Sexual Selection and Animal Genitalia

Description

As Darwin first pointed out, two distinct evolutionary processes have contributed to the diversity of form and function in plants and animals: natural selection and sexual selection. In this book William Eberhard presents a new theory that explains male genitalic evolution as a result of sexual selection. From flatworms to fish, from moths to rodents, animal genitalia display an extraordinary variety of baroque morphologies. Not only are the forms varied, they have diverged rapidly in the course of evolution. Why such strange forms and such rapid divergence? These questions have puzzled evolutionary biologists and animal taxonomists for over a century, and several hypotheses have been proposed. Eberhard shows that none of the explanations is adequate and proposes a new hypothesis. He views genitalia as courtship devices that function in the competition for mates by influencing the females’ choices of fathers for their offspring. To the extent that male genitalic structures affect female choices, male genitalia are subject to the same type of runaway selection as that on structures, such as the peacock’s tail, used in precopulatory courtship. Eberhard’s hypothesis can explain the fact that in a vast range of animals, from nematodes to mammals, male genitalia tend to be more complex than female genitalia, are often more elaborate than would be required for simply introducing sperm into the female’s body, and have diverged rapidly and are thus highly species-specific in form. Although the emphasis is on theoretical explanations, many examples are presented of the vast diversity of animal genitalia: squids with arms whose tips break off and swim around inside the female after introducing an explosive, grenade-like sperm packet into her; flatworms that have rows of penes despite the presence of only a single female aperture; damselflies that give their mates contraceptive douches prior to inseminating them; and female seahorses with penes. 1. An Introduction to the Problem Operational Definitions A Survey of Animal Genitalia Genitalic Extravagance 2. Previous Hypotheses: Their Status to Date Lock and Key Genitalic Recognition Pleiotropism Mechanical Conflict of Interest Summary 3. Tests of the Lock and Key and Genitalic Recognition Hypotheses Lock and Key Species Isolation 4. Tests of the Other Hypotheses and a Summary Pleiotropism Arnold’s Modification Mechanical Conflict of Interest Summary 5. Sexual Selection by Female Choice The Function of Copulation The Female Choice Hypothesis Effects of Female Choice on Genitalia Mechanical Fit of Genitalia Genitalia and Overall Male Fitness Major Evolutionary Patterns That Are Explained Relationships to Other Hypotheses 6. Male–Male Genitalic–Genitalic Competition Sperm Displacement Clasping Devices Male Adaptations That Short-cut Female Choice Topology of Female Reproductive Systems Too Summary 7. Female Discretion after Genitalic–Genitalic Contact Intromission Insemination Sperm Transport Sperm Destruction, Storage, and Activation Egg Maturation and Fetal Nourishment Oviposition Rate Tendency to Remate Summary 8. Remating by Females Strict Behavioral Monogamy Induced by Males Copulatory Plugs Antiaphrodisiacs Short-lived Females Monogamy in Termites Determining Remating Frequencies The Best Field Data on Remating Frequencies Heliconius Butterflies: A Test Case Sex Ratios and Sperm Exhaustion Summary 9. Apparent Contradictions of the Female Choice Hypothesis Interspecific Differences in Female Genitalia Lack of Species Specificity in Genitalia High Paternal Investment in Offspring Intraspecific Uniformity of Genitalia Spermatophore Complexity and Male–Female Contact Infrequency of Postcopulatory Courtship Patterns of Genitalic Change Summary 10. Use of Genitalia as Stimulators External Movements Intromission without Insemination Movements within the Female Indirect Evidence 11. Specialized Nongenitalic Male Structures 12. Conclusions and an Overview References Index