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What are the colors in a rainbow? Humans and animals might give different answers. While scientists have long known how people can see colors, more recent discoveries have revealed that other animals may perceive a world beyond visible colors.

Color is the perception of light wavelengths that enter the eyes. Different wavelengths of light correspond to different colors: red light has the longest wavelengths and violet the shortest (1). When light hits an object, the object absorbs some light and reflects the rest. The reflected light that enters the eye then strikes the retina, a layer of light-sensitive tissue at the back of the eye that contains two groups of photoreceptors (cells that can detect light): rods and cones (2). 

Rods detect not colors but shapes, movement, and changes in light intensity via a photopigment (a pigment that reacts to light) called rhodopsin. Humans typically use rods when navigating in low light (2, 3, 4).

Cones detect both color and fine details in objects, although they require bright light to do so, which is why humans can’t see colors well in the dark. To detect different colors, humans use three different types of cone cells. Each type of cone uses a different photopsin (cone photopigment) that optimally detects visible light wavelengths within the red, green, or blue range (the maximum peak of these ranges occur at 560, 530, and 426 nanometers respectively). When the photoreceptors detect light, they send a signal to the visual cortex of the brain. The brain then combines the signals sent by red, green, and blue cones to generate the colors of the object in view. Humans use three different sets of cones to see. Therefore, human vision is classified as trichromatic (2, 3, 4, 5, 6).

Trichromacy isn’t the only way to see the world. In some cases of color blindness, such as anomalous trichromacy, one or more photopsins in the eye may have a different sensitivity to its assigned color range than normal. In dichromatic color blindness, only two of the three photopsins function; the missing color is not perceived (5).

Among mammals, humans are special for possessing three types of cones. Most mammals (including dogs and mice) are dichromatic and see the world through only blue and green cones, a vision analogous to red-green colorblindness (7, 8).

The mantis shrimp has a staggering twelve photoreceptors in its eyes. Despite the vast range of colors that one might think a mantis shrimp can see, scientists have found that mantis shrimps are only able to differentiate between colors up to a 15-25 nanometer difference in wavelength with a 50% accuracy. By comparison, humans can distinguish between colors as close as 5 nanometers apart with a 50% accuracy. Scientists are still researching why the mantis shrimp, with so many photoreceptors, appears to not be able to distinguish colors clearly. One hypothesis suggests that, rather than using its vision to compare different colors as humans do, mantis shrimps use their influx of color information to recognize desired colors instead. This may assist mantis shrimps to stab or club quickly passing prey (9, 10).

Humans do not have access to all of nature’s colors. Bees, while trichromats, use blue, green, and UV sensitive cones to see the world. Many birds, fish, and some reptiles are tetrachromats; they utilize cones that detect red, green, blue, and UV light (7). Animals with UV cones can detect ultraviolet light, and studies have found that some animals, such as hummingbirds, use their UV vision to find sources of food (11).

Some plants may have evolved to take advantage of animals’ ability to see UV light. Certain flowers have petals with regions that either absorb or reflect UV light. While the flower may appear to be a single color to humans on the visible light spectrum, UV photography reveals that pollinators, like bees, that can perceive UV light will see a “nectar guide” outlined on the flower. Thus, plants with UV-reflecting petals may have a reproductive advantage over their competitors (12, 13).

More eye-opening discoveries surely await scientists who continue to explore animal senses. The rainbow may never seem quite the same again.

 

References:

  1. “Visible Light.” NASA, https://science.nasa.gov/ems/09_visiblelight.
  2. “How Humans See In Color.” American Academy of Ophthalmology, 8 Jun. 2017, https://www.aao.org/eye-health/tips-prevention/how-humans-see-in-color.
  3. “The Retina.” Neuroscience For Kids, https://faculty.washington.edu/chudler/retina.html.
  4. “Rods & Cones.” RIT Center for Imaging Science, https://www.cis.rit.edu/people/faculty/montag/vandplite/pages/chap_9/ch9p1.html.
  5. Purves, D, et al. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates, 2001.
  6. Berg, JM, et al. Biochemistry. 5th edition. New York: W H Freeman, 2002.
  7. Kelley, Laura. “Inside the Colourful World of Animal Vision.” The Conversation, 6 Nov. 2014, https://theconversation.com/inside-the-colourful-world-of-animal-vision-30878.
  8. Stromberg, Joseph. “New Study Shows That Dogs Use Color Vision After All.” Smithsonian Magazine, 17 Jul. 2013, https://www.smithsonianmag.com/science-nature/new-study-shows-that-dogs-use-color-vision-after-all-13168563/.
  9. Braun, Ashley, “Mantis Shrimp Vision Is Not As Mindblowing As You’ve Been Told.” Popular Science, 19, Jun. 2014, https://www.popsci.com/blog-network/ladybits/mantis-shrimp-vision-not-mindblowing-you%E2%80%99ve-been-told/.
  10.  Yong, Ed. “The Mantis Shrimp Sees Like A Satellite.” National Geographic, 23 Jan. 2014, https://www.nationalgeographic.com/science/phenomena/2014/01/23/the-mantis-shrimp-sees-like-a-satellite/.
  11. Greenwood, Veronique. “Hummingbirds Navigate an Ultraviolet World We Never See.” The New York Times, 19 Jun. 2020, https://www.nytimes.com/2020/06/19/science/hummingbirds-color-vision.html.
  12. “Visual Cues.” US Forest Service, https://www.fs.fed.us/wildflowers/pollinators/Plant_Strategies/visualcues.shtml.
  13. Klomber, Yannick, et al. “The role of ultraviolet reflectance and pattern in the pollination system of Hypoxis camerooniana (Hypoxidaceae).” AoB Plants, no. 11, 23 Sep. 2019, plz057, https://doi.org/10.1093/aobpla/plz057.