Abstract

Background. Physical, visual, chemical, and auditory cues signalling fighting ability
have independently evolved in many animal taxa as a means to resolve conflicts without
escalating to physical aggression. Facial width-to-height ratio (fWHR, i.e., the relative
width to height of the face) has been associated with dominance-related phenotypes
both in humans and in other primates. In humans, faces with a larger fWHR are
perceived as more aggressive.
Methods. We examined fWHR variation among 11 species of the genus Macaca.
Macaques have been grouped into four distinct categories, from despotic to tolerant,
based on their female dominance style. Female dominance style is related to intra and
inter-sexual competition in both males and females and the result of different
evolutionary pressure across species. We used female dominance style as a proxy of
intra-/inter-sexual competition to test the occurrence of correlated evolution between
competitive regimes and dominance-related phenotypes. fWHR was calculated from
145 2D-photographs of male and female adult macaques.
Results. We found no phylogenetic signal on the differences in fWHR across species
in the two sexes. However, fWHR was greater, in females and males, in species
characterised by despotic female dominance style than in tolerant species.
Discussion. Our results suggest that dominance-related phenotypes are related to
differences in competitive regimes and intensity of inter- and intra-sexual selection
across species.