Testosterone implants increase song but not aggression in male Lapland longspurs

Lapland longspurs, Calcarius lapponicus, are tundra-nesting arctic birds with an extremely short breeding season. Males show a pronounced brief peak of plasma testosterone early in the season. We studied the effect of exogenous testosterone on song and aggression in free-living male Lapland longspurs. We gave high-testosterone implants to 16 birds, low-testosterone implants to seven birds, and empty or no implants to 33 birds. The implants resulted in significantly different plasma testosterone levels during incubation (high-testosterone mean=20.79+/-3.4 ng/ml, low-testosterone mean=2.54 +/- 0.5 ng/ml, control mean=0.53 +/- 0.1 ng/ml). High-testosterone birds had larger cloacal protuberances than controls, but low-testosterone birds did not. We measured aggression and song with simulated territorial intrusions during incubation, a mean of 12.03 +/- 1.45 days after the insertion of implants. High-testosterone birds and low-testosterone birds were no more aggressive than controls. However, high-testosterone birds, but not low-testosterone birds, were significantly more likely than controls to sing. These results suggest that the high, brief testosterone peak of Lapland longspurs may be related more to song than to territorial aggression. Alternatively, both aggression and song may be testosterone-influenced, but aggression may be more readily suppressed during incubation. The hormone-behaviour patterns demonstrated in this study may be adaptations for breeding in the short arctic summer. (C) 1997 The Association for the Study of Animal Behaviour.