Abstract
Bird song is often cited as a classic example of a sexually-selected ornament, in part because historically it has been considered a primarily male trait. Recent evidence that females also sing in many songbird species and that sexual dimorphism in song is often the result of losses in females rather than gains in males therefore appears to challenge our understanding of the evolution of bird song through sexual selection. Here I propose that these new findings do not necessarily contradict previous research, but rather they disagree with some of our assumptions about the evolution of sexual dimorphisms in general and female song in particular. These include misconceptions that current patterns of elaboration and diversity in each sex reflect past rates of change and that levels of sexual dimorphism necessarily reflect levels of sexual selection. Using New World blackbirds (Icteridae) as an example, I critically evaluate these past assumptions in light of new phylogenetic evidence. Understanding the mechanisms underlying such sexually dimorphic traits requires a clear understanding of their evolutionary histories. Only then can we begin to ask the right questions.
Highlights
Male songbirds sing to attract mates and compete with rivals, and since Darwin these elaborate vocalizations have been widely acknowledged to have evolved through sexual selection (Darwin, 1859; Andersson, 1994)
Female singing might be widely underreported because many species with female song have sexually monomorphic plumage and year-round territorial defense by both sexes, so female singers could be frequently mistaken for males (Price et al, 2008)
Female song probably existed in the ancestor of all modern songbirds, suggesting that the absence or presence of female singing among many current taxa is the outcome of either past losses in females or the evolutionary maintenance of this behavior in both sexes (Odom et al, 2014). These findings call into question the long-standing assumption that sexual dimorphism in song is the outcome of sexual selection acting on males. Does this suggest that previous researchers, including Darwin himself, have been wrong about the evolution of male bird song through sexual selection? Here I contend that these new findings are surprising not because they contradict previous results but because they disagree with some of the assumptions we tend to make regarding the evolution of sexual dimorphisms in general and female song in particular, assumptions that may not be accurate
Summary
Male songbirds sing to attract mates and compete with rivals, and since Darwin these elaborate vocalizations have been widely acknowledged to have evolved through sexual selection (Darwin, 1859; Andersson, 1994). I contend that these new findings are surprising not because they contradict previous results but because they disagree with some of the assumptions we tend to make regarding the evolution of sexual dimorphisms in general and female song in particular, assumptions that may not be accurate These include: (1) viewing sexual dimorphism as a discrete rather than continuously variable attribute, and underestimating its prevalence across taxa; (2) conflating evolutionary rates of change with levels of selection; (3) conflating current levels of elaboration and divergence in each sex with past evolutionary rates; (4) assuming that levels of sexual dimorphism necessarily reflect levels of sexual selection; and (5) viewing decreases in dimorphism, in song or in other traits, as evolutionary gains in one sex rather than losses of sex-specific developmental mechanisms. Song may occur in both sexes but may reflect selection pressures and patterns of evolutionary change that are sex-specific
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