Sperm competition is the idea that if the sperm of more than one male is present in the reproductive tract of an ovulating female, the sperm themselves compete to fertilize the egg. What could this possibly have to do with human sexuality? Before we get to that, there’s a lot to unwrap.
If you’ve heard a narrative of how humans find mates, it probably goes something like: “Women trade the promise of fidelity to men in exchange for meat, shelter, status, protection, etc. by building monogamous relationships. This is because women have a higher minimal investment for sex: nine months of pregnancy and a couple of years of additional vulnerability in breastfeeding and carrying around a baby compared to a man’s minimal investment of a few minutes of pleasure. Men, in turn, get access to sexual pleasure and can ensure paternity by keeping a close eye on their mates.”
A consequence of this narrative is that, effectively, women are whores and men only care about pleasure and paternity. I’m not a fan of this theory, and neither are husband/wife duo Christopher Ryan and Cacilda Jethá, co-authors of Sex at Dawn: How We Mate, Why We Stay, and What It Means for Modern Relationships.
Says Ryan, “I’m arguing against the shame that’s associated with desires, specifically the idea that if you love your partner but are attracted to other people, there’s something wrong with you, something wrong with your partner, or there’s something wrong with your marriage. A lot of families are fractured by unrealistic expectations that are based upon this false vision of human sexuality.”
Ryan and Jethá argue that oppositional relationship of “trading fidelity for stuff” has less to do with the evolution of anatomically-modern humans (who’ve been around for the last 200,000 years) and everything to do with the restructuring of human societies due to agriculture (which has existed for only 10,000 years).
Before agriculture, people lived in hunter-gatherer groups characterized by “fierce egalitarianism.” According to Ryan, “They demanded that things be shared. All the things that are commonly thought of as being traded to women for fidelity are actually shared widely among these societies.” Living in a foraging context, this is simply the best way to mitigate risk.
Ryan and Jethá argue that this sharing behavior extended to sexuality, which evolved up until the introduction of agriculture as a means of establishing and maintaining complex, flexible social systems as a matter of survival. The evidence for this argument comes from the social and sexual behaviors of other ape species, and the anatomy and instincts with which evolution has equipped humans.
Chimps and bonobos are more closely related to humans than they are to any other primate including gorillas, orangutans, and gibbons. Humans are more closely related to chimps and bonobos than are African and Indian elephants. Male chimps, bonobos, and humans are all 10 to 20 percent bigger than females, unlike gorillas where males are easily twice as big. Humans and bonobos are the only animals that have sex face-to-face.
When they have their sexual swellings, chimps and bonobos mate one to four times per hour with up to a dozen males per day. Chimps have sexual swellings through roughly 40 percent of their menstrual cycle, bonobos 90 percent. Humans are the only species on the planet where females are available for sex throughout their menstrual cycle (and they don’t exhibit sexual swellings at all). Sex continues whether a woman is menstruating, post-menopausal, pregnant, or breastfeeding. This is vanishingly rare among mammals.
Humans, chimps, and bonobos all have external testicles, which keep sperm cells cool and ready for ejaculation. Ryan describes this akin to “being a spare beer fridge in the garage. It’s out there keeping it cool because you know something could happen at any minute and you want to be ready.” The average human has sex about 1,000 times for every baby birthed. We share that ratio with chimps and bonobos. Gorillas, orangutans, and gibbons, who are more typical of mammals, have sex only about a dozen times per birth.
Humans have the longest, thickest, and most flexible penises of any living primate. The number of thrusts per sexual encounter ranges 10-500 over a span of 4-7 minutes before ejaculation. All this creates a vacuum in the female’s reproductive tract which removes the semen of other males. The Daily Beast also talked to author Jesse Bering of “Why is the Penis Shaped Like That?” who explained that the “mushroom-shaped head [of the penis] is the critical feature defining sexuality and has been designed over the course of natural selection to remove the sperm cells of males that have had sex with that same female within about 48 hours,” the average lifespan of a human sperm cell. Chimp and bonobo penises are, instead, thin and conical. While they thrust (presumably for stimulation), the average copulation is only 15 seconds in bonobos and 7 seconds in chimps.
“After a man ejaculates,” Bering continues, “he becomes impotent fairly quickly and further thrusting is actually unpleasant. The assumption is that that way he’s not dis-servicing his own conception odds. If he keeps thrusting he’s just removing his own sperm cells.” Continuing this idea of thrusts aiding sperm competition, “the longer we’re away from our partners, like on a business trip, the more intense the reunion sex tends to be,” Bering says. “The argument is that at some subconscious level, you’re thinking that you couldn’t monitor the sexual behavior of your partner during that time and they were more likely have sex with a competitor, so that’s why your arousal is going to be more intense.” Intense movements are more effective in removing whatever was there before.
Human ejaculation consists of 3-9 “spurts.” The chemical composition of early spurts differs from latter ones, suggesting functional differences. From Sex at Dawn, “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).”
Female instincts and anatomy are also important. From Sex at Dawn, “After an orgasm, a woman may be anticipating a dozen more.” Women are usually louder than men in the sack, and the sound of a woman having an orgasm is notoriously difficult for heterosexual men to ignore. Primatologists refer to such noises as “female copulatory vocalizations” which serve to inform other potential mates to “come hither.”
The complexity of the human cervix suggests it evolved to filter the sperm of various males, serving as an obstacle course in which only the most robust sperm can complete. The men with the most robust sperm tend also to have the best immune systems. Thus Darwinian selection still takes place among a troop of primates all sharing sex—it just happens at different level.
Intriguing as it is, this theory of human sexuality is not without its critics. Professor of anthropology at University of Nebraska-Lincoln Raymond Hames described Sex at Dawn as “a piece of crap.” According to Hames, “The book has been hammered pretty well by anthropologists who know the literature. That males are not sexually jealous and don’t mind their wives having other partners…what we know from hunter-gatherers is one of the most common causes of physical violence is sexual jealousy; men who’ve been cuckolded, or their wife has left them, etc.”
Ryan responds by saying, “Yes, there’s jealousy; yes, there’s insecurity; but some societies extenuate that, and celebrate it and call it love; and other societies consider it ridiculous and try to minimize it because it is disruptive and destructive to the fabric of that society. What we’re saying is that hunter-gatherer societies are the latter, and most agricultural societies are the former.”
“To argue that our ancestors were sexual omnivores is no more a criticism of monogamy than to argue that our ancestors were dietary omnivores is a criticism of vegetarianism,” Ryan says in a TED talk. “Everyone has to respond to the modern world. But the body does have its inherent evolved trajectories…a person can do what they want but not want what they want.”