The 2D:4D digit ratio and social behaviour in wild female chacma baboons (Papio ursinus) in relation to dominance, aggression, interest in infants, affiliation and heritability
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Prenatal androgens are responsible for sex differences in behaviour and morphology in many species, causing changes in neural structure and function that persist throughout life. Some variation in the expression of behaviour between individuals of the same sex can also be attributed to differences in exposure to prenatal sex hormones. 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 female chacma baboons (Papio ursinus). We validated a new method to measure 2D:4D indirectly using digital photographs and computer-assisted image analysis software. There was a strong correlation between 2D:4D ratio and dominance rank amongst female baboons. Low 2D:4D ratios were associated with high rank, lower submission rates and higher rates of non-contact and contact aggression. This is consistent with the hypothesis that prenatal androgens are linked to the expression of these behaviours in female baboons, although it was not possible to separate the effects of PAE and dominance rank on some rank-related behaviours. The 2D:4D ratio did not correlate with interest in infants or with the rate of affiliative behaviours, possibly because these behaviours are more affected by ovarian hormones in adult life than by PAE. Finally, mean 2D:4D ratios were positively correlated in six mother/infant pairs, consistent with a heritable basis for the 2D:4D ratio in primates. We suggest that PAE contribute significantly to the patterning of social relationships in female primates.
KeywordsHormone Primate Human Development ImageJ
Thanks go to all C.A.R.E staff and volunteers for their support with data collection. We are grateful to Ian and Retha Gaigher for permission to work at Lajuma and to Pete Tomlin for assistance with the baboons. Also, we thank the editor and reviewers for their helpful comments which improved the manuscript. Funding for the data collection equipment and software was provided by Durham University’s Research Capital Equipment Fund.
All fieldwork was approved by the Life Sciences Ethical Review Process Committee at Durham University, UK, and the Department of Anthropology Ethics Committee. All work at Lajuma was conducted with permission from the Limpopo Department of Economic Development, Environment and Tourism, South Africa.
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