In species with internal fertilization, females can favour certain males over others, not only before mating but also within the female’s reproductive tract after mating. Here, we ask whether such directional post-mating (that is, cryptic) female mate choice can also occur in species with external fertilization. Using an in vitro sperm competition experiment, we demonstrate that female ovarian fluid (ovarian fluid) changes the outcome of sperm competition by decreasing the importance of sperm number thereby increasing the relative importance of sperm velocity. We further show that ovarian fluid does not differentially affect sperm from alternative male phenotypes, but generally enhances sperm velocity, motility, straightness and chemoattraction. Under natural conditions, female ovarian fluid likely increases the paternity of the preferred parental male phenotype, as these males release fewer but faster sperm. These results imply females have greater control over fertilization and potential to exert selection on males in species with external fertilization than previously thought possible.

Males that exhibit alternative reproductive tactics (ARTs) often differ in the risk of sperm competition and the energetic trade-offs they experience. The resulting patterns of selection could lead to between-tactic differences in ejaculate traits. Despite extensive research on male ARTs, there is no comprehensive review of whether and what differences in sperm traits exist between male ARTs. We review existing theory on ejaculate evolution relevant to ARTs and then conduct a comprehensive vote-counting review of the empirical data comparing sperm traits between males adopting ARTs. Despite the general expectation that sneaker males should produce sperm that are more competitive (e.g. higher quality or performance), we find that existing theory does not predict explicitly how males adopting ARTs should differ in sperm traits. The majority of studies find no significant difference in sperm performance traits between dominant and sneaker males. However, when there is a difference, sneaker males tend to have higher sperm performance trait values than dominant males. We propose ways that future theoretical and empirical research can improve our understanding of the evolution of ejaculate traits in ARTs. We then highlight how studying ejaculate traits in species with ARTs will improve our broader knowledge of ejaculate evolution. This article is part of the theme issue ‘Fifty years of sperm competition’.

Sexual selection has long been acknowledged as an important evolutionary force, capable of shaping phenotypes ranging from fascinating and unusual displays to cryptic traits whose function is only uncovered by careful study. Yet, despite decades of research, reaching a consensus definition of the term ‘sexual selection’ has proved difficult. Here we explore why arriving at a unifying definition of sexual selection is so hard. While some researchers have argued about whether sexual selection should be considered a form of natural selection, we concentrate on where the line between sexual selection and other forms of selection falls. We focus on identifying the ‘grey zones’ of sexual selection by illustrating cases in which application of the term ‘sexual selection’ would be considered controversial or ambiguous. We believe that clarifying why sexual selection is so difficult to define is an essential first step forward towards greater clarity, and if possible towards reaching a consensus definition. We suggest that a more nuanced perspective may be necessary, particularly one that specifies for cases of ‘sexual selection’ why the term is used or whether they fall into a grey zone.

Individuals should defend sites when the expected benefits of the territory exceed the cost of defense. However, if territory qulaity is unpredictable or difficult to assess, the expected pattern of territorial behavior is less clear. In a Mediterranean wrasse, Symphodus ocellatus, mating success is skewed with 2% of nesting males getting more than 20% of the spawning success. Yet, variation in mating success is not explained by any known physical characteristic of males or their territories. Instead, females prefer nests with a recent history of mating success because males are less likely to desert the offspring she leaves behind. Thus, territory quality is transient and determined by interactions between the sexes. I measured the frequency of territorial takeovers and the uncertainty in mating success among days at a nest. Observations indicated that S. ocellatus males usurped their neighbor's successful nests when males were unsuccessful and larger than their successful neighbor. Sites that achieved mating success had a significantly higher probability (0.84) of remaining sucessful between consecutive days than unsuccessful territories had of becoming successful (0.30). Unsuccessful males obtained higher and more certain fitness returns if they usurped a successful neighbor's territory. Interactions within and between the sexes drive uncertainty in success, which influences territorial behavior in this species.