One size doesn't fit all: female-male interactions on the path to fertilization

Extensive research indicates that fertilization outcomes are shaped by individual female and male traits that reflect their intrinsic quality. Yet, surprisingly little is known about the influence of interactions between the sexes and their adaptive significance in either externally or internally fertilizing species. Here, we review empirical evidence on how female-male interactions influence each stage of the fertilization process, including sperm transfer, transport, storage, chemoattraction and fertilization. We also address the challenges of examining female-male interaction effects within a realistic biological context and why research in this area lags behind understanding the role of individual sex-specific traits. While relatively little data are currently available to address interactive effects between the sexes and their impact on the fertilization process, what is presently known suggests these effects are likely to be more common across the animal tree of life than appreciated. Future research will help to not only identify these interactions, but their understanding can also help to explain the maintenance of genetic variation and inform applied studies of fertility.