Taking into account that the human being has his eyes placed in a frontal position, so that their areas of vision overlap, facilitating binocular vision; that thanks to this we are able to calculate distances with our eyes; and that we are capable of distinguishing a chromatic circle based on three colors —except for color blindness—, we can say that we have a relatively good visual system .
Inherited from our arboreal ancestors, who benefited from full color vision to choose which fruits to eat, and the calculation of distances helped them to know which branch they could jump to without falling. Of all our senses, sight is the most developed.
However, we are by no means the animal species with the best eyesight . There is a long list of animals that surpass us. In this list we will highlight three.
Kestrel, ultraviolet vision
In the retina of our eyes there are two types of light receptor cells, rods and cones. The rods are cells specialized in capturing the intensity of light , and it is what allows us to see in dim light. The cones , meanwhile, are excited at certain wavelengths, and therefore allow us to see colors . The cones need a relatively high intensity of light to activate, for this reason, when there is little light we distinguish objects but it is difficult for us to see colors.
In the human eye there are three types of cones, the L , which perceive red light, the S , which are responsible for the wavelengths corresponding to blue, and the M , which are excited by green light. Our brain therefore composes all colors based on these three colors: red, green and blue . We have trichromatic vision.
But not all animals share these receptors. Some have fewer receptors, or have them distributed differently, which makes their color vision different from ours. And there are animals that have more types of cones.
Birds have six types of cones . Three are very similar to humans, therefore they recognize the colors red, green and blue. But the other three correspond to capacities that mammals —and therefore, humans— cannot enjoy.
One of the extra types of cones senses violet radiation . The other two types form a double cone structure that act as a single functional unit, and serve to detect motion . Mammals lack these double cones, and to perceive movement we use the rest of the receptors; this greater specialization of birds makes them more effective at detecting moving objects.
But the most important thing is that fourth color that birds are able to see, and mammals are not. Those violet cones add a new dimension to our basic tricolor scheme, generating a broader and much richer spectrum of hues than our own. In certain species, such as the kestrel, the violet cones have become specialized for vision in the ultraviolet spectrum , an electromagnetic range totally invisible to us.
It has often been proposed that this ability to see ultraviolet may be related to the detection of rodent urine from the air, something that could give these expert hunters a huge advantage in locating their prey. Let us remember that kestrels have the ability to remain suspended in the air, without moving from their spot, compensating their flight speed with the speed of the wind and constantly reorienting their bodies, with their heads completely immobile, watching the ground.
However, some research suggests that the urine of voles and voles does not have adequate refractance in the ultraviolet spectrum, and is unlikely to serve as a visual signal for kestrels. The role of ultraviolet vision in raptors therefore remains unclear .
The octopus, polarized vision
The octopus is a fascinating animal , capable of impressive feats. Among his many exceptional abilities, perhaps one of the most surprising is his ability to perceive the polarization of light.
Octopuses can easily be trained to choose objects with a certain pattern—a shape, color, brightness, contrast, or even quantity—and transfer the learning to select objects that have similar but not identical patterns.
In the mid-1990s, a group of researchers wondered if octopuses would be able to detect the contrast produced by variations in polarization orientation, how much contrast they could recognize, and if they could relate objects based on their polarization patterns of light.
Octopuses were not only able to differentiate between polarized and unpolarized light , but they could also differentiate polarization contrasts with a difference of only 20 ° . They had no difficulty establishing relationships between objects with a similar polarization pattern.
Under water, the wavelength reflected by a body, and therefore the apparent color, depends on the depth, however, the orientation of the polarization vector reflected by said object remains constant, even in very low light conditions . Consequently, for octopuses, this ability to discern polarization and to internalize and compare different polarizations is useful for identifying objects underwater , regardless of the amount of incoming light or apparent color.
The stomatopod, the vision of the superhero
The most complex visual system known of all animals is that of the stomatopods, also called sea mantises . And perhaps, of all the species in the group, the case of Neogonodactylus oerstedii is the most impressive.
It has at least 16 different types of photoreceptor cells – four times more than humans -, of which half are related to color perception. In our eyes, each cone—red, green, or blue, remember—has only one visual pigment, called opsin , that is excited by a certain wavelength. However, in the 8 photoreceptors dedicated to stomatopod color perception there are a total of 33 opsins .
Of these, 20 are sensitive to short wavelengths, and cover three areas of red, three of orange, four of yellow and up to ten of different shades of green; another 10 are sensitive to medium wavelengths, and include four receptors for cyan, two for blue, and four for violet; the remaining three opsins receive different wavelengths of ultraviolet.
Of course, if there are 33 pigments in only 8 types of photoreceptors for color perception, some cells have several opsins and not just one. Only the receptors that perceive ultraviolet have a single pigment; the rest of the photoreceptor units of wavelengths within the visible spectrum have two or more opsins. It is the nervous system that discriminates which of the colors is being excited in each case, based not only on the excited receptors, but also on their position. And it is that the same photoreceptor unit can have different pigments at different heights .
They also have other photoreceptors intended for tasks other than color detection —and which use mixtures of a multitude of pigments for this purpose—. These animals have specialized cells for the perception of light intensity, movement, and polarization .
In addition, each individual eye – which can move independently – has three distinct parts: a central section, like a belt, that separates two hemispheres. Color vision is centered in the central band, and the hemispheres are used to recognize shapes and movement. This allows them to generate a different image in each ocular hemisphere, so they have full binocular vision with each eye and can calculate distances by looking in two different directions at the same time.
REFERENCES:
Kram, Y. A. et al. 2010. Avian Cone Photoreceptors Tile the Retina as Five Independent, Self-Organizing Mosaics. PLoS ONE, 5(2), e8992. DOI: 10.1371/journal.pone.0008992
Lind, O. et al. 2013. Ultraviolet sensitivity and colour vision in raptor foraging. Journal of Experimental Biology, 216(10), 1819-1826. DOI: 10.1242/jeb.082834
Portero, M. L. et al. 2020. Exceptional diversity of opsin expression patterns in Neogonodactylus oerstedii (Stomatopoda) retinas. Proceedings of the National Academy of Sciences, 117(16), 8948-8957. DOI: 10.1073/pnas.1917303117
Shashar, N. et al. 1996. Polarization contrast vision in Octopus. Journal of Experimental Biology, 199(4), 999-1004. DOI: 10.1242/jeb.199.4.999
