HOW TO COMPENSATE FOR COSTLY SEXUALLY SELECTED TAILS - THE ORIGIN OF SEXUALLY DIMORPHIC WINGS IN LONG-TAILED BIRDS

Recent work on birds suggests that certain morphological differences b etween the sexes may have evolved as an indirect consequence of sexual selection because they offset the cost of bearing extravagant ornamen ts used for fighting or mate attraction. For example, long-tailed male sunbirds and widowbirds also have longer wings than females, perhaps to compensate for the aerodynamic costs of tail elaboration. We used c omparative data from 57 species to investigate whether this link betwe en sexual dimorphism in wing and tail length is widespread among long- tailed birds. We found that within long-tailed families, variation in the extent of tail dimorphism was associated with corresponding variat ion in wing dimorphism. One nonfunctional explanation of this result i s simply that the growth of wings and tails is controlled by a common developmental mechanism, such that long-tailed individuals inevitably grow long wings as well. However, this hypothesis cannot account for a second pattern in our data set: as predicted by aerodynamic theory, w e found that, comparing across long-tailed families, sexual dimorphism in wing length varied with tail shape as well as with sex differences in tail length. Thus, wing dimorphism was generally greater in specie s with aerodynamically costly graduated tails than in birds with cheap er, streamer-shaped tails. This result was not caused by confounding p hylogenetic effects, because it persisted when phylogeny was controlle d for, using an independent comparisons method. Our findings therefore confirm that certain aspects of sexual dimorphism may sometimes have evolved through selection for traits that reduce the costs of elaborat e sexually selected characters. We suggest that future work aimed at u nderstanding sexual selection by investigating patterns of sexual dimo rphism should attempt to differentiate between the direct and indirect consequences of sexual selection.