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2018 Abstracts

Destined to Dominate? Sexual Dimorphism in Rhesus Monkeys' 2D:4D Ratio and the Role of Prenatal Androgens in Alpha Males and Females

John Capitanio; Elizabeth Wood; Alexander Baxter; Ashley Cameron, Brigham Young University

Prenatal androgen exposure (PAE) plays a pivotal role in masculinizing the brain. PAE also affects other parts of the body, and induces growth in the ring finger. Consequently, the pointer-to-ring-finger digit length ratio (2D:4D ratio) is frequently used to assess PAE. Though this phenotype is sexually dimorphic across primate species, preliminary data in a small number of primate species indicates that the effect of PAE on digit ratio may be in the opposite direction when comparing human and nonhuman primates. In humans, males typically show lower 2D:4D ratios than do females, while in nonhuman primates, males exhibit a higher 2D:4D ratio. Research in humans shows that females rate men with low 2D:4D ratios as more dominant when compared to men with high 2D:4D ratios. Similarly, in nonhuman primates with strict dominance-rank hierarchies, digit ratio is correlated with adult female social status. We investigated whether rhesus macaques’ (Macaca mulatta) digit ratio shows the same sex-differentiated pattern that is present in other nonhuman primates, and whether individual differences in 2D:4D ratio are correlated with social status in this species. Subjects were 446 rhesus monkeys (316 females, 130 males) housed indoors and outdoors at the California National Primate Research Center. Subjects’ 2D:4D ratios were measured between 3-24 years of age (M = 7.49). To determine subjects’ social status, trained colony management staff observed each social group and ranked subjects based on aggressive and submissive interactions with others in their group, recording win and losses during direct competition. For analyses, alpha status was compared with lower status stratified by tertials (high, medium, and low). T-tests with sex as the independent variable showed that 2D:4D ratio in rhesus monkeys is consistent with the pattern seen in other nonhuman primates, with males exhibiting higher 2D:4D ratio than females, possibly indicating an organizational effect of PAE. To our knowledge, this is the first study to demonstrate a significant sex difference in rhesus monkey digit ratio. Subjects’ left-hand digit ratio also differed by rank: across both sexes, alpha males and females had significantly higher 2D:4D ratio than all other subjects, though the three rank tertials did not differ in average left-hand digit ratio. Our data suggest that higher levels of PAE are associated with higher rank-attainment in rhesus monkeys, likely through its effect on traits that lead to assertiveness and dominance.