Social instability increases plasma testosterone in a year-round territorial neotropical bird

In many vertebrates, elevated levels of plasma testosterone (T) are importa nt for reproduction and territorial aggression. However, many tropical bird s reproduce and defend territories while plasma T-levels are basal. We stud ied how aggression and T-levels are regulated in male neotropical spotted a ntbirds, which defend territories year-round in the Panamanian rainforest. Although spotted antbirds reproduce seasonally, T-levels of individual male s often remained at baseline (0.2 ng ml(-1)) throughout the year, even in c ourting males. On the other hand, T-levels were elevated (maximally to 1.57 ng ml(-1)) during periods of social instability at any time of the year, e ven when males had entirely regressed gonads. Experimental territorial intr usions (broadcast of conspecific song) confirmed these observations by show ing that T-levels increased after about two hours of playback time. Our dat a suggest that spotted antbirds avoided the potential costs associated with constantly high plasma T-levels (e.g. increased mortality rates). Contrary to temperate-zone birds, spotted antbirds had the potential to react to so cial challenges with an increase of plasma T year-round. These results are, to our knowledge, presently unique, but may apply to many vertebrate speci es that inhabit the tropics.