A. Balmford et al., HOW TO COMPENSATE FOR COSTLY SEXUALLY SELECTED TAILS - THE ORIGIN OF SEXUALLY DIMORPHIC WINGS IN LONG-TAILED BIRDS, Evolution, 48(4), 1994, pp. 1062-1070
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.