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Q.  I live near Charleston, SC, and spotted a blue anole sunning himself. It was fully blue. I am familiar with our common green and brown anoles, but this one made me think twice. His tail had been severed and he looked rather thin. I live near an elementary school and am concerned that it may be an escaped pet rather than a “native.” He did not scurry off when I took its picture. Please let me know if you believe it is a lost pet or if more blue anoles may start appearing.

A. Carolina anoles often turn blue soon after dying. The photo you sent appears to be a very sick, dying or already dead anole, so the blue skin color has already proceeded to dominate. The recently broken tail suggests it may have been attacked by a predator.

Blue is an uncommon color among animals and is unknown as a true pigment in any flying or terrestrial animal except for a few butterflies native to South America and Africa. The blue color in male baboons and large monkeys known as mandrills as well as in blue jays, bluebirds and blue-eyed people is a consequence of the structural arrangement of cells that absorb other wavelengths but reflect blue. Green anoles and common green snakes appear green in color when alive because the reflected blue mixes with yellow pigments. When the lizard or snake dies, the yellow pigment does also and only the blue is visible to an observer.

Gray squirrels, blue jays and garden toads have bellies that are paler than their backs. Many more animals, including fish and reptiles, also have contrasting body coloration above and below. Why are so many animals a different color on their belly than on their back? Why don’t animals have the same color over all of their body?

A phenomenon, called countershading, is when body coloration above is darker than the underside. In its simplest terms, lighting from above creates a shadow on the abdominal region, giving the perception of a more uniform body color that would be less obvious to another animal. Whether predator or prey, most animals do not want to be conspicuous, and one biological explanation is that countershading adds to visual anonymity by making depth perception more difficult for an observer.

As with most general rules about biological traits, a variety of other explanations have been offered for some color contrasts among animals. For example, some prey that live in forest litter or mucky swamp habitats have dark upper bodies and bright bellies. One hypothesis proposed is that the underside is only displayed as a decoy when the animal is disturbed by a predator and trying to escape. Several eastern snakes offer excellent examples of decoy behavior. Black swamp snakes and mud snakes have shiny black bodies but are bright red on their bellies. Ringneck snakes, common in hardwood forests, are also black above but bright yellow beneath. If a searching bird or mammal predator should undercover any of these, it will see a bright red or yellow flash, but the retreating snake will be black. If the decoy strategy works, the predator is looking for a light-colored animal while the dark-bodied prey escapes unobserved beneath the swamp muck or forest soil. The decoy hypothesis is difficult to document with scientific experiments but is a plausible explanation for the observed color differences.

Another hypothesis for why lighter colors prevail on the underside of some animals is that melanophores, dark pigment cells, require energy to produce and maintain. Therefore, natural selection has favored individuals not investing unnecessary resources in creating dark coloration where it is not needed. The evolutionary time required for such processes could take millions of generations and is not likely to be confirmed with simple scientific experiments. Nonetheless, speculating about why animals are the way they are can be interesting and entertaining.

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The blue coloration on the belly of a six-lined racerunner lizard is caused by the structure of cells. No terrestrial vertebrates are known to produce blue pigment. Photo courtesy Jake Zadik