Studies Upon the Biological Significance of Animal Coloration. I

AUTHOR'S ABSTRACT OF THIS PAPER ISSUED BY THE BIBLIOGRAPHIC SERVICE AUGUST 11. STUDIES UPON THE BIOLOGICAL SIGNIFICANCE OF ANIMAL COLORATION I. THE COLORS AND COLOR CHANGES OF WEST INDIAN REEF-FISHES W. H. LONGLEY Goucher College, and Department of Marine Biology, Carnegie Institution of Washington EIGHT FIGURES (ONE PLATE) CONTENTS Introduction. .................................. ........................... 533 Detailed statement of results. ............................................. 536 Countershading. ........................... ........................... 536 Color change ............................... ........................... 542 Correlation of color with habit.. ........~. ........................... 555 Discussion of results.. ......................... ........................... 583 Summary. ..................................... ........................... 596 Bibliography. ................................... ........................... 597 INTRODUCTION Zoologists have long been uncertain how to interpret the vivid coloration of some animals. Many consider the advantage it confers measured by its conspicuousness. Others deny its utility; ascribe its appearance to the vagaries of metabolism; regard it as an expression of tendencies determined by racial constitution, or refer it largely to the action of external factors. To only a few the conspiquousness of animals of high color seems too lightly assumed, and they maintain that belief in its existence rests chiefly upon failure to appreciate the obliterative effect of bright or strongly contrasted hues when they are displayed under natural conditions. Thus it is apparent that confusion prevails, and that new methods are required to rehabilitate a subject of investigation that has fallen somewhat into disrepute. 533 534 W. H. LONGLEY ,4 study of the interrelations of the gulfweed fauna was in progress, when the prevalence of Sargassum colors upon animals living in it focussed attention almost at once upon issues raised by this agreement. It appears that no less than 12 species (4 fishes, 2 crabs, 2 shrimps, 3 gastropods, and a planarian) are marked with the brown and yellow of the algae, with or without the white of their adherent bryozoan skeletons and worm tubes. No animals normally restricted to the floating weed distinctly display other colors, and the convergent evolution of the group is indicated in addition by the fact that several have organs so shapen that their resemblance to parts of the plants can scarcely be ignored. These observations suggest many problems whose direct pre- sentation reinvests them with the freshness and interest they possessed when pioneers of Darwinism first attempted their solution. Among them few seem more fundamental, than why in some habitats all the animals should wear dull colors, while in others conspicuousness should appear at a premium, and no hues be too gaudy to serve in attaining it. It is noticeable that in the former case the animals’ colors tend to repeat those of their surroundings. In the present in- stance one is impressed besides by the fact that agreement in color is correlated with coincidence in range, and exaggerated resemblance to surrounding objects with intimate association with them; for the gulfweed animals are essentially inhabitants of small oceanic islands, and are cut off from racial points of origin by open water. But, if a tendency toward uniformity in coloration characterizes groups of sluggish animals living in surroundings whose color-characters are relatively invariable, it is possible that the colors of more active ones living under less uniform conditions may obey the same laws of distribution without the fact being obvious. For in one case all the colors which the animals wear may surround them at dl times, while in the other this may not be so, and the fact of repetition may thereby escape detection. Hence, it was accepted as a working hypothesis that, in general, the colors which may appear upon animals are rather strictly conditioned by the environment in which they live. THE COLORATION OF REEF FISHES 535 The diversity of coloration occurring among reef fishes led to the testing of this assumption by its application to them, but in such an undertaking no advantage is to be won through multiplying sporadic instances of ‘protective’ or ‘warning’ coloration which may attract one’s attention upon the reefs. It is necessary rather, if any should appear to exist, to determine the system or law of distribution of color among the fishes of a given region, in the hope of discovering relations of general validity, facts which, regardless of preconceived opinion, shall make the same appeal to all observers to whose attention they may come. It is not clear that there is more than one way to approach such a problem: the foundation of all later researches, pursued by whatever method, must be laid by long continued study of living specimens under absolutely normal conditions. It may be an incorrect conclusion based upon ignorance of other animals, but tropical reef-fishes seem superior material upon which to carry out such a preliminary study as the present,. In the richness of their coloring they vie with birds and butterflies. Many species may be observed in one circumscribed region. Of these a fair proportion are common. The great majority of them are not very shy and are little disturbed by observation from a boat, and perhaps as little, or even less, by one wading among them waist-deep in water or going down among them at greater depths with diving equipment. They possess, as a class, large power of color-change, which, correctly interpreted, is of the highest significance, for it provides an unmistakable clue to much that has remained obscure. They may be studied in a perfectly natural environment, unaffected by human activities as most terrestrial habitats are. Finally, their food is such that its nature may usually be determined without difficulty from analyses of stomach contents, and conclusions drawn from observation of the creatures’ movements be checked readily against information of this sort and validated or disproven. Regarding methods of observation little need benotedin detail; such information as is necessary or of interest will appear in its appropriate place with the statement of results obtained. The fishes have been viewed from every angle above and below, by 536 W. H. LONGLEY day and night, free and confined, wholly beyond the control of the observer, or deliberately led from place to place upon the reefs. So many of the Teleosts that are common at the Tortugas during the summer have been studied, that it is believed that false conclusions flowing from the study of unrepresentative material have been avoided. The minimum amount of apparatus has been used with full appreciation of the fact, that under such circumstances charges of manipulation may not be urged with any such show of reason, as that with which they have been directed against Thayer’s clever devices The research has been in progress for about five years, yet no one realizes more clearly than the writer that the intensive study of one class of animals in a single region, though the species studied be considerably more than a hundred, and the stations at which observations were made, more than a thousand miles apart, is little more than a reconnoissance in the field of so large a problem. Still it seems that two conclusions may be safely accepted: There is method in Nature’s madness, and that method is perfectly demonstrable by approved and conservative modes of analysis. The results achieved speak for themselves. They might have been obtained with great difficulty, if at all, with other support than that of The Carnegie Institution of Washington, yet owe such merit as they possess, far less to material assistance gener- ously placed at my disposal, than to the unfailing interest and support of Dr. Alfred G. Mayer, Director of the Department of Marine Biology, to whom I remain permanently indebted. DETAILED STATEMENT OF RESULTS Countershading Countershading is the definite gradation of pigment from darkest on the mid-dorsal, or upper, to lightest on the mid- ventral, or lower line. Its effect in destroying the solid appearance of an object when it is exposed to uniform lighting from the direction of its darkest surface is clearly demonstrated by Thayer’s countershaded models. Nevertheless, F. C. Selous THE COLORATION OF REEF FISHES 537 (’08),the well-known English hunter of big game in Africa, states that he is quite sure that to a South African Bushman there is in nature no such thing as protective coloration. In so far as this remark has reference to the effect of countershading (and it must have such reference, since the part is in- cluded in the whole), Mr. Selous is affirming an apparent impossi- bility. The native tracker may be vastly superior to his white master in his ability to interpret certain sense-impressions, but whatever reduces their distinctness militates against his success, and this is exactly the effect of countershading. For of much light falling upon the darker surface of a strongly countershaded object little is reflected, while of the little reaching its shaded underparts a mere fraction is absorbed. But, if much subtracted from much and little from little leave the same net result, no eye, savage or civilized, can gather from the appearance of the countershaded object that evidence of its solidity upon which every observer is forced largely to rely. Thayer’s theory of obliterative coloration was begot!ten through understanding the necessary effect of countershading. Even if it should appear that errors have been made in its application it remains an eminent contribution to the literature of its subject; yet Dewar and Finn (’09) fail to review it upon its merits, and justify their reference to it by the fact that it was enunciated in reputable journals, and has been noted with approval by a leading student of the color problem. They intimate that this is an hypothesis based upon the assumption that animals see with an artist’s eye. Their view is apparently in part a direct result of Mr.

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