According to the challenge hypothesis, the testosterone level correlates with the mating related aggression in seasonal breeders. In the breeding season, the testosterone level baseline is higher compared to the non-breeding season. The high baseline in breeding season is said to be necessary for successful reproductive behavior. While this seasonal change is due to environmental clues, social clues from conspecifics can additional increase the testosterone level in the begin of breeding season leading to more aggressive behavior in defending territory or finding mating partners. But how is that in non-aggressive species like the tufted capuchin monkey? Jessica W. Lynch, Toni E. Ziegler and Karen B. Strier published 2002 a study were they analyzed the seasonal changes in fecal testosterone levels of wild male tufted capuchin monkeys. As expected, all male individuals had their highest testosterone level in the period when the adult females were fertile. However, despite the high testosterone level, the aggression level in the group was low. Interestingly, the highest aggression level could be seen in the non-breeding season when the testosterone level was low in all individuals, but these aggressions were not mating related but related to food competition. Moreover, the testosterone level of the top-ranked male, who is favored as mating partner by the females, did not differ from the other males. However, the alpha male just achieved the position three month before the begin of the study. Therefore, it is not sure if on long timescale the alpha male will produce more testosterone as the others. Sure is that there are exceptions to the challenge hypothesis and a high testosterone level does not have to lead to higher aggression levels... at least in tufted capuchin monkeys. Individual and Seasonal Variation in Fecal Testosterone and Cortisol Levels of Wild Male Tufted Capuchin Monkeys, Cebus apella nigritus
Jessica W. Lynch, Toni E. Ziegler and Karen B.Strier Hormones and Behavior 41, 275–287 (2002) doi:10.1006/hbeh.2002.1772
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