TESTOSTERONE gradually declines with age, right? Not for the Ariaal - subsistence pastoralists living in northern Kenya. They experience a decline in levels of the male hormone only when they get married. The finding provides a social and evolutionary explanation for the decrease in testosterone, rather than an age-related one.
Ariaal men remain single "warriors" until they are around 30, at which time they marry one or more women. Peter Gray of the University of Nevada, Las Vegas, and colleagues measured testosterone in 205 Ariaal men and found that those with one wife had lower levels of the hormone than unmarried men, and men with more than one wife had the lowest levels of all (Current Anthropology, DOI: 10.1086/522061). "Testosterone levels are lower among married men probably because they are investing less in mating effort," he says. Or to put it another way, they no longer have to compete for mates.
That link between mating effort and testosterone is made clearer by the fact that the Ariaal have an "aloof" marital system: apart from sex, husbands and wives have very little to do with each other, and men are minimally involved in childcare. In a separate study of 203 married Ariaal men, only three participants cited their wife or wives as a source of emotional support.
18 October 2007, original piece at NewScientist.com news service