Tactical dimorphism: the interplay between body shape and mating behaviour in the swordtail Xiphophorus multilineatus (Cyprinodontiformes: Poeciliidae)

Alternative reproductive tactics (ARTs) are characterized by dimorphism for reproductive phenotypes (i.e. tactical dimorphism). Tactical dimorphism often evolves as a response to differing phenotypic optima for a shared reproductive trait between members of the same sex in a species. We characterized dimorphism for body shape between ARTs in the swordtail fish Xiphophorus multilineatus and investigated the relationship between body shape and mating behaviours. Xiphophorus multilineatus possesses two ARTs, a courter male that uses only courtship to attract females, and a sneaker male that is contextually plastic and uses both courtship and force copulatory sneak chases. We found that courter and sneaker males are dimorphic for body shape; however, we did not detect a significant difference in the allometric relationship between body shape and size. Additionally, we found that more fusiform sneaker males were more likely to use sneak-chase behaviour in the absence of a competitor. These results suggest that the ARTs in X. multilineatus have different phenotypic optima for body shape and that a fusiform body shape might be more optimal for sneak-chase behaviour. Understanding tactical dimorphism in this species will allow us to explore further how the ARTs evolved and the potential for intralocus tactical conflict to constrain the evolution of each ART from reaching their respective optima.