Maximum Gape in Developing Macaca mulatta

Maximum gape and bite force are two key metrics of primate craniodental performance, but without mitigating adaptations, large gapes are associated with reduced bite force. Primate craniodental morphology undergoes functional transitions across development, but how these performance variables vary with life-history transitions is unknown.

Maximum gapes of 211 rhesus macaques (Macaca mulatta) spanning infancy to post-prime developmental stages from Cayo Santiago (Puerto Rico) were collected in vivo, and jaw leverage, a contributor to bite force, was calculated from skeletal measurements of the same individuals post-mortem. Allometric relationships were tested using a combination of OLS regressions and mixed-effects models with developmental stage and sex as interaction terms.

In vivo gape did not differ from isometry relative to body mass and jaw length across developmental stages by sex, but jaw leverage was significantly lower in larger compared to smaller macaques. The largest developmental differences occurred between pre-prime and prime individuals. Canine length scaled with positive allometry against body mass and jaw length in males but not females, while maximum gape scaled with negative allometry relative to canine length in both sexes.

These results indicate that macaques undergo a functional tradeoff between jaw leverage and gape around the onset of sexual dimorphism. Pre-prime macaques exhibit relatively greater jaw leverage, whereas adults maintain proportionate gape relative to body and jaw size at reduced mechanical advantage. The eruption of large canines in males does not influence linear gape outside of size differences captured by body mass and jaw length.