Colors. It’s a simple concept, really. Light, which is a colorless form of electromagnetic radiation that exhibits the behaviors of a wave, travels at different wavelengths and reflects off the surfaces of objects to activate cells in our retina. Some types of light, such as ultraviolet light, X-rays, and gamma rays are not visible to the human eye, because they simply do not fall under the portion of the electromagnetic spectrum that we can see. Even when the wavelengths fall in the range of visible light, however, we are still unable to see all of them. This is because some surfaces of objects absorb certain wavelengths better than others, and the wavelengths that are not as well absorbed are the colors we identify the object as.
The colors visible to us are quite clear. The primary colors of light, which are red, blue, and green, combine in varying amounts to produce the rest of the colors in the visible light spectrum. At this point, one may raise the question: why, then, do other animals perceive light differently from us? Bees detest shades of black, and dogs are unable to distinguish between red and green. Why is this so? If all objects reflect light off their surfaces in a relatively consistent manner, and the colors that certain wavelengths represent stay constant, why is there such a vast difference in the types and colors of light that different species see?
The answer lies in the varying structures of our eyes. More specifically, the types of cone cells that are located in the retina of vertebrate animals determine the range of colors we can see. Humans, who have three color cone types in their eyes, are able to detect and respond to light with wavelengths from approximately 380 to 740 nanometers. Comparatively, dogs only have two color cone types in their retina, which is responsible for their red-green colorblindness. Birds, on the other hand, have four color cone types, allowing them to see a wider spectrum of light than both humans and dogs. While this has been a known fact for some time, exactly how birds see color has remained a mystery. But now, through the intensive study one team at Harvard University has conducted on wild hummingbirds, the answer has become more clear.
Photo credits: Business Insider (LINK)
Caption: A visualization of the drastic difference in color recognition between dogs and humans (left: dog, right: human)
Many animals, including humans, have the ability to see non-spectral colors, which are colors that are unable to be produced by a monochromatic light and require two or more color cones to be discernible. Scientists have predicted that birds, with a greater number of color cones, are able to see a larger range of non-spectral colors compared to humans. To test this hypothesis, the team of researchers used behavioral experiments involving shades from natural plumage and plants, eventually reaching the conclusion that wild hummingbirds could indeed perceive non-spectral colors such as UV+red and UV+green.
Moreover, apart from the affirmation that birds could indeed see non-spectral colors in the ultraviolet spectrum — which has wavelengths that fall between 10 and 400 nanometers — the scientists also made an unexpected discovery: the extra color cone that birds have could cause them to see more colors as non-spectral than humans. While it has been a known fact that many species of reptiles and aquatic vertebrates, including common types of fish, exhibit a similar trait, this ability was not yet explored in hummingbirds.
The researchers, having now established solid proof that wild hummingbirds can discriminate a broader range of colors and identify a larger portion of them as non-spectral than human beings can, are now hoping to examine how this color-distinguishing ability affects their behavior, such as how it may aid hummingbirds in their foraging endeavors.
The addition of a single type of color cone allows hummingbirds to see the world from an almost entirely different viewpoint than many other animals. However, they are far from the most generously endowed — mantis shrimp, the subject of my article for the third issue, have 16 types of color cones in their retina, and the vivid colors that characterize their world are but imaginative elements in our eyes. In the large electromagnetic spectrum, the range of light that is visible to us is startlingly limited, and this discovery is but a reiteration of this fact.
Works Cited
“Colours of Light.” Science Learning Hub, Science Learning Hub, 2020, www.sciencelearn.org.nz/resources/47-colours-of-light.
Greenwood, Veronique. “Hummingbirds Navigate an Ultraviolet World We Never See.” The New York Times, The New York Times, 19 June 2020, www.nytimes.com/2020/06/19/science/hummingbirds-color-vision.html.
“How Do We See Color?” Pantone, Pantone LLC, 2020, www.pantone.com/color-intelligence/articles/technical/how-do-we-see-color.
Stoddard, Mary Caswell, et al. “Wild Hummingbirds Discriminate Nonspectral Colors.” Proceedings of the National Academy of Sciences of the United States of America, HighWire, 28 Apr. 2020, www.pnas.org/content/early/2020/06/09/1919377117.
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