I have collaborated with Frank Marlowe and Peter Ellison to investigate the reproductive ecology of human males. This work has focused on the role of the steroid hormone testosterone as a modulator of male mating effort. Recent studies have suggested that men in committed romantic relationships exhibit lower levels of testosterone than single men. Furthermore, men intending to invest in their children show significant decreases in testosterone during the period surrounding parturition. The existence of long-term pair bonds and paternal care in humans is a clear difference between our species and chimpanzees, and these factors have been hypothesized to play an important role in human evolution.
I have conducted fieldwork in Tanzania to examine the relationship between testosterone and paternal care in two populations: Hadza foragers and Datoga pastoralists. These are interesting groups in which to investigate this relationship because they live in close proximity, but exhibit strikingly different patterns of paternal care. I found that high levels of paternal care by Hadza fathers were associated with decreased testosterone in comparison with non-fathers, but that no such difference between fathers and non-fathers was evident in Datoga men, who provide minimal direct care.