Effect of male body size on sperm precedence in the polyandrous butterfly Pieris napi L. (Lepidoptera: Pieridae)

Male Lepidoptera produce an ejaculate during copulation that contains both sperm and accessory gland nutrients and may function as paternal investment and/or male mating effort. Several studies have examined how ejaculates function as paternal investment, but few have determined the influence of sperm competition on male investment. This study examines the effect of male body size on sperm precedence in the polyandrous butterfly Pieris napi L. We used male body mass as an indicator of the size of ejaculate transferred and found that relative male size had a significant effect on paternity. The offspring of twice-mated females showed a low incidence of mixed paternity. Larger males obtained the majority of fertilizations, and the degree of second-male sperm precedence was influenced by relative body size of mating males. In general, second mates obtained fewer fertilizations the larger the size of the first mate. The interval between the first and second mating was influenced by the size of the first male mate. Females first mated to small males remated sooner than females first mated to large males. Our results suggest that large males may have a selective advantage over small males when both a male's fertilization success and a female's refractory period are influenced by the size of ejaculate transferred. Furthermore, the effect of male body size on the proportion of offspring sired lends support to the hypothesis that sperm competition has played a major role in the evolution of ejaculate size.