Condition-dependent sexual traits and social dominance in the house finch

Elaboration of costly sexual traits can reduce investment in other aspects of reproduction, such as parental care or intrasexual competition, which may lead to the evolution of alternative mating tactics. In house finches (Carpodacus mexicanus), less elaborately ornamented (dull) males tend to dominate more elaborated (redder) males, but redder males pair earlier and invest more in parental care. This suggests that males may pursue alternative parental or competitive tactics, depending on the elaboration of their sexual trait. Elevation of testosterone, a hormone that is closely associated with condition in male house finches, influences dominance and sexual behaviors but is antagonistic to parental behaviors. We tested the hypothesis that the higher dominance status of dull males reflects an alternative testosterone-dependent mating tactic. First, we experimentally manipulated the testosterone levels of captive males and measured the effect on dominance rank, and second, we measured the association of testosterone elevation and plumage hue in free-living males. We found that, as predicted, testosterone elevation increased dominance rank in captive males. However, in free-living males, testosterone levels were higher in redder males, suggesting that testosterone is dissociated from dominance status under natural circumstances. This may be because the context of social interactions and the higher motivation of dull males to access food resources have a stronger influence on the outcome of dominance interactions than does the physiological effects of testosterone elevation. In turn, the strong positive correlation between testosterone levels and plumage elaboration likely reflects the common condition dependence of these traits.