Effect of testosterone on male traits and behaviour in juvenile pheasants

Recent studies have shown that female choice and male-male interactions in the common pheasant (Phasianus colchicus) are influenced by features of som e male ornaments. It is still not clear what females gain by choosing bette r ornamented males nor why the aggressiveness of a male decreases when he e ncounters a better ornamented opponent. In this paper we report the results of the administration of testosterone (T) to 36 juvenile male pheasants. T he aim of the study was to investigate the extent to which T affects the ma le traits that have an influence on female choice and male-male competition . We found that T positively affected the vertical size of the wattle and b ody weight, but had no effect on the spurs. T administration also affected male agonistic behaviour; as expected, it increased male aggressiveness, ma le rank and the number of male-male interactions. Our data support the idea that vertical wattle size can be a reliable phenotypic indicator of male a ndrogen levels. Thus in view of the immunocompetence handicap hypothesis, t his male ornament can signal the male's condition to conspecifics.