Natural and sexual selection often have differing effects on the expression of male characteristics. One characteristic that appears to be influenced by both types of selection is the sword in green swordtails. The sword is a colorful extension at the bottom of the tail fin, and only male swordtails have swords. Female swordtails prefer males with long swords; one reason for this preference is that their ancestor preferred swords, even before males expressed swords. Thus, females have a pre-established bias favoring swords. The sword also affects competition for mates; males with long swords more often win contests for females. Yet, long swords also appear to be disadvantageous to males; in natural populations in which there are fish predators, males have shorter swords than in populations without these predators, suggesting that long swords may increase a male's risk of predation. If swords are costly to males because they make males more conspicuous to predators, females that associate with males with long swords may be more vulnerable to predation than females who associate with males with short swords. It does appears that the presence of a predator can influence a female's interest in a male with a long sword; when females see a predator attack a male with a long sword, they decrease their interest in long swords, and will then spend more time with a male with a short sword. Thus, the preference for long swords shows behavioral plasticity. Predation could also affect the importance of the sword in contests between males for females. The goal is to more fully understand the role predation plays in modifying both male and female responses to the sword. This will be accomplished by investigating the response of predators, female swordtails, and male swordtails to long swords. Plasticity in the sword response will also be studied. This research is important because despite a growing body of evidence indicating that pre-established biases can affect the subsequent expression of conspicuous characteristics, little is knowledge about how such biases currently function. With the proposed research, the swordtail system holds promise to shed light on broad questions concerning not only plasticity in behaviors that are important in mating, but also plasticity in receiver responses in general.
This project investigated the response of predator fish to the colorful morphology of prey, and the antipredator behavior of male and female prey. This study suggests that predator fish respond to the greatest degree to male prey with the most elaborate expression of a morphological trait called a sword. A sword consists of a set of colorful rays towards the base of the caudal fin, which can grow beyond the caudal margin in adults. Sword length is highly correlated with male body length; larger males have longer swords. Adult females do not have swords. As predators respond most strongly to males with the longest swords, longer swords likely increase male predation risk. This study found that males reduce their level of activity when they have recently seen a dangerous predator, most likely in order to decrease the likelihood of a predatory attack. Females prefer males with longer swords, and this preference appears to stem from a preexisting bias. As swords are conspicuously colored, and cichlids prefer males with longer swords, females may be at greater risk when they are in association with long-sworded males compared to short-sworded males. It appears from this study that females decrease the amount of time spent with a longer-sworded male after observing a large predator nearby. Thus, the female preference for a long sword appears to change when a predator is near. This project contributes to our understanding of selection on conspicuous male traits. This project has broader impacts as well. Multiple graduate and undergraduate students received training during the course of this project. In addition, an education exhibit about predators and prey was developed and experienced by the general public in two venues: at the Wonderlab Children's Science Museum in Bloomington Indiana, and at DNA Day at the Henry Doorly Zoo in Omaha Nebraska.
