It is believed that 90 percent of information is received by humans through vision. If this is true, then the eyes of other animals perceive 150 and all 200 percent of the information. Compared to ours, of course.
During embryonic development, the human eye seems to turn inside out. So the primary front side of the retina faces away from the pupil in the opposite direction, and the light overcomes the thickness of other cells before reaching the photoreceptors. A nerve passes right through the retina, forming a blind spot. Because of it, objects that are right in front of the eyes suddenly become invisible. We see the world in color, and our color perception is called trichromatic: from the Greek “three” and “chroma” (color).
If we compare it to the colorful senses of many animals, our chromatism is more likely to come from the word “limp.” It so happened that our distant ancestors, the first placental mammals, lived literally in the shadow of dinosaurs. They probably preferred not to come out at all while the daytime guard of terrible predators was on duty. Small nocturnal animals did not need all the colors of the world. So they lost half of the color receptors – cones – that their reptilian ancestors had. Whales and seals, who mastered the water element, as well as nocturnal primates, completely lost their color perception – their world became monochromatic, black and white.
It’s hard to be a horse. Like most placental mammals, horses are color blind, and their visual acuity is two times worse than that of humans. But thanks to the large eyes (above: 3.4 centimeters in diameter versus 2.4 in humans) located on the sides of the head and the elongated pupil, the horse’s panoramic view is 300 degrees (in humans – 190), and at night it sees much better than we do.
Color vision is nothing more than the ability to distinguish between the wave spectra of light. Most placental mammals remain dichromatic: they lack cones that are sensitive to the long-wave part of the spectrum, i.e., to red. Everything appears to them either ultraviolet-green (rodents) or blue-green (horses, cows, cats, dogs). Like colorblind people. This is the name given to people for whom red and green look the same, and there are no shades at all. Instead of, say, yellow-green, they see white, gray, or just yellow. This defect doesn’t bother many people, and until kindergartens and schools introduced mandatory color perception testing, a person could live their whole life without even realizing that they were different.
The English chemist John Dalton was the first to try to understand the nature of this phenomenon at the end of the eighteenth century. He noticed unusual sensations of colors in himself and his brother: the pelargonium flower, which seemed sky blue in daylight, turned almost yellow in candlelight. (In fact, pelargonium was pink.) Dalton decided that he had a blue filter and bequeathed his eyes for research. After the scientist’s death in 1844, his physician Joseph Ransom performed an autopsy and found nothing unusual in the vitreous, cornea, or lens. Only 150 years later, the remains of Dalton’s eyes were studied by molecular biologists. They discovered the absence of the gene encoding opsin, which perceives the green part of the spectrum. Opsin is a protein part of the pigment; the other part is chromophore, a vitamin A derivative. The chromophore changes its structure under the influence of light, and opsin captures this chemical signal and transmits it further to the optic nerve of the brain.
On average, 2 percent of people suffer from color blindness. This disease is hereditary and is associated with malfunctions in the X chromosome, where the genes encoding two different opsins are located close to each other. Among men who have only one such chromosome, the number of dichromatics is higher – up to 8 percent. The same genetic patterns are observed in our closest relatives, other Old World primates. But in some species of South American monkeys, all males are colorblind.
Among mammals, only primates regained trichromatic vision 35-40 million years ago. The stimulus for the newly acquired color vision was the way of life associated with the treetops in search of fruit. After all, unripe green fruits are not only not very tasty, but often poisonous, unlike ripe, juicy and sweet, red and bright yellow fruits. And the precision of jumping from branch to branch is impossible without a developed brain, which plays an important role in processing visual signals and, therefore, in the correct perception of color. At the chemical and genetic level, everything was resolved quite simply – thanks to the emergence of two different genes based on one responsible for the synthesis of cone opsin, which perceives medium (green) waves. “Replace some amino acids with others in just three of the 348 sites of the opsin molecule, and color perception will shift by 30 nanometers,” says neuroscientist Gerald Jacobs of the University of California, Santa Barbara. – “This is quite enough to see an additional spectrum: the difference between the red and green spectral peaks is exactly 30 nanometers.
Birds see red-green and ultraviolet-greenish-red shades that we cannot even imagine.
How quickly can such a genetic change spread? Very quickly. As already mentioned, there are on average two color-blind people per 100 people, and on Pingelap Atoll in Micronesia, 75 out of 700 people do not recognize red. There, only 20 people survived the typhoon of 1775. One of them was color blind, but very prolific…
Let’s go back to the time of the appearance of mammals. If at the beginning of their evolution they lost half of their color receptors and were left with two types of cones, then their ancestors had tetrachromatic vision? This is indeed true. Almost all other vertebrates-fish, amphibians, reptiles, and birds-have richer color perception than we do. We are used to considering red, green, and blue as primary colors; the other hundred shades are their derivatives. This color scale was created by our light-sensitive pigments. They are most susceptible to light waves with peaks of about 560 (red), 530 (green) and 420 (blue) nanometers. And, for example, birds can also see ultraviolet (370-390 nanometers). In the multicolor and ultraviolet light, they see partners, fruits, and flowers that seem to us to be monochromatic. They see red-green and ultraviolet-greenish-red shades that we cannot even imagine. In addition, birds and reptiles have color filters in their eyes – colored oil droplets. These filters narrow the areas of the spectrum perceived by each pigment, and thus multiply the number of visible colors. A bird will never confuse an orange-yellow caterpillar with a yellowish-orange one.
Over the past decade, scientists have revolutionized our understanding of insect abilities. It turned out that bees can memorize and recognize people’s faces!
Not only birds can boast of ultraviolet color perception. In bees, this ability was discovered in the XIX century, and the famous ethologist Karl von Frisch in 1914 figured out how to find out how many colors these Hymenoptera see using colored and gray (different shades) paper squares. However, Frisch was unable to determine how bees actually perceive red or yellow. “Today, we can take a bee, implant a microelectrode into its 5-μm photoreceptor, then direct a beam of light of a particular spectrum into its eye and measure the potential difference that occurs in the cage,” says zoologist Lars Chittka of the University of London. Together with his colleagues, over the past decade, he has revolutionized our understanding of insect abilities. It turned out that bees can memorize and recognize people’s faces!
For this purpose, the same von Frisch squares were used, but instead of paints, they were covered with photographic portraits of people. On the third attempt, most of the bees correctly chose the face that had been smeared with honey in previous experiments instead of the one covered with bitter quinine. Only the inverted portraits caused difficulties. (It seems that Winnie the Pooh, in order to deceive the bees, had to hang upside down on a balloon instead of pretending to be a cloud.) But humans are no better at this task. So, to recognize faces, it is not necessary to have special parts of the brain, as neuropsychologists suggest? Even tiny brains can do a lot. After all, bees that thought longer made more accurate choices. When Lars Chittka and installation artist Julian Walker placed reproductions of paintings by Vincent Van Gogh, Paul Gauguin, Fernand Leger, and Patrick Caulfield in front of bees bred in artificial conditions and never seen real flowers, most of the insects chose Van Gogh’s Sunflowers. Art critics were already talking about the fact that even bees can distinguish real artists, but the experimenters cooled their ardor: the bees were primarily interested in contrasting combinations of colors and the most attractive colors for them.
A caged sky. Most arthropods, including bees, have compound, or faceted, eyes (above). The final image in such an eye resembles a spherical mosaic (bottom right). Almost all bees do not perceive red, but they can see ultraviolet light.
The color scale of bees consists of the ultraviolet, blue and green spectrums (340, 440 and 530 nanometers, respectively). These insects see the world as follows: purple poppies, whose petals contain an almost invisible blue tint, appear to them in ultraviolet; lilac bellflower – ultraviolet blue; dark pink Ivan-tea – blue; pale pink rose hips and white clover – bluish green; light yellow meadowfoam – green; and dark yellow rapeseed – greenish ultraviolet. Of course, all of these are our ideas of “bee” colors. The crystalline lens, which does not transmit ultraviolet rays, prevents us from seeing the world in true bee colors.
However… in 1923, the French artist Claude Monet had the lens of his right eye removed along with cataracts, and he could see ultraviolet light with this eye. Among his paintings of the following years there are paired landscapes that are very different from each other by the combination of colors. Art historians believe that he painted them under different lighting conditions. Or maybe he covered one eye or the other in turn… The earth bumblebees on the island of Sardinia had a similar story to the Micronesians of Pingelap Atoll. However, they did not become color-blind, but, on the contrary, gained the ability to see red. And they began to perceive the world in four spectrums – like many butterflies, beetles, dragonflies and flies.
Butterflies also have more complex cases of color vision – up to five spectra, and their eyes have additional pigments, light filters. Accuracy in choosing the right shade is necessary for lepidopterans to detect the freshest and youngest leaves for laying eggs, from which voracious caterpillars will hatch. Sometimes you can tell what colors a butterfly sees by its wings: the color of its wings is determined by the same pigments that perceive colors in its eyes. It happens that additional eyes appear on the penis, and they can see. And if the wings of males and females are noticeably different, as, for example, in the case of damselflies, it is because males and females look at the world with different eyes. However, in some birds, even the right and left eyes perceive color differently. Now that it has become clear that the brighter the animals look, the brighter their perception of the world is, it is enough to look around to notice the bright coloring of birds’ plumage, insect wings, lizard and frog skins. One can only envy them. Flowering plants, adapting to the vision of their pollinators and seed carriers, also resemble a rainbow, not a seven-colored one, but a much more colorful one invisible to us.
For a long time, it was believed that the autumn colors of the forest were only a consequence of the destruction of the green pigment chlorophyll in dying leaves. That’s when the initially hidden yellow carotenoids appear. But red anthocyanins begin to be produced by trees in the fall. “Anthocyanins are released simultaneously with phenols that are dangerous for leaf-eating insects such as aphids,” says entomologist Marco Archetti of the University of Basel about his research. – “And since aphids are able to distinguish red from green, although they apparently do not have special photoreceptors, they try to avoid red leaves, and the tree is freed from uninvited guests.”
Fish, especially those that live in shallow water, can compete with birds and butterflies in terms of color – and they can distinguish between many colors. For cichlids living in large African lakes, the difference in color perception even became the basis for further evolution: in Lake Victoria, species with red scales are prolific, and in Lake Nyasa, species with blue and purple scales.
By the way, cichlids have hexachromatic vision: their eyes distinguish between ultraviolet, violet, blue, blue-green, green, and red spectra. The latter, the long-wave spectrum, is best distributed in the muddy waters of Lake Victoria, which is why red fish predominate there. And the changes in color, of course, are based on genetic changes, primarily related to the genes encoding opsins. Six spectral types of light-sensitive cells are far from the limit: in mantis crayfish, there are 16 of them, and 10 or 12 of them are used for color perception! We can only envy them, but, unfortunately, we will never know even approximately what this arthropod sees.
And why does it need to see all this? In the sea, the long-wave (red) part of the spectrum is absorbed within a dozen meters, then it is the turn of medium (green) waves, and short (blue) waves penetrate the deepest. That’s why shallow water looks turquoise to us, and the open sea looks blue. The spectral difference between the upper and lower layers of water could stimulate the appearance of at least two different photopigments.
But why do fish and other marine life need to distinguish red? Many ocean inhabitants prefer it because they fluoresce – emit a red glow. In the Red Sea, which is so beloved by divers, sea needles, dogs, gobies, gobies, as well as some algae, sponges, corals and ophiuchus are capable of this. The blue element, if you look at it through the eyes of fish, really turns out to be red. Even in the depths of many kilometers, where not a single solar photon penetrates, fish are in no hurry to part with their color vision. Using red and orange signal flashes, dragonfish (stomia) find their mates at a distance of several meters. Unfortunately, they can’t go any further.
One of these fish, the malacosta, has adapted the green pigment of plants, chlorophyll, which is part of light-sensitive cells, to perceive red light. The malacost receives chlorophyll with its food – paddlefoot crustaceans, and they, in turn, feed on unicellular algae. In order to avoid getting caught in the teeth of a predator, the fish release counter sprays that distort the body contour. And the ocean floor itself sometimes resembles a waking night city: a passing diamond stingray sways its fins with frequent strokes, and the bamboo corals blaze with advertising neon, among which the “dimensional” lights of ophiuchus, sea spiders and sea lilies flash. Giant squids with their gigantic eyes (27 centimeters in diameter, even a blue whale’s is 2.5 times smaller) can see a sperm whale at 600 meters depth. This is because, diving through clouds of plankton, this toothed whale causes microorganisms to glow. The squid will not have time to escape, but it will be able to meet the enemy fully armed.
How do you understand what you can see, say, at a depth of 400 meters? It’s very simple: take a walk in the woods on a moonlit night. The illumination in such a forest is 100 million times lower than in an open field on a cloudless sunny day. On a moonless, but starry night, it is even 100 times lower, as at a depth of 600-700 meters. At best, we can distinguish the vague outlines of the nearest objects, and no colors. But the fast-winged butterflies, the pansies, which fly out to drink nectar at dusk, and the gecko lizards, which hunt at night, are guided by color.
“Color night vision devices as different as the faceted eyes of the damselfly and the chambered eyes of the gecko have one thing in common,” explains neuroscientist Almut Kelber of Lund University. – “Both have a special mirror-like cell lining behind the retina. This mirror reflects the light lost by photoreceptors and directs it back directly to these cells.” That’s why the eyes of moths sparkle in the dark when a flashlight beam is directed at the moth. A cat’s eyes also glow. And they have the same lining. In addition, a slit pupil and the close proximity of the retina to the lens help it to catch invisible night light. But the cat cannot distinguish colors in the dark.
Like a cat in the dark. A cat needs a narrow pupil (above) to prevent light particles – photons – from flying back into space after being reflected from the lining of the fundus. It is the shape of the pupil and eyeball, as well as the reflective lining, that help the cat see objects well in the dark (below).
All this is not the end of the eyes’ capabilities. For example, there is a lot of life near the deep-sea “black smokers”, which spew 350-degree geysers. Numerous shrimps and crabs live here. In order not to get lost in the cold lifelessness of the warm oases surrounding them on all sides, they have adapted to see infrared radiation (700-1000 nanometers) emanating from hot solutions. But not only that: zoologists have discovered pigments in the eyes of these crustaceans that are susceptible to green light. Medium-length light waves do not penetrate to this depth. So, the source of the green glow should be sought in the “smokers”. Geophysicists have discovered it: myriads of gas bubbles released during the eruption of black smokers explode and emit green light. This phenomenon is called sonoluminescence. On land, pythons and rattle snakes can see infrared light. Such a snake has a pair of pits on its head, which are arranged almost the same way as real vertebrate eyes: only the lens is missing. This is a kind of thermal imager: heat emanating from the body of, for example, a mouse, enters the pit and excites sensitive cells that can distinguish temperature differences of a thousandth of a degree Kelvin.
The brain processes the information it receives, compares it with the information received through the eyes, and assembles a fairly clear image of the mouse. Light rays differ not only in their spectrum: passing through the atmosphere, reflecting off a smooth water surface or glossy leaves, they polarize. While in an ordinary beam of light, electromagnetic waves oscillate in any field planes perpendicular to its propagation, in a polarized beam, most waves oscillate in the same plane. And many insects and birds have adapted to see polarized light to find its source: the sun during the day and the moon at night. Of course, in clear weather there is no such need, but when the whole sky is covered with clouds, it is not easy to determine where the luminary is now.
