Female reproductive fluid increases the opportunities for postmating sexual selection by prolonging the egg fertilization window

Female reproductive fluid (the fluid that surrounds the eggs) has attracted increasing attention for its role in fertilization and postmating sexual selection through its effects on sperm traits. Surprisingly, however, only a few studies have investigated the effects of the female reproductive fluid on the eggs. Yet these effects might offer great potential to affect fertilization dynamics by, for example, increasing the opportunities for postmating sexual selection. Here, we determined whether the female reproductive fluid, by extending the egg fertilization window (the time available for egg fertilization), could also increase the opportunities for multiple paternity. Using the zebrafish (Danio rerio), we first tested the prediction that female reproductive fluid increases the egg fertilization window; then, using a split-brood design with the sperm of two males added at different time points after egg activation, we tested whether the degree of multiple paternity varies in the presence or absence of female reproductive fluid. Our results reveal the potential of female reproductive fluid to increase multiple paternity through its effects on the egg fertilization window, thus broadening our knowledge of how female mechanisms affect postmating sexual selection in externally fertilizing species.