Black Pigmentation: Adaptation for Concealment or Heat Conservation? A recent report by Hamilton and ward the environment. Under these conditions, excessive heat loss from birds is prevented chiefly by the notable Heppner (/) on some possible func- insulative qualities of their feathers. Interpreting the observations of Ham- velvement picture evoked by use of the concept of endothermy, and in conjunction with tions of surface coloration (albedo) in birds once again states the possible inof physiological processes with varying light or energy absorption in organisms having differing coefficients of reflectivity. Because the theoretical conclusions that might be drawn from these data could be interpreted as disproving or ilton and Heppner, according to the the insulative effectiveness of bird plumage,I find it scarcely possible that even the notable metabolic economy amounting to 22.9 percent could be attributed to the transfer into the body of surface heat impinging on black face coloration, and its function under natural selection and in evolution, I am feathers. Such transfer is implied by the statement, “These results indicate that homeothermic animals can absorb and utilize radiant solar energy and conservation obtained (/) in insolated process.” Is it not equally possible that the heated surface might reverse the at least as denigrating the classical views of the definitive function of sur- constrained to argue that despite the demonstrable 22.9 percent metabolic blackened birds, other conclusions can be derived from the experimental re- sults on white andartificially blackened Australian zebra finches (Poephila castanopsis). Many workers have repeatedly noted and confirmed the selective value of concealing albedos and coloration. Of course even extremes in albedos might serve one, two, or even more selec- tively effective roles in nature; nonetheless, even if one (or more) function is served, this fact does not byitself require substitution of one for the other, and especially not of a lesser use for a greater one, nor especially for the overlooked, effects. but probably definitive In this instance difficulties chiefly due to semantics may have contributed to the conflicting viewpoints and interpretations of otherwise unarguable experimental observations. As long as in- that dark pigmentation facilitates this normal thermal gradient and thusinterpose a barrier to an otherwise extrava- the mating activities. Blackness in conjunction with the notable penalties of heat absorption and overheating would processes of heat movement. In the absence of this thermal information and with due regard to the minute amounts of solar energy available at dawn and dusk, when external heat is supposedly needed and used, the feathers, regardless of color, might even occlude the needed supplementary external heat precisely when it would be most bene- ficial. An additional conclusion is drawn by the authors, “The same evidence is applicable to the coloration of man. Dark human skin coloration may maximize the absorption of solar radia- In discussions of energy conservation used, there will probably be additional conflicts in the interpretation of data. and body temperature it is semantically advantageous to substitute the dynamically expressive terms endotherm and endothermic (2) which direct attention to the source of energy and heat, for homoiotherm, which merely denotes a more or less static condition resulting from endothermy. This usage has particular value in the comparison of the internally heated mammals and birds with the externally heated reptiles and amphibia (the ectotherms) or with the basking heliotherms. Under the usual conditions prevailing in and around endotherms, and par- ticularly in birds, the thermal gradient usually slopes steeply from the body to1340 diurnal creatures often must generate surface layer of feathers, it is impossible to follow and evaluate the precise measurements of the body at the skin surface, and outwardly to the insolated ture, as at dawn and dusk in otherwise hot climates.” It seems probable that just because confusing words 10 hours a or more, over a period of at several months each year. And hot hours of intense insolation are precisely those when many In the absence of any temperature gant heat loss? tion in situations where energy must or burden for as many as day, least these also internal heat while foraging, evading enemies, defending territories, and even in conducting respiratory cooling, or, are appropriate Under the conditions in which dark pigmentation is supposed to exert a favorable effect it would be necessary to have a readily changeable albedo from light-absorbing dark hue, to a heat-reflecting white so as to modulate the absorption of environmental heat. Without such a mechanism black pigment would be a distinct disadvantage, not only in direct proportion to its efficiency for heating during the fleeting “dawn” and “dusk” hour or so, but also according to the number of hot hours per day. In hot climates where the sunlight is very intense, a dark skin might cause a serious heat be expended to maintain body tempera- solar energy is so effectively absorbed by the dark skins of most tropical races, a low albedo (which might not be helpful even for brief periods at dawn and dusk) also would expose the possessor to the handicap of an excessive external heat load for all the rest of the daylight hours. This alternative effect raises a serious objection to the theoretical energy-conserving benefits of blackness in man or finch. To meit seems scarcely possible that maximizing the absorption of solar radiation in animals living in hot climates in summation could have any but a del- eterious effect. except for man, carrying on most of seem to outweigh the evanescent bene- fits by at least ten or more times. But even the suggested benefits at dawn and dusk of surface heating as a result of black color seem exceedingly dubi- ous, since the light intensities and the total insolated energy at these critical hours are, except for total darkness, at their lowest levels. The facts concerning the effects of black pigmentation on energy conservation can be ascertained only by plotting the metabolic and external heat conditions throughout a 24-hour period under natural radiative and behavioral circumstances. If some other explanation for black plumage and pelage and for the dark skins of humanbeings in tropical countries must be sought, then that of concealment seems most plausible. Viewed from this classical interpretation of natural selection via effective concealment, the surface coloration or albedo which absorbs most of the incident light and reflects the least, simultane- ously results in providing minimum visibility in an object. Thus dark or black bodies that reflect almost none of the incident light rays that would be necessary for stimulation of the retinal cells of a potential predator, will be least visible or invisible. Furthermore, under crepuscular conditions when predation is usually most intense, a black object is more apt to be overlooked than one that reflects light. If a dark object is SCIENCE, VOL. 158