What makes a mountain lion’s vision so different from a human’s? How do scientists know whether animals can see colors and which ones they detect?
Ed Yong’s An Immense World reveals fascinating details about how animals see color and light. From deep-sea creatures that navigate in darkness to birds that can spot prey from a mile away, the animal kingdom showcases an incredible range of visual abilities that differ dramatically from human perception.
Keep reading to discover the remarkable ways animals perceive their world through sight, and how evolution has shaped these diverse visual capabilities.
How Animals See Color and Light
When it comes to sight—the ability to interpret the environment based on how light interacts with it—the way animals see color and light differs from the ways humans do. Yong details the fascinating distinctions.
Color
Color is subjective. We determine color by detecting and comparing different wavelengths of light. Objects aren’t inherently any particular color; rather, our photoreceptors (light-detecting cells), neurons (nerve cells), and brains turn light into our perception of color. Yong outlines the ways animals perceive color.
1) Animals see color to varying degrees. Many animals, such as raccoons and whales, are monochromats, meaning they can only see in shades of gray. (This is because they have only one type of color-detecting cone cell, which makes them unable to compare different wavelengths of light.) Other animals are dichromats, meaning they see two colors, along with gray. Dogs and horses, for example, see in shades of blue, yellow, and gray. Humans and other primates are trichromats. Dichromats can see only about 1% of the colors that trichromats can see. Birds, reptiles, insects, and freshwater fish are tetrachromats, meaning they can probably see hundreds of millions of colors, many times what humans can see.
Yong explains that one reason primates may have evolved to see color so vividly is to help them spot the reds, oranges, and yellows of ripe fruit (or nutritious young leaves) against a backdrop of green foliage.
| How Do Scientists Know That Animals Can See Color? To demonstrate how scientists gauge animals’ ability to see color, Yong gives the example of an experiment in which dogs point with their noses to differently colored panels. Besides this type of behavioral experiment, scientists can tell whether an animal can see color by looking into its eyes—at its photoreceptors, or the light-detecting cells in the retina. There are two types of photoreceptors involved in sight: rods and cones. Rods work at low levels of light, such as at night, and don’t help with color vision. Cones require much higher levels of light and are used to see color. If an animal has more than one type of cone in its eyes, it can see some color. Trichromats like humans have three types of cones, which detect three types of light wavelengths: blue, green, and red. Other animals might have more or fewer types of cones. |
2) Just as animals’ eyes evolve to see certain colors, coloration evolves depending on the eyes that are viewing them. For example, once primates evolved trichromacy and could see red, they also began evolving areas of skin that could flush with blood to convey meaning to others of their species. Yong says that even flowers evolved their colors to suit bees’ eyes, rather than the other way around. Many of the colors we see in nature result from the way animals see each other.
(Shortform note: In fact, a 2024 study showed that color vision evolved in animals more than 100 million years before the emergence of colorful fruits and flowers. Color vision evolved in animals about 500 million years ago. However, brightly colored fruits didn’t appear until around 350 million years ago, and colorful flowers emerged about 200 million years ago. While the reason for this delay is unclear, the purpose of vibrant colors in plants is not: Colorful fruits evolved as a strategy to attract animals for seed dissemination; vibrantly colored flowers evolved to attract insects that can disperse pollen.)
3) Unlike humans, most animals can see ultraviolet (UV) light. Animals use UV markings to identify all kinds of things humans can’t, such as the gender of other songbirds, the location of lichen on a UV-reflective, snow-covered hill, or an attractive male fish (some male swordtail fish have UV patterns that are appealing to females but invisible to the fish’s predators, which can’t see UV light).
(Shortform note: In 2024, scientists developed a new video recording technique to more accurately replicate animals’ UV vision. They used a beam splitter that splits light into human-visible and ultraviolet wavelengths. Then two cameras—one sensitive to UV light and one standard—recorded each of the beams separately (and simultaneously). Next, a set of algorithms combined the two recordings and produced videos that show how different animals view moving colors. Seeing UV light the way animals see it allows scientists to do things such as developing bird-safe windows or minimizing the effects of light pollution on insects.)
Light
Humans have two eyes on their heads that face forward, but Yong says that’s not normal for most animals. Animals can have hundreds of eyes, which can be located anywhere on their bodies. Although almost all animals have some form of eyes, some animals see only light and dark, while others see in sharp detail at great distances. For example, a starfish has simple eyes at the tips of its arms that can sense predators, but can’t see color or detail, while an eagle can spot a rat a mile away (birds of prey are the only animals with vision sharper than humans). Yong details other ways that animals see light differently.
1) Animals with high visual acuity (sharpness), don’t have high visual sensitivity (ability to see in low or no light), and vice versa. For example, humans can see the patterns on a deer in the forest on a sunny day, but would barely be able to see the forest itself on a dark night. Mountain lions, on the other hand, can’t see stripes, spots, or other patterns on their prey, but their eyes are sensitive enough to hunt deer in low light at dusk and dawn.
2) Different types of animals have different fields of vision. Vultures, bald eagles, and other large birds of prey (raptors) frequently collide with wind turbines, even though they have excellent visual acuity. Yong explains that this is because raptors, with eyes on either side of their head, have visual fields that extend to each side of their head but not above or below it. They tilt their heads downward when they fly so their blind spots are directly ahead of them; as a result, they can’t see what’s right in front of them.
Humans often misinterpret animal behavior because we don’t understand the way they see. Yong gives the example of a viral video of a male pheasant doing a mating dance for a female, who appears to look off to the side. Viewers found the video amusing because the female seemed completely uninterested in the male’s overzealous display. In truth, the female was not looking away from the male, but was looking directly at him with her side-facing visual field.
Similarly, cows may appear to be bored or lacking in curiosity because they rarely turn their heads to look at you. But their visual fields wrap all the way around their heads, so in reality, they don’t need to turn to see you approaching from the side or even from behind. (Shortform note: Farmers must take cows’ vision, and therefore behavior, into consideration when handling them. Due to their panoramic vision, cows may kick if approached from behind, but it can be hard to move them forward because their poor depth perception makes them balk at shadows.)
3) Animals also see at different “speeds,” meaning some process visual cues faster than others. For example, the killer fly sees at super-fast speeds because its prey are other flies and insects, which it needs to hunt at rapid speeds.
4) In the deep sea, marine animals’ eyes have adapted to see in total darkness. The lights of a submarine can blind them or cause them to panic and even kill themselves. As a result, scientists created a camera that uses red light invisible to deep-sea creatures and lures them with blue LED lights that mimic a glowing jellyfish. This allowed scientists to study deep-sea creatures without harming them or altering their natural behavior.
| Variation in Animal Eyesight Is the Result of Evolution As with all variations in the way different animal species perceive the world, variations in the way various animals see are the result of evolution. As Charles Darwin explained in his seminal 1859 work, On the Origin of Species, species develop gradually through the natural process of evolution, in which animals are most likely to inherit the traits that give them the best chance of survival and reproduction. Through evolution, animals adapt both to their environments and to their ecosystems. Adaptation to a specific environment explains why, for example, deep-sea creatures evolved to see in the dark and raptors evolved to scan the ground below them for prey. It also explains why raptors collide into wind turbines: because wind turbines didn’t exist until the late 1800s, and it takes many species thousands or millions of years to evolve physical traits or behaviors that allow them to survive in their environment. Similarly, because all organisms evolve alongside each other, they adapt to the presence of other organisms in their ecosystems. The killer flies’ super-fast vision is an example of this: It evolved to be able to spot its fast-moving prey. By the same token, mountain lions evolved visual sensitivity but not visual acuity because they need to be able to spot their prey in low light—but don’t need to see its patterns to know it’ll make a good meal. |
