Browsing by Author "Cooper, Eve"

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    Greater variability in rhesus macaque (Macaca mulatta) endocranial volume among males
    (2022-03) Colby, Abigail; DeCasien, Alex; Cooper, Eve; Higham, James
    The greater male variability hypothesis posits that males exhibit more physical and behavioral variability than females. This pattern is observed across mammalian species and is especially pronounced in sexually selected traits. Greater variability in males likely reflects some combination of evolutionary mechanisms (e.g. balancing or disruptive selection) and developmental mechanisms (e.g. sexually dimorphic developmental schedules) that produce and maintain inter-individual variability. However, research investigating this phenomenon in brain size and structure is almost exclusive to humans. To address this gap in knowledge, we investigate sex differences in variability and heritability of relative and absolute endocranial volume (ECV) in a pedigreed sample of 542 (300F/242M) rhesus macaques using generalized linear mixed models that control for pairwise relatedness (i.e. an ‘animal model’). We found that males display more variable relative ECVs (phenotypic variance: male mean=0.646 [0.578,0.693], female mean=0.503 [0.457,0.540]) and absolute ECVs (phenotypic variance: male mean=0.670 [0.598,0.717], female mean=0.533 [0.481,0.568]). These findings are consistent with research in humans and chimpanzees. However, males do not display significantly lower heritability estimates and sex differences in phenotypic variance are not driven by environmental variance for relative (heritability: female mean=0.658 [0.568,0.778], male mean=0.626 [0.522,0.809]; environmental: female mean=0.171 [0.109,0.211], male mean=0.241 [0.119,0.302]) or absolute (heritability: female mean=0.683 [0.613,0.806], male mean=0.667 [0.578,0.827]; environmental: female mean=0.168 [0.112,0.210], male mean=0.223 [0.115,0.283]) ECV. These results suggest that, in rhesus macaques, greater male variability in brain size is likely driven by balancing or disruptive selection, rather than greater susceptibility to environmental effects during their relatively extended development.