New insights from female bird song: towards an integrated approach to studying male and female communication roles

Abstract

Historically, bird song has been regarded as a sex-specific signalling trait; males sing to attract females and females drive the evolution of signal exaggeration by preferring males with ever more complex songs. This view provides no functional role for female song. Historic geographical research biases generalized pronounced sex differences of phylogenetically derived northern temperate zone songbirds to all songbirds. However, we now know that female song is common and that both sexes probably sang in the ancestor of modern songbirds. This calls for research on adaptive explanations and mechanisms regulating female song, and a reassessment of questions and approaches to identify selection pressures driving song elaboration in both sexes and subsequent loss of female song in some clades. In this short review and perspective we highlight newly emerging questions and propose a research framework to investigate female song and song sex differences across species. We encourage experimental tests of mechanism, ontogeny, and function integrated with comparative evolutionary analyses. Moreover, we discuss the wider implications of female bird song research for our understanding of male and female communication roles.

Females don’t always sing in response to male song, but when they do, they sing to males with higher pitched songs

“…Sexually monomorphic songs are expected to fulfil a similar function between the 376 sexes (Riebel et al, 2019), and thus could be used for both intrasexual and intersexual 377 territory defence. But in a species where the female sings a fraction of what the male 378 does, it seems she chooses carefully when she will use her song for the purpose of 379 intersexual territory defence.…”

“…The prevalence of female song in the tropics has been attributed to sex role 78 convergence in locations where birds are resident and defend territories year-round 79 (Slater and Mann, 2004), but this view has been challenged by studies suggesting 80 female song is ancestral in songbirds (Odom et al, 2014, Riebel et al, 2019. So 81 while songs may represent ornaments attractive to the opposite sex, their primary 82 function in females is thought to be in intrasexual competition in species defending 83 territories year-round from same-sex rivals (Tobias et al, 2011, Tobias et al, 2012b.…”

Analysis of female song provides insight into the evolution of sex differences in a widely studied songbird

“…Darwin, 1801, p. 396; Hedrick & Temeles, 1989; Lande, 1980; Ng et al, 2019). However, owing to historical biases, studies of the drivers of dimorphism in sexual signaling traits have traditionally focused on male signals, and most approaches assume that sexual selection has promoted dimorphism via elaboration of male traits (Alexander V. Badyaev & Hill, 2003; Freed, 2000; Langmore, 1998; Riebel, 2016; Riebel et al, 2005, 2019; Rosvall, 2011). Yet, meta-analyses of the strength of sexual selection on male traits report moderate effect sizes (Jennions et al, 2012), with evidence that males are often under variable selection pressures within and across breeding seasons (Chaine & Lyon, 2008; Kingsolver et al, 2012; Robinson et al, 2008; Steele et al, 2011), have trait values near optima (Evans, 1998; Rodríguez et al, 2006), or that the magnitude of trait dimorphism may not reflect the strength of current selection on males (Miller et al, 2016).…”

“…Indeed, dimorphism in signals can be caused by a range of selection pressures on both males and females, resulting in exaggeration or reduction of a variety of sex-specific signals (Bell & Zamudio, 2012a; Price, 2015a; Wiens, 2001). To better understand how divergent selection operating between the sexes drives dimorphism, we require holistic approaches that fully describe the traits of both males and females (Riebel et al, 2019). Ideally, studies should include multiple signaling traits that mediate inter- and intra-sexual interactions (Bro-Jørgensen, 2010; Hebets et al, 2016; Hebets & Papaj, 2004; Partan & Marler, 2005), as well as their fitness consequences.…”

“…Likewise, many studies have examined the fitness correlates of male song (Catchpole & Slater, 2003; Gil & Gahr, 2002), but only a handful have looked at how female song impacts fitness (Brunton et al, 2016; Cain et al, 2015; Kleindorfer et al, 2016b; Krieg & Getty, 2016a). Although integrated approaches can best untangle the relative contributions of selection on males and females to these signaling modalities (Hebets et al, 2016; Riebel et al, 2019), researchers have seldom included both visual and acoustic phenotypes from both sexes within the same studies. Moreover, to our knowledge, no study has directly tested whether the pattern of dimorphism in vocal and visualsexual signaling traits is better predicted by contemporary selection on males (as is often assumed) or females.…”