Natural versus sexual selection: predation risk in relation to body size and sexual ornaments in the green swordtail

In general, we assume that natural (predation-mediated) and sexual selection have opposing effects on the evolution of characters that serve as ornaments. Males of most swordtail fishes (genus Xiphophorus) have an elongation of the caudal fin known as the sword that is used to attract females, as it increases apparent body size of males. By increasing apparent body size, the sword may also attract the attention of predators or decrease the likelihood that a predator will attack. Using the green swordtail, Xiphophorus hellerii, we evaluated separately the effect of body size and the presence of the sword on the likelihood of being attacked by a predator. We conducted preference tests using a sympatric cichlid, Thorichthys ellioti, as a predator. For the effect of body size we used live pairs of male swordtails of different sizes without swords (surgically removed) as stimuli, and for the effect of the sword we used videos of male swordtails with and without their sword (digitally removed). We found no effect of body size, but the cichlid predators directed more bites towards individuals with swords. Despite their benefits in terms of attracting mates, the sword seems to increase the risk of being attacked by a predator and thus represents a trade-off between natural selection (mediated by predation) and sexual selection.