Reconstructing the Evolution of Sexual Dichromatism: Current Color Diversity Does Not Reflect Past Rates of Male and Female Change

Abstract

Males of sexually dimorphic species often appear more divergent among taxa than do females, so it is often assumed that evolutionary changes have occurred primarily in males. Yet, sexual dimorphisms can result from historical changes in either or both of the sexes, and few previous studies have investigated such patterns using phylogenetic methods. Here, we describe the evolution of male and female plumage colors in the grackles and allies (Icteridae), a songbird clade with a broad range in levels of sexual dichromatism. Using a model of avian perceptual color space, we calculated color distances within and among taxa on a molecular phylogeny. Our results show that female plumage colors have changed more dramatically than male colors in the evolutionary past, yet male colors are significantly more divergent among species today. Historical increases in dichromatism have involved changes in both sexes, whereas decreases in dichromatism have nearly always involved females evolving rapidly to look like males. Dichromatism is also associated with mating system in this group, with monogamous taxa tending to exhibit relatively low levels of sexual dichromatism. Our findings suggest that, despite appearances, female plumage evolution plays a more prominent role in sexual dichromatism than is generally assumed. K E Y W O R D S :Icterid, phylogeny, plumage evolution, sexual dimorphism, sexual selection, vision.

Rethinking our assumptions about the evolution of bird song and other sexually dimorphic signals

“…This again parallels studies of sexual dichromatism in birds, which show that striking male-female color differences are often the outcome of past losses of conspicuous plumage colors in females rather than gains of these colors in males (Irwin, 1994;Burns, 1998;Wiens, 2001;Hofmann et al, 2008;Price and Eaton, 2014). Dramatic female changes have also occurred when dichromatism has decreased, with females evolving rapidly to look like males and males changing comparatively little over time (Johnson et al, 2013;Price and Eaton, 2014). Based on these patterns, one might assume that selection on females has been stronger than selection on males.…”

“…The New World blackbirds again provide an example. Although many species in this clade are considered sexually monochromatic (Jaramillo and Burke, 1999), nearly all exhibit at least some sexual color differences based on thresholds of avian color discrimination, and across taxa the sexes vary from nearly indistinguishable to strikingly different along a continuous range (Hofmann et al, 2008;Price and Eaton, 2014).…”

“…As an example, numerous studies show that the songs and conspicuous red epaulets of male redwinged blackbirds (Agelaius phoeniceus) are important sexuallyselected signals that clearly influence male success (Searcy and Yasukawa, 1995). Nevertheless, phylogenetic analyses including other species in the Agelaius genus suggest that the buzzy songs, brightly colored epaulets, and glossy black plumage of males are ancestral traits that have changed relatively little since the common ancestor of this clade (Barker et al, 2008;Price and Eaton, 2014). In contrast, female vocalizations and colors differ substantially among species (Jaramillo and Burke, 1999), some appearing very similar to males (A. assimilis, A. xanthomus, A. humeralis) and others strikingly different (A. phoeniceus, A. tricolor), reflecting significant past changes.…”

“…Phylogenetic reconstructions of male and female plumage color evolution show that the sexes exhibit very different modes of change, with male colors diverging steadily over time and female colors changing periodically and relatively rapidly either away from or toward male patterns, resulting in respective increases or decreases in dichromatism (Johnson et al, 2013;Price and Eaton, 2014). These sex-specific evolutionary modes might reflect different mechanisms of selection, with male colors continuously changing through sexual selection (Prum, 1997;Price and Whalen, 2009) and female colors showing a more punctuated pattern of change through natural or social selection, perhaps based on levels of intra-sexual competition or the need for female crypsis near the nest (Martin and Badyaev, 1996;Amundsen, 2000;Soler and Moreno, 2012;Tobias et al, 2012).…”

“…The New World blackbirds have served as a model clade for investigating a variety of such sexual dimorphisms, including the evolution of male-female differences in color (Irwin, 1994;Hofmann et al, 2008;Friedman et al, 2009;Price and Eaton, 2014), size (Webster, 1992), and song (Price, 2009;Odom et al, 2015). Here I focus on this avian group in particular to examine how our new understanding of the evolutionary history of bird song compares with that of other sexually dimorphic traits, especially sexual dichromatism, and how these patterns line up with our past assumptions.…”