As modern humans moved north into environments with longer winters, women were less able to feed themselves and their children through food gathering. They thus became more dependent on food from their male partners. For men, this greatly increased the cost of having a wife and children, thus making polygyny prohibitively expensive for all but the ablest hunters.
Initially, this situation came about by men and women pushing their respective envelopes of behavioral plasticity. It may not have been the happiest of situations, but circumstances left no other choice. Over time, however, natural selection should have improved things by favoring men who were less predisposed to polygyny and more predisposed to provide for their wives and children.
How? Apparently, by lowering testosterone levels in men once they’ve entered a pair bond. This has been shown by findings recently presented at this year’s annual meeting of the American Association of Physical Anthropologists. According to Shur et al. (2008):
Numerous studies reveal a negative correlation between testosterone concentration and paternal care in diverse mammals including nonhuman primates and humans. Several researchers suggest that spousal investment accounts for the lower testosterone of married men compared to unmarried men, but findings that the lowest testosterone levels are observed in married men with children implicate paternal care as particularly relevant. Thus testosterone reduction may reflect a facultative shift in male reproductive strategy from intrasexual competition and copulation to care of young.
This hypothesis was tested with wild olive baboons, among whom lactating females form close “friendships” with their male partners.
In contrast to control males, male friends experienced a decrease in testosterone level coinciding with the birth of their female friends’ infants. Male friends also maintained a lower basal testosterone level than did control males during the lactation period of their female friends. Testosterone levels in male friends increased gradually corresponding with developing infant independence.
This finding may explain the marked differences in testosterone levels we see in humans, particularly between tropical and non-tropical populations. These levels seem to decrease wherever men compete less keenly for mates (because polygyny is less common) and wherever they invest more in parenting. Lowering the level of testosterone seems to lower the threshold for expression of paternal investment.
If the testosterone level has fallen in some populations because of selection for paternal investment, we should see evidence of such selection elsewhere, e.g., altered spatial distribution of testosterone receptors in the brain, more mental space dedicated to parenting behavior, etc.
Shur, M.D., Palombit, R.A., and Whitten, P.L. (2008). Association between male testosterone and friendship formation with lactating females in wild olive baboons (Papio hamadryas anubis). Program of the 77th Annual Meeting of the American Association of Physical Anthropologists, p. 193. http://www.physanth.org/annmeet/aapa2008/AAPA2008abstracts.pdf