Seasonal variation in male testosterone levels in a tropical bird with year-round territoriality

Testosterone is important in mediating investment in competing activities such as territoriality, parental care, and maintenance behavior. Most studies of testosterone function have focused on temperate species and less is known about the role of testosterone in territoriality or variation in mating systems of tropical species. Results of studies of tropical species with year-round territoriality indicate that territorial aggression during the non-breeding season is maintained with low levels of testosterone, and increased levels of testosterone in males during the breeding season may increase mating opportunities or aid in competition for mates. We studied seasonal variation in testosterone levels of male Red-throated Ant-tanagers (Habia fuscicauda), a socially monogamous species with year-round territoriality and with high levels of extra-pair matings (41% of young), to determine if testosterone levels increased during the breeding season. We captured males during the non-breeding and breeding seasons and collected blood samples for hormone analysis. We found that mean testosterone concentrations were low during the non-breeding season (0.18 0.05 [SD] ng/ml, range = 0.11-0.31 ng/ml), and significantly higher during the breeding season (2.37 2.47 ng/ml, range = 0.14-6.28 ng/ml). Testosterone levels of breeding males were not related to aggression levels as measured by attack rates toward a stuffed decoy or singing rates during simulated territorial intrusions. These results suggest that the higher testosterone levels of breeding male Red-throated Ant-tanagers may be important in an extra-pair mating context, possibly in display behavior or mate attraction, but additional study is needed to clarify the role of testosterone during the breeding season.