© iStockphoto/Jonas AnderssonWhite-faced Capuchin (Cebus capucinus) checking its nails.
Research at the universities of Liverpool and Oxford into the finger length of primate species has revealed that cooperative behavior is linked to exposure to hormone levels in the womb.
The hormones, called androgens, are important in the development of masculine characteristics such as aggression and strength. It is also thought that prenatal androgens affect finger length during development in the womb. High levels of androgens, such as testosterone, increase the length of the fourth finger in comparison to the second finger. Scientists used finger ratios as an indicator of the levels of exposure to the hormone and compared this data with social behaviour in primate groups.
The team found that Old World monkeys, such as baboons and rhesus macaques, have a longer fourth finger in comparison to the second finger, which suggests 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 behaviour.
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.