Experimental analyses of sexual and natural selection on short tails in a polygynous warbler

We believe that no experimental study has yet tested Darwin's idea that, as well as generating trait elaboration, intersexual selection might sometime s drive sex-biased trait reduction. Here we present the results of two expe riments exploring the negative relationship between tail length and reprodu ctive success in male golden-headed cisticolas (Cisticola exilis). In the f irst experiment, artificially shortening a male's tail produced a dramatic increase in his reproductive success, measured as either the number of fema les nesting or number of chicks fledged on his territory. A second experime nt, in which manipulated birds were flown through a maze, revealed that sho rt tails also impose costs by reducing aerodynamic performance during slow- speed foraging flight. Because tail shortening yields reproductive benefits and viability costs, we conclude it has evolved via sexual selection. Dise ntangling exactly how short tails enhance male reproductive success is more difficult. Male-male competition appears partly responsible: aerodynamic t heory predicts that tail reduction enhances high-speed flight and, in line with this, shortened-tail males spent more time engaged in high-speed aeria l chases of rivals and defended higher-quality territories. However, shorte ned-tail males had higher reproductive success independent of territory qua lity and spent more lime in aerial displays which may be directed at female s. This suggests that tail shortening is also favoured via female choice ba sed on male phenotype.