Coming in at hundreds or thousands of copulations per child born, human beings outcopulate even chimpanzees and bonobos—and are far beyond gorillas and gibbons. When the average duration of each copulation is factored in, the sheer amount of time spent in sexual activity by human beings easily surpasses that of any other primate—even if we agree to ignore all our fantasizing, dreaming, and masturbating.
The evidence that sperm competition played a role in human evolution is simply overwhelming. In the words of one researcher, “Without sperm warfare during human evolution, men would have tiny genitals and produce few sperm.. There would be no thrusting during intercourse, no sex dreams or fantasies, no masturbation, and we would each feel like having intercourse only a dozen or so times in our entire lives.. Sex and society, art and literature—in fact, the whole of human culture—would be different.” We can add to this list the fact that men and women would be the same height and weight (if monogamous) or that men would likely be twice the size of women (if polygynous).
Just as Darwin’s famous finches in the Galapagos evolved different beak structures for cracking different seeds, related species often evolve different mechanisms for sperm competition. The sexual evolution of chimps and bonobos followed a strategy dependent upon repeated ejaculations of small but highly concentrated deposits of sperm cells, while humans evolved an approach featuring:
• a penis designed to pull back preexisting sperm, with extended, repeated thrusting;
• less frequent (compared to chimps and bonobos) but larger ejaculates;
• testicular volume and libido far beyond what’s needed for monogamous or polygynous mating;
• rapid-reaction DNA controlling development of testicular tissue, this DNA apparently being absent in monogamous or polygynous primates;
• overall sperm content per ejaculate—even today—in the range of chimps and bonobos; and
• the precarious location of the testicles in a vulnerable external scrotum, associated with promiscuous mating.
