Adaptive variation in testosterone levels in response to immune activation: empirical and theoretical perspectives.

High testosterone levels reflect investment in male reproductive effort through the ability to produce and maintain muscle tissue and thus augment mate attraction and competitive ability. However, high testosterone levels can also compromise survivorship by increasing risk of prostate cancer, production of oxygen radicals, risk of injury due to hormonally-augmented behaviors such as aggression, violence and risk taking, reduced tissue and organ maintenance, negative energy balance from adipose tissue catabolism, and suppression of immune functions. Here, I briefly discuss how inter- and intra-individual variation in human male testosterone levels is likely an adaptive mechanism that facilitates the allocation of metabolic resources, particularly in response to injury, illness or otherwise immune activation. Maintaining low testosterone levels in resource-limited and/or high pathogen-risk environments may avoid some immunosuppression and suspend energetically-expensive anabolic functions. Augmenting testosterone levels in the presence of fertile and receptive mates, areas of high food resource availability, and low disease risk habitats will function to maximize lifetime reproductive success.