1School of Archaeology, Classics & Egyptology, University of Liverpool, 2Department of Anthropology, University of Central Florida, 3Department of Biology, University of Pennsylvania, 4Department of Human Evolutionary Biology, Harvard University, 5Department of Comparative and Developmental Psychology, Max Planck Institute for Evolutionary Anthropology, 6Department of Evolutionary Anthropology, Duke University
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There is extensive indirect evidence from human studies showing that the second-to-fourth digit ratio (2D:4D) in adults reflects the action of prenatal androgen effects (PAE) on physical and behavioural development. Primate individuals and species that exhibit high levels of competitive and status-related behaviours have been found to have lower 2D:4D, implying higher PAE. These kinds of investigations, however, can potentially be confounded by the effects of rearing environment. Here we address this issue by comparing datasets between captive and wild groups belonging to three species.
Our comparative sample included Macaca mulatta (305 zoo and 87 free-ranging), Pan paniscus (25 zoo and 43 wild-born, sanctuary-living), and Pan troglodytes (252 zoo and 88 wild-born, sanctuary-living). Results indicated a significant interaction between captivity and side: zoo residents had lower 2D:4D (F=11.6, p<0.001), which was most marked on the left side. There was also a significant interaction between species and sex (F=3.7, p=0.025), such that chimpanzees had a significantly greater sex difference than rhesus macaques (difference=0.02, t=2.7, p=0.007), while bonobos were intermediate.
In conclusion, left hand 2D:4D may be reduced by environments characterizing zoo-living compared with free-ranging, possibly due to differences in maternal stressors. These effects are more marked in the left hand. This contrasts with the human pattern where 2D:4D is lower in the right hand. Furthermore, the more marked sex-difference in 2D:4D in male-dominant chimpanzees, compared to bonobos and macaques, might suggest that PAE plays a role in prenatally programming female dominance in these species.