Evidence of intralocus sexual conflict: physically and hormonally masculine individuals have more attractive brothers relative to sisters
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Fig. 1
The y axis reflects standardized betas. Higher anthropometric masculinity/femininity scores indicate greater masculinity, whereas lower masculinity/femininity scores indicate greater femininity. Positive betas thus indicate positive relationships between individual masculinity and sibling mate value, whereas negative betas indicate negative relationships between masculinity and sibling mate value. A significant interaction between sibling sex and anthropometric masculinity/femininity is depicted, such that individual masculinity increases brothers' attractiveness relative to sisters'. Effects of body fat and sibling age were controlled.
Fig. 2
The y axis reflects standardized betas. Positive betas indicate a positive relationship between individual estrogen level and sibling mate value, whereas negative betas indicate a negative relationship between estrogen and sibling mate value. A significant three-way interaction among sibling sex, participant sex and E is depicted: Evidence for IASC is stronger in women than in men (i.e., stronger when E is measured in female participants than when measured in male participants). Body fat, sibling age, T and interactions between body fat and E and between body fat and T were controlled.
Abstract
Intralocus sexual conflict (IASC) occurs when sex-specific selection favors genes that increase fitness in one sex and decrease fitness in the other sex. The current study was designed to explore whether IASC occurs in humans. In a sample of siblings, we identified and measured sexually dimorphic traits and hormones within each sex that are related to fitness and are likely coded for by antagonistic genes: waist-to-hip ratio (WHR) and breast size in women, WHR and bicep size (an index of muscularity) in men, and estradiol (E) and testosterone (T) in both sexes. If these traits and hormones are coded for by genes under IASC, masculine or feminine expression of traits and hormones should differentially predict brothers' and sisters' fitness. Consistent with an IASC model, both men and women who were physically masculine for their sex reported higher mate value brothers relative to sisters. Similarly, in normal-weight individuals, E levels positively predicted the mate value of sisters relative to brothers and T levels positively predicted the mate value of brothers relative to sisters. We found no evidence that individuals with indicators of high genetic quality (i.e., physically masculine men and physically feminine women) share high mate value with all siblings, regardless of sibling sex. Results are novel and demonstrate for the first time that intralocus conflict in humans may influence the fitness of related individuals.
Keywords:
Intralocus conflict, Sexually antagonistic selection, Masculinity, Femininity, Estrogen, Testosterone, Sexual dimorphismTo access this article, please choose from the options below
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