Most people even have big eyes

I can see what you can not see…

Where human eyes only see black and buzzards, hawks and co. Have long since fallen into their feathers because of the darkness, it just looks a bit gloomy for nocturnal animals. This is because their eyes are constructed very differently from the eyes of diurnal beings. Nocturnal animals have much larger eyes compared to their body size than humans and diurnal animals. In addition, they have pupils that can open very wide in the twilight in order to capture any available residual light.

When rays of light have passed through the pupil, they are focused in the eye on the retina. The retina is connected to the optic nerve, which directs the light impulses to the brain. The structure of the retina is extremely complicated, it consists of 10 different layers and more nerve cells collect on it than anywhere else in the body.

The two most important receptor cells there are rods and suppositories. Suppositories ensure that we can perceive details and colors. The rods are more sensitive to light and therefore have a lower spatial resolution. They react primarily to light and dark stimuli. In the eyes of nocturnal animals, the rods are especially active. Most nocturnal animals therefore have only a few suppositories.

Nocturnal animals have even more tricks to capture every ray of light at dusk or at night. Some animals, such as cats, have a mirror-like layer in the back of the eye, the tapetum. The tapetum acts as a kind of residual light intensifier. The light rays are reflected, pass the retina a second time and can be evaluated again by the uvula. This is also the reason why cat eyes glow in the dark. By the way, the color of the wallpaper varies from animal to animal. Researchers can also identify the animals at night by the color of the glow in their eyes.

During the day, nocturnal animals have to protect their sensitive eyes from the glaring sunlight. For this purpose, some animals have a kind of eye patch that slides in front of the eye and protects it. Most animals, including us humans, simply make the pupil smaller. This can be done in a number of ways. In humans, for example, the pupil retains its circular shape and simply becomes smaller. Or, as is widespread in the animal kingdom, it pushes together on the sides and becomes slit-shaped. Depending on the species, there are horizontal, vertical and diagonal slots.

For example owls:

Owl eyes © IMSI Masterclips

Owls have such excellent night vision that even on a moonless night they can spot a mouse tripping through the undergrowth more than 100 meters away. To accomplish such a feat, owls have very special eyes: First, the animals' unusual eye shape ensures a broader surface of the retina. Second, the owl's iris can expand enormously, the opening can be up to two times larger than that of a hawk. The eyes also have special lenses. These are able to enlarge visual information like a telescope. No movement, no matter how tiny, escapes the keen owl's eye at night.

For example the cat:

Cat eyes © IMSI Masterclips

The cat's eyes are very special. Because the elegant predators can see in the dark and in the twilight. 400,000 sensory cells per square millimeter have the task of capturing every movement, no matter how small, from a 180 degree viewing angle. During the day, when it is very bright, the cat's pupil contracts to form a vertical slit. When it slowly gets dark, the pupil expands to capture every available sparkle of residual light. For this purpose, the eye is equipped with strongly curved lenses. Your built-in "residual light amplifier" in the eye, the tapetum layer also helps cats to spot potential prey in the dark. But there must always be some residual light. Even cats cannot see anything in total darkness.

For a long time it was thought that cats could only see the world in black and white at night. But recent research shows that cats and other nocturnal animals should be able to see not only in color, but also clearly in the twilight. And this is how it works: colors have different wavelengths and are therefore refracted differently by the lenses of the eye. In humans, the iris contracts and makes the pupil smaller in order to bring all of the hues with their different wavelengths into focus.

Since cats open their irises wide in the evening, it was previously thought that they could only see different shades of gray. But the cats use a very special trick to be able to see both colored and sharp in the evening: the lenses of their eyes are structured like onions. Each of the bowl-shaped lens areas has its own degree of refraction. This is caused by the fact that the protein molecules of the lens cells are packed with different densities. With the help of these lens shells, the different wavelengths of colored light can be refracted in such a way that their color is retained and they still hit the retina sharply. No longer stands in the way of the nocturnal hunt in all colors ...

Status: 07/26/2000

July 26, 2000