that E2 governs aggressive behavior during the non-mating season. It is unknown however if this is just specific to birds, or if this extends to other animal species. These examples all culminate in a challenge hypothesis model which focuses on the role of testosterone on aggression in the breeding season. The challenge hypothesis most likely cannot be applied to the non-breeding season since, as mentioned above, there is most likely a mechanism independent of testosterone governing aggression in the non-mating season. A sigmoidal relationship between testosterone plasma levels and male-male aggression is observed under the challenge hypothesis when the birds’ testosterone levels were above seasonal breeding testosterone baseline levels. If birds remained at the seasonal breeding baseline levels during the breeding season, then there is not a significant difference observed in male-male aggression. In addition, there is a negative, sigmoidal relationship between testosterone levels in the birds and the amount of parental care provided when parents are above the seasonal breeding testosterone baseline levels. As such, the relationship between testosterone plasma levels and male-male aggression is context-specific to the species. Figure 2 and 3 describe the relationships observed of many single- or double-brooded bird species, from male western gulls to male turkeys. In other animals The challenge hypothesis has been used to describe the testosterone levels in other species to certain social stimuli. The challenge hypothesis predicts the testosterone influence on aggressive male-male interactions between male northern fence lizards. This reinforces the challenge hypothesis by showing rapid changes in aggressive behaviors of the lizards do not correlate with testosterone concentrations. Yet, over the mating season, the intensity of the behavior and the levels of testosterone levels yielded a positive correlation. Research has also shown the challenge hypothesis applies to specific monogamous fish species, with a greater correlation in species with stronger pair bonding. In addition, the challenge hypothesis has been adapted to primate species. In 2004, Martin N. Muller and Richard W. Wrangham applied a modified challenge hypothesis to chimpanzees. Similar to the original hypothesis, they predicted that there would be increased male-male aggressive interaction when a receptive and fertile female chimpanzee was present. Muller and Wrangham also correctly predicted the testosterone levels of more dominant chimpanzees to be higher as compared to lower status chimpanzees. Therefore, chimpanzees significantly increased both testosterone levels and aggressive male-male interactions when receptive and fertile females presented sexual swellings. This study also highlighted how