Data from: Is male rhesus macaque facial coloration under intrasexual selection?

Exaggerated male traits can evolve under intra- or intersexual selection,
but it remains less clear how often both mechanisms act together on trait
evolution. While the males of many anthropoid primate species exhibit
colorful signals that appear to be badges of status under intrasexual
selection, the red facial coloration of male rhesus macaques (Macaca
mulatta) appears to have evolved primarily under intersexual selection and
female mate choice. Nonetheless, experiments show that red color is
salient to males, raising the question of whether the signal may also be
under intrasexual selection. Here, we examine whether males express this
signal more strongly in competitive contexts. Facial images were collected
on all 15 adult males of a free-ranging social group during the peak of
the mating season, and coloration was quantified using visual models.
Results show that males more similar in facial redness were more likely to
interact aggressively than more dissimilar ones, suggesting that color may
be involved in the assessment of rivals. Furthermore, males exhibited
darker coloration on days they were observed copulating, and dominance
rank predicted facial redness only on copulating days, suggesting that
coloration may also advertise motivation to defend a mate. Male rhesus
macaque facial coloration may thus mediate agonistic interactions with
rivals during competition over reproductive opportunities, such that it is
under both inter- and intrasexual selection. However, color differences
were small, raising perceptibility questions. It remains possible that
color variation reflects differences in male condition, which in turn
alter investment towards male–male competition and mating effort.