Sex Differences in the Capuchin Monkey Brain

Many nonhuman primate species show sex differences in behavior, which suggests that there may also be sex differences in brain organization. In order to better understand the evolution of sex differences in the human brain, it is important to investigate neuroanatomical sex differences in other primate species. The capuchin monkey (Sapajus [Cebus] apella) is a neotropical primate species that shares several behavioral characteristics with humans that are relevant to social organization. Using magnetic resonance imaging, we conducted a whole-brain volumetric analysis of gray matter using voxel-based morphometry on T1-weighted scans from 20 capuchin monkeys (15 female, 5 male). While males showed significant expansion in areas of the hypothalamus, we found that females showed expansion in the early visual cortex, the cerebellum, and higher-order areas across the occipital and temporal cortex. Several of these regions showed non-overlapping ranges between males and females. We then investigated sex differences in white matter tracts using tract-based spatial statistical analysis on fractional anisotropy (FA) images from the T1-weighted scans. We found that females showed significantly higher FA than males in the right cerebral hemisphere, specifically in regions of frontal-parietal white matter, and these values were also non-overlapping. We suggest that distinct socioecological niches which male and female capuchins occupy could be associated with the sex differences found in cortical gray and white matter. The sex differences in neuroanatomy that we report here appear more pronounced than what is typically observed in humans. These differences could be associated with human adaptations for prolonged neurodevelopmental trajectories and increased plasticity.

Learning Objectives:

1. Identify differences that exist in the capuchin monkey male and female social environment.

2. Understand which type of brain tissue is analyzed with voxel-based morphometry and tract-based spatial statistical analysis.

3. Examine whether selection pressures on social behavior drive sexual differentiation in primate neuroanatomy.