A recent article by Trumble et al. (2012) investigates testosterone levels in the Tsimane, hunter-gatherer society in the Bolivian Amazon. They organised a competitive football match between eight different Tsimane villages (Seriously, how cool is that – organising a football match as part of your research?), tested testosterone levels of the (male) players men before and after a match and compared these to testosterone levels in an age-matched male US sample. The authors have two key-findings: On the one hand, both groups show increased testosterone levels in a competitive football match. On the other hand, baseline testosterone levels are lower in the Tsimane.
Why might that be? According to the challenge hypothesis, testosterone enhances physical performance and therefore is short term release is beneficial for an individual under stress. But even more interesting in my view is their finding that the Tsimane also have lover baseline levels of Testosterone. The authors point to the downsides of high testosterone. As the hormone makes an individual build more muscle and burn fat faster, a high testosterone baseline requires more energy.
Hunter-gatherer societies are under much greater stress from diseases and parasites, compared to industrialised societies. But since testosterone is a relatively costly hormone, it can actually lower chances of survival in these conditions and selected against.
These findings give an interesting account of human evolution. For one, it shows that there is considerable variation even between humans, and that humans are still under considerable selective pressures. Orthodox evolutionary psychology however, holds that our bodies are essentially the product of natural selection in our Environment of Evolutionary Adaptedness, which would be found in the pleistocene. Trumble et al. show that a considerable amount of variation has developed since then, and the much higher baseline testosterone levels are a relatively recent development, since we can assume that environmental stressors only started to decline with the introduction of agriculture, and possibly even later.
This has also been part of the recent critique of evolutionary psychology, brought forward by Bolhuis et al. (2011). They argue that Evolutionary Psychology needs to be reassessed. In particular, evolution seems to work much faster than assumed. Consequentially, the developmental time lag between the development of a feature and it’s Environment of Evolutionary Adaptedness is smaller. This further strengthens the case for gene-culture co-evolution, since genes would be able to keep track of a fast-evolving culture. The paper therefore argues that we need to depart from classical tenets of evolutionary psychology, but has also received mixed reviews.
Trumble et al.‘s findings could present an interesting account of co-evolution, but this is preliminary only and needs further research. Bigger than the scientific contribution is probably its value as a tool in challenging our pop-evolutionary understanding of our (male) ancestors as testosterone driven muscle men, because that’s not what environment demands. Our past evolution is no excuse for current behaviour, but that we knew already.
Bolhuis, J. J., Brown, G. R., Richardson, R. C., and Laland, K. N. (2011). Darwin in mind: New opportunities for evolutionary psychology. PLoS Biol, 9(7):1–8.
Trumble, B. C., Cummings, D., von Rueden, C., O’Connor, K. A., Smith, E. A., Gurven, M., and Kaplan, H. (2012). Physical competition increases testosterone among amazonian forager- horticulturalists: a test of the ‘challenge hypothesis’. Proceedings of the Royal Society B: Biological Sciences.