Sexual selection has often been invoked to explain extravagant morphological and behavioural adaptations that function to increase mating success. Much is known about the effects of inter-sexual selection, operating through female mate choice, in shaping animal signals. The role of intra-sexual selection has been less clear. We report the first evidence for the co-evolution of signal complexity and sexual size dimorphism (SSD), which is characteristically produced by high levels of male-male competition. We use two complementary comparative methods to reveal that the use of complex signals is associated with sexual size dimorphism in extant species, and that historical increases in complexity have occurred in regions of a phylogenetic tree characterised by high levels of pre-existing size dimorphism. We suggest that signal complexity has evolved to improve opponent assessment under conditions of high male-male competition. Our findings suggest that intra-sexual selection may play an important, and previously underestimated, role in the evolution of communicative systems.

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