Modularity in the evolution of visual signals associated with aggressive displays

Abstract

Interactions between conspecifics commonly involve the use of stereotyped display movements, which can vary markedly between species. Theoretically, sexual selection by female choice can lead to large differences between species, but sexual selection by male competition may result in more limited diversification. Here, we evaluate display evolution in the aggressive signals of 10 leaf warbler species. Using high-speed videography of territorial behavior, we quantify differences in wing motion intensity and form. We find that both the rate of wing motion and the form of the display remain similar across species, which we attribute to an effective signal maintained through multiple speciation events. Differences among species arise though discrete additions to the behavioral repertoire (three species), loss of display (one species) and the presence of a pale patch on the wing. While some habitats differ discretely and dramatically in light intensity, this cannot account for all the differences in display behavior. We conclude that display evolution proceeds largely in a modular fashion. The basic conventional signal is maintained across species, enabling modifications to appear without loss of efficacy.