Simulating winning in the wild--the behavioral and hormonal response of black redstarts to single and repeated territorial challenges of high and low intensity

In many vertebrates testosterone increases during aggressive interactions and the surges in this hormone may be responsible for the winner effect. So far studies on this relationship have been done in captivity only, because simulating a winning situation for a territory owner in the field is difficult. However, an increasing number of studies show that territorial aggression is not necessarily accompanied by elevated testosterone after a single simulated territorial intrusion (STI) and therefore it has been proposed that STIs may even create a losing experience. We examined whether free-living male black redstarts (Phoenicurus ochruros) show changes in androgens, corticosterone and behavior following repeated STIs of high or low intensity and in contrast to being challenged only once. Repeated intrusions had no influence on androgen and corticosterone levels regardless of intrusion intensity. In contrast, the behavioral response changed over days depending on the intensity of the intrusion. Only birds challenged with high-level intruders approached the decoy significantly faster during the third intrusion than during the first one, stayed closer to the decoy, and sang more songs than males challenged with low-level intruders. Thus, although black redstarts reacted differently to STIs varying in frequency and intensity, these behavioral differences were not reflected in androgen or corticosterone levels. Our data show that it is unlikely that STIs induce a losing experience. Furthermore, they indicate that a hormonal effect of winning an encounter may not be universal in vertebrates and may depend on the ecological or life-history context.