Rates of signal evolution are associated with the nature of interspecific communication

Abstract

Some signals vary greatly, whereas others are remarkably similar across distantly related species. Here, we ask how the suite of receivers and information communicated correlates with signal evolution by comparing 2 different signals across the same set of species. Within the Old World leaf warblers (Phylloscopidae), each species utters 2 acoustically distinct alarm calls. The first, termed a "general" call, is used in interactions with conspecifics as a well as during confrontations with predators and nest-parasitic cuckoos. The second, termed a "rasp" call, is primarily used in the presence of nest-parasitic cuckoos. The rasp call precedes aggressive attacks on cuckoos and attracts surrounding heterospecifics that are also potential hosts. The general call attracts a wide range of species threatened by predators, including those that are not cuckoo hosts. Acoustic features of general calls evolve >5× faster than rasp calls. We argue that rasp calls show strong stasis because they have a restricted function as aggressive antiparasite signals, whereas multiple contexts and receivers have promoted divergence in general calls. These results support the idea that variation in the suite of receivers is a powerful force affecting signal evolution.