Figure 1.

Stimuli used for the experiments. Each horizontal pair of images represents 2 versions of the same face, with its color manipulated to be either a dark red or pale pink rhesus color as exhibited by males at the peak of the mating season. During the experiments, faces of 2 different males were presented simultaneously to subjects as illustrated by the arrows; the relative position (right vs. left) of each coloration was randomized and balanced, for a total of 12 possible combinations.