Songs versus colours versus horns: what explains the diversity of sexually selected traits?

Abstract

Papers on sexual selection often highlight the incredible diversity of sexually selected traits across animals. Yet, few studies have tried to explain why this diversity evolved. Animals use many different types of traits to attract mates and outcompete rivals, including colours, songs, and horns, but it remains unclear why, for example, some taxa have songs, others have colours, and others horns. Here, we first conduct a systematic survey of the basic diversity and distribution of different types of sexually selected signals and weapons across the animal Tree of Life. Based on this survey, we describe seven major patterns in trait diversity and distributions. We then discuss 10 unanswered questions raised by these patterns, and how they might be addressed. One major pattern is that most types of sexually selected signals and weapons are apparently absent from most animal phyla (88%), in contrast to the conventional wisdom that a diversity of sexually selected traits is present across animals. Furthermore, most trait diversity is clustered in Arthropoda and Chordata, but only within certain clades. Within these clades, many different types of traits have evolved, and many types appear to have evolved repeatedly. By contrast, other major arthropod and chordate clades appear to lack all or most trait types, and similar patterns are repeated at smaller phylogenetic scales (e.g. within insects). Although most research on sexual selection focuses on female choice, we find similar numbers of traits (among sampled species) are involved in male contests (44%) and female choice (55%). Overall, these patterns are largely unexplained and unexplored, as are many other fundamental questions about the evolution of these traits. We suggest that understanding the diversity of sexually selected traits may require a shift towards macroevolutionary studies at relatively deep timescales (e.g. tens to hundreds of millions of years ago).

Animal Preferences: Implications of Sexual Selection Research for Empirical Aesthetics

“…This is an interesting question that does not, as far as I know, have a definitive answer. As it so happens, not all biological taxa do rely on mate choices (although almost all plants and animals do), and most species that evolve sexually selected traits appear to exhibit only a very limited number of traits and behaviors (Wiens & Tuschhoff, 2020). It remains unclear why this is case, although sexually selected traits seem more abundant in species with intense competition for access to reproduction, suggesting that certain conditions must be met for mate choice to evolve as a mechanism of sexual selection (Wiens & Tuschhoff, 2020).…”

Performance, but not size, of hindleg weaponry is sexually dimorphic in the giant mesquite bug (Thasus neocalifornicus)

“…Animal weapons are remarkably diverse structures, and have widely variable sizes and shapes within and between species (Emlen, 2008;Wiens & Tuschhoff, 2020). Bearing a large weapon usually increases the fitness of the sex that engages in territorial contests (Husak, Lappin, & Bussche, 2009;Lailvaux & Husak, 2014).…”

“…One of the most common forms of weapon sexual dimorphism comes from the disproportional increase in sizes of preexisting structures or appendages. For example, throughout Coleoptera, the males of many species have longer forearms and hindlegs used to grapple with competitors during aggression (Katsuki, Yokoi, Funakoshi, & Oota, 2014;Kojima & Lin, 2017;Rink, Altwegrg, Edwards, Bowie, & Colville, 2019;Wiens & Tuschhoff, 2020). Similar weapon size dimorphisms are common in other arthropods (e.g., crustacean claws, weevil rostrums) and vertebrates (e.g., tortoise gular horns, gorilla forearms), demonstrating the apparent competitive benefit of increased weapon size (Rico-guevara & Hurme, 2018; Vieira & Peixoto, 2013).…”

Genomic heritability of song and condition in wild singing mice

“…Few traits pose such interesting and challenging examples of phenotypic evolution as male sexual signals. Advertisement displays are present across diverse taxonomic groups (Wiens and Tuschhoff, 2020), vary dramatically within and among species (within: Dubuc et al, 2014; Izzo and Tibbetts, 2015; Karin et al, 2018; between: Ingram et al, 2016; Vanhooydonck et al, 2009), and evolve under both natural and sexual selection (Andersson, 1981; Endler, 1992). Heritable variation is not merely a substrate for this diversity but underlies common explanations for exaggerated male traits.…”