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.

Ornaments in Females

In this chapter I want to explore the role of ornamental traits in females. They pose a bit of a conundrum, as they are not really predicted to exist—at least until recently. From a simple sexual selection point of view, female ornaments should be selected against by males because inconspicuous females suffer less predation and are more likely to care for their offspring successfully. Yet, countless species show ornaments in females. Are they adaptations of some kind or just the side effect of a genetic correlation? And what information do female ornaments convey to males?

Mechanism of Male Choice: Sex Ratios

In this short chapter I want to discuss the role of sex ratios in choosiness. So far, we have mostly reviewed intrinsic reasons for male choosiness to be expressed such as male investment and female quality; however, sex ratios may also be important drivers of choosiness. Sex ratios are important in population biology and influence the evolution and structure of mating systems. Most important for the purpose of this book is that they can change quickly in time and space. Male and female choice are sensitive to such changes and can lead to situations where females are choosy when they are rare in a population but change to courtship and competition when males are rare. There are not many examples of this process, but there are likely some that have been overlooked. Interestingly, the majority of data on preferences are collected using binary choice tests, which almost always represent a 2:1 sex ratio. Furthermore, sex ratios do not take into account differences in mate quality, as all adult individuals are classified as either male or female without making any further distinction.

Female–Female Competition

There is no disagreement that males compete over access to reproductive opportunities. This competition can be covert or take the form of physical fights, which are easily observed in many species. Males take considerable risks when they fight, presumably because the price—access to reproduction—is so high. But what about females? They also often compete for males, but in less risky ways. They also seem to compete less directly for individual males, but more indirectly for resources that males can provide. Nonetheless, female competition is more important in shaping sexual selection than previously thought.

Female and Male Mate Choice

Females choose mating partners for three main reasons: direct benefits, indirect benefits, and compatibility, either genetic or social. In this chapter I am not trying to look at all angles of mate choice, but to give a short overview of female choice to provide a basis for a comparison with male choice. This will highlight what studies are needed to reach a more complete picture of sexual selection. I would summarize the chapter like this: it’s the ecology, stupid.

Sexual Selection, Mate Choice, and Competition for Mates

Well over a century ago Charles Darwin redefined biology and introduced the theory of natural selection. One of the problems he encountered was the existence of traits, mainly in males, that seemed to defy the principles of natural selection: they did not aid its bearers in survival and were often outright detrimental. Darwin solved this conundrum by introducing sexual selection. Unlike natural selection where all individuals compete with each other for survival and reproduction, in sexual selection individuals within each sex compete with each other for reproduction. In the original formulation of the principle, Darwin recognized two mechanisms for this. Males would compete with each other for access to females, and females would choose mating partners of their preference. In this opening chapter I want to introduce the topics to be covered in the book, define some basic terms that we will need to understand the subject matter, and define the questions to be asked. My aim for this book is to summarize our growing, yet still comparatively limited empirical knowledge and theory, and to provide suggestions for future research. What interests me most is the relationship between the four forms of sexual selection and their consequences.

What Males Choose: Differences in Female Quality

In female choice, differences in male quality are very important. Males display to females to provide information often via costly ornaments. Females also differ in quality, but what they display to males is less clear. Also, how males evaluate differences in female quality is not well understood. From the literature on male mate choice one might conclude that female fecundity is the most important feature a female can display to a choosy male, but I argue that there must be many more features of females that are important in male mate choice, maybe even indirect benefits.