Eyes on the eyes Science

Many animals see more than humans

The crustacean senses colors, ultraviolet radiation, infrared and polarization of light.

Human color vision is one of the best in the animal kingdom. It is thanks to the three types of pin cells in our eyes that specialize in different wavelengths of light and distinguish the primary colors of yellow, red, and blue. For example, a dog only has pin cells that are sensitive to yellow and blue, and it does not distinguish a red ball from a green grass, for example.

The accuracy of color vision tells about lifestyles. Man, like many other monkeys, is active during the day. The ability to find ripe berries and fruits has already benefited our distant ancestors. The opposite is true of the dog: wolves often move in the twilight of the morning and evening, and red-green blindness then does not interfere with predation.

At least the dove has better color vision than humans. It distinguishes up to millions of color tones. Some species of crustaceans have the widest selection of pin cells, which have their own pins for as many as 16 different wavelengths.

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In addition to colors, both crustaceans and pulps detect ultraviolet radiation with a shorter wavelength than light visible to the human eye.

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Ultraviolet vision is common in birds. They take advantage of it in many ways since the coloring of the feather suit. For example, it allows hawks to separate rodent urine in the field and perhaps even ascends the rising air currents above the field.

Cold air is denser than warm air. Their interface probably looks like a bird’s eye that reflects UV radiation in the same way that light in our eyes is reflected from the water surface.

The world of the crustacean would look alien to us.

Some animals even see the polarization of light. Usually, light particles vibrate randomly in all directions, but the surface of water or ice, for example, can reflect or refract light so that the particles continue to vibrate in parallel.

In this species, too, the longest stalk is drawn by the crustacean. It is known to be the only animal that senses ambient polarized light in which light particles oscillate in a corkscrew orbit. What seafood information is used for is a mystery.

In addition to the crustacean, some frogs and fish also emit infrared radiation with their eyes, a kind of red light whose wavelength exceeds human vision. Infrared vision is common in freshwater diving species, probably because the water in it is often cloudy.

When salmon embark on a spawning migration, their eyes are sensitized to infrared light on their way from seawater to the river of birth.

Many other animals, from snakes to mosquitoes, accurately detect infrared radiation with a specialized sensory organ. Because it is not actually an eye, they do not experience infrared as a sight-like visual perception, but rather as a tactile sensation.

Even a person can sometimes feel the heat of infrared radiation on their skin.

The eyes of a dragonfly consist of up to 30,000 partial eyes.

The eyes of a dragonfly consist of up to 30,000 partial eyes.

The gaze can reach behind your back

The size and shape of the eyes adjust the size of the eye.

The narrowness of my own field of vision becomes clear when I sit in the car on the bench with a driver: I need mirrors to look back and to the sides. Human vision covers 180 degrees, that is, the area of ​​about a semicircle.

The dragonfly’s field of vision is double that, as its roaring eyes almost reach the head. In addition to size, the insect’s gaze is broadened by the structure of the eyes. The numerous small lenses in the mesh collect light from a wider area than our single-lens eyeballs.

The field of view of a mallard is almost as wide as that of a dragonfly. Not even the bun crumbs that drop to its tail side go unnoticed. Wide vision is allowed by the location of the eyes on opposite sides of the head. The same is true of other prey, which must keep an eye on threats approaching from all directions.

The downside of the wide field of vision is that the field of vision common to the eyes is narrow and the depth of vision is poor. A bird of prey raging from a moving lunch would often starve the eyes of a duck. Indeed, the eyes of many predators tap closer to each other than the prey.

Blind corners are missing from the field of view of the mallard.

Blind corners are missing from the field of view of the mallard.

In humans, the owl’s eye position is not related to predation but to climbing. Extensive in-depth vision has helped the primate’s common ancestors assess when to jump to the next branch and when not.

The accuracy of vision is also affected by the structure of the retina. Man has one point of accurate vision with only pin cells of accurate vision. The hawk has many of them, the dog none.

As the owl rotates its head and the chameleon its eyes, they seek a solution between the size and accuracy of the field of view. Because the chameleon is able to move its eyes independently, it can observe an area of ​​nearly 360 degrees. If it is hit by a delicious bug, the chameleon literally takes advantage of the depth of sight in the turn of his eye and lassos his victim with his tongue.

Perhaps the most creative compromise in eye placement has been developed by the four-eyed fish. It has two eyeballs, each split in two: the lower part sees below the water surface and the upper part above the surface. This allows the fish to search for food and observe threats from the air and water at the same time.

In addition to the location of the eyes, the field of vision is shaped by the pupil, through which light enters. The cat is known for its viral eyes. Similar pupils are found in several ambush predatory beasts. The narrow pupil obscures the edges of the field of view, but sharpens the visual perception in the depth direction. It helps attack the prey with just the right dimension.

Large cats do not have the viral eyes of a domestic cat.

Large cats do not have the viral eyes of a domestic cat.

Interestingly, pupils are round in large beasts such as tigers and wolves. In addition to the size of the animal, the difference is probably related to the fact that they combine their prey by chasing.

The pupils are also streaky in many grazers, the gap just flattened in a different direction than in beasts. The benefit is the opposite: through the wide pupil, the world appears as a wide panorama. As the horse bends over to hump the hay, its eyes turn in its pits so that the pupils remain in the suit with the horizon.

The author is the editor of Science Nature.

You can read the full article in Science Nature issue 8/2021 or digilehdet.fi:stä, if you are a subscriber to a Sanoma magazine.


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