Signal Evolution and Species Coexistence: Visual Signals as Mediators of Interspecific Interactions

Abstract

Species coexistence is a key step in diversification, so learning about the traits that allow closely related species to live together is important for understanding the origin and maintenance of biodiversity. Signals, specifically, can allow individuals to avoid costly interactions, but we still have much to learn about between-species signaling. My work focuses on signal evolution in birds, asking which signals play important roles in encounters with members of other, closely related species, and addressing how these signals reduce unfavourable aggressive and mating interactions. Here, I use citizen scientist-captured photographs and videos to broadly characterize the visual signals that birds use in aggressive interactions with other species (chapter 2). I find that birds often use the same signals in between-species aggressive interactions as they do when competing with members of their own species, and that these signals often emphasize traits associated with aggressive intent or fighting ability, like weapons and size, which can act as a shared language among species. Next I conduct a comparative analysis to ask whether colour pattern differences are associated with between-species dominance status, exploring a means by which information beyond species membership might be conveyed through signaling (chapter 3). I find that black colouration, in particular, is associated with between-species behavioural dominance, especially in regions that appear to be most important for aggressive signaling. Finally, I test the contributions of competing alternative hypotheses in driving colour pattern divergence among co-occurring species using tightly controlled field experiments (chapters 4 and 5). These experiments suggest that increased colour pattern divergence among sympatric species is more likely to reduce the frequency of mixed matings than the frequency of between-species aggression. Together, this work shows that similar selective pressures may drive both within and between-species signal evolution and paints a new picture of the key role that visual signals play in mediating interspecific interactions among coexisting species, thus influencing community structure and ongoing diversification.