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Color Changes in Fish, Frogs and Lizards
Nature Bulletin No. 706   February 23, 1963
Forest Preserve District of Cook County
Seymour Simon, President
David H. Thompson, Senior Naturalist

COLOR CHANGES IN FISH, FROGS AND LIZARDS
Proverbially, the chameleon changes its color to suit every mood or situation. Hence the word is also an expression of contempt for a person who is fickle or changeable in character.

That little lizard which is peddled at circuses or sold in pet shops under the name of "chameleon" is not the true Chameleon of the Old World tropics but the Anole, a native of the warm humid regions of our southeastern states. It is able to change from green to brown, or reverse, with some intermediate colors. Ordinarily it is pale green when quite warm or after it has been in the dark. In bright light or at low temperatures it is brown. Contrary to popular belief a brown anole may be found on a green leaf or a green one on brown bark.

All of our common frogs exhibit differences in skin color. In an hour or two a little Tree Frog can change from a vivid grass green, through a pattern of alternate blotches of gray and green, to a pale ashy gray with dark markings. Just as in the lizards, these responses do not necessarily harmonize with the frog's surroundings.

Fish of many kinds, on the other hand, change the pigmentation of their skins to match the color of their background more or less closely. In the 1930's a series of experiments was carried out with the little Silver- mouthed Minnow, a common fish of small streams and ditches in central Illinois. Placed in a black-bottomed pan they became a dark slate gray; in a white pan they faded to a pale straw color. When dipped from the black to the white pan, or vice versa, the skin color changed completely within two minutes. No matter how often it was repeated, they changed as quickly as the first time. However, when they were allowed to swim from a black background, or a white one, into a pan painted half white and half black, 85 percent of them chose the background to which they were already adapted.

The advantage to a fish of being able to match the color of its background was proved by experiments with the little Mosquito Fish, or Gambusia. For example, when fish from a white tank were added to an equal number in a black tank, a penguin quickly devoured three times as many of former as of the camouflaged dark ones. The value of this protective coloration was shown still more strikingly in other tests when a heron or a game fish was used as the predator.

The champion of all imitators is the Flounder, a fish which lies flat on the bottom in shallow coastal waters. Not only does it adjust its skin color through the various shades of gray but it accurately duplicates the pattern of mud, sand, gravel, stone and shell bottoms. Checkerboard and polka dot patterns are not imitated so exactly. Strangest of all, black, off-white, blue, green, yellow, orange, pink and shades of brown -- but not red -- are excellently matched. However, some of these changes take a long time.

The color changes in all of these animals are, for the most part, the result of changes in certain star-shaped pigment-bearing cells in the skin, called chromatophores. Different ones contain granules of black, yellow, orange or red pigment. In a dark-colored animal, for example, the chromatophores with black pigment have the granules scattered throughout the cell. When the skin fades these granules become concentrated into a microscopic dot, leaving the remainder of the cell colorless. This exposes chromatophores with other colors. Still deeper in the skin are glistening silvery cells that reflect light like a mirror. The control of these color changes involves the retina of the eye, nervous system, the hormones in the blood, and so forth. The story becomes very complicated.

Scotch plaid frustrates even a flounder.


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