Ultraviolet Colors?

While we humans see a wide variety of colors from red to violet, we see only part of the electromagnetic spectrum (from about 390 nanometers - the violet end - to about 780 nanometers - the red end of the spectrum). Other animals see different parts of the spectrum. Even other mammals see different parts of the electromagnetic spectrum than we do. For example, deer are not able to see orange and red except as shades of gray, so hunters use clothing of these colors to be visible to other hunters but not to deer. Dogs, once thought to be colorblind, actually are now thought to see about the same part of the color spectrum as humans that are red-green colorblind. Conversely, some insects and birds see colors in the ultraviolet end of the spectrum that are invisible to humans.

Plants use colors in the ultraviolet to communicate with insects and birds to the benefit of all involved. Ultraviolet photos show more patterning on some flowers than similar photos taken in the (human) visible spectrum. These patterns are called honey or nectar guides and they direct pollinators to the parts of the flower where nectar and pollen are available. The pollinators eat the nectar and carry the pollen to the next flower. Researchers from Cornell have discovered that in some plants these same ultraviolet colors may warn herbivores to stay away because the plants contain anti-feedant chemicals.

A short article in Science* profiled a study of the ultraviolet markings on butterflies.When researchers used ultraviolet absorbent paint to obscure the white (to the human eye) dots on the wings of male African satyrid butterflies (Bicyclus anynana), females ignored them, but flocked to males whose normal ultraviolet markings were not obscured. Researchers believe the males that retained their ultraviolet patches look younger and healthier to the females.

Scientists now believe that increases in ultraviolet light due to atmospheric ozone layer thinning are unlikely to affect color vision in insects. However, models show that bees probably cannot identify pure ultraviolet colors under changing illumination conditions, so plants use a combination of colors in both the visible (to humans) and ultraviolet ranges to communicate with them. Next time you see a beautiful flower, realize that you may only be seeing some of its colors, and appreciate pollinators. We have all these beautifully colored flowers because plants need to attract their attention!

*Couzin, J. Science 309, 242 (2005).