Isabelline coloration: a heretofore unrecognized chromatic aberration in bighorn sheep

Coloration is among the characteristics noted first by observers, and color has a strong influence on how animals are perceived. Abnormal coloration has been reported in a variety of taxa, but less frequently among mammals than other classes of vertebrates. Chromatic disorders occurring among Mammalia include albinism, leucism, piebaldism, melanism, xanthism, erythrism, and isabellinism, but only leucism, melanism, and piebaldism have been confirmed in bighorn sheep (Ovis canadensis); ‘albinism’ has been reported in that taxon, but in the absence confirmatory evidence. Bighorn sheep possessing white, or extremely light-colored pelage, have been described in the literature as albinistic, leucistic, or simply as ‘white’. Isabellinism is a genetic anomaly that manifests as sandy- or cream-colored pelage that appears washed-out and otherwise would be dark in color. Some reports of albinistic, leucistic, or ‘white’ bighorn sheep, however, likely represent animals that were of isabelline coloration. This is the first paper to ascribe this unusual color morph to bighorn sheep, and I (1) provide photographs, references, and other evidence of what appears to be isabellinism in that species; (2) explain why those observing ‘white’ bighorn sheep likely have failed to recognize isabelline coloration; (3) discuss the evolutionary consequences of that anomalous condition in terms of fitness and future adaptation; (4) emphasize the role of what may appear to be meaningless observations in terms of their potential ramifications for understanding or explaining phenomena that heretofore have not been fully explored; and (5) encourage readers to recognize that natural history observations have potentially important implications for the interpretation of adaptation and natural selection, and can provide new insight into prior conclusions or future applications.