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.

The better, the choosier: a meta-analysis on inter-individual variation of male mate choice

Male mate choice occurs in several animal species, but we know little about the factors that influence the expression of this behaviour. Males vary in their capacity to acquire mates (i.e. male quality), which could be crucial to male mate choice expression but it is often overlooked. Using a meta-analytical approach, we explore inter-individual variation in the expression of male mate choice by comparing the mating investment of males of different qualities and phenotypes to high- and low-quality females. We used two datasets that together contained information from 60 empirical studies, comprising 52 species. We found that males of all qualities and phenotypes prefer high-quality females, but differ in the strength of such preference. High- and medium-quality males are choosier than low-quality males. Similarly, males that are larger or in greater body condition are choosier than their counterparts. In contrast, male body mass and age are not associated with changes in male mate choice. We also show that experimental design may influence our understanding of male mating investment patterns, which may limit the generalization of our findings. Nonetheless, we argue that male quality may be an important feature in the expression of male mate choice.

Colourful traits in female birds relate to individual condition, reproductive performance and male-mate preferences: a meta-analytic approach

Colourful traits in females are suggested to have evolved and be maintained by sexual selection. Although several studies have evaluated this idea, support is still equivocal. Evidence has been compiled in reviews, and a handful of quantitative syntheses has explored cumulative support for the link between condition and specific colour traits in males and females. However, understanding the potential function of females' colourful traits in sexual communication has not been the primary focus of any of those previous studies. Here, using a meta-analytic approach, we find that evidence from empirical studies in birds supports the idea that colourful female ornaments are positively associated with residual mass and immune response, clutch size and male-mate preferences. Hence, colourful traits in female birds likely evolved and are maintained by sexual selection as condition-dependent signals.

Male Mate Choice in Mosquitofish: Personality Outweighs Body Size

Background Despite its important implications in behavioural and evolutionary ecology, male mate choice has been little studied, and the relative contribution of personality and morphological traits remains largely unknown. Using standard two-choice mating trials, we studied whether personality traits (i.e. shyness and activity) and body size of both sexes affect mate choice in male mosquitofish Gambusia affinis. Results Both shyness and activity in males were significantly repeatable and constituted a behavioural syndrome. No overall directional preference for large (or small) females with the same activity levels was detected because larger males preferred larger females and smaller males chose smaller females. However, males spent more time associating with active females regardless of their body lengths and had an enhanced preference for inactive females when they increased activity levels. We also found that more proactive (bolder and more active) males had stronger preferences for more active females. Conclusions Our study supports the importance of body size in male mate choice but highlights that personality traits may outweigh body size preferences when males choose mating partners.

Synthesis and Outlook

In this final chapter I want to briefly recap what I presented in the previous chapters and provide a few ideas on what might be done in the future to move the field forward. All three factors I discussed as relevant in male mate choice—male investment in reproduction, sex ratios, and variability in partner quality—are still emerging fields in sexual selection research and need more theoretical and empirical work. I suggest that variability in female quality is more important and more complex than currently known. The same is true for sex ratios. On the other hand, I suggest that sheer investment in gametes may be a little less important than currently assumed. Most importantly we need to explore the interactions of these three pathways to male mate choice. Female competition and also female ornamentation are still somewhat enigmatic and both topics are likely to grow in importance for our understanding of sexual selection. I think considering male and female choice together, as well as female and male competition will ultimately provide a more complete picture of Darwinian sexual selection.

Examples of Male Mate Choice

In this chapter, what interests me most is how often male mate choice has already been documented, independent of the underlying mechanism. I am not concerned with the origin of the description: some authors express some degree of surprise that they found male preferences; other studies are motivated by theory. I also want to highlight that there is a continuum from no male contribution to the offspring to male contributions that are larger than the female contribution. Furthermore, there are differences in female quality at different levels, which can contribute to the evolution of male choice. There are many studies that infer differences in female fecundity as underlying male choice, but females can differ in many more aspects—just like males.

Male Investment and Male Choice

In this short chapter I revisit the role that male investment might play in the evolution of male mate choice. Simply put, one might expect with more male investment into reproduction, they may be more selective about with whom they mate. If the male investment is larger than the female investment sex roles are expected to flip. This is, however, not common. But even if males almost never invest as much or more than females, in many species they do invest somewhat, or even heavily. Should this lead to choosiness in males? I will provide a few examples where choosiness may be linked to investment in males.

Male Choice, Female Competition, and Female Ornaments in Sexual Selection

When Darwin first proposed sexual selection theory he suggested two mechanisms: competition among males and choice by females. There is no doubt that these mechanisms are immensely important, but their mirror images have been largely underappreciated so far. In fact, males choose as well and females compete. Males choose based on female quality, often selecting mating partners that are more fecund. But male choice is also associated with changes in the sex ratio of a population and males can be choosy when they are rare. Furthermore, males sometimes invest heavily into reproduction and that too can be associated with male choice. That females compete with another, although less often with open aggression, is another understudied phenomenon. Finally, we now know that females are often ornamented, but are these ornaments under sexual selection by males? This book tries to review what we know and point to what we don’t know while pointing out the connections between male mate choice and female competition for a more complete view of sexual selection.