cones, allowing us to see a broad spectrum of colors with sensitivity peaking at red, blue, and green. It is also thought that cats might have all three types as well, but the number and distribution of each type is vastly different in comparison.
In behavioral tests, it was concluded that some cones are sensitive to green and blue, but none are to the color red. Feline photoreceptors were most sensitive to wavelengths in the blue-violet and greenish-yellow ranges of the visible light spectrum. It was also speculated that they might be able to see a little bit of green as well. In short, this means that cats are mostly red-green colorblind.
A feline’s inability to see the vibrant hues that humans enjoy evolved for a reason. Instead of noticing a variety of colors, the retina picks up other differences such as brightness, patterns, shape, and size. These details are much more important for cats when hunting.
Cats, as crepuscular creatures, are most active at dawn and dusk. As such, cats must be resourceful due to the limited availability of light. Instead of seeing the fine detail and rich colors that humans have, they have a highly developed night vision. It allows all cat species to pick up and amplify any faint light from the stars and moon to see.
Like mentioned before, there are more photoreceptor rod cells than cone cells in a retina of the eye. Furthermore, these light sensitive cells are packed tightly in the fovea, the most sensitive area of the retina. In cats, the cones are spread all over their retinas, instead of being concentrated in the fovea like humans. This means that cats can only a general and not a very detailed picture of their surroundings during the day.
In general, humans can see much better in full daylight as the rods in a cat’s eye becomes too overloaded. To adjust to the brightness, cats evolved the ability to contract their pupils to narrow vertical slits, less than 1/32 of an inch thick. They can also reduce the amount of light by half-shutting their eyes. Limiting the amount of light that can enter the eye protects their sensitive retinas from being overwhelmed with light.
At night, however, cats are able to dilate their pupils, providing more surface area for the light to enter. Along with an increased pupil are increased lens, preventing distortion at the edges, and a strongly curved cornea, allowing the light to be focused most closely on the retina. The more nocturnal an animal is, the more pronounced the cornea.
In addition to the highly specialized pupil and retina is the tapetum lucidum. Any incoming light that misses the receptor cells in the retina bounce off of the tapetum back through the retina, which can strike a receptor cell from behind. This enhances the sensitivity of the eye by up to 40%. It also gives the cat its characteristic green eye shine whenever a light is shone into its eyes in the dark. Humans did not develop this kind of structure and any unabsorbed light from the retina is absorbed at the back of the eye. It is also theorized that the tapetum may also shift the wavelengths of light that cats can see, making prey or other objects silhouetted against a night sky more prominent.
Lastly, outer characteristics of a cat’s face can affect their vision. For example, the white stripes under a lion’s eyes help reflect faint light into the eyes, maximizing the amount of light entering the eye. This characteristic is a good indication that the species is nocturnal. And for cats who are more active during the day, like the cheetah, have black tear marks reducing the glare so that they could hunt in daylight hours.
Fun Fact: Outdoor cats are slightly longsighted, whereas indoor cats are short-sighted.
Fun Fact: To compensate for the inability to focus on an object within a foot, cats can swing their whiskers forward to provide a 3D tactile “picture” of objects that are right in front of their noses
References + For More Reading
Cat Sense by John Bradshaw
Smithsonian Answer Book: Cats by John Seindensticker
Cats (Eyewitness Companions) by Bruce Dr. Fogle
The Natural History of Cats by Claire Necker
Evolution’s Witness: How Eyes Evolved by Ivan R. Schwab