Hidden Colors: You can’t see them, but lizards can!

Unlike humans, many animals have (UV) vision, but what exactly does that mean? You can think of UV vision as the “opposite” of color blindness.  For example, people suffering from red-green color blindness cannot distinguish red and green colors.

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Photos: Colblindor (www.color-blindness.com)

They would not be able to read the numbers “8” and “5” in the above plates.

Why not? Defective or missing green- or red-sensitive cones. Human eyes contain two types of light-detecting cells (known as “photoreceptors”), rods and cones. Rods are more sensitive to light and used in low-light conditions, but only cones are able to detect color. Humans have three types of cone cells, each sensitive to a specific color of light (red, green, or blue). These cells work together to allow us to detect all the colors in the visible spectrum. Color blindness is a vision problem in which one of these cones is missing or doesn’t work properly. Red-green color blindness is by far the most common type of color vision deficiency and generally inherited, but color blindness can also develop over time as a result of aging or eye problems and injuries.

On the other hand, animals that are able to see light reflected in the UV region of the color spectrum have an extra type of photoreceptor cell that is particularly sensitive to ultraviolet light. In the same way that having a missing or defective photoreceptor changes the colors that colorblind people see, having an UV photoreceptor changes the colors those animals can see. While the capacity to see in the UV was long-neglected, studies in recent years have shown that UV vision and coloration is widespread in non-mammalian vertebrates, including many species of birds, reptiles, insects, and fishes. The below pictures simulate how different animals with different

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Photos: Dr. Klaus Schmitt, Weinheim, Germany, uvir.eu.

combinations of photoreceptors would view the same flower. As you can see, humans (which normally have receptors sensitive to red (R), green (G), and blue (B) light) would perceive this flower very differently than would a bee or a bird.

So why does this all matter? Color is commonly used as a signal in animal communication. For example, I investigated communication in the common wall lizard (pictured below) in the Île-de-France department of France.

 

Photo: Cécile Vande Maele

 

But until recently, most studies on lizard visual communication have neglected the UV component of color signals. This phenomenon can largely be attributed to humans’ inability to perceive such colors (“out of sight, out of mind”). Now, studies are finding that UV coloration is key to understanding many lizard interactions. In the Augrabies flat lizard, for example, UV throat color is a consistent predictor of fighting ability. In my work with the common wall lizard, I was interested in determining how spots on the lizards’ flanks (two of which are indicated by red arrows in the photo below) are used as a signal in competition between males. While the exact function of the spots in signaling remains unclear, my study showed that common wall lizards 1) have spots that reflect light in the UV region of the color spectrum, and 2) have receptors that are capable of detecting UV light.

gabbielizard

These experiments on lizards illustrate the importance of considering animals’ sensory systems (i.e. vision, auditory, olfaction) when carrying out behavior experiments. If you were give a person with red-green color blindness and a person with unimpaired color vision, would you expect to get the same result? No. And the same is true in comparing our visual perception of the world with that of other animals, such as the common wall lizard.

 

 

Sources:

Alonso-Alvarez, C., Doutrelant, C., & Sorci, G. (2004). Ultraviolet reflectance affects male-male interactions in the blue tit (Parus caeruleus ultramarinus). Behavioral Ecology, 15(5), 805-809.

Bastiaans, E., Morinaga, G., Gaytán, J. G. C., Marshall, J. C., & Sinervo, B. (2013). Male aggression varies with throat color in 2 distinct populations of the mesquite lizard. Behavioral Ecology, 24(4), 968-981.

Bennett, A. T. D., Cuthill, I. C., & Norris, K. J. (1994). Sexual selection and the mismeasure of color. American Naturalist, 144(5), 848-860.

Bowmaker, J. K. (2008). Evolution of vertebrate visual pigments. Vision research, 48(20), 2022-2041.

Hecht, E. (1987). Optics (2nd ed.). Addison-Wesley.

Martin, M. (2013). Fonctions et maintien de la variabilité de la coloration ultraviolette chez les lacertidae (unpublished doctoral dissertation). Ecole Doctorale: Diversité du vivant, Université Pierre et Marie Curie, Paris.

WebMD (2015). Eye Health Center: Color Blindness – Topic Overview. Retrieved from http://www.webmd.com/eye-health/tc/color-blindness-topic-overview.

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