Cryptic female choice (CFC) does not necessarily involve discriminative responses in the female nervous system to sperm from multiple mates. Even without any active sperm-choice mechanisms, polyandrous females can gain genetic benefits by having an arena in which genetically superior sperm are “automatically ” sorted. In this chapter, possible mechanisms in this CFC category, termed “indirect CFC, ” are reviewed. A simple theoretical model is developed to examine the hypothesis that females obtain genetic benefits by allowing only partial displacement of stored sperm by subsequent mates. The model predicts that such restricted sperm displacement automatically grants genetic benefits when genetically superior males copulate more times per encounter with the female than less fit males. The promiscuous earwig species, Euborellia plebeja, provides an empirical example of this type of indirect CFC. The elongated female sperm-storage organs allow only partial removal and displacement of stored sperm by shorter male genitalia, resulting in a 20 % gain in paternity per mating. In staged mating trials, large males dominated male-male competition for burrows housing females, resulting in a significant increase in paternity by repeated matings with the same female. A numerical simulation based on this mating pattern showed that restricted sperm displacement (~20 % per mating) is optimal for females to accumulate sperm from larger males. Given that male body size is heritable, females were estimated to gain a 1.4 % increase in their sons’ mating success as a genetic benefit. Advantages and disadvantages of indirect CFC are discussed and compared with precopulatory mate choice and direct CFC.
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