The asymmetrical cost of tail elongation in red-billed streamertails

Abstract

Ornaments used in signalling should vary in size with signaller quality. It has also been suggested that high-quality individuals should be less susceptible to asymmetrical development than low-quality individuals. It has been claimed that the degree of asymmetry in ornaments could be used by receivers to assess signaller quality. If these claims are correct, high-quality individuals should produce large symmetrical ornaments, and low-quality individuals small, asymmetrical ones. Negative relations between tail length and asymmetry have been demonstrated and taken as support for these predictions. However, these results could occur if individuals growing long tails were constrained to more symmetrical development. This mechanical constraints hypothesis predicts that the costs of a given amount of asymmetry should increase with increasing tail length and, when tails are asymmetrical, the costs of increasing tail length should rise faster than when symmetrical. To test these predictions, we manipulated tail streamer length and symmetry of male red-billed streamertails (Trochilus polymus) which were released through a maze forcing the birds to manoeuvre to avoid collision. We show that if streamer length is experimentally reduced then manipulations of symmetry have no effect on the ability of the birds to negotiate this maze. However, when streamers are experimentally elongated, the time taken to negotiate the maze and the number of collisions increase with the degree of the asymmetry manipulation. This demonstrates that individuals producing long tails may be forced into greater symmetry to minimize costs.