Latitudinal cline in reproductive traits in the red flour beetle Tribolium castaneum

Several previous studies have reported geographic variation and/or latitudinal clines of morphological sexual characteristics, but there are few studies that consider reproductive traits that are not morphological. Here, we measured the proportion of females fertilized by males, frequency of reproductive failure in males, and number of female copulations of the red flour beetle Tribolium castaneum collected from fields in Japan to investigate the relationship between reproductive traits and latitude. Our results show substantial differences in the reproductive traits of both sexes among field populations. We identified latitudinal clines for reproductive traits in males, but not females. Moreover, female, but not male, reproductive traits were correlated with body size. Our study suggests that selection for male reproductive traits varies with latitude in T. castaneum . Significance statement It has been established that latitudinal gradients are frequently observed in the traits of numerous animal species, of which body size is a prime example. Analogously, latitudinal gradients have been documented in the reproductive traits of several animal species, indicating that sexual selection may be influenced by latitude, although the specific understanding thereof remains elusive. In this study, we quantified the male and female reproductive traits and analyzed their relationship with latitude in the red flour beetle Tribolium castaneum . Our findings indicate that male mating efficacy was superior at lower than at higher latitudes, but no discernible latitudinal gradient was observed in female mating frequency. These results imply that selective pressure on males of this species is stronger at lower than at higher latitudes.