Sex role similarity and sexual selection predict male and female song
elaboration and dimorphism in fairy-wrens
Historically, bird song complexity was thought to evolve primarily
through sexual selection on males, yet in many species both sexes sing.
Previous research suggests competition for mates and resources during
short, synchronous breeding seasons leads to more elaborate male songs
at high latitudes. In contrast, we expect male-female song dimorphism
and elaboration to be more similar at lower latitudes because longer
breeding seasons and year-round territoriality yield similar social
selection pressures in both sexes. However, studies seldom take both
selective pressures and sexes into account. We examined song elaboration
and sexual dimorphism in 15 populations of nine fairy-wren species
(Maluridae), a Southern Hemisphere clade with female song. We compared
song elaboration and sexual song dimorphism to latitude and life history
variables tied to sexual and social selection pressures and sex roles.
Our results suggest that song elaboration evolved in part due to sexual
competition in males: male song variability was more positively
correlated with temperate breeding and greater breeding synchrony than
female song. We also found strong evidence that sex-role similarity
contributed to male-female song similarity: male and female songs were
shorter and more similar when parental care was more equal and when male
survival was high. Contrary to Northern Hemisphere latitudinal patterns,
songs were less dimorphic at higher, temperate latitudes. These results
suggest that selection on song can be sex-specific, with male song
elaboration favored in contexts coincident with sexual selection.
However, selection pressures associated with sex-role similarity also
appear to constrain sex specific song evolution and song dimorphism.