Cryptic Female Choices

The last chapter focused on the effects of sexual selection, via sperm competition, upon the evolution of the male genitalia in mammals, with special reference to the question of human reproduction and evolution. However, in human beings as in other mammals, it is the female's reproductive tract which constitutes the arena for sperm competition. Spermatozoa must traverse anatomical and physiological sieves and barriers in the vagina, cervix, uterus, uterotubal junction, and oviduct before encountering an ovum, enclosed in its vestments (Figure 4.1). Once it is acknowledged that the female's anatomy and physiology may influence the fate of spermatozoa, it is logical to ask whether such factors might act differently upon the ejaculates of rival males. If, for example, one male possesses a more advantageous penile morphology, or copulatory pattern, or a biochemically more effective mixture of accessory glandular secretions, then the female's reproductive system might preferentially receive and transport his spermatozoa. It is this hidden female potential which William Eberhard refers to as cryptic female choice. Although Eberhard was not the first scientist to employ this term (see Thornhill 1983) he provided the major synthesis of this important concept (Eberhard 1985; 1996). Cryptic female choice may partially explain, for example, why sexual selection has favoured the evolution of complex phallic morphologies in animals in which females mate with multiple partners. The phallus may function as an internal courtship device under such conditions.

Much more is known about possible mechanisms of cryptic female choice in insects and some other invertebrates than is the case for mammals. As an example, in the beetle Chelymorpha alternans the male's intromittent organ bears a long flagellum.

The flagellum is threaded into the female's sper-mathecal duct, and sperm pass along it to gain access to her storage organs. If a male's flagellum is shortened surgically, then the female ejects more of his gametes after mating has occurred. Males with longer flagella are more successful in fertilizing ova and achieve greater reproductive success (Eberhard 1996). The muscles of the female's sperm storage organ play a crucial role in selective ejection of spermatozoa (Rodriguez 1994; Simmons 2001). Mechanisms underlying cryptic female choice have been explored in several insect species (e.g. in the cowpea weevil: Wilson et al. 1997; damsel flies: Cordoba-Aguilar 1999; and the yellow dungfly: Ward, 2000). Although equivalent investigations of mammals are few in number and limited in scope, there are two issues which are worthy of consideration in relation to the origins of human sexuality. Firstly, has cryptic female choice played any role in moulding the evolution of phallic morphology in relation to stimulation of the female during copulation? Specifically, is orgasm in the human female significant in terms of sperm transport and fertility, and might penile morphology in the human male have evolved in relation to mechanisms of female orgasm? Secondly, does female genital morphology in mammals correlate in predictable ways with their mating systems and the likelihood of sperm competition between males? Specifically, is the length of the oviduct subject to sexual selection via cryptic female choice, and, if so, how does the size of the human oviduct compare with that of other mammals?

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100 Pregnancy Tips

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