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.