male testosterone and aggression levels rise only when males are in the presence of parous females. This is because nulliparous females are less attractive to males, and they are not guarded, meaning there is little competition. This evidence suggests that the increase in testosterone is related to only aggression – not sexual activity – as male chimpanzees mate equally with both parous and nulliparous females. Currently, no research has specified a relationship between the modified challenge hypothesis and human behavior, yet, many testosterone/human behavior studies support the modified hypothesis applying to human primates. Cornerstones Mating effort versus parenting effort A fundamental feature of male life history is the tradeoff between the energy devoted to male-male competition and mate attraction (mating effort) versus that allocated to raising offspring (parenting effort). There is a trade off because decreased paternal effort caused by increased testosterone dramatically decreases reproductive success, due to decreased parental care and protection for the offspring. Therefore, to maximise reproductive success, the optimal balance between the two must be found. The challenge hypothesis proposes testosterone as the key physiological mechanism underlying this tradeoff. When the opportunity to reproduce arises—namely, the species enters the breeding season or females enter estrus—males should exhibit a rise in testosterone levels to facilitate sexual behavior. This will be characterized by increased mating effort and decreased parenting effort, as investment in the former may be incompatible with parental care due to insufficient time and energy to engage in all of these facets of reproductive effort. Research on nonhuman species has found that testosterone levels are positively associated with mating effort and negatively related to parenting effort. Moreover, experimental manipulations have revealed a causal role of testosterone, such that elevations in testosterone result in increased mating effort and decreased parenting effort. Paternal care The challenge hypothesis makes different predictions regarding testosterone secretion for species in which males exhibit paternal care versus those in which males do not. When aggressive interactions among males arise in species that exhibit paternal care, testosterone levels are expected to be elevated. Males are predicted to exhibit an increase in testosterone to Level C (physiological maximum), but only during periods of territory establishment, male-male challenges, or when females are fertile so that paternal care is not compromised. When aggression is minimal, specifically during parenting, testosterone levels should decrease to Level B (breeding baseline). Level B represents the minimal levels of testosterone required for