Dissociation of testosterone and aggressive behavior during the breeding season in male chestnut-collared longspurs, Calcarius ornatus

In many species of birds, expression of male–male aggression during breeding elicits increased secretion of testosterone. Elevations in testosterone in turn enhance expression of aggressive behavior in appropriate contexts. However, in other species, the relationship between elevated plasma testosterone and aggressive behavior is subtle or nonexistent. We examined the relationship between high testosterone and male–male aggression in the chestnut-collared longspur, a species in which males exhibit a peak in testosterone early in the breeding season that declines to a breeding baseline after nests are initiated. Elevated testosterone enhances sexual behavior after nests are initiated, but does not affect care of nestlings. We used simulated territorial intrusions (STIs) to test (1) if males increased testosterone secretion after an aggressive interaction early in the breeding season (2) if expression of aggressive behavior declined as testosterone levels declined to the breeding baseline, and (3) if experimentally elevated testosterone enhanced aggression when testosterone levels were at the breeding baseline. Plasma testosterone was unrelated to duration of STI before nests were initiated. In addition, although males were significantly more aggression prior to nest initiation than during incubation, elevation of plasma testosterone to pre-nesting levels did not enhance aggression during incubation. Thus, although elevated plasma testosterone and expression of aggressive behavior appear to overlap temporally, high plasma testosterone and male–male aggression may not be fully coupled in this species. We suggest that high plasma testosterone may be more important in eliciting and maintaining sexual behavior than aggressive behavior in this species.