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.
Author(s): William G. Eberhard
Publisher: Harvard University Press
Year: 1985
Language: English
Pages: 244
City: Cambridge,MA / London
Tags: quahaha
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
