Sylvian fissure asymmetry in capuchin monkeys (Cebus apella).

Asymmetries of the sylvian fissure (SF) are believed to reflect an enlargement of the posterior temporal lobe, particularly a region that corresponds to part of Wernicke's area in humans. In nonhuman primates the homologue to the region may be involved in the discrimination and processing of species-specific vocalisations. As capuchin monkeys are large-brained, socially complex primates with a rich vocal repertoire, it was hypothesised that they would display asymmetry of the SF. We used high-resolution 3T MRI scans to investigate this asymmetry in 17 brown capuchin monkeys (Cebus apella; 9 males, 8 females). Results indicated a trend towards population-level leftward asymmetry in the lateral region of the SF. Post hoc analyses revealed significant sex differences in SF asymmetry, with females displaying a population-level leftward asymmetry of the lateral region of the SF. Age was a significant mediator of the effects of sex on asymmetry of the lateral region of the SF. These results provide evidence that capuchin monkeys display sex differences in the asymmetry of the SF and show developmental changes in hemispheric lateralisation.