Mutual mate choice in the Asiatic toad, Bufo gargarizans, exerts stabilizing selection on body size

Sexual selection is critical to both reproduction and evolution. The effects of male–male competition and female choice regarding body size have been examined in a large number of taxa, including toad species. Males and females have different optimal reproductive strategies, achieving breeding advantages in discrepant ways. Further, the relative contributions of intra- and inter-sexual size selection vary among species. Thus, to understand the mechanisms affecting mating success, it is important to consider both male–male and male–female interactions simultaneously and elucidate their interrelationship. In this study, we measured body sizes of all mated and unmated individuals in a population of Asiatic toad (Bufo gargarizans) and counted fertilized eggs of several clutches. Based on correlation and regression techniques, we tested for female choice and intra-sexual competition among males relative to body size gradients, and we compared the relative importance of these two processes for mating success. Our results reveal that male–male competition and female choice simultaneously contribute to sexual selection in toads. Furthermore, both interactions are most intense among smaller toads. The synergistic trends of male–male competition and female choice support the mutual mate choice hypothesis and works to stabilize body size in B. gargarizans. Normal distributions of breeding success relative to body size were detected for both sexes, suggesting that medium-sized individuals enjoy a reproductive advantage in the population studied.