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Challenge hypothesis
The challenge hypothesis outlines the dynamic relationship between testosterone and aggression in mating contexts. It proposes that testosterone promotes aggression when it would be beneficial for reproduction, such as mate guarding, or strategies designed to prevent the encroachment of intrasexual rivals.〔Buss, D. M. (2002). Human mate guarding. ''Neuroendocronology Letters Special Issue, 23'', 23-29.〕 The challenge hypothesis predicts that seasonal patterns in testosterone levels are a function of mating system (monogamy versus polygyny), paternal care, and male-male aggression in seasonal breeders.
The pattern between testosterone and aggression was first observed in seasonally breeding birds, where testosterone levels rise modestly with the onset of the breeding season to support basic reproductive functions. However, during periods of heightened male aggression, testosterone levels increase further to a maximum physiological level. This additional boost in testosterone appears to facilitate male-male aggression, particularly during territory formation and mate guarding, and is also characterized by a lack of paternal care.〔Wingfield, J. C., Hegner, R. E., Dufty, A. M., & Ball, G. F. (1990). The 'challenge hypothesis': Theoretical implications for patterns of testosterone secretion, mating systems and breeding strategies. ''American Naturalist, 136'', 829-846.〕 The challenge hypothesis has come to explain patterns of testosterone production as predictive of aggression across more than 60 species.〔Wingfield, J.C., Jacobs, J.D., Tramontin, A.D., Perfito, N., Meddle, S., Maney, D.L., Soma, K. (2000). Toward an ecological basis of hormone-behavior interactions in reproduction of birds. In: Wallen, K., Schneider, J. (Eds.), ''Reproduction in Context''. MIT Press, Cambridge, MA, pp. 85–128.〕
==Patterns of testosterone==
The challenge hypothesis presents a three-level model at which testosterone may be present in circulation. The first level (Level A) represents the baseline level of testosterone during the non-breeding season. Level A is presumed to maintain feedback regulation of both GnRH and gonadotropin release, which are key factors in testosterone production. The next level (Level B) is a regulated, seasonal breeding baseline. This level is sufficient for the expression of reproductive behaviors in seasonal breeders and the development of some secondary sex characteristics. Level B is induced by environmental cues, such as length of day. The highest level (Level C) represents the physiological testosterone maximum and is reached through social stimulation, such as male-male aggression. The challenge hypothesis proposes that social stimulation which leads to this rise in testosterone above breeding baseline serves to increase the frequency and intensity of aggression in males, particularly for competing with other males or interacting with sexually receptive females.〔Goymann, W., Landys, M. M., Wingfield, J. C. (2007). Distinguishing seasonal androgen responses from male-male androgen responsiveness—Revisiting the challenge hypothesis. ''Hormones and Behavior, 51'', 463-476.〕
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