Evolution of sexual communication in a swordtail fish, Xiphophorus birchmanni

Both sexual selection and environmental pressures shape communication systems. I explored the evolution of sexual communication in a swordtail fish, Xiphophorus birchmanni (Pisces: Poeciliidae). This species uses both visual and chemical signals to detect, assess, and compete for mates, and is greatly influenced by its trophic, physical, and social environments. The first set of experiments investigates how resource availability affects mate choice. Females preferred pheromones of well-fed males to food-deprived males, and this preference was specific to potential mates, and not conspecific females. Female mating behavior is also related to her access to resources: hungry females showed stronger mating preferences for well-fed males, and responded faster to cues in the environment. In the second set of experiments, I examined the role of the chemical environment on olfactory detection of cues and species recognition. Females use chemical cues to avoid mating with heterospecifics; however, when tested in water from disturbed streams or containing supplemental humic acid, females no longer discriminated between cues. The effect of the humic acid persisted days after initial exposure, and did not affect their motivation to mate. Anthropogenic disturbance of natural streams may thus have led to hybridization with X. malinche because mechanisms of species recognition were hindered. The third set of experiments focused on how communication has evolved in differing social climates with respect to intraspecific interactions. I identified three visual traits used by females in mate assessment: body size, barring pattern, and dorsal fin size. The combination of female preferences for these traits, however, was in conflict with allometric growth of males, as the most preferred male phenotype does not exist in nature. Further experiments showed that selection for male traits is greatly influenced by male competition. Large dorsal fins were avoided by females but were used to intimidate potential rivals, and thus were displayed during courtship.