More News:

February 13, 2015

The science of everything: Monogamy

Since the Middle Ages, Valentine's Day — actually named after multiple martyred saints — has been a holiday associated with love. As children, we gave away valentines willy-nilly to every classmate, perhaps treasuring the one signed by a secret crush. But as adults, Valentine's Day firmly becomes a couple's holiday, with parties of two crowding the city's choice restaurants, bars and museums. 

  • With this inaugural column, introduces a new series called “The Science of Everything,” an opportunity for science journalist Meeri Kim, Ph.D., to explore the how and why of everyday things, from monogamy to movies to MSG.

Let's look at the bigger, biological puzzle that this holiday represents: Why do so many humans feel the urge to pair off? Taking a quick inventory of our fellow mammal brethren, less than 10 percent of all mammalian species are monogamous, in that it is common for two individuals to mate exclusively. In primates, the number is a bit higher — 10 to 15 percent of all species — but it still reveals that the majority enjoy a more free-wheeling lifestyle. 

So why did mankind tread down the less-traveled, monogamous path, and when did the shift happen?

Of course, humans are by no means exclusively monogamous. There are polygamy and polyamory, and cheating is quite rampant within our species. Before finding that so-called “special someone,” most of us play the field and date around. 

“There's no one fixed pattern. We're not somehow fixed into monogamy,” said Temple University anthropology professor Leonard O. Greenfield, who discusses the evolution of monogamous mating relationships in humans and other primates in several of his courses. “For instance, the Bible is full of all these guys that are making out like bandits with large numbers of wives.”

In most societies, however, the prevailing assumption is that the majority of the population remains more or less serially monogamous. The idea is reinforced through the social conventions of marriage and the nuclear family. 

Scientists have no direct way to find out when our ancestors started pairing off, but they can look at animals living today to gather some clues. A year-and-a-half ago, two independent studies were published that aimed to find out why monogamy had evolved in mammals. One, released by the Proceedings of the National Academy of Sciences, argued for the infanticide avoidance hypothesis: The imminent threat of violence from rival males led females to choose partners that would stick around and protect the family.

The other study, published in the journal Science, instead found that as females began to spread out due to limited food resources, males had a harder time keeping track of multiple mates. Instead of running around and trying to protect their harem, it was just easier and less dangerous to stick with one mate.

Of course, the results of these studies — both based on tracing back the evolutionary tree based on the DNA sequences of animals alive today — might not apply to humans at all. 

A different peek into our ancestors' relationship history may be given by sexual dimorphism, or the differences in the appearance of males and females of the same species. In primates, a general rule is this: the bigger the ratio between male/female body size, the more polygamous their mating relationships will be. 

“When there is extreme sexual dimorphism, when males might be twice the size of females, you have harems,” said Greenfield. 

In the primate world, generally, the closer the ratio between male and female body size, the more monogamous the relationship. (File illustration)

Such size disparity is seen in gorillas, where males must fight each other to acquire and defend their multiple females. In the gorilla world, being a massive, hulking male specimen is crucial. That is quite a stark difference from gibbon society, where males and females are more or less the same size, and individuals tend to couple up long term. 

Some researchers believe we can apply this reasoning to our hominid ancestors, using fossils to look back in time at whether males back then were friends or foes — and what that could mean for mating relationships. 

Australopithecus, an early hominid that became extinct about 2 million years ago, had a moderate amount of dimorphism. Male/female difference in body size was about 50 percent; in other words, if a typical female weighed 60 pounds, a male would be a decent amount heavier at 90 pounds. 

Fast forward to the time of Homo erectus. For this hominid species, who lived 1.89 million to 143,000 years ago, both males and females were much larger than the diminutive Australopithecus. But differences in body size between the sexes has diminished — for instance, 130 pounds for a female, 135 for a male — possibly hinting that competition among males was no longer a big issue. 

Perhaps coupling up, similar to how we do today, has its origins in this era?

“There's a lot of consensus that pair-bonding — in other words, love, such a powerful feeling for someone else — has to be rooted in our biology,” said anthropologist Eduardo Fernandez-Duque, a former professor at the University of Pennsylvania who now teaches at Yale University. “Most likely, it was present a long, long time ago.”

He finds discussing pair-bonding in humans much more interesting than monogamy. 

“There's nothing natural suggesting monogamy in humans, which is very different than saying there's nothing natural for pair-bonding,” said Fernandez-Duque. “During Valentine's Day, we tend to think about love — anyone who has loved knows that with love comes anxiety and extreme pain when that relationship stops.” 

Such strong pair-bonds may have helped our ancestors improve survival rates of their offspring, whose bigger brains required more time to develop and left the little ones helpless for longer. The male hunted and foraged for food to bring back to the mother, who would have had her hands full with baby. Raising a child together made it harder for both parties to fool around with others and caused them to naturally fall into a monogamous setup. 

“As babies take longer and require more to mature, that's an impetus for males to be monogamous,” Greenfield said. “The common element in monogamy seems to be that one parent isn't sufficient to deal with the needs of offspring — the idea is you need two parents around.”

Despite the prevalence of pair-bonding in human society, we don't know all that much about its evolutionary and biological origins. While the unmistakable bond between mother and baby has been studied extensively, there is a surprising lack of scientific literature about love among adults. 

“I was surprised at how little scientific research has been done on the topic,” Fernandez-Duque said. “There's a lot of talk on the sexual part of the relationships, but I'm convinced that what drives human relationships is not the sex — it's the bonding, the emotional connection.”

And so the reason romantic love is so deeply rooted within ourselves, inspiring the creation of countless works of art, music and literature, could be that it actually helped get us where we are today as a species. Despite the commercialism and mushiness of Valentine's Day, having one day per year to celebrate love doesn't seem so bad now, does it?