Sexual ornament size and the cost of fluctuating asymmetry

Given that extravagant secondary sexual characters, i. e. ornaments, give males a mating advantage through male-male competition or female choice, all males should benefit from growing very large ornaments. If sexual characters are to function as reliable advertisements of male quality, there must be a limit for males in their ability to grow or maintain large ornaments because of associated fitness costs, which may be more prominent in low than in high quality individuals. I used fluctuating asymmetry in ornament size as a measure of the ability of individuals to achieve identical development of the ornament on both sides of their body. Long-tailed males of the sexually size dimorphic swallow Hirundo rustica had less asymmetrical tails than had shorter-tailed individuals. The cost of fluctuating asymmetry in tail size in terms of manoeuvrability during flight was estimated directly by manipulating the degree of asymmetry and thereafter recording the number of times swallows hit semi-partitions in a flight cage. Both the degree of asymmetry caused by experimental treatment as well as the natural degree of asymmetry negatively affected manoeuvrability during flight, suggesting that asymmetry per se and inherent differences between individuals affected their flight performance. Female mate choice may be optimized by relying on fluctuating asymmetry in male ornaments, such as the tails of male swallows, being likely to reflect individual properties of direct fitness value.