Popular Science Monthly/Volume 37/August 1890/The Uses of Animal Color
By EDWARD B. POULTON, M. A., F. R. S.
COLOR, as such, is not necessarily of any value to an organism. Organic substances frequently possess a chemical and physical structure which causes certain light-waves to be absorbed; or, the elements of tissues may be so arranged that light is scattered, or interference colors are produced. Thus blood is red, fat is white, and the external surface of the air-bladder in certain fishes has a metallic luster, like silver. In such cases there is no reason why we should inquire as to the use or meaning of the color in the animal economy; the color, as such, has no more meaning than it has in a crystal of sulphate of copper or iron. Such colors are the incidental results of chemical or physical structure, which is valuable to the organism on its own account. This argument will be still further enforced if we remember that the colors in question are, strictly speaking, not colors at all. Blood and fat are so constituted that they will be red and white, respectively, in the presence of light, but they can not be said to possess these colors in their normal position, buried beneath the opaque surface of an animal.
The existence of non-significant colors is, nevertheless, most important, for they form the material out of which natural or sexual selection can create significant colors. Thus the color of blood may be made use of for "complexion," while fat may be employed to produce white markings, as in certain insect larvae. The yellow, brown, and red fatty matters of the connective tissue are accumulated beneath the skin in patches, so as to produce patterns.
All animal color must have been originally non-significant, for, although selective agencies have found manifold uses for color, this fact can never have accounted for its first appearance. It has, however, been shown that this first appearance presents no difficulty, for color is always liable to occur as an incidental result. This is even true of the various substances which seem to be specially set apart for the production of color in animals; for pigments occur abundantly in the internal organs and tissues of many forms. The brilliant colors of some of the lower organisms are probably also non-significant. In all higher animals, however, the colors on the surface of the body have been significant for a vast period of time, so that their amount, their arrangement in patterns, their varying tints, and their relation to the different parts of the body, have all been determined by natural selection through innumerable generations. Because the origin of all pigments is to be found in the incidental result of the chemical and physical nature of organic compounds, it by no means follows that incidental or non-significant colors would have appeared at all on the surface of most animals. And we find as a matter of fact that such colors tend to disappear altogether, directly they cease to be useful, as in cave-dwelling animals. On the other hand, the non-significant color of blood or of fat would persist undiminished in such forms.
Just as natural selection may develop an appearance which harmonizes with the surroundings, out of the material provided by non-significant color, the same agency may lead to the disappearance of the latter when it impedes the success of an animal in the struggle for existence. Thus the red color of blood has disappeared in certain transparent fishes, which are thereby concealed from their enemies. Among the manifold possible variations of nature is that of a fish with colorless blood, which can, nevertheless, efficiently perform all the duties of this fluid. While such a variation would be no advantage to the great majority of vertebrates, it would be very beneficial to a fish which was already difficult to detect on the surface of the ocean on account of its transparency.
Colors may be useful in many ways, and are therefore always liable to be turned to account in one direction or another. They may be of direct physiological value to the organism, or may assist in the struggle for existence by deluding other species, or by aiding the individuals of the same species, or they may be intimately connected with courtship.
The color of chlorophyl, which causes the green appearance of vegetation, must be intimately connected with the important changes which take place in this substance in the presence of light. It is well known that under these circumstances carbon dioxide (popularly called "carbonic acid") can be split up, and its carbon made to unite with the elements of water, forming organic substance. Although this process has been much studied, it is still very imperfectly understood. It is clear, however, that the color of chlorophyl, involving the special absorption of certain lightwaves, has some direct bearing upon the changes which occur,
No equally clear instance has been proved to occur in the animal kingdom, except in those few forms which resemble plants in possessing chlorophyl. Dr. Hickson, however, believes that among corals "the most widely distributed colors will eventually be proved to be allied to chlorophyl, . . . and perform a very similar if not precisely identical physiological function." It is much to be desired that this interesting suggestion, which Dr. Hickson supports by many arguments, may be thoroughly tested as soon as possible.
In the very common association of colored substances with the important function of respiration, it is clear that the color is not more than incidental; while the fish with transparent blood shows that color is not indispensable for the due performance of the function. Pigment is, however, of direct importance for vision; it is always present in the eyes of animals, except in the case of albinos, and it is said that even they possess the essential visual pigment associated with the termination of the optic nerve (retinal purple).
The difference between the physiological importance of color in animals and plants is well shown by the fact that a true albino variety (not merely a variegated example) of a green plant could not live for any length of time.
There are, however, certain cases among animals in which it is extremely probable that color is of direct physiological value. It is well known that dark colors readily absorb radiant heat, while light colors do so with difficulty. For this reason black clothes are most trying, and white most comfortable, in the hottest weather. Conversely, a dark surface readily parts with heat by radiation, while a white surface retains heat far more effectually.
A few writers had suggested that these principles may explain the colors of certain animals, but the question was first fully entered upon in Lord Walsingham's presidential address to the Yorkshire Naturalists' Union in 1885. The predominance of dark varieties of insects and white varieties of birds and mammals in northern latitudes is connected with these facts. "Birds and animals living through the winter naturally require to retain in their bodies a sufficient amount of heat to enable them to maintain their existence, with unreduced vitality, against the severities of the climate. Insects, on the contrary, require rapidly to take advantage of transient gleams of sunshine during the short summer season, and may be content to sink into a dormant condition so soon as they have secured the reproduction of their species; only to be revived in some instances by a return of exceptionally favorable conditions."
It would be fatal for the temperature of one of the higher vertebrates to sink a few degrees below the normal, except in the case of certain species, such as the dormouse, etc., which have the power of hibernating in a dormant condition; such animals were once called "warm-blooded," but are now more correctly termed "homothermic," because it is the constancy of the temperature which is so important, and which must be maintained whether the surrounding medium be colder or warmer than themselves. Other animals with an inconstant temperature are now correctly called "poikilothermic" rather than "cold-blooded."
Lord Walsingham's conclusions appear to be supported by the fact that young dark-colored caterpillars, like those of the emperor moth (Saturnia carpini), or tortoise-shell butterfly (Vanessa urticæ), seek the light side of a glass cylinder, and always change their position when the cylinder is turned round. The question needs further investigation, and much might be learned by interposing various screens between such larvae and the light, thus cutting off different sets of light-waves.
The most important support to the hypothesis is found in an experiment made by Lord Walsingham, in which several Lepidoptera of different colors were placed on a surface of snow exposed to bright sunshine; in half an hour the snow beneath the darker insects showed distinct signs of melting, but no effects were seen beneath the others. The differences were further brought out in the course of two hours, when the darkest insect of the lot, a black geometer, the chimney-sweeper (Odezia chærophyllata), "had decidedly won the downward race among them."
It is therefore certain that the absorption of radiant heat is favored by the dark colors of northern insects, and it is in every way probable that they are benefited by the warmth received in this way. We can not, however, as yet assert that such dark colors are not also advantageous for concealment or some other purpose.
The white appearance of arctic birds and mammals must be advantageous for concealment in a region so largely covered with snow, but it is very probable that advantage is also secured by checking the loss of heat through radiation.
Thus Lord Walsingham's experiments and conclusions seem to prove that colors are sometimes of direct physiological value to animals, although a great deal more work must be done before we can safely estimate the proportion which this advantage bears to others also conferred by the same colors.
By far the most wide-spread use of color is to assist an animal in escaping from its enemies or in capturing its prey; the former is protective, the latter aggressive. It is probable that these were the first uses to which non-significant colors were put. The resemblances are of various kinds; the commonest cases are those of simple concealment. The animal passes undetected by resembling some common object which is of no interest to its enemies or prey respectively, or by harmonizing with the general effect of its surroundings; the former is special, the latter general resemblance, and both may be protective or aggressive. Among the most interesting special aggressive resemblances are the cases of alluring coloring, in which the animal, or some part of it, resembles an object which is attractive to its prey.
Mimicry is in reality a very important section of special resemblance. The animal gains advantage by a superficial resemblance to some other, and generally very different, species which is well known and dreaded because of some unpleasant quality, such as a sting or an offensive taste or smell, etc., or it may even be protected from the animal it resembles: this is protective mimicry. When, however, the animal resembles another so as to be able to injure the latter or some other form which accompanies it or is not afraid of it, the mimicry is aggressive....
When an animal possesses an unpleasant attribute, it is often to its advantage to advertise the fact as publicly as possible. In this way it escapes a great deal of experimental "tasting." The conspicuous patterns and strongly contrasted colors which serve as the signal of danger or inedibility are known as warning colors. In other cases such colors or markings enable individuals of the same species easily to follow those in front to a place of safety, or assist them in keeping together when safety depends upon numbers. It is these warning colors which are nearly always the objects of protective mimicry.
Finally, in the highest animals, the vertebrata and many of the most specialized invertebrate groups, we have some evidence for the existence of an æsthetic sense. Darwin believed that this sense was brought into play in courtship, and that colors and pattern have been gradually modified by the preference of the females for the most beautiful males; he believed that such sexual selection accounts for many of the most beautiful features possessed by animals, viz., those which are especially displayed during courtship.