An animal’s colors and patterns serve many different purposes. For example, some of these purposes are to make them stand out in finding mates and to warn other predators that they are poisonous. But for lurking predators such as tigers, the ability to be invisible to their prey determines whether they will eat their dinner or starve. So why are tigers orange when there are so many other possible colors?
For humans, that’s a good question, given that orange is a color used for items that need to be ultra-visible, like traffic cones and safety vests. To our eyes, orange stands out in most environments, making tigers relatively easy to spot.
But that’s because we have what’s called trichromatic color vision. When light from the outside world enters the eye, it hits a thin layer at the back called the retina. The retina processes light using two types of light receptors: rods and cones. Rods only detect differences in light and dark, not color, and are mostly used in dim light. Cones are what we use for color perception, and most people have three types: cones for blue, green, and red. This is why our vision is called trichromatic: we can see the three primary colors and their colorful combinations. We share this viewpoint with the monkeys.
However, most terrestrial mammals, including dogs, cats, horses, and deer, have bicolor color vision. This means that their retinas contain cones for only two colors: blue and green. People who only receive information from the blue and green cones are considered colorblind and cannot distinguish between shades of red and green. The same is probably true for dichromatic animals.
Terrestrial mammals such as deer are the tiger’s main prey, and their bicolor vision means they don’t see the predator as orange, they see them as green. This makes it much more difficult to spot a tiger when it is prowling behind a bush or crouching in the grass.
“In essence, it’s evolutionarily easier for the animal to be brown or orange for it to be green than because of the biomolecular nature of its structure,” says John Fennell, lecturer in animal detection and biometrics at Bristol Veterinary School in the United Kingdom. “In fact, the only known green mammal is a sloth, and its fur isn’t actually green. It’s the color of an alga growing on its fur.”
Fennell used artificial intelligence to identify ideal coloring and ideal patterns for hiding in a variety of environments. In 2018, her work was featured on BBC One’s “Anials Behaving Badly”.
But given that evolution tends to favor traits that help a species survive, why didn’t prey animals develop orange eyesight?
“You would imagine that in an evolutionary arms race, an improvement in visual perception would provide its prey with better visual systems in the first place. However, there doesn’t seem to be any evolutionary pressure for deer, especially the tiger’s main prey, to become trichromatic. This is probably because the tiger is orange,” Fennell said. It is because he does not know that he is a dichromate.”
