Pregnancy coloration in macaques may act as a warning signal to reduce antagonism by conspecifics

Instances of bright, hormonally induced coloration among females during gestation have been reported in a few reptile and primate genera. Gravid coloration in lizards has been linked to female aggression but the influence of color changes associated with pregnancy has not yet been experimentally pursued for primates. As a first step to determine whether the crimson to magenta hues common to pregnancy coloration in rhesus macaques (Macaca mulatta) contains information, to which conspecifics of either sex attend, we evaluated whether male and female rhesus macaques discriminate between pregnant and non-pregnant female faces. To these ends, we presented 19 adult rhesus macaques with color-manipulated digital images of female faces where pregnancy coloration was present or absent, and measured visual attention and behavioral reactions. Males were significantly more attentive to female faces with pregnancy coloration over those without pregnancy coloration. Both sexes engaged in higher levels of appeasement behavior toward stimulus with pregnancy coloration, and males showing signs of anxiety did so exclusively when exposed to faces with pregnancy coloration. Our results suggest that pregnancy coloration might be an attention grabbing stimulus to males and a warning stimulus to both male and female rhesus macaques. The findings provide a comparative perspective on the use of color in intra-specific communication, and suggest similarity in female similarity in signalling properties in distantly related taxa.