When some animals enter estrus or courtship, certain parts of the body become bright and eye-catching, such as birds and tropical fish. In primates, there are similar situations, such as female baboons and chimpanzees in estrus, whose butts are swollen and appear prominently red. Interestingly, scientists have found that this act of courtship through "discoloration" is closely related to the visual evolution of animals.
Japanese macaque
The retina of a living organism is covered with a large number of cone cells, and it is it that helps the biometrics to recognize colors and perceive bright lights in order to observe different colors. Simply put, the more types and numbers of cone cells there are, the richer the colors that organisms can see. Taking humans as an example, the world we see actually benefits from the three kinds of cone cells that perceive red light, green light and blue light, so humans are also called "panchromatic visionaries".
Different species of insects have the most cone cells, they can see the world is more colorful than we are; most birds have four-color vision, in addition to the red light, green light, blue light that humans can perceive, they can also perceive ultraviolet rays, so humans see birds and birds look at birds are completely different effects, and their eyes are colorful with each other.
Primates are slightly more complex than birds, and are divided into proto-monkey and ape-monkey, of which apes are divided into Old World Monkey and New World Monkey. Proto-monkeys are considered to be the most primitive primates, including tree shrews, lemurs, skinny monkeys and tarsiers, which retain the characteristics of their ancestors' nocturnal travel, and in the dark of the night, the role of vision is far less than that of hearing and smell, so after a long period of evolution, they have lost the ability to perceive color while having a strong sense of hearing and smell.
Tarsiers
Studies have also shown that proto-monkeys usually have only two kinds of cone cells, and there is no stereoscopic vision, which is why they do not attract the opposite sex with "discoloration" during estrus, but by emitting unique odors, such as the ring-tailed lemurs living in Madagascar, when in estrus, the glands in multiple positions on the body can emit special smells, and rub the glands with a large tail to help the odor spread, thus gaining the favor of the opposite sex.
Ring-tailed lemur
New World monkey refers to monkeys mainly distributed in the forests of Central and South America, such as tamarin monkeys and capuchin monkeys, they generally have a developed sense of smell, but also have three-dimensional vision, but whether it is tricolor vision "varies from monkey to monkey", such as howler monkeys have three kinds of cone cells, and humans are the same as "panchromatic vision", and night monkeys have only two kinds of cone cells like proto-monkeys, which are red-green blindness. Unlike proto-monkeys, New World monkeys often do not rely on smell, but rather use special behaviors, such as howler monkeys that express love by spitting out their tongues, while capuchin monkeys are more special, throwing stones at the object of their hearts to convey a signal of love.
Capuchin monkeys
Most of the "red-ass monkeys" we often call "red-ass monkeys" belong to the Old World monkeys, including colobus monkeys, macaques, baboons, mountain rays, as well as orangutans, gorillas, chimpanzees, etc., of course, including us humans, these species have tricolor vision, but the sense of smell is not so sensitive. In other words, species that "change color" by their butts are more adept at using vision to attract bright colors, while their sense of smell deteriorates.
mandrill
Scientists through computer simulations have found that in primates, this change in the Old World monkey occurred about 23 million years ago, when a protein of an enzyme related to the sense of smell in the primate began to mutate and gradually lost its role in the evolutionary process, that is, since then, the role of vision in choosing a mate has become larger, which also explains why most Old World monkey females will have bright red butts during estrus or courtship, while this change is not found in protosthes monkeys and New World monkeys. So they rely more on smell or behavior to find a mate.
Whether or not to rely on vision for courtship is in fact the embodiment of the principle of "use in and out" in the evolutionary process, which is closely related to the environment in which species live. The original monkeys have been active at night for a long time, without the skill of distinguishing colors, and their keen sense of hearing and smell is necessary for survival, which determines the sense of smell that they mobilize when they live and reproduce. The Old World monkey, on the other hand, became more and more important in the diurnal way of living, three-dimensional vision and the ability to distinguish colors, and when the sense of smell did not have a decisive role in successful reproduction, it began to degenerate, and the way of courtship changed from smelling smell to seeing color. So the next time you go to the zoo to see monkeys (by the way, the most common monkey in a zoo is macaques), their reddish butts are definitely not meant to be seen, but their counterparts know how to appreciate the red.