Mating strategies are as varied as the creatures that make up the animal kingdom, but most fall into one of two broad categories: monogamous or polygamous. Penguins mate for life; walruses prefer to play the field. Our close relatives in the primate family are a mixed bag: Chimps and baboons are polygamous, while marmosets like to settle down. Humans, unlike so many other species, are both, according to a new study in Biology Letters.
Men are typically cast as the more promiscuous sex, but the study shows that both males and females breakdown into two groups: those who favor monogamous relationships and those who prefer to practice promiscuity. “It’s very, very rare to find two competing phenotypes that can both be sustained in a population,” says Rafael Wlodarski, a postdoctoral researcher at the University of Oxford and lead author of the paper.
Males tended to fall into two groups: those with much longer ring fingers, who may be more inclined to be promiscuous, and those with index and ring fingers of a similar size, who may be more likely to prefer monogamous relationships.
To elucidate these co-existing strategies, Wlodarski used a questionnaire to collect data on courtship behavior from nearly 600 individuals. The survey measured participants’ sociosexuality, or inclinations toward promiscuity, with questions like, “With how many different partners have you had sexual intercourse on one and only one occasion?” Wlodarski then used statistical tests to find out where men and women fell on the promiscuous and monogamous spectrums.
“When we ran these analytical tests it seemed to be that within males and within females we found that the distribution is best described by the presence of two potential populations,” Wlodarski says. This study is the first to suggest that women also cluster into either promiscuous or monogamous groups.
The study also looked at the ratio of index finger length to ring finger length in 1,314 British individuals. This ratio is a known indicator of testosterone exposure in the womb, which may lead to increased promiscuity as an adult. Males tended to fall into two groups: those with much longer ring fingers, who may be more inclined to be promiscuous, and those with index and ring fingers of a similar size, who may be more likely to prefer monogamous relationships. Finger length can’t predict individual behavior, though; the relationship between finger length and mating strategy only emerges with large datasets.
“At this stage, there’s no way of predicting whether someone will preferentially pursue one strategy or another,” Wlodarski says. “You can’t just look at someone’s digit ratio and say, ‘You’re definitely promiscuous.’”
If future studies show that the same behavioral breakdown exists in other cultures, it could provide a unique insight into the behavior of our distant ancestors. “Unlike every other primate, we can’t tell from our physiology whether in our evolutionary past we were monogamous or promiscuous,” Wlodarski says. In most species, certain physiological traits are associated with either polygamous or monogamous strategies, meaning you can tell just by looking at an animal what kind of mating strategy it likely practices. Polygamous male primates, for example, tend to be much bigger than females. “This kind of explains why we’re such strange creatures compared to other primates,” Wlodarski says.