Most species in the genus Xiphophorus (swordtail fish) have males that vary greatly in body size. Females prefer the larger males for mating. One species, X. pygmaeus, was thought to lack large males, but, surprisingly, female X. pygmaeus still prefer large males: they are more attracted to big males of other species than they are to their own small males. Dr. Ryan recently discovered that large male X. pygmaeus exist at two sites, although all other sites lack large males. At sites without large males, females are more attracted to the large conspecific males from other sites than to the small males from their own sites. However, females from sites with large males did not show any discrimination based on male body size. Dr. Ryan and his colleagues will investigate the factors that cause large body size in this species; preliminary results suggest it does not stem from the same genetic mechanism that causes large size in other swordtails. The researchers will also attempt to determine why females at sites with large males do not prefer large males, as do females from other sites. The results will bear on several areas of animal behavior and evolution. They will indicate alternative mechanisms for how species evolve large body size and the consequences for behaviors correlated with body size. They will indicate if female mating preferences for traits absent in the species might exist, perhaps because these preferences were inherited from ancestral species in which males had the trait under question. The results will also indicate if such inherited preferences could be lost if these preferences begin to entail costs of mating. Finally, this research will provide badly needed information about the natural history and biogeography of a species that is a spectacular component of the Mexican fish fauna but whose range is restricted to a few small tributaries.
