Chapter 9

Reproductive Behavior

1. Darwin’s sexual selection theory explains why certain traits can spread through populations even though these attributes appear to lower individual survival, a result that would seem disfavored by natural selection. But sexual selection can overcome natural selection for improved survival if the sexually selected characteristics promote success in securing mates.

2. Sexual selection results in sex differences in reproductive behavior and morphology. These standard sex roles arise because males usually make huge numbers of very small gametes and often try to fertilize as many eggs as possible, while providing little or no care for their offspring. In contrast, females make fewer, larger gametes and often provide parental care as well. As a result, receptive females are scarce, becoming the focus of male competition while ensuring that females have many potential partners to choose among.

3. Typically, males compete with each other for mates while trying to convince females to mate with them. The intrasexual or competition-for-mates component of sexual selection has led to the evolution of many characteristic male reproductive behaviors, including a readiness to fight over females or for social dominance. In addition, this form of sexual selection has resulted in the evolution of mating tactics that enable some males to make the best of bad situations created by their rivals as well as the ability of males to give their sperm an edge in the competition for egg fertilizations, especially by guarding their mates for some time after copulation.

4. In a typical species, females exercise mate choice (intersexual selection) because they control egg production, egg fertilization, and the care of offspring. Females often seek direct benefits in the form of parental care or access to resources. Males of some species seek to win favor with females by offering them material benefits, including nuptial gifts or parental care. In these cases, males with better gifts or superior paternal capabilities often produce more descendants.

5. Mate choice by females occurs even in some species in which males provide no material benefits of any sort. In these cases, females seek indirect genetic benefits that enhance the viability of their offspring (good genes model). However, extravagant male features could spread through a population in which even arbitrary elements of male appearance or behavior became the basis for female preferences. Exaggerated variants of these elements could be selected strictly because females preferred to mate with individuals that had them (runaway selection model). Yet another possibility is that the extreme ornaments of males evolve as a result of a cycle of conflict between the sexes, with males selected for ever-improved ability to exploit female perceptual systems and females selected to resist those males ever more resolutely (chase-away selection model).

6. Interactions between the sexes can be viewed as a mix of cooperation and conflict as males seek to win fertilizations in a game whose rules are set by the reproductive mechanisms of females. Conflict between the sexes (sexual conflict) is widespread and includes sexual harassment, manipulation, and forced copulations, which often result in sexual arms races and demonstrate that what is adaptive for one sex may be harmful to the other.