Mating partners often have conflicting interests when copulating [1-3]. One of the major agents affecting female mating partners is seminal fluid, transferred along with sperm. The role of seminal fluid proteins (SFPs) in reproductive success is well studied in separate-sexed animals [4, 5] but is much less so in simultaneous hermaphrodites . The latter potentially have a unique target to exploit for the sperm donor's own benefit: the male function of their mating partners [7, 8]. Here we show that, in the great pond snail Lymnaea stagnalis, receipt of specific SFPs reduces both sperm transfer and paternity success in a subsequent insemination event. Lowering investment in the mating partner's male function constitutes a novel role for SFPs. This demonstrates for the first time that hermaphrodites alter their mates' male as well as female reproductive output . Although it remains to be tested whether this represents mate manipulation or an adaptive response of recipients , our findings identify male investment as a new target for postcopulatory sexual selection . © 2014 Elsevier Ltd.