a compromise between costs and benefits of increased testosterone levels, as increased levels were tightly timed around days of female estrus. This shows that there is an optimum length of time to have high testosterone levels when considering the costs and benefits. Male-male aggression It has long been known that testosterone increases aggressive behavior. While castration tends to decrease the frequency of aggression in birds and replacement therapy with testosterone increases aggression, aggression and testosterone are not always directly related. The challenge hypothesis proposes that testosterone is most immediately related to aggression when associated with reproduction, such as mate-guarding. An increase in male-male aggression in the reproductive context as related to testosterone is strongest in situations of social instability, or challenges from another male for a territory or access to mates. The relationship between aggression and testosterone can be understood in light of the three-level model of testosterone as proposed by the challenge hypothesis. As testosterone reaches Level B, or breeding baseline, there is minimal increase in aggression. As testosterone increases above Level B and approaches Level C, male-male aggression rapidly increases. Continuous breeders The challenge hypothesis was established based upon data examining seasonal breeders. There are many species, however, who are continuous breeders—namely, species that breed year-round and whose mating periods are distributed throughout the year (e.g., humans). In continuous breeders, females are sexually receptive during estrus, at which time ovarian follicles are maturing and ovulation can occur. Evidence of ovulation, the phase during which conception is most probable, is advertised to males among many non-human primates via swelling and redness of the genitalia. Support for the challenge hypothesis has been found in continuous breeders. For example, research on chimpanzees demonstrated that males became more aggressive during periods when females displayed signs of ovulation. Moreover, male chimpanzees engaged in chases and attacks almost 2.5 times more frequently when in groups containing sexually receptive females. Implications for humans The predictions of the challenge hypothesis as applied to continuous breeders partially rests upon males' ability to detect when females are sexually receptive. In contrast to females of many animal species who advertise when they are sexually receptive, human females do not exhibit cues but are said to conceal ovulation. While the challenge hypothesis has not been examined in humans, some have proposed that the predictions of the challenge hypothesis may apply. Several lines of converging evidence in the human literature suggest