Sexually selected signal attracted females before deterring aggression in rival males

Many sexually selected male signals elicit responses in males and females and, as a result, are affected by selection due to both mate choice and competition for mates. It has been proposed that because signals used in aggressive interactions may also be reliable indicators of mate quality, these dual functioning traits initially evolved in the context of male–male competition and were later co-opted by females to assess potential mates. Our phylogenetic reconstruction of the ancestral states of male and female responses to the sexually selected trait ‘vertical bars’ in the northern swordtail fish, Xiphophorus spp., suggests that female response to the bars was present before male response to the bars evolved. This is the first evidence that males may have co-opted a signal for use in male competition that was initially used by females in mate choice. We suggest that dual functioning, sexually selected traits are as likely to initially evolve in the context of female preference as in the context of male–male competition. In addition, we propose that eavesdropping may be an important mechanism by which signals are co-opted, with behavioural syndromes potentially playing a key role in the transition of the signal from a context of courtship to one of aggression.