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White zebra finches exposed to artificial sunlight used an average of 22.9 percent less energy after they were dyed black. The hypothesis that black homeotherm coloration functions primarily to maximize absorption of radiant solar energy is suggested. This hypothesis may explain the dark skin pigmentation of certain human populations.

The surface color of homeotherms is commonly thought to have two possible functions: concealment (Cott 1957) and communication (Lorenz 1935; Tinbergen 1948). Hamilton and Heppner (1967) proposed a third possible function for dark, or black, homeotherm coloration: the reduction of the metabolic cost of homeothermy by maximization of absorption of radiant solar energy. This hypothesis proposed that the integument o,f black animals absorbs more of the visible and near infrared components of solar energy than does the integument of lighter-colored animals, and that this greater absorption may permit them to reduce their heat 108s~ when ambient temperatures are below their lower critical temperature. Hamilton and Heppner reported that blackdyed, white Zebra Finches (Poephilu castan-Otis) demonstrated a significant metabolic economy over undyed birds when both were exposed to artificial sunlight while in a moderately cold (10%) metabolism chamber. I now report the results of experiments relating the metabolic differences between blackened and white birds observed by Hamilton and Heppner to the difference in amount of radiant energy absorbed by the black and white birds in our experiments, and discuss a physical mechanism whereby dark-colored birds might achieve a metabolic advantage over lighter counterparts. DETERMINATION OF DIFFERENCES IN ABSORBED ENERGY Of paramount importance is the quantitative relationship between the difference in requirements for chemical energy of the white and blackened birds, and the different rates of energy absorption from the artificial sun. This relationship can be determined by comparing the difference in heat production, expressed in calories, between the white and blackened birds, with the difference in the amount of energy absorbed at the surface of birds in the two color conditions.

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