As an enduring mystery of the human condition, it has been praised and damned in equal measure. It is a bridge over the abyss of isolation, but a bit like croquet, too: easy enough to grasp the rules, but a hard game to enjoy.
Now scientists in Britain have taken the puzzle of monogamy and boiled it down to one big question: how did it come about in the first place? In a new study they claim finally to have an answer.
"Humans have ended up monogamous to some extent, it's the predominant way we live," said Christopher Opie, an anthropologist at University College, London. "What we have now is an evolutionary pathway for the emergence of monogamy."
How males and females come together is as varied as anything in nature. Around 90% of birds live in pairs, but less than three percent of mammals do. Humans and other primates fall somewhere in the middle, with a quarter of species pairing up, according to Opie.
What leads some species to monogamy and others not has prompted scientists to come up with three possible explanations. One says that when offspring are demanding, two parents might be better than one. The second, "mate guarding", proposes that males need to stay close to their mates in order to ward off the attentions of rival males. The third is that males stick with females to defend their offspring against the violence of other males. There is an evolutionary incentive for new males to kill existing offspring because without a baby to care for, females become fertile again - so intruding males can impregnate them.
Opie's team drew up a plan to find out which hypothesis most likely led to monogamy. They took a family tree of 230 animals, including lemurs, bushbabies, monkeys, apes and modern humans, and collected information on their mating behaviour, rates of infanticide and paternal care.
Next, they used a computer to simulate evolution from 75m years ago to modern day. As the simulation ran forwards in time, it showed how monogamy rose and fell for different species. Having run the simulation millions of times, they found that the evolution of monogamy in primates was preceded by one thing only: infanticide by males.
"You do not get monogamy unless you already have infanticide, and you do not get a switch to paternal care if you don't already have monogamy," said Opie. The research is published in the journal Proceedings of the National Academy of Sciences.
Opie says the findings are linked to increases in brain size. In primates, brain size expanded as social groups grew larger. That meant mothers were infertile for longer, and more males were close by. For males, infanticide was a strategy to make females fertile again. Monogamy was a defence against that violence.
"Monogamy is only one strategy for dealing with infanticide. But it's not the only one. Chimps mate with all the males in their group to confuse paternity so males won't attack. But in others, humans included, males stick with females to protect them," Opie said. Once a species becomes monogamous, paternal care and other behaviours evolve that help offspring to thrive, he said.
Robin Dunbar, a co-author on the paper and director of the social and evolutionary neuroscience at Oxford University, said: "It highlights the risk of infanticide and harassment for primates in general and it simply gets worse and worse as you get bigger brains, because the reproductive cycle gets longer."
Dunbar believes that monogamy evolved in our human ancestors only after modern forms emerged 200,000 years ago, and perhaps in the past 100,000 years.
For some scientists, however, the mystery of monogamy remains unsolved. Dr Maren Huck, who studies animal behaviour at the University of Derby, said the findings should be treated with "extreme caution". She argues that the authors have labelled some animals as monogamous that are not in the wild, and made some false assumptions about the impact of infanticide. "Owl monkeys give birth once a year. So if the female loses her offspring, she will not resume oestrous again until the next breeding season the following year. Hence, killing the offspring would not hasten reproduction," she said.
"I also wonder how the authors classify social monogamy', given that they claim that 25% of species are monogamous, which seems very high to me. Very few old world monkeys, for example, are monogamous."
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