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Why They Stray: The Evolutionary Advantages of Infidelity

It went against most everything scientists thought they knew at the time, but when Cornell University researchers studied the mating habits of bobolinks 30 years ago, they saw that it wasn’t just the males who were fooling around on the side. The females were too—and for good reason.
Studying the bobolink, researchers discovered primary nests housing youngsters sired by more than one male. (Photo: David Watkins/Shutterstock)

Studying the bobolink, researchers discovered primary nests housing youngsters sired by more than one male. (Photo: David Watkins/Shutterstock)

I hadn’t thought much about bird sex in a long time—30 years. But as I stood on a dirt road in the Canaan Valley National Wildlife Refuge in West Virginia this past summer, instinct (or perhaps muscle memory) took over. I lifted my binoculars to my eyes, listened for a distinctive bubbly song, and scanned the fence posts in the adjacent field. Sure enough, two male bobolinks were perched a few posts apart singing like mad to keep the other away from his territory.

Typical male behavior, I thought.

Years ago, as a wildlife biology major at Cornell University, I spent an entire summer watching a field of bobolinks do their thing. Males fiercely guarded their territories and females chose mates based on some mysterious combination of alluring song and impressive real estate. The mated pairs built nests and raised their broods together in seemingly monogamous bliss. On the surface, it was a Father Knows Best kind of scenario straight out of the Eisenhower era. But all of us on the research team watching knew that some of those daddies were fooling around on the side. We called the mistresses “secondary” and “tertiary” females. They raised their young on father’s territory, but he never acknowledged their existence. Instead, he doted faithfully on his “primary” female and helped her feed the brood.

Evolution isn’t influenced by the sanctity of marriage. The only thing that matters in evolution is whether a particular trait increases the likelihood of producing greater numbers of offspring that, in turn, will survive and reproduce as well.

As I watched a few bobolinks posture again this past summer for the first time since then, I suddenly saw things a bit differently. The fence posts morphed into bar stools and the bobolinks became men in some rowdy roadhouse. And I wondered: How did our research on bobolink sex influence thinking about human sex? This wasn’t a random question. Thirty years earlier, our team discovered something just beginning to be recognized: The mommy birds were fooling around on the side too.

Ever since Charles Darwin put his ideas about sexual selection down on paper in 1871 (in The Descent of Man and Selection in Relation to Sex) biologists had been reinforcing conventional thinking about female sexuality. Namely, that males of most species compete for as many females as they can get, that their “investment” in mating is low (sperm and copulation are energetically cheap compared with eggs and pregnancy), and therefore it’s to their advantage to seek out as many sexual partners as possible. Females, the thinking went, would gain no such advantage from having more than one sexual partner. Instead, it made evolutionary sense for females to choose one really good mate and put her eggs in one basket (figuratively and literally).

Scientists quibbled over the details and tweaked these ideas over the decades, but didn’t challenge them much. A.J. Bateman seemed to prove this point in 1948 with his classic study of fruit flies: Male fruit flies that mated with many females had more offspring that those who mated with few. But he found no such advantage for female fruit flies. Robert Trivers added consideration of parental investment to the discussion in 1972, noting that the sex that invests more in raising offspring would be choosy about mates, and the sex that invests less would compete with others of their gender for partners. But since females of most species—including humans—typically invest more time and energy in their offspring than males, scientific thinking didn’t change all that much.

Until DNA analysis.

By the mid-1980s, when I was at Cornell, biologists could compare genes across individuals through a technique called electrophoresis, in which an electrical current is passed through a gel containing blood or tissue. The current causes different versions of DNA to migrate through the gel at different speeds. For the bobolink study, that meant my professor, Thomas Gavin, could accurately identify related individuals simply by taking blood samples from every bird and nestling in the field. Those samples revealed that some of the primary nests housed youngsters sired by more than one male. Siblings, in some cases, had two different daddies.

Other ornithologists began looking for this phenomenon as well and finding similar results across species—white-crowned sparrows, indigo buntings, and more. As the evidence for female infidelity in birds mounted, evolutionary biologists were forced to re-adjust their thinking.

“The genetic data from the bird studies really turned things upside down,” says anthropologist Brooke Scelza of the University of California-Los Angeles. “All of a sudden it was clear that females were [choosing more than one mate] and presumably, if they’re doing it this commonly across species, there are some benefits.” In 1968 the famed ornithologist David Lack proclaimed that more than 90 percent of passerine bird species were monogamous and that polyandry (where females mated with more than one male) was unknown. By 2002, those figures essentially were reversed. In a review of the scientific literature that year, Simon Griffith of Oxford and colleagues calculated that—while on the surface social monogamy is common—true genetic monogamy exists in only 14 percent of passerine species. Genetic polyandry occurs in 86 percent.

The question became why. What benefits do these female birds (and, as soon revealed, many presumably monogamous female mammals) gain from having multiple mates? And what does that mean for a “socially monogamous” species like humans?

Infidelity among human couples is nothing new. Data from Western societies shows that it ranges from 20 to 50 percent. Could there be an evolutionary advantage to having an affair? After all, evolution isn’t influenced by the sanctity of marriage. The only thing that matters in evolution is whether a particular trait increases the likelihood of producing greater numbers of offspring that, in turn, will survive and reproduce as well. But in the West, data about reproductive success is confounded by widespread use of contraceptives. The few limited data sets available suggest that, despite high rates of infidelity, only about one to 10 percent of offspring result from those liaisons.

Scelza has tried to answer this question by looking at non-Western, natural fertility societies such as the Himba of Namibia. The Himba marry, but extra-marital affairs are common and widely acknowledged. In confidential interviews with more than a hundred Himba women, Scelza calculated in 2011 that, conservatively, more than 17 percent of Himba offspring resulted from extra-marital affairs and almost a third of married women had children resulting from those affairs. Women with these “extra-pair” children had higher rates of reproductive success—that is, more offspring—than those who did not. Interestingly, all of the extra-marital offspring occurred with women whose marriages had been arranged. None of them occurred with “love match” marriages.

Monique Borgerhoff Mulder of the University of California-Davis found in 2009 that Pimbwe women in Tanzania increased their reproductive success by marrying multiple times throughout their reproductive years. Men who married multiple times did not.

The evolutionary theories about why females (whether birds or people) might benefit from multiple mates apply across species: Mating with multiple partners increases the genetic diversity of a female’s offspring, increasing the odds that at least some will survive regardless of changing environmental conditions. It can help a female acquire so-called “good genes” for her young—the superior genes found in some males, but not all. It provides a hedge against possible infertility in her social mate. And it can help her access valuable resources such as food that additional mates might control. Still, although the theories are sound, Scelza says proof that multiple mates lead to greater reproductive success for women remains thin.

“I would say that the evidence is pretty good that there are benefits to females in a lot of (non-human) species,” Scelza says. “It is possible and maybe probable that those benefits will exist for humans as well. But the jury is still out.”

And, of course, culture matters. Many indigenous societies in lowland South America believe that children can have more than one biological father, and both men and women have multiple sexual partners. Often, all of the recognized fathers help provide for their partners’ children—a clear advantage to those kids. Stephen Beckerman of Penn State found that among the Bari people of Venezuela, children with more than one male provider were more likely to survive childhood than those with just one. Conversely, the Mosuo of China have been called a society without fathers. Mosuo women and children live with the mother’s family, and the women receive male “visitors” at night. But men don’t raise their children. Instead, women rely on support from their families, freeing them up (one could theorize) to diversify their offspring’s gene pool.

None of this is to say that males—whether bobolinks or men—don’t compete for females. Or that males might not realize even greater benefits from having multiple partners than their mates. But it does suggest that Darwin and his scientific descendants missed half the picture. Perhaps they were just products of their time, suffering from a little masculine wishful thinking. Darwin would surely recognize that evolution doesn’t discriminate. With a little DNA evidence, I suspect, neither would he.