Craniodental features in male Mandrillus may signal size and fitness: an allometric approach.

According to a hypothesis in the broader mammalian literature, secondary sexual characteristics that have evolved to signal fitness and size to other conspecifics should exhibit positive allometry across adult males within a species. Here this hypothesis is tested in the genus Mandrillus. The overbuilding of bony features in larger individuals necessitates a functional explanation as bone is metabolically expensive to produce and maintain. Canine size and size of the maxillary ridge are scaled against a body size surrogate in intraspecific samples of male Mandrillus sphinx (mandrills) and Mandrillus leucophaeus (drills). Areal dimensions are weighted more heavily as they represent the size of a feature as it is viewed by individuals. Measures of the maxillary ridge and canine tooth are significantly correlated with the size surrogate and scale with positive allometry in both samples supporting the hypothesis that these features function to advertise a male's body size and fitness to other males competing for mates and potential discerning females. This is the first study in primates to test for intraspecific positive allometric scaling of bony facial features in adult males based on a theory of fitness signaling and sexual selection.

Plasticity of mandibular biomineralization in myostatin-deficient mice.

Compared with the normal or wild-type condition, knockout mice lacking myostatin (Mstn), a negative regulator of skeletal muscle growth, develop significant increases in relative masticatory muscle mass as well as the ability to generate higher maximal muscle forces. Wild-type and myostatin-deficient mice were compared to assess the postweaning influence of elevated masticatory loads because of increased jaw-adductor muscle and bite forces on the biomineralization of mandibular cortical bone and dental tissues. Microcomputed tomography (microCT) was used to quantify bone density at a series of equidistant external and internal sites in coronal sections for two symphysis and two corpus locations. Discriminant function analyses and nonparametric ANOVAs were used to characterize variation in biomineralization within and between loading cohorts. Multivariate analyses indicated that 95% of the myostatin-deficient mice and 95% of the normal mice could be distinguished based on biomineralization values at both symphysis and corpus sections. At the corpus, ANOVAs suggest that between-group differences are due to the tendency for cortical bone mineralization to be higher in myostatin-deficient mice, coupled with higher levels of dental biomineralization in normal mice. At the symphysis, ANOVAs indicate that between-group differences are related to significantly elevated bone-density levels along the articular surface and external cortical bone in the knockout mice. Both patterns, especially those for the symphysis, appear because of the postweaning effects of increased masticatory stresses in the knockout mice versus normal mice. The greater number of symphyseal differences suggest that bone along this jaw joint may be characterized by elevated plasticity. Significant differences in bone-density levels between normal and myostatin-deficient mice, coupled with the multivariate differences in patterns of plasticity between the corpus and symphysis, underscore the need for a comprehensive analysis of the plasticity of masticatory tissues vis-à-vis altered mechanical loads.