Testosterone and year-round territorial aggression in a tropical bird.

Testosterone (T) regulates avian behaviors such as song and aggression during the breeding season. However, the role of T in year-round territorial birds is still enigmatic, especially in tropical birds. Spotted antbirds (Hylophylax n. naevioides) defend territories in the Panamanian rainforest year-round but have low plasma T levels (0.1-0.2 ng/ml), except during brief periods of social challenges. We manipulated T action in captive male Spotted antbirds to test whether this hormone is involved in the regulation of song and aggression. T-implants increased plasma androgen levels (T and dihydrotestosterone) and enhanced song in nonbreeding males. During a staged male-male encounter, T-implanted males sang more and were more aggressive than controls. In a second experiment, we blocked the two known T actions: its binding to androgen receptors and its conversion into estradiol by the enzyme aromatase. For this, we administered the androgen receptor antagonist flutamide (Flut) in combination with the aromatase inhibitor 1-4-6 androstatrien-3, 17-dione (ATD) to birds in breeding condition. ATD + Flut treatment significantly elevated plasma levels of luteinizing hormone, presumably via the lack of T feedback from its receptors. ATD + Flut-treated birds gave fewer spontaneous songs than control-implanted males. During staged male-male encounters, ATD + Flut-treated males did not sing at all and showed reduced aggressive behavior. Our data indicate that T can regulate aggressive behavior in these tropical birds. Although plasma T levels can be low year-round, Spotted antbirds may use T either by secreting it briefly during social challenges, by having a high sensitivity to T action, or by enzymatically converting circulating T precursors directly at the site of action. We hypothesize that plasma T levels are kept low in these year-round territorial birds to avoid potentially detrimental effects of tonically elevated T. Future treatment of nonbreeding birds with ATD + Flut will reveal whether T is indeed involved year-round in regulating aggressive behavior.