The 2D:4D ratio & social behaviour in female chacma baboons: Dominance, Aggression, Affiliation & Interest in infants

Abstract

Prenatal androgens are responsible for sex differences in behaviour and morphology in many primates and cause changes in neural structure and function that persist throughout the animal’s life. Some of the variation in the expression of behaviour between individuals of the same sex can be attributed to individual differences in exposure to prenatal sex hormones. The phases in development when prenatal androgens have masculinising and defeminising effects on the brain, and consequently on behaviour, are also the phases when digit growth is influenced by androgen and oestrogen receptor activity. Thus, the ratio of the second and fourth digits (2D:4D ratio) is a proposed biomarker for prenatal androgen effects (PAE). Through assessment of 2D:4D ratios, this study aimed to investigate the relationship between inferred PAE and social behaviours in wild female chacma baboons (Papio hamadryas ursinus). 2D:4D ratios were measured indirectly for 20 adult and five adolescent females using digital photographs and computer-assisted image analysis software (ImageJ). Low 2D:4D ratios (high inferred PAE) were associated with high rank, lower rates of submission and with higher rates of non-contact and contact aggression among females. The 2D:4D ratio correlated positively with submission and negatively with dominance and aggression suggesting that PAE are linked to the expression of these behaviours in female baboons and likely in other cercopithecine primates. The 2D:4D ratio did not correlate with rate of behaviours indicating females’ interest in infants or with rate of affiliation among females possibly because these behaviours are regulated by ovarian hormones in adult life rather than by PAE. Finally, mean 2D:4D ratios were positively correlated in six mother/infant pairs. These preliminary results suggest that the 2D:4D ratio may be heritable in baboons and other primates.