Male mate choice, female competition, and female ornaments as components of sexual selection

The significant role of post-pairing male behavior on the evolution of male preferences and female traits

Existing sexual selection theory postulates that a sufficiently large variation in female fecundity or other direct benefits are fundamental for generating male mate choice. In this study, we suggest that, in addition to pre-pairing preferences, choosy males can also have different post-pairing behaviors, a factor which has been comparatively overlooked by previous studies. We found that both male preferences and female traits could evolve much more easily than previously expected when the choosy males that paired with unpreferred females would allocate more efforts to seeking additional post-pairing mating opportunities. Furthermore, a costly female trait could evolve when there was a trade-off between seeking additional mating and paternal care investment within social pair for choosy males. Finally, a costly male preference and a costly female trait might still evolve and reach a stable polymorphic state in the population, which might give rise to a high variability in male choice and female traits in nature. We suggest that male mate choice may be even more common than expected, which needs to be verified empirically.

Lyu and colleagues use theoretical models to examine the relationship between post-pairing male behaviors and female trait evolution. They find that male mate choosiness can be surprisingly influential on the evolution of costly female traits, but also that costly male preference and costly female traits could evolve to a polymorphic equilibrium under certain conditions.

Validation of the three-chamber strategy for studying mate choice in medaka

The three-chamber experiment, in which one test animal can choose between two animals placed in physically inaccessible compartments, is a widely adopted strategy for studying sexual preference in animals. Medaka, a small freshwater teleost, is an emerging model for dissecting the neurological/physiological mechanisms underlying mate choice for which intriguing findings have been accumulating. The three-chamber strategy has rarely been adopted in this species; therefore, here we investigated its validity using medaka colour variants that mate assortatively. First, a total of 551 movies, in which a test male and two choice females interacted for 30 min under a free-swimming condition, were manually analysed. The sexual preference of the males, calculated as a courtship ratio, was highly consistent between human observers (r > 0.96), supporting the objectivity of this manual-counting strategy. Second, we tested two types of three-chamber apparatuses, in which choice fish were presented in either a face-to-face or side-by-side location. Test fish (regardless of sex) spent most of the time associating with choice fish in the compartments. However, their sexual preference, calculated as an association ratio, was poorly reproduced when the locations of the choice fish were swapped. Third, the sexual preferences of males quantified using the manual-counting and either of the three-chamber strategies did not correlate (r = 0.147 or 0.297). Hence, we concluded that, even for individuals of a species like medaka, which spawn every day, sexual preference could not be reliably evaluated using the three-chamber strategy. Optimization of the protocol may solve this problem; however, the explanation for the observation that animals that are ready for spawning persist with never-accessible mating partners must be reconsidered.

Considering Gender-Biased Assumptions in Evolutionary Biology

Many organisms studied by evolutionary biologists have different sexes, and the evolution of separate sexes and sexual dimorphisms in morphology and behaviour are central questions in evolutionary biology. Considering scientists to be embedded in a social and cultural context, we are also subjected to the risk of gender-biased assumptions and stereotypical thinking to appear when working on topics related to sexual reproduction and sexual dimorphism. Here we present, for continued discussion, a set of good-practice guidelines aimed at (1) helping to improve researchers’ awareness of gender-biased assumptions underlying language use, generalizations, and interpretation of observations; and (2) providing recommendations to increase transparency, avoid problematic terminology, and improve study designs.

Synthetic progestin etonogestrel negatively affects mating behavior and reproduction in Endler's guppies (Poecilia wingei)

High rates of progestins consumption in the form of active ingredients in women's oral contraceptives and other hormonal preparations may lead to their increased concentrations in aquatic environments and subsequent harmful effect on fish reproduction. The objective of the present study was to assess the effect of etonogestrel, a third-generation synthetic progestin, on the reproductive behavior, fertility, gonads histology, and secondary sexual characteristics of male and female Endler's guppies (Poecilia wingei). Fish were subjected for 34 days to two concentrations of etonogestrel, including one possibly environmentally relevant (3.2 ng L^-1) and one sublethal (320 ng L^-1) concentration. A mating behavior study was subsequently conducted and revealed that the treatment with etonogestrel significantly reduced mating frequency in the exposed fish compared to controls. All the exposed females were unable to reproduce. In addition, female fish exposed to the highest level of etonogestrel were masculinized, as their anal fins and body coloration showed patterns similar to those of male fish. Etonogestrel-exposed females also had fewer developed oocytes. In conclusion, the low etonogestrel concentration (3.2 ng L^-1) led to a reduction of mating activity in males without effect on their reproductive success, but it completely inhibited reproduction in females. Exposure to etonogestrel clearly has more severe consequences for females than males.