Chapter 2 Summary

  1. The original function of sex—and its only function in many species—is reproduction. The reasons why many species rely on sexual rather than asexual reproduction are disputed. Two general theories have been presented. First, sexual reproduction may promote the elimination of harmful mutations. Second, by mixing genes from different individuals, sexual reproduction may foster the selection of advantageous traits.
  2. Natural selection has caused gametes to diverge into female and male forms. Female gametes are large and contain nutrients; male gametes are small and motile. Natural selection also acts to keep the ratio of the sexes near equality in most species, because any imbalance favors animals that have offspring of the minority sex.
  3. Sex may be determined by chromosomal mechanisms, as in mammals, or by the temperature at which eggs are incubated, as in many reptiles.
  4. Sexual selection, driven by competition for mates, has led to different morphological and behavioral traits in males and females. Because females generally invest more than males in reproduction, males often compete among themselves for access to females. This competition may select for large, aggressive individuals.
  5. Females often choose among males. Their choices may be based on morphological features such as symmetry, display feathers, or antlers, or on behavioral traits such as the offering of gifts. Some female choice seems aimed at forcing males to make a greater investment in reproduction than they otherwise would. In species in which males do make significant investments, males become choosier, and females become more competitive.
  6. A wide variety of relationship styles exist. Animals may engage in sex without establishing any social bond, or they may bond in socially monogamous or polygamous relationships. Polygamy usually involves one male and several females (polygyny); the reverse arrangement (polyandry) is rare.
  7. In many socially monogamous or polygynous species, both males and females engage in sex outside these social structures. Promiscuity has obvious benefits for males in terms of increased numbers of offspring. For a female, promiscuity may offer a range of benefits: It may help her gain resources from males, it may give her access to high-quality genes, or it may favorably influence the behavior of males toward her or her offspring.
  8. Forced copulation has been observed in many species. In a few species, this behavior is clearly adaptive, increasing the male’s likelihood of having offspring.
  9. Because close relatives share many genes, evolution has led to altruistic behavior among relatives, including behavior in the reproductive domain. Some altruistic reproductive behavior, such as adoption of orphans, is less easily explained in evolutionary terms.
  10. Sexual behavior has developed other functions besides reproduction. Bonobos offer a striking example: In this species, much sex takes place when the female is incapable of becoming pregnant, or between individuals of the same sex. Bonobo sex is directed not only toward reproduction, but also toward the avoidance or resolution of conflicts and the establishment of social bonds.