We examined the function of the vertical bar pattern on male swordtails (Xiphophorus multilinneatus) as a signal in both male-male competition and female choice. This pattern had previously been described as an aggressive signal because males intensified the bars during male-male encounters in the laboratory. Our field observations supported this observation and also showed that bars intensified when males courted females. The intensity of bars was correlated with access to females in the field. Within the size range of males that have bars, however, neither bar number nor male size appeared to influence access to females. We used freeze-branding to remove the bars from males in the laboratory so that we could control for characters correlated with bar intensity, and tested males and females separately so that we could separate the influence of these two components of sexual selection. We compared the responses of males and females to males that had their bars removed and control males freeze-branded between the bars. Test males responded more aggressively to males without bars as compared to control males. In addition, females showed a preference for control males over males that had their bars removed. These results suggest that the bars may function as a signal that deters rival males and attracts females.
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Morris et al. (1995) studied this question.