Meeting Abstract
Complex animal displays evolve amidst a tug-of-war between multiple evolutionary processes, each of which has profound consequences on phenotypic elaboration and variability. Chief among these are sexual selection, which typically favors signal elaboration. However, morphological and physiological constraints can simultaneously limit elaboration and variability. How does sexual selection contend with constraint to shape signal design? Studies that address this question typically do so only by assessing how sexual selection or morphological constraint influence signal evolution, but rarely both. Here we examine how constraint and selection differentially act on multiple components of a complex social signal: woodpecker drumming. This highly-physical acoustic display is produced when individuals rapidly hit their bill on a hard surface, and is primarily used in territorial competition by over 200 species worldwide. Every species exhibits a unique combination of drum characteristics encompassing the number of beats in the drum (drum length) and how fast those beats are produced (drum speed). In this study, we explore these two drum characteristics each undergo a unique macroevolutionary trajectory. First, we find that drum speed is constrained by morphology—smaller species can produce faster drums than smaller ones—while drum length is not. As a consequence, drum speed is significantly less variable than drum length on within- and between-species scales. Moreover, intrasexual selection appears to preferentially elaborate drum length, which is unconstrained by morphology. Taken together, these results offer a remarkably clear look at how constraint and selection interact when shaping a complex display: when constraint closes one route to phenotypic elaboration, sexual selection instead operates on a less-constrained signal component.
