Effects of artificial selection for moving activity on male reproductive traits and leg length in the red flour beetle Tribolium castaneum

Movement has a profound effect on male reproduction. Males with higher levels of moving activity are predicted to have higher mating success, because they are encounter more females. Males with lower migratory activity and lower encounter rates with females are predicted to invest more in maximizing post-mating fertilization success than in increasing mating success. In insects, the legs are important appendages not only for movement, but also for mating, and sexual dimorphism in leg morphology has been observed in various species. In male legs, the morphology advantageous for movement may also be disadvantageous for grasping a female during mating, suggesting that male legs are formed via various selection pressures. Males with lower moving activity, which are likely to invest more in increasing fertilization success, are expected to exhibit different leg morphologies from males with higher moving activity. In this study, we investigated the response of reproductive traits and leg morphology to artificial selection for walking activity in the red flour beetle Tribolium castaneum Herbst. The results indicated that males from strains selected for lower moving activity had significantly longer legs than those from strains selected for higher moving activity. The former males may use their long legs to grasp females, thus prolonging mating. In addition, males selected for lower moving activity exhibited greater fertilization success, suggesting that long legs are important for male reproduction. Based on the results of our studies, I discuss the evolution of sexual size dimorphism of legs, and interspecific variation in the degree thereof.