4. Sex roles and stereotypes

‘Sex roles and stereotypes’ examines the notion, implicit in many of the original ideas about sexual selection, that males and females have natural ‘roles’ with characteristic behaviour associated with each sex. It also explores further the reasons behind deviations from the ‘typical’ sex roles in mate choice and in mating competition. Are there ‘standard’ male and female roles in both humans and other animal species? One version of sex roles holds that males are generally dominant and females submissive, stemming from the way that sexual selection favours different behaviours in each sex. This could mean that sexual selection dictates particular behaviours in males and females. But in fact, sexual behaviour is extraordinarily varied in nature.

2. Mating systems, or who goes with whom, and for how long

‘Mating systems, or who goes with whom, and for how long’ examines the variation in how males and females associate during the breeding season, ranging from brief couplings with multiple partners to lifelong monogamy. It also shows how the discovery that females mate with many partners, even in supposedly monogamous species such as songbirds, was made possible by modern genetic techniques. Variation in mating systems holds considerable implications for the operation of sexual selection. The way that animal mating systems have been explained historically is outlined before considering how a more contemporary understanding of genetic and social relationships has reshaped our thinking and how understanding a species’ mating system can have practical applications.

5. Sexual selection after mating

Darwin viewed sexual selection as a process that ended with mate acquisition, assuming that females are fundamentally monogamous, mating with just one male. ‘Sexual selection after mating’, however, shows this assumption to be false. Sexual selection continues long after the physical act of mating is over, as sperm compete inside a female’s reproductive tract and females bias the paternity of their young by selectively using sperm from particular males. Multiple mating by females has turned out to be ubiquitous across animal taxa. The far-reaching evolutionary consequences of sperm competition and cryptic female choice for the evolution of reproductive traits are examined, from the gametes themselves to the adult organisms producing them.

3. Choosing from the field of competitors

There is now unequivocal evidence for Darwin’s long-rejected suggestion that females choose among potential mates based on their secondary sexual traits. This has shifted attention from the question of whether females exercised mate choice, to why they should exhibit the mating preferences they do. ‘Choosing from the field of competitors’ gives a contemporary view of mate choice and its occurrence in a wide variety of animal species, along with some examples of what females are choosing and why parasites and disease may play a role in the evolution of extravagant secondary sexual characteristics. Several theoretical models are discussed along with the role of sexual selection in the evolution of humans.

1. Darwin’s other big idea

The idea that males and females often look, sound, smell, and behave differently is uncontroversial. Where those differences came from, however, and what role they play in various species—including humans—is not. ‘Darwin’s other big idea’ outlines Charles Darwin’s sexual selection theory: differential reproduction based on sexual competition, whether between the members of one sex for access to the other, or by selection of particular mating partners. Differences in reproductive parts directly involved in sperm or egg production—primary sexual characteristics—were relatively easy to explain. The other kinds of sexual differences, he proposed, could evolve in one of two ways: male–male competition resulting in weapons, or female choice resulting in ornaments, but this was highly controversial.

6. Sexual conflict

The reproductive interests of males and females will almost always differ, for example over whether to mate and how often, when to produce offspring and how many, or how much to invest in each offspring. Whenever the reproductive interests of males and females differ, opposing selection on males and females to achieve their preferred outcome will generate sexually antagonistic selection. Such sexual conflict is reflected in differences in the appearance and behaviour of the sexes as each evolves to gain the advantage in a fitness ‘arms race’. ‘Sexual conflict’ explores the evolutionary consequences of these arms races in the context of sexual selection as it occurs both before and after mating.

8. Conclusions, and where to from here

‘Conclusions, and where to from here’ concludes that animals are far more variable in their sexual behaviour than is commonly believed, which means that they do not set a model for natural human behaviour, regardless of how we define it. Females are not always the more caring sex, males are not always more competitive than females, and mate choice and competition take many forms. Under the same selective pressures—to survive long enough to reproduce and to pass on genes to subsequent generations—animals have evolved many different solutions. New areas of sexual selection research include the use of genomics to understand the evolution of alternative reproductive behaviour and for better understanding sexual conflict.

7. How sex makes species survive

How new species come about is one of the most fundamental questions in biology. And because that process involves reproduction, and because reproduction often (though not always) means sexual selection, it should come as no surprise that sexual selection has often been thought to play a role in speciation—the way that new species arise and are maintained. ‘How sex makes species survive’ explains that sexual selection, and the dizzying array of ornaments and weapons it produces, is now recognized to be far more than simply a means for females to recognize a male of the appropriate species. It can also be the engine that drives new species to come about.