High levels of androgens, such as testosterone, increase the length of the fourth finger in comparison to the second finger. Some archaeologists and anthropologists are using finger ratios as an indicator of the levels of exposure to the hormone and recently compared this data with social behavior in primate groups.
They found that Old World monkeys, such as baboons and rhesus macaques, have a longer fourth finger in comparison to the second finger, which suggests to the researchers that they have been exposed to high levels of prenatal androgens. These species tend to be highly competitive and promiscuous, which suggests that exposure to a lot of androgens before birth could be linked to the expression of this behavior.
Other species, such as gibbons and many New World species, have digit ratios that suggest low levels of prenatal androgen exposure. These species were monogamous and less competitive than Old World monkeys.
They say their results show that Great Apes, such as orangutans and chimpanzees, expressed a different finger ratio and suggest that early androgen exposure is lower in this groups compared to Old World monkeys. Lower androgen levels could help explain why Great Apes show high levels of male cooperation and tolerance.
Emma Nelson, from the University of Liverpool's School of Archaeology, Classics and Egyptology, said, "It is thought that prenatal androgens affect the genes responsible for the development of fingers, toes and the reproductive system. High androgen levels from a foetus or mother during pregnancy, may alter gene function and lead to subtle changes in relative digit length and the functioning of the reproductive system. Finger ratios do not change very much after birth and appear to tell us something about how very early androgens affect adult behaviour, particularly behaviour linked to mating and reproduction."
Dr Susanne Shultz, from the Institute of Cognitive and Evolutionary Anthropology at the University of Oxford, said, "Humans are unique in that they live in large multi-male, multi-female groups, but maintain strong bonds and show high levels of group cooperation in both males and females. In most other species males are competitive rather than co-operative. Research from finger ratios may help us understand more clearly the development of human sociality and its evolutionary origins."