Are people meant to pair off faithfully, or did nature design us to be serial philanderers, forever wrestling with the constraints of marriage?
Philosophers, priests and pop musicians have all had their say. Now an evolutionary biologist from the University of Cambridge has entered the debate with a novel attempt to measure how monogamous humans really are and how we compare with the rest of the animal kingdom.
Humans, the results suggest, are not complete angels but we do sit near the top of the league table of lifelong devotion.
The study, published in the Proceedings of the Royal Society B, examined the mating behaviour of 35 species of mammals. Humanity was represented by data from 103 separate populations, ranging from modern societies to groups that lived as long ago as the Neolithic period.
Dr Mark Dyble compared siblings who shared both parents with half-siblings — species that mate in exclusive pairs produce full siblings; those that mate promiscuously do not.
For modern humans, he used family records; for ancient ones he drew on DNA from archaeological sites, including Neolithic settlements in Anatolia, Turkey, and Bronze Age burial grounds in Europe, dating back as far as about 6,000 years.
The results gave humans an average “monogamy rating” of 66 per cent — meaning two thirds of the sibling pairs, on average, were full siblings. That put us in seventh place, comfortably above meerkats (60 per cent), but trailing beavers (73 per cent). In part, the human score reflects societies that have allowed polygamous marriage.
Beavers ranked just ahead of humans at number six
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However, Dyble argued that the figure was high enough for humans to qualify as a predominantly monogamous species.
“There is a premier league of monogamy, in which humans sit comfortably, while the vast majority of other mammals take a far more promiscuous approach to mating,” he said. “The finding that human rates of full siblings overlap with the range seen in socially monogamous mammals lends further weight to the view that monogamy is the dominant mating pattern for our species.”
Interestingly, both ancient and modern humans had similar scores. The top-ranked creature was the California deermouse, at 100 per cent — suggesting that these rodents very seldom stray, though their perfect score probably reflects a shortage of data as much as spotless fidelity. African wild dogs came next at 85 per cent — their packs are organised around long-term breeding pairs.
Meerkats are more promiscuous than humans and mole rats, according to the study
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Only one primate ranked higher than humans: the moustached tamarin, a small Amazonian monkey, which had a full-sibling rate of 78 per cent.
Our nearest evolutionary cousins were placed much lower. Chimpanzees managed a score of 4 per cent, which was on a par with dolphins. Mountain gorillas, where a dominant male mates with many females, reached 6 per cent. Scotland’s Soay sheep languished at the very bottom, their promiscuous mating habits earning them a miserly 0.6 per cent.
Why would our species rank so high? We cannot know for sure, but Dyble points to the effort required to raise a human infant. Our children are demanding creatures, born helpless, slow to mature and equipped with energy-hungry brains that take years to develop to a point where they can fend for themselves. Pair-bonding encourages paternal investment, helping to meet these demands.
Overall, the data may help correct both romantic idealism and cynical tropes about “natural” infidelity. Humans are not perfectly faithful, nor are they wired for sexual anarchy. By mammalian standards, we are remarkably monogamous — most of the time.
The Monogamy League Table top tenCalifornia deermouse: 100 per centAfrican wild dog: 85 per cent Damaraland mole rat: 79.5 per centMoustached tamarin: 77.6 per centEthiopian wolf: 76.5 per centEurasian beaver: 72.9 per centWhite-handed gibbon: 63.5 per centAnd the bottom ten… Bottlenose dolphin: 4.1 per centVervet monkey: 4 per centSavannah baboon: 3.7 per centKiller whale: 3.3 per centAntarctic fur seal: 2.9 per centJapanese macaque: 2.3 per centRhesus macaque: 1.1 per centCelebes crested macaque: 0.8 per cent