Wednesday, October 10, 2007

Of birds & men: testosterone edition   posted by Razib @ 10/10/2007 01:20:00 PM

Natural Variation in a Testosterone-Mediated Trade-Off between Mating Effort and Parental Effort:
Male birds frequently face a trade-off between acquiring mates and caring for offspring. Hormone manipulation studies indicate that testosterone often mediates this trade-off, increasing mating effort while decreasing parental effort. Little is known, however, about individual covariation between testosterone and relevant behavior on which selection might act. Using wild, male dark-eyed juncos,,,we measured individual variation in testosterone levels before and after standardized injections of gonadotropin-releasing hormone (GnRH)...We correlated these testosterone increases with behavioral measures of mating and parental effort...Males that showed higher postchallenge testosterone displayed more territorial behavior, and males that produced higher testosterone increases above initial levels displayed reduced parental behavior. Initial testosterone levels were positively but nonsignificantly correlated with aggression but did not predict parental behavior. These relationships suggest that natural variation in testosterone, specifically the production of short-term increases, may underlie individual variation in the mating effort/parental effort trade-off. We discuss the implications of these results for the evolution of hormonally mediated trade-offs.

The results of this paper are not surprising, though not as clear as one would hope for. That being said, man is not a bird. So this paper, Testosterone and Marriage among Ariaal Men of Northern Kenya is of more interest:

Recent studies suggest that differential human male investment in mating (male-male competition and mate-seeking behavior) and parenting effort may be associated with variation in testosterone levels. The Ariaal present an interesting test case because marital relations tend to be aloof and direct paternal care minimal by cross-cultural standards. Polygyny is prevalent and increases with age, and the age-set system highly structures the transition to marriage. A test of the effect of marital status on testosterone levels among the Ariaal involved 205 men aged 20 and older from a settled agropastoral community and nomadic populations. Each participant provided morning and afternoon saliva samples in which testosterone levels were measured, provided demographic background during interviews, and had anthropometrics taken. As predicted, during the dynamic ages (20–39) of transition from life as a bachelor and warrior to monogamous marriage, men with one wife had significantly lower testosterone levels than unmarried men. Contrary to prediction, however, polygynously married men did not have higher testosterone levels than their monogamously married counterparts. While variation in testosterone may be associated with mating effort in young Ariaal men, political networks and wealth may be better predictors of marital status in older men.

ScienceDaily has more detail on this study. Man is obviously not a bird, but all men are not the Ariaal of Kenya, so one has to be cautious about the generalizing from one case study. That being said, the peculiarity of the results here point to the problem that human societies are so complicated in terms of countervailing parameters that epiphenomenally they can throw biosocial expectations for a loop. Virpi Lummaa's finding that the presence of grandfather's may reduce a child's fitness for example is a curious result which may simply be a sociological epiphenomenon which may take biology a while to catch up which point social systems may have changed radically so as to make the issue moot. In the big-picture philosophical context the flexibility of social systems which characterizes our species, and is likely one of our adaptive traits, may be seen as a net wash. But in terms of proximate examination and understanding of human societies it obviously matters quite a bit.

In the press there are often stories about men with powerful jaws being more attractive. Later there are follow ups which suggest that softer faced men are preferred except when women are fertile. From these data researchers often draw very general conclusions, but the polymoprhic tendency of human behavior and physique might suggest to us that the temporally protean nature of our societies and the wide scope for a host of stable niches across their enormous spatial spans may mean that these general explanations are ultimately irrelevant. Human variation could very well be a function of the constant balancing effects of changing social circumstances to which biological evolution can never be optimized. It isn't that our species is post-evolutionary, rather, the gene-culture co-evolutionary arms race is a more much complex and turbid dynamic than current models can conceptualize with clarity.

Addendum: Some dynamics are simple enough; it seems clear that genes did catch up with cattle culture across most of Eurasia.