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Q. You once wrote that blue pigment is not known to exist in any terrestrial or freshwater animals with backbones. I found a blue bullfrog in a marsh in Virginia. It was mostly blue with some gray blotches; it did not have green anywhere on its body. How do you explain the appearance of a frog like this?

A. It’s true about the absence of blue pigment. But the perceived color of a plant or animal is one of the simplest yet most complex biological attributes of any species. You saw a blue frog and yet you didn’t. The answer lies in a mix of genetics, physics and the visual abilities of people.

Part of the answer lies in the structural arrangement of cells in some animals that absorb other wavelengths but reflect blue ones. In other words, the blue we see is not because of the animal’s pigment but because of biomechanical configurations in the outer layers of the body. If we see red or yellow, there is red or yellow pigment. Animals produce a variety of red and yellow pigments that vary biochemically. Not so for blue. We normally see a bullfrog as green because of the structural blue plus the presence of a yellow pigment. When the structural blue (i.e., the reflected wavelength) in a frog combines with its yellow pigment, we’re back to the grade school secondary color green. However, a rare genetic condition known as axanthism occurs in some animals, including frogs. An axanthic animal is one in which the biochemical pathways that produce yellow pigment do not function properly. When a bullfrog with structural blue is missing its yellow pigment, a blue frog appears on the scene. The blue frog phenomenon has been reported occasionally in green treefrogs in the Southeast and in other members of the bullfrog family. Usually only a portion of the body is blue. A totally blue bullfrog is quite rare.

 Color is complicated, and explaining wavelengths and the physical processes that produce color is a job for a physicist, not an ecologist. However, certain observations are readily apparent. When we mix paint, the so-called primary colors—red, blue and yellow—work fine to produce the standard secondary colors of orange, green and purple. I learned this finger painting in kindergarten. But when physicists discuss the properties of light, the primary colors are red, blue and green. And sure enough, if you shine stage lights onto a wall, red and green produce yellow, not brown!

Despite the conundrum of which primary colors produce which secondary ones, an ecologist can report certain biological facts about color. Colors we cannot see are those with the shortest (ultraviolet) and longest (infrared) wavelengths, although we may see everything in between. Honeybees and numerous other pollinator insects see some flowers as ultraviolet, a shade of alien purple you won’t find on any color wheel. Humans can see the ultraviolet flower itself. But to us it appears white. At the other end of the light spectrum, rattlesnakes, copperheads and cottonmouths can detect infrared with the heat-sensitive pit that is between each eye and nostril. Within the human range of visible colors, that is how we can see a blue frog in the absence of blue pigment.

Structural blue coloration of feathers appears in many birds. A common blue jay feather can demonstrate that the blue is not a pigment. Hold the feather up toward the sun or a fluorescent light. It will look brownish rather than blue because the light is no longer being reflected from it. If the blue were a pigment, the feather would be the same color from any angle. You can’t hold a blue bullfrog up to check for reflective light so you’ll have to take science’s word for why it’s blue (but not really blue). Which should in no way detract from your enjoyment of nature’s wonders.

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A blue bullfrog found alongside a freshwater marsh in Virginia is a true oddity of nature resulting from the absence of yellow pigment, presumably from a genetic abnormality. Photo courtesy Patricia West