Testosterone and year-round territorial aggression in a tropical bird

Testosterone (T) regulates avian behaviors such as song and aggression duri ng the breeding season. However, the role of T in year-round territorial bi rds is still enigmatic, especially in tropical birds. Spotted antbirds (Hyl ophylax 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 pe riods of social challenges. We manipulated T action in captive male Spotted antbirds to test whether this hormone is involved in the regulation of son g and aggression. T-implants increased plasma androgen levels (T and dihydr otestosterone) 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 b inding to androgen receptors and its conversion into estradiol by the enzym e aromatase. For this, we administered the androgen receptor antagonist flu tamide (Flut) in combination with the aromatase inhibitor 1-4-6 androstatri en-3,17-dione (ATD) to birds in breeding condition. ATD + Flut treatment si gnificantly elevated plasma levels of luteinizing hormone, presumably via t he lack of T feedback from its receptors. ATD + Flut-treated birds gave few er 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 beha vior in these tropical birds. Although plasma T levels can be low year-roun d, Spatted antbirds may use T either by secreting it briefly during social challenges, by having a high sensitivity to T action, or by enzymatically c onverting circulating T precursors directly at the site of action. We hypot hesize that: plasma T levels are kept low in these year-round territorial b irds to avoid potentially detrimental effects of tonically elevated T. Futu re treatment of nonbreeding birds with ATD + Flut will reveal whether T is indeed involved year-round in regulatin aggressive behavior. (C) 2000 Acade mic Press.