Male–male competition and the sword in male swordtails, Xiphophorus helleri

Male green swordtails, Xiphophorus helleri, express a secondary sexual trait, the sword, which is favoured by female mate choice. However, the role of the sword as a signal in male–male competition has received little attention. We studied the effect of both male body size and sword length on the outcome of male–male competition. Although body size and natural sword length were positively correlated, only body size had a significant effect on competitive success. To examine the independent effect of sword length on competitive success, males were matched for body size and natural swords were replaced with plastic caudal attachments simulating swords. The attachments were of equal size but were painted to approximate swords of different lengths. When matched for body size, males with longer apparent swords won significantly more contests than males with shorter apparent swords. These data suggest that, controlling for body size, longer-sworded males experience greater competitive success. Thus, in green swordtails, it appears that both male–male competition and female choice may have played a role in the evolution of longer swords.