Data supporting the study "Sperm performance in the race for fertilization, the influence of female reproductive fluid"

In studies of sperm competition, particularly in external fertilizers, the importance of the fertilisation environment on the paternity distribution among rival males often goes overlooked. The female reproductive fluid (FRF), produced and released by the females, creates the microenvironment that sperm encounter on their quest for fertilisation and can generate paternity biases by affecting key traits in sperm competition. Yet, whether there is a direct link between FRF effects on sperm traits and its effect on competitive fertilisation dynamics remains to be explored. Here, using the zebrafish <i>Danio rerio</i>, we compare within-female paternity share among two rival males and predictors of fertilisation success (i.e., sperm traits) in the presence/absence of FRF. Our results reveal a direct link between the direction and magnitude of the effect of FRF on sperm traits and the change in the competitive fertilisation success of each male. This study unequivocally demonstrates that the FRF directly mediates post-mating female control through its differential effect on sperm performance, and that the FRF’s effect on sperm quality alone is sufficient to predict the magnitude of the fitness effects. These findings urge to consider FRF in studies of fertilisation, to avoid biases resulting from an exclusive focus on sperm traits.