• colour change;
  • sexual selection;
  • signal;
  • Xiphophorus

This paper reports a new aspect of male signalling in the green swordtail Xiphophorus hellerii. Males are highly ornamented and possess a sword (an extension of the caudal fin), as well as a lateral stripe that has been shown to be a badge of status. Males of one natural population, however, were found to quickly change the colour of their lateral stripe to red when socially dominant over other males and black when subdominant, thereby exploiting the female preference for red stripes while dominant, and avoiding the costs of male aggression while subdominant.