It was a moment that smashed assumptions with the force of a wrecking ball. She approached the sexy older male who seemed to arrive from out of nowhere, his black-and-white coat gleaming in the light. She put herself directly in his path, shook her head provocatively, then turned and bent over to “present” herself to him. She eagerly pressed her backside against his groin and the two gyrated against each other. Spectators watched as the two built toward a climax, and then they went their separate ways. The sexual display took the scientific community completely by surprise.
When primatologist Sarah Hrdy described this behavior among female hanuman langurs—or Semnopithecus entellus, a monkey species from western India—in the late 1970s, it erupted into the kind of controversy usually reserved for lewd performances at the MTV Video Music Awards. Ever since Darwin there had been an assumption among evolutionary biologists that females were coy and choosy in their sexual behavior while males were the ardent, promiscuous sex. Even though important advances in gender equality have been achieved since then, “most Darwinian models of human origins incorporate females only as passive objects of male competition,” wrote biological anthropologists Craig Stanford and John Allen as the 20th century came to a close. And yet these female langurs were observed actively pursuing males from neighboring troops while, according to the prevailing theory, they should have been chaste rather than chasing. What was even more surprising was that they would exhibit these sexual advances at any stage in their estrous cycle, sometimes even when they were already pregnant.
“Under some circumstances,” Hrdy wrote in her classic 1977 book The Langurs of Abu, “females are continuously sexually receptive, a pattern previously thought to occur only among human females.” Primatologists refer to langur societies as polygynous, in that they are composed of multifemale, single-male groups. Darwin’s theory of sexual selection held that these females should choose the most impressive male in their troop to ensure the hereditary success of their offspring. But here was clear evidence that females would actively engage in “adulterous solicitations” with males from other societies. As Hrdy revealed to a scandalized scientific community, the genetic benefits that came from seeking extra-pair matings—while maintaining the support of an existing partner—meant that evolution could favor females who choose to cheat.
More than 30 years of subsequent research has confirmed Hrdy’s findings and expanded on them to reveal that females in many primate species, humans included, engage in a diversity of sexual strategies to enhance their overall reproductive success. For example, in saddle-backed tamarins, females will solicit sex from multiple males who will each help to care for her offspring.* Female mouse lemurs will mate with up to seven males during a single night. Capuchin monkeys will seek out mating opportunities in the early stages of their pregnancy, presumably to confuse males about paternity. And bonobo females will have sex with everybody at pretty much any time they feel like it.
In the latest addition, Brooke Scelza, a human behavioral ecologist at the University of California–Los Angeles, contends in Evolutionary Anthropology that not only do human females seek out multiple sexual partners as an evolutionary strategy, they opportunistically shift that strategy depending on the environmental context (more on that below). In other words, female sexuality is not so much blindly promiscuous as it is pragmatic.
Of course, in an earlier era the scientific paradigm for understanding sex was much more rigid. In 1948, a balding and near-sighted English geneticist by the name of Angus Bateman published one of the most influential papers ever written on the evolution of sexual behavior. After studying patterns of inheritance among offspring in the common fruit fly, Drosophila melanogaster, Bateman concluded that the division between ardent males and coy females was “an almost universal attribute of sexual reproduction” across the entire animal kingdom. Bateman reasoned that, because females produce dramatically fewer eggs than males do sperm, and because eggs were physiologically more expensive, female reproductive success would not increase by mating with more than one male. Instead, females should focus on choosing the “best” male that they could and then directing their energy toward raising offspring. On the other hand, males who mated with multiple females would be expected to greatly increase their own reproductive success because the benefit outweighed the cost of production. Sex, like economics, was a question of quantity versus quality.
There was only one problem: Bateman got it wrong. In June 2012, UCLA biologist Patricia Gowaty and colleagues replicated Bateman’s study only to find that he had come to faulty conclusions because his methodology was severely flawed. Without modern genetic analysis at his disposal, Bateman conducted his trials with males and females of known mutant strains whose offspring could be easily identified. However, he counted only offspring that had two mutations—one from each parent—in order to be certain of a given fly’s reproductive success. This approach resulted in a biased sample because flies with some mutations were less likely to survive than those with others. In the end, the premiere study on sexual selection—which had been cited by more than 2,000 peer-reviewed papers and textbooks—contained a fatal flaw that would have been easily identified had the study been replicated sometime in the preceding 64 years. How could this happen?
“Our worldviews constrain our imaginations,” Gowaty said after her study was published in the Proceedings for the National Academy of Sciences. “For some people, Bateman's result was so comforting that it wasn't worth challenging. I think people just accepted it.” The uncomfortable implication is that Bateman’s paradigm was so widely cited because it conformed to assumptions about how female sexuality ought to be. These assumptions were constructed over a long history and had infiltrated Western culture so completely as to be nearly invisible.