In the development of brilliantly-coloured areas of bare skin the monkeys stand alone among the Mammalia. The hues displayed are remarkable for their brilliancy, and this varies in intensity, waxing and waning with the varying moods of their possessors, and attaining their maximum during periods of sexual excitement. Blue, green, red, and violet are the dominant colours, and these are confined to the face, buttocks, and genital organs. The same hues are commonly present in both sexes, though in the female they are less brilliant. Normally the male appears to be unconscious of the conspicuous patches of colour, but when under the irrepressible stimulus of sexual excitement he seems to endeavour to make the utmost possible capital out of such adornments, more especially presenting his buttocks to his mate in an apparent endeavour to stimulate her desire. In some species, as with the Baboons for example, the naked area of this hinder part of the body is a much more conspicuous feature in the female than in the male, becoming enormously swollen and carunculated, and from its vivid red colour presents a positively revolting appearance, according to our standard of what is beautiful. The most vividly coloured species of all is the Mandrill, which, in this matter exceeds all other living Mammals. The face, in the male, is produced forward to give the head a dog-like shape, while the whole of the upper surface of the muzzle has been transformed into a swollen, deeply fluted mass by the excessive inflation of the underlying bone. The bare skin covering this is of a brilliant cobalt blue, with lines of violet in the furrows, while the nose is of a bright scarlet. The naked skin of the buttocks, and the genital organs, are suffused with brilliant tints of scarlet and blue. In spite of the purity and brilliance of the coloration the effect is to make the creature really hideous. Of the display Cuvier writes: “La partie postérieure du corps n’est ni moins extraordinaire ni moins révoltante. Sous une courte queue sans cesse relevée est un anus entouré d’un gros bourrelet d’écarlate; de larges fesses nues, que l’animal semble montrer sans cesse avec autant de lascivité que d’impudence, sont colorées d’un rose vif nuancé sur les côtés de lilas et de bleu. Les parties genitales enfin sont d’un rouge de feu d’autant plus tranché qu’elles sont absolument nues, et qu’elles viennent a la suite d’un abdomen revêtu de poils blancs.”