, &. i detectable at all, it is only against a light or reflective background. My observations (3) in hot climates lead me to believe that even during the daylight hours man, other mammals, and birds frequent the shade during the heat of the day, and that in the somnolent midday hours and in shady retreats either a color matching with the environment or one with low light reflectivity would offer maximum concealment. Despite the fact that black color also might expose its possessor to seri- a drab concealing color a met the most effective one for visual communication in sexual or group behawwr. Two opposing pressures can thus act in the selection of color. The camouflaged animal must have some compensation, either behavioral, physiological, or mor- phological, to overcome its communication handicap. Conversely, the brightly colored animal must have compensa- tion for its greater exposure to predation. We would argue that in additron to the pressures exerted by the ous thermal problems in deserts and need to avoid predation amd to com- significant that the sedentary inhabitants of black lava landscapes, the hot- servation, may be able ito influence color. In sOme cases, energy conserva- other intensely insolated areas, it is test known environments, have evolved a concealing dark-to-black coloration whereas their adult, varietal, or subspecific equivalents living only a short distance away, sometimes only a few meters, still match the prevailing desert pallor of their environment. Even if color or albedo matching changes occur sometime during the ontogeny of cer- tain reptiles and insects, this requisite matching and concealment despite the entailed exposure to heat, would constitute even more emphatic evidence of the definitive factor in the evolution of light-absorbing surfaces. Other arguments in favor of the theory of concealment rather than for any definitive physiological advantages (these doubtless occur but are less important) have been proposed (3), but it seems probable that need for concealment almost invariably takes precedence over any attendant metabolic benefits or dangers and that this may have been true for the evolution of some prototypal human skin color. R. B. COWLES Department of Biological Sciences, University of California, Santa Barbara References 1. W. J. Hamilton III and F. Heppner, Science 155, 196 (1967). 2. R. B. Cowles, ibid. 135, 670 (1962). 3. , Amer, Natur, 93, 283 (1959). 16 March 1967 The direction of a species’ evolution is guided by thetotal effect of all the selective forces acting upon it. An op- timum response to one selective force can be disadvantageous with respect to another, and a compromise may be required. An animal’s color is a case in point. Predator pressure may force selection for a concealing color. However, 8 DECEMBER 1967 municate, a third pressure, energy con- tion may be of prime immportance: in others it may be of no importance. We do not think that any ome of these factors is “definitive” for all animals, nor do we rule out the possibility that other forces can influence color under certain conditions. Only study of a particular animal can show which, if any, of these pressures is of primary importance for that animal. We stated that “homoiothermic ani- mals can absorb and witilize radiant solar energy.” We believe that the ex- perimental results show that this is a reasonable conclusion; the means by which the energy absorbed by the sur- face is translated into a metabolic economy is still subject to inwestigation. Cowles’s suggestion of a tTeversal of thermal gradient may weil be correct, and does not rule out our conclusion. animals are relatively more conspicuous than species that match the background, and some other explanation for their blackness must be offered. Nor does the principle of concealment by background matching change at low light intensities, as Cowles implies. Discontinuities and contrasts with the environment continueto be effective stimuli at night. However, at night our own eyes provide less reliable information about the situation. There are relatively few black nocturnal birds and mammals. Most nocturnal animals heavily involved in predator-prey relationships are background matching rather than black. Since these nocturnal animals generally obtain shelter during the day and are thus not exposed to “the nota- ble penalties of high [radiation] absorption and overheating” we are led to ask why they have adopted their back- ground-matching colors rather than black, For these reasons Cowles’s conclusion that black coloration is in the vast majority of cases an adaptation to concealment seems vulnerable. Cowles is probably right in stating that a sunbathing black animal would face a heat load problem at midday in the tropics. The solution to this problem, as he pointed out, is to get out of the sun. In the shade, color is probably irrelevant to heat exchange, since both black and white animals have similar radiative properties in the far infrared, as Kelly, Bond, and Heit- man (J) observed. In the early hours We found the term homoiotherm to be of the day, solar radiation may enable supply of these cues. The basic prin- This correction in no way influences homototherms not only to reduce mainmore useful than endotherm in this tenance metabolism requirements but particular case because we needed a also to restore body temperatures which term to describe a state of relative have fallen during the night. In hot clitemperature constancy wethout making a commitment to the source of _mates, particularly at dawn, a wide variety of sunbathing homoiotherms heat. can be found, many of them orienting Cowles’s suggestion that the surface black surfaces to the sun. which absorbs the most mcident light We wish to take this opportunity to is the least effective visual stimulus does correct an error in the second sentence not seem to consider that discontinuiof the caption of Table 1. It should ties and contrasts are important visual read, “Units are milliliters of oxygen stimuli, and in most terrestrial environper gram of body weight per four.” ments black animals prewide a rich ciple of concealment by mmatching the general background appears to be that maximum concealment is obtained by reflecting the same quantity and quality of light as the background. In some the conclusions from the experiment. WILLIAM J. HAMILTON ITI* FRANK HEPPNER Department of Zoology, University of California, Davis special environments, such as lava flows, burnt vegetation, or dark soils, this may involve dark or even black colora- tion, but black birds, mammals, and men are not restricted to these environ- ments. In other environments black Reference 1, C. F. Kelly, T. E. Bond, H. Heitman, Ecology 35, 362 (1954). * Present address: P.O. Box 4186, Nairobi, Kenya, East Africa. 27 October 1967 1341