Greater variability in rhesus macaque (Macaca mulatta) endocranial volume among males than females

The greater male variability (GMV) hypothesis proposes that traits are
more variable among males than females, and is supported by numerous
empirical studies. Interestingly, GMV is also observed for human brain
size and internal brain structure, a pattern which may have implications
for sex-biased neurological and psychiatric conditions. A better
understanding of neuroanatomical variability in nonhuman primates may
illuminate whether certain species are appropriate models for these
conditions. Here, we tested for sex differences in the variability of
endocranial volume (ECV, a proxy for brain size) in a sample 542 rhesus
macaques (Macaca mulatta) from a large pedigreed free-ranging population.
We also examined the components of phenotypic variance (additive genetic
and residual variance) to tease apart the potential drivers of sex
differences in variability. Our results suggest that males exhibit more
variable ECVs, and that this pattern reflects either balancing/disruptive
selection on male behaviour (associated with alternative male mating
strategies) or sex chromosome effects (associated with mosaic patterns of
X chromosome gene expression in females), rather than extended
neurodevelopment among males. This represents evidence of GMV for brain
size in a nonhuman primate species and highlights the potential of rhesus
macaques as a model for sex-biased brain-based disorders.