Sex at Dawn полностью

An example they don’t discuss in their book, but might consider for future editions, is testicular size. Testicles can change size almost in the blink of an eye—blue or otherwise. In some species of lemurs (small, nocturnal primates), testicular volume changes seasonally, swelling up in breeding season and then shrinking back down in the off-season, like a beach ball with a slow leak.⁴

Testicular tissue in humans, chimps, and bonobos (but, interestingly, not gorillas) is controlled by DNA that responds unusually rapidly to environmental changes. Writing in Nature, geneticists Gerald Wyckoff, Hurng-Yi Wang, and Chung-I Wu report, “Rapid evolution of male reproductive genes is ... quite notable in the lineages to human and chimpanzee.” They go on to note that the rapid response of these genes may well be associated with mating systems: “The contrast is intriguing in light of the socio-sexual behaviors of African apes. Whereas modern chimpanzees and bonobos are clearly promiscuous with ample chance of multiple insemination, ovulating female gorillas seem much less likely to be multiply inseminated.”⁵

Let that soak in for a moment. Humans, chimps, and bonobos—but not gorillas—show “accelerated evolution of genes involved in sperm and seminal fluid production” associated with “multiple insemination.” The genes associated with the development of human, chimp, and bonobo testicles are highly responsive to adaptive pressures, far more so than these same genes in gorillas, where females typically mate with only one male.

Because they are composed entirely of soft tissue, testicles don’t leave a trace in fossils. So, while defenders of the standard narrative assume that human testicular volume has remained constant for millennia, it’s now clear this assumption may be wrong.

Wyckoff, Wang, and Wu confirm a prediction made by biologist Roger Short back in 1979, when he wrote, “Testis size might be expected to respond rapidly to selection pressures. One of the most intense forms of selection will be found in promiscuous mating systems..”⁶ Geoffrey Miller agrees: “Heritable differences in sperm quality and sperm delivery equipment will be under intense selection.” Finally, evolutionary biologist Lynn Margulis and her co-author Dorion Sagan reason that men’s “souped-up genitals” backed by “a lot of spermatic firepower” would be worthwhile only if there were “some sort of race or contest. Otherwise,” they write, “they seem excessive.”

Souped-up genitals. Spermatic firepower. Now we’re talking!

Indications of spermatic firepower are evident in the differences between a man’s first spurts and his last. A human ejaculation typically consists of anywhere from three to nine spurts. Researchers who somehow managed to capture “split ejaculates” for analysis found that the first spurts contain chemicals that protect against various kinds of chemical attack. What sort of chemical attack? Aside from leucocytes and antigens present in a woman’s reproductive tract (more on that later), they protect the sperm from the chemicals in the latter spurts of other men’s ejaculate. These final spurts contain a spermicidal substance that slows the advance of any latecomers. In other words, competing sperm from other men seems to be anticipated in the chemistry of men’s semen, both in the early spurts (protective) and in the later spurts (attacking).⁸

The importance of sperm competition has been debated in scientific conferences and academic journals for the past few decades as if it were a new discovery, but several centuries BCE, Aristotle and his predecessors noted that if a bitch copulated with two dogs during a single fertile period, she could produce a litter of pups fathered by one or both of them. And consider the story of Heracles and Iphicles: the night preceding Amphitryon’s marriage to Alcmene, Zeus disguised himself as Amphitryon and slept with the bride-to-be. The following night, Amphitryon consummated her marriage. Alcmene had twins: Iphicles (fathered by Amphitryon) and Heracles (fathered by Zeus). Clearly, the ancient Greeks had an inkling of sperm competition.