Masculine Dimensions

Abstract The male genitalia evolved as an exquisitely complex, integrated system for the production, storage, and delivery of sperm (and accessory glandular secretions) to females during sexual activity. Studies of invertebrates have provided ample evidence for effects of sexual selection upon the evolution of male genitalic morphology and physiology (Eberhard 1985; 1996; Simmons 2001; Poiani 2006). The same evolutionary principles may be applied in comparative studies of the genitalia in the mammals, including human beings. The reproductive systems of male mammals are structurally diverse, with remarkable differences in their accessory reproductive glands and penile morphologies. At least some of this diversity may have been influenced by sexual selection, whether via sperm competition or cryptic female choice. The question of cryptic female choice will be deferred until the next chapter, where possible effects of sexual selection upon the evolution of the female genitalia are discussed. Here I shall explore the effects of sperm competition upon the evolution of the male reproductive system in mammals, in order to address a number of questions.