Bonobos have a more human-like second-to-fourth finger length ratio (2D:4D) than chimpanzees: a hypothesized indication of lower prenatal androgens

Abstract

The ratio of the second-to-fourth finger lengths (2D:4D) has been proposed as an indicator of prenatal sex differentiation. However, 2D:4D has not been studied in the closest living human relatives, chimpanzees ( Pan troglodytes) and bonobos ( Pan paniscus). We report the results from 79 chimpanzees and 39 bonobos of both sexes, including infants, juveniles, and adults. We observed the expected sex difference in 2D:4D, and substantially higher, more human-like, 2D:4D in bonobos than chimpanzees. Previous research indicates that sex differences in 2D:4D result from differences in prenatal sex hormone levels. We hypothesize that the species difference in 2D:4D between bonobos and chimpanzees suggests a possible role for early exposure to sex hormones in the development of behavioral differences between the two species.