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Across taxa, sexually selected traits are more variable in the target sex than 1) the same trait in the opposite sex or 2) non-sexually selected traits, likely due to their condition-dependent expression. In humans, males show greater variability in certain cognitive abilities and brain structures that 1) may facilitate intra- or intersexual competition and 2) are greater/larger in males on average, suggesting these traits may also have been subject to sexual selection. This study investigates sex differences in brain structure variability in chimpanzees. Although male chimpanzees exhibit strong intrasexual competition, reproductive skew is reduced by female mate choice and male coercion. In vivo MRI scans were collected from 226 (135F/91M) individuals and surface areas were calculated for 25 cortical sulci. Outliers for each sex and sulcus were removed prior to analysis. We measured sex differences in variability by calculating the ratio of male-to-female standard deviations of MCMCglmm residuals, controlling for age, rearing condition, scanner type, and kinship. We tested for significant sex differences through permutation. We find that males are significantly more variable at the cingulate (ratio=1.18;p=0.043), middle-frontal (ratio=1.36;p=0.001), occipital-lateral (ratio=1.20;p=0.029), occipital- temporal-marginal (ratio=1.8;p=0.006), superior-temporal (ratio=1.36;p<0.001), subcentral-posterior (ratio=1.62;p=0.033), and superior-parietal (ratio=1.21;p=0.028) sulci. These regions are associated with social perception, face recognition, and motion prediction. Females are more variable at the medio-parietal-occipital sulcus (ratio=0.78;p=0.009), a region associated with planning. This is the first study to demonstrate greater male variability in brain structure in a nonhuman primate species, and suggests sexual selection may lead to greater variability in male cognition across taxa.
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Vascularization underlies differences in sexually selected skin coloration in a wild primate
Abstract Male reproductive competition can select for condition‐dependent, conspicuous traits that signal some aspect of fighting ability and facilitate assessment of potential rivals. However, the underlying mechanisms that link the signal to a male's current condition are difficult to investigate in wild populations, often requiring invasive experimental manipulation. Here, we use digital photographs and chest skin samples to investigate the mechanisms of a visual signal used in male competition in a wild primate, the red chest patch in geladas (Theropithecus gelada). We analysed photographs collected during natural (n = 144) and anaesthetized conditions (n = 38) to understand variability in male and female chest redness, and we used chest skin biopsies (n = 38) to explore sex differences in gene expression. Male and female geladas showed similar average redness, but males exhibited a wider within‐individual range in redness under natural conditions. These sex differences were also reflected at the molecular level, with 10.5% of genes exhibiting significant sex differences in expression. Subadult males exhibited intermediate gene expression patterns between adult males and females, pointing to mechanisms underlying the development of the red chest patch. We found that genes more highly expressed in males were associated with blood vessel development and maintenance but not with androgen or oestrogen activity. Together, our results suggest male gelada redness variability is driven by increased blood vessel branching in the chest skin, providing a potential link between male chest redness and current condition as increased blood circulation to exposed skin could lead to heat loss in the cold, high‐altitude environment of geladas.
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- PAR ID:
- 10420816
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Molecular Ecology
- Volume:
- 32
- Issue:
- 15
- ISSN:
- 0962-1083
- Page Range / eLocation ID:
- p. 4401-4411
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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