Patterns of Phenotypic and Genetic Correlation in the Jaws, Teeth, and Postcrania of Rhesus Macaques

Abstract Body : Rhesus macaques and many other primate species use their canine teeth in agonistic interactions beyond the typical masticatory function of the dentition. Occlusion, gape, and movement at the temporomandibular joint all contribute to the masticatory and social functions of the dentition and depend upon the morphology of the jaws and teeth. The goal of this project was to compare genetic and environmental contributions to the morphology of these interacting features in a sexually dimorphic primate population and to develop a model against which other primates can be compared. We used linear measurements from the teeth, mandibles, and postcrania of 362 rhesus macaques (183 females, 179 males) from the Caribbean National Primate Research Center to assess patterns of phenotypic and genetic correlation. Phenotypic correlations were estimated separately for males and female using age-corrected data in R version 4.2.1. Heritabilities and genetic correlation matrices were estimated using maximum likelihood estimation in SOLAR with sex, age, and sex-by-age interaction as covariates using a pedigree of 15,440 individuals. Phenotypic and genetic correlation matrices were compared statistically to test matrices with different modular structures using random skewers analyses. Hierarchical clustering was also used to summarize correlation matrix structure. Male and female phenotypic correlation matrices were most similar to matrices containing distinct variational modules for canine size, postcanine size, jaw size, and postcranial size. The genetic correlation matrix was, however, most similar to a pattern of weak integration across all variables. Hierarchical clustering grouped maxillary canine dimensions with jaw and postcranial measurements in males, but not in females or in the genetic correlation matrix. Together, these results provide evidence of environmental contributions to covariance between dental and skeletal traits, some of which are greater in males than females. We hypothesize several explanations for this observation including the longer period of canine development in males and impacts of sex hormones on body, jaw, and canine growth and development. Finally, we discuss the potential role of genetic and environmental covariance in the evolution of body and canine size dimorphism in primates and in the evolution of body size dimorphism and canine reduction in the hominin lineage.