Intrasexual selection predicts the evolution of signal complexity in lizards

Sexual selection has often been invoked in explaining extravagant morpholog ical and behavioural adaptations that function to increase mating success. Much is known about the effects of intersexual selection, which operates th rough female mate choice, in shaping animal signals. The role of intrasexua l selection has been less clear. We report on the first evidence for the co evolution of signal complexity and sexual size dimorphism (SSD), which is c haracteristically produced by high levels of male-male competition. We used two complementary comparative methods in order to reveal that the use of c omplex signals is associated with SSD in extant species and that historical increases in complexity have occurred in regions of a phylogenetic tree ch aracterized by high levels of pre-existing size dimorphism. We suggest that signal complexity has evolved in order to improve opponent assessment unde r conditions of high male-male competition. Our findings suggest that intra sexual selection may play an important and previously underestimated role i n the evolution of communicative systems.