Popular Science Monthly/Volume 43/August 1893/The Color Changes of Frogs
By Prof. CLARENCE M. WEED.
ONE who, with observant eye, leisurely paddles among the water lilies of an inland lake must often notice how closely the colors of the various frogs resting upon or among the lily pads resemble their environment. In the open sunshine, where light green is the prevailing tint, the colors of the frogs closely approximate it, but in the dark and shady recesses of the forest-bordered banks the batrachians are dull, deep brown, with darker spots scattered over their bodies. These are the effects as seen from above. If one were to dive beneath the water and look upward, he would see in either case only the whitish undersides of their bodies and legs—if, indeed, these were visible against the general lightness of the upper world.
It is evident that this resemblance to environment might result in either of two ways: first, the light-colored frogs might seek the light surroundings and the dark ones the dark surroundings; or, second, the frogs, provided they had the power, might change their color to agree with the environment. The latter method would, of course, from the frog's point of view, be decidedly the more desirable, saving him much exertion in seeking safe retreats; and recent researches have shown that this is in fact the method adopted.
The ability of the tree frogs, or "tree toads" (Hylidæ), to change their color has long been known, but precise studies of the color changes of the common ground frogs (Ranidæ) have Fig. 1.—Wood Frog. Adult. only been made comparatively recently, and as yet the record is far from complete. A few years ago Dr. Fickert, of Tübingen, experimented with the color adaptability of the common European frog (Rana temporaria): "Three frogs approximately similar in color were placed in three glass vessels, of which the first stood on a black, the second on a green, and the third on a white surface, being surrounded up to a height of some five centimetres with the same color. After about an hour and a half the frog a on the black surface was the darkest, b on the white the lightest, while the frog c surrounded by green was intermediate in color between the two. Hereupon the frog a was transferred to the glass on the white, frog b into the one on the black surface. After three quarters of an hour they were again examined, and a was the lightest, b the darkest. Then c and b were interchanged, and in a quarter of an hour c was the darkest, while b was intermediate in color between c and a. When, finally, b and a were interchanged, a change of coloring appeared immediately; b became light again, and a took the intermediate tint between b and c."
A similar but less complete experiment with the same species of frog was made many years previously by Sir Joseph Lister, who found that "a frog caught in a recess in a black rock was itself almost black, but after it had been kept for about an hour on white flagstones in the sun was found to be dusky yellow, with dark spots here and there. It was then placed again in the hollow of the rock, and in a quarter of an hour had resumed its former darkness.
I have recently made a number of observations upon the commoner New England frogs which show that our species possess the power of color adaptation to a large extent. The prettiest of our frogs is the common wood frog (Rana sylvatica) a pale red-dish-brown species, nearly an inch and a half long when adult (Fig. 1), but very often found in the smaller immature condition (Fig. 2). It is most commonly seen on the carpets of pine needles in the woods, where its color is precisely like that of the bed of needles on which it lives. When found on the fields and meadows away from the woods it is seldom reddish brown, being usually either light fawn color or dark brown.
A fine large wood frog was brought to my laboratory August 8th, and placed in a glass vivarium near a window. I began to study its color changes August 11th, at noon, adopting as a color standard the plates in Ridgeway's admirable Nomenclature of Colors, and the figures in parenthesis hereafter refer to those plates.
At the time mentioned the frog was light fawn color (III, 22) on the back. That night it escaped from the vivarium and wandered about the laboratory, being found the next day at 1 p. m. It was then much darker than before, the fawn color having changed to Van Dyke brown (III, 5),and the sides being dark clove brown (III, 2). Mr. Sylvatica was next placed in a dry glass Fig. 2.—Wood Frog. Immature. jar, and put in a corner of the room with a white wall on two sides of it. Three days later (August 15th, 11 a. m.) it was an extremely light fawn color on the back (III, 22, but lighter), with the sides very light drab, approaching écru drab (III, 21).
A little water was next placed in the bottom of the jar, and it was put beside a blackboard, where it was left until August 23d. The frog was then cinnamon color (III, 20). with sides dark drab. I then placed it in an open window on a whitish bottom, and the next day it was light brown. At 2 p. m., August 24th, I put it on a jet-black shelf, with black surroundings. Forty-five minutes later it was very dark, nearly mummy brown (III, 10), but darker. At 3 p. m., while it was still so dark, I put it back in the window with white surroundings; at 3.05 it was considerably lighter brown, at 3.10 much lighter, and at 3.15 it had become cinnamon-colored—a very marked change thus occurring in fifteen minutes.
These experiments were repeated a number of times with several different individuals, and similar results were obtained.
The common green frog (Rana clamata) has the power of changing its color to a considerable extent. Specimens kept for some time amid light surroundings became of a very light green color—even lighter than apple green—while if placed amid a black environment they become very dark. The leopard frog, or spotted frog (R. virescens), is not able to change its appearance so completely, the permanent color markings preventing; but the green ground color varies somewhat. The few observations I have been able to make on the bullfrog (R. catesbiana) indicate that its ability in this direction is very similar to that of the green frog.
The power of color change is also present to a decided extent in our common toad (Bufo lentiginosus). A very large specimen of this species was found in wet grass June 1st, at 11 p. m. It was then of a light wax-yellow color. It was brought to the laboratory and put in a glass jar on a black shelf. Twenty-four hours later it was very much darker, being tawny olive brown. Three days later it had become still darker, being almost clove brown. A similar power has been observed in the European toad.
It is conceivable that these color changes might occur in either of two ways: First, by the direct action of the light reflected from the surroundings upon the pigment cells of the skins, and second, by an indirect action through the eye of the animal. The second method is the one involved. Experiments have shown that, when blinded, a frog does not change its color to agree with the environment. Mr. Poulton describes the process of change by saying that "certain kinds of light act as specific stimuli to the eye of the animal, and differing nervous impulses pass from this organ along the optic nerve to the brain. The brain being thus indirectly stimulated in a peculiar manner by various kinds of reflected light, originates different impulses, which pass from it along the nerves distributed to the skin, and cause varying states of concentration of the pigment in the cells. . . . The pigment cells in the skin are often of various colors, and are arranged in layers, so that very different effects may be produced by concentration in certain cells, leading to the appearance of those of another color, or to a combined effect due to the colors of two or more kinds of cells."
Probably the most important advantage derived by the frogs from their power of color change is that of concealment from birds and other enemies. Many of the larger waders devour these animals whenever opportunity offers, and a protective resemblance would help greatly in escaping detection. In the case of the wood frog, I suspect that the resemblance to the carpet of pine needles helps to preserve them from birds of other kinds—the hawks and owls. Last summer I placed a wood frog in a cage containing a red-tailed hawk (Buteo borealis), and it was immediately gobbled up by the bird.
It also seems likely that the resemblance to the environment may be of benefit to the species in enabling it more readily to obtain its insect food, but in the present state of our knowledge of the vision of insects one can not place very much stress upon this phase of the subject.