Testosterone-mediated behavior and muscle mass convey advantages to male fitness. However, high-testosterone phenotypes entail energetic and immunologic costs: the physiological and somatic effects of testosterone require significant caloric maintenance, and may disrupt immune function. Human studies of acute testosterone change have focused on male-male competition among healthy young men in industrialized populations with higher, potentially evolutionarily novel, levels of testosterone compared to environmentally stressed subsistence populations. These studies provide little information about other fitness enhancing hormone-behavior interactions in evolutionarily relevant environments. The overarching objective of this dissertation is to examine the conditions andselective pressures that resulted in flexible testosterone response to environmental and social stimuli. Field andlaboratory studies were conducted among US and Tsimane men to examine the relative dynamics of acute testosterone change for men engaged in: 1) fasting; 2) physical competition (soccer); 3) non-competitive physically-intensive food production (tree-chopping); 4) non-physical competition (soccer penalty kicks); and 5) a semi-competitive physical food production activity (hunting). Despite lower basal testosterone levels, Tsimane forager-horticulturalists express relative increases in testosterone similar to those seen in industrialized populations. Physical challenges (soccer, tree-chopping, hunting) increased salivary testosterone more than a non-physical challenge (penalty kicks). A competitive challenge (soccer) resulted in increased testosterone, but did not augment testosterone as much as physically intensive food production (tree-chopping). These studies underscore the importance of moving beyond a unidimensional focus on competitive interactions in order to understand the importance of acute testosterone-behavior interactions during other reproductively important activities, providing a broader understanding of human male life-histories and adaptive variation.
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