Opposite effects of learning cause asymmetric mate preferences in hybridizing species

How do females decide which males to accept or reject as potential mates when the individuals encountered are unfamiliar and may be either heterospecifics or conspecifics? Learning often influences the development of mate preferences. Experience with particular phenotypes often positively biases preference for that phenotype. However, experience can also induce aversion. We studied the effect of short-term experience with unfamiliar conspecific, heterospecific, or hybrid males on mate preferences of females of 2 swordtail fish species with native habitats,which differ in both ecology and effective population size. After exposure to males for a week, we tested the females' preferences for male olfactory cues. Both species shifted their mate preferences, but in opposite directions. Female Xiphophorus.birchmanni, living in larger populations, increased their preference for familiar phenotypes, whereas female X. malinche, from smaller, island-like populations, showed an inverse effect of familiarity, namely a decreased preference for newly familiarized males. The pattern of opposite effects of learning on mate choice mirrors with that seen in the evolution of reinforcement of genetic preferences in continental and island populations. Diametrically opposed shifts in preference can thus arise from the same social experience, causing asymmetry in the species' conspecific mate preferences.