Sex Differences in Rhesus Monkeys' Digit Ratio (2D:4D Ratio) and Its Association With Maternal Social Dominance Rank.

ABSTRACT

Prenatal androgen exposure (PAE) plays a pivotal role in masculinizing the developing body and brain, and extreme exposure may contribute to autism, anxiety disorder and schizophrenia. One commonly used biomarker for PAE is the pointer-to-ring-finger digit length (2D4D) ratio. Although this biomarker is widely used in human studies, relatively few studies have investigated 2D4D ratio in nonhuman primates, particularly rhesus macaques (Macaca mulatta), one of the most commonly used animals in biomedical research. Thus far, data suggest that sexual dimorphism in 2D4D ratio may be in the opposite direction in some monkey species, when compared to the pattern exhibited by humans and great apes. Using a large sample size, we investigated whether rhesus monkeys' 2D4D ratio shows the same sex-differentiated pattern present in other Old World monkey species. We also investigated whether individual differences in 2D4D ratio are associated with the social dominance rank of subjects' mothers during pregnancy, and the social dominance rank the subjects attained as adults. Subjects were 335 rhesus monkeys between 3 years and 24 years of age (M = 6.6). Maternal dominance rank during pregnancy and subjects' adult dominance rank were categorized into tertiles (high, middle and low). Results showed that, across both hands, male rhesus monkeys exhibited higher 2D4D ratio than females, a pattern consistent with other monkey species and a reversal from the pattern typically observed in humans and apes. This sex difference was modulated by maternal dominance rank, with female offspring of high-ranking and middle-ranking mothers exhibiting masculinized 2D4D ratio, indicating that maternal dominance rank during pregnancy may influence levels of PAE. There was no association between subjects' 2D4D ratio and the social dominance rank they attained as adults. These findings show a consistent sex difference in Old World monkeys' 2D4D ratio that diverges from the pattern observed in apes and humans, and suggest maternal social dominance rank modulates PAE in rhesus monkeys.