Spermatophore displacement and male fertilization success in the ground beetle

Sperm displacement is an effective male mating tactic of sperm competition to ensure paternity. However, the effectiveness of sperm displacement may depend on defensive mating tactics employed by rival males and also may be affected by females. To test this hypothesis, I conducted double-mating experiments with microsatellite paternity analyses using the ground beetle Carabus insulicola. First, I examined how often spermatophore displacement by the second male occurs. Spermatophore deposition success of the second male was high (85%) as was that of the first male (100%), in which 80 % of the second males displaced the first male’s spermatophore, suggesting that spermatophore displacement is a highly effective offensive tactic of sperm competition in C. insulicola. Second, I examined the effects of postcopulatory guarding by the first male and remating interval and sperm utilization pattern of the female on fertilization success of the second male. P2 values were moderate on average (0.30–0.57) and negatively correlated with postcopulatory guarding duration of the first male. Sperm use by the female was temporally nonrandom, such that sperm of the second male tended to be used first, although there was no significant effect of female-remating interval. These results suggest that the effectiveness of spermatophore displacement depends on postcopulatory guarding by the first male and the sperm utilization pattern of the female. These findings provide empirical evidence for the conflicting nature of selection for male offensive and defensive tactics in sperm competition and significant contribution of females therein. Key words: genitalia, microsatellite, P2, sexual selection, sperm competition. [Behav Ecol 18:628–634 (2007)] Sperm competition is a strong agent of sexual selection inpromiscuous species and results in various morphological