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Resumo

Testosterone (T) plays a key role in the increase and maintenance of muscle mass and bone density in adult men. Life history theory predicts that environmental stress may prompt a reallocation of such investments to those functions critical to survival. We tested this hypothesis in two studies of rural Bolivian adult men by comparing free T levels and circadian rhythms during late winter, which is especially severe, to those in less arduous seasons. For each pair of salivary TAM/TPM samples (collected in a ∼ 12‐h period), circadian rhythm was considered classic (CCLASSIC) if TAM > 110%TPM, reverse (CREVERSE) if TPM > 110%TAM, and flat (CFLAT) otherwise. We tested the hypotheses that mean TAM > mean TPM and that mean TLW TOTHER‐PM (A: P = 0.035, B: P = 0.0005) and TOTHER‐AM > TLW‐AM (A: P = 0.054, B: P = 0.007); TPM did not vary seasonally, and T diurnality was not significant during late winter. T diurnality varied substantially between days within an individual, between individuals and between seasons, but neither T levels nor diurnality varied with age. These patterns may reflect the seasonally varying but unscheduled, life‐long, strenuous physical labor that typifies many non‐industrialized economies. These results also suggest that single morning samples may substantially underestimate peak circulating T for an individual and, most importantly, that exogenous signals may moderate diurnality and the trajectory of age‐related change in the male gonadal axis. Am. J. Hum. Biol., 2009. © 2009 Wiley‐Liss, Inc.

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