Color vision is the ability to distinguish different wavelengths of electromagnetic radiation. Color vision relies on a brain perception mechanism that treats light with different wavelengths as different visual stimuli (e.g., colors). Usual color insensitive photoreceptors (the rods in human eyes) only react to the presence or absence of light and do not distinguish between specific wavelengths.

We can argue that colors are not real—they are “synthesized” by our brain to distinguish light with different wavelengths. While rods give us the ability to detect the presence and intensity of light (and thus allow our brain to construct the picture of the world around us), specific detection of different wavelengths through independent channels gives our view of the world additional high resolution. For instance, red and green colors look like near identical shades of grey in black and white photos.

An animal with black and white vision alone won’t be able to make a distinction between, let’s say, a green and red apple, and won’t know which one tastes better before trying them both based on color. Evolutionary biologists believe that human ancestors developed color vision to facilitate the identification of ripe fruits, which would obviously provide an advantage in the competitive natural world.

Why certain wavelengths are paired with certain colors remains a mystery. Technically, color is an illusion created by our brain. Therefore, it is not clear if other animals see colors the same way we see them. It is likely that, due to shared evolutionary history, other vertebrates see the world colored similarly to how we see it. But color vision is quite common across the vast animal kingdom: insects, arachnids, and cephalopods are able to distinguish colors.

What kind of colors do these animals see?

Human color vision relies on three photoreceptors that detect primary colors—red, green, and blue. However, some people lack red photoreceptors (they are “bichromates”) or have an additional photoreceptor that detects somewhere between red and green colors (“tetrachromates”). Obviously, having only 3 photoreceptors doesn’t limit our ability to distinguish other colors.

Each photoreceptor can absorb a rather broad range of wavelengths of light. To distinguish a specific color, the brain compares and quantitatively analyses the data from all three photoreceptors. And our brain does this remarkably successfully—some research indicates that we can distinguish colors that correspond to wavelength differences of just 1 nanometer.

This scheme works in largely the same way in most higher vertebrate animals that have color vision. Although the ability to distinguish between specific shades varies significantly between the species, with humans having one of the best color distinguishing abilities.

However, invertebrates that have developed color vision (and vision in general) completely independently from us demonstrate remarkably different approaches to color detection and processing. These animals can have a exceptionally large number of color receptors. The mantis shrimp, for instance, has 12 different types of photoreceptors. The common bluebottle butterfly has even more—15 receptors.

Does it mean that these animals can see additional colors unimaginable to us? Perhaps yes. Some of their photoreceptors operate in a rather narrow region of light spectrum. For instance, they can have 4-5 photoreceptors sensitive in the green region of the visual spectrum. This means that for these animals the different shades of green may appear as different as blue and red colors appear to our eyes! Again, the evolutionary advantages of such adaptations are obvious for an animal living among the trees and grasses where most objects, as we see them, are colored in various shades of green.

Researchers tried to test if a more complicated set of visual receptors provide any advantages for animals when it comes to the distinguishing between main colors. The findings show that this is not necessarily the case, at least not for the mantis shrimp. Despite the impressive array of receptors detecting light in a much broader part of the electromagnetic spectrum compared to humans, the shrimp’s ability to distinguish between colors that great in comparison to us. However, they determine the colors fast. This is probably more important for practical purposes, as mantis shrimps are predators. A large number of photoreceptors allows for their quick activation at specific wavelengths of light and thus communicate directly to the brain what specific wavelength was detected. In comparison, humans have to assess and quantify the signals from all three photoreceptors to decide on a specific color. This requires more time and energy.

Apart from employing a different number of photoreceptors to sense light of specific wavelengths, some animals can detect light that we humans are completely unable to see. For example, many birds and insects can see in the UV part of the spectrum. Bumblebees, for instance, have three photoreceptors absorbing in the UV, blue, and green regions of the spectrum. This makes them trichromates, like humans, but with the spectral sensitivity shifted to the blue end of the spectrum. The ability to detect UV light explains why some flowers have patterns visible only in this part of the spectrum. These patterns attract pollinating insects, which have an ability to see in this spectral region.

A number of animals can detect infrared light (the long wavelength radiation) emitted by heated objects and bodies. This ability significantly facilitates hunting for snakes that are usually looking for small warm-blooded prey. Seeing them through IR detecting receptors is, thus, a great tool for slow-moving reptiles. The photoreceptors sensitive to IR radiation in snakes are located not in their eye but in “pit organs” located between the eyes and nostrils. The result is still the same: snakes can color objects according to their surface temperature.

As this brief article shows, we humans can see and analyze only a small portion of the visual information available to other creatures. Next time you see a humble fly, think about how different it perceives the same things you are both looking at!

References:

Skorupski P, Chittka L (2010) Photoreceptor Spectral Sensitivity in the Bumblebee, Bombus impatiens (Hymenoptera: Apidae). PLoS ONE 5(8): e12049. doi: 10.1371/journal.pone.0012049

Thoen HH, How MJ, Chiou TH, Marshall J. (2014) A different form of color vision in mantis shrimp. Science 343(6169):411-3. doi: 10.1126/science.1245824

Chen P-J, Awata H, Matsushita A, Yang E-C and Arikawa K (2016) Extreme Spectral Richness in the Eye of the Common Bluebottle Butterfly, Graphium sarpedon. Front. Ecol. Evol. 4:18. doi: 10.3389/fevo.2016.00018

Arikawa, K., Iwanaga, T., Wakakuwa, M., & Kinoshita, M. (2017) Unique Temporal Expression of Triplicated Long-Wavelength Opsins in Developing Butterfly Eyes. Frontiers in Neural Circuits, 11, 96. doi: 10.3389/fncir.2017.00096

Image: https://pixabay.com/en/butterfly-3d-blue-mushroom-forest-2049567/