Evolutionary consequences of non-random mating: do large males increase offspring fitness in the anuran Bufo bufo?

Abstract

The purpose of my study was to determine whether male body size, a trait known to be important to mating success, covaries with offspring performance. I tested the effects of male body size on the performance of Bufo bufo tadpoles reared at two food levels by mating large, small, and naturally-mated males to the same females. Survival of tadpoles in the high-food environment was affected by male size class, but in the opposite way to that expected. Tadpoles sired by large males had the lowest survival, and those sired by small males the highest. Neither body size at metamorphosis nor larval period were affected by male size class alone, but male size interacted with the female contribution: tadpoles sired by large males had short larval periods and large size at metamorphosis with some females,but long larval periods and small body sizes with others. Food level had a significant effect on both size at metamorphosis and larval period, and interacted with female contribution, but not male size class. This indicated that female contribution to tadpoles was dependent on food level, but that the effects of male size were not differentially expressed by tadpoles at the two food levels. My results indicate that traits with a direct effect on offspring fitness are not enhanced by large male body size, yet some males and females produced offspring with significantly better performance. I suggest that evolutionary change in this mating system is unlikely to occur through the non-random mating of males based on body size alone.