According to Stockley (2020) Promiscuous mating by females leads to male competition. In some species, males are able to deposit a mating plug during copulation. This study revealed that the 'mating plug' simultaneously inhibits the sperm of rival males while promotes the transports their own sperm without having to physically mate guard the female.
Stockley, P., Franco, C., Claydon, A. J., Davidson, A., Hammond, D. E., Brownridge, P. J., ... & Beynon, R. J. (2020). Revealing mechanisms of mating plug function under sexual selection. Proceedings of the National Academy of Sciences, 117(44), 27465-27473.