By F. Perry Wilson, MD, MSCE

Humans, as a species, are pretty special. Maybe that seems obvious, but the truth is, the more we learn about ourselves and other species on this planet, the less unique we seem to be. There was a long period of time when Homo sapiens were thought to be unique due to our tool use. And then we found chimpanzees, dolphins, and even crows using tools. We share 98% of our DNA with chimps and bonobos. We share about 60% of our genes with fruit flies. Killer whales have culturally-shared hunting behaviors. Asian elephants bury dead calves.

We are born like other animals, get sick like other animals, die like other animals. Of course, the complexity of our society is galaxies away from those of other species. And our biology and psychology have both facilitated that complexity and adapted to it.

One way we have adapted doesn’t get talked about too much. But the adaptation is so unusual, so out-of-place considering the type of animal that we are, it must have been incredibly important. That adaptation? Monogamy.

Yes, monogamy is on my mind this week, not because we’ve hung the mistletoe and thereby increased the odds of random kisses, but because of this paper, “Human monogamy in mammalian context” appearing in the Proceedings of the Royal Society B journal, which systematically evaluated just how monogamous humans and 34 other mammalian species are. The results are fascinating.

Some context here. There is no perfect way to measure the degree of monogamy of a species. For humans, we can look at genealogies, genetics, surveys, and self-reports. For other animals, we can do some work with genetics and even direct observation.

In this new paper, Mark Dyble, an evolutionary anthropologist from Cambridge, takes a pretty clever approach. He looks at the proportion of siblings who are half-siblings.

I’ll tell you how humans fit in a bit, but let’s think through this for a minute. Imagine a species that is 100% monogamous; once they mate, they mate for life. No other partners. An example here would be the California deermouse — cute little guy. In such a species, every sibling would be a full sibling.

Now let’s think about the other end of the spectrum, a species that essentially mates randomly. The fact that a pair has mated before has no impact on whether they mate again in the future. On Dr Dyble’s list of species, the Soay sheep is the closest to a random mater. In the case of the Soay sheep, just 0.6% of siblings are full siblings.

How does this sibling-based metric hold up compared to observation? Actually, extraordinarily well. Scientists have already classified these 34 species as socially monogamous or non-monogamous based on behavioral observation.

And the demarcation is quite clear. Socially monogamous mammals have a full-sibling rate of around 70%. Social non-monogamous mammals have a full sibling rate of around 10%. There is little overlap here. The full-sibling rate seems to be a reliable indicator of social monogamy.

Now this is where it gets interesting. Let’s look at the percentage of full siblings in our closest relatives — the great apes. The chimpanzee? 4.1%. Mountain gorilla? 6.2%. These are clearly non-monogamous species. That means the monogamy in humans is a relatively recent evolutionary development.

Of course, human monogamy is influenced by human culture. You’ve probably heard the statistic that 85% of pre-industrial societies permitted polygyny— a man being paired with more than one woman at the same time. Between that fact and the clear non-monogamous nature of our primate ancestors, you might assume that our relatively monogamous lifestyles in the present are rather new, a top-down cultural imposition rather than the biologic norm.

But the study shows quite the opposite. It included data from 103 separate human populations pulled both from archaeological and ethnographical datasets. It included studies using ancient DNA from bronze age and even neolithic sites.

And here’s the thing, across the board — regardless of whether the culture was in the stone age, in the pre-industrial period with its storied tolerance for polygyny, or more modern — the full sibling rate was just under 70%.

That puts Homo sapiens firmly in the monogamous mammal class. In fact, in the full ranking, we’re near the top, just above the white-handed gibbon and just below the Eurasian beaver.

Now, I should draw an important distinction here. This sibling thing describes what we might call reproductive monogamy — monogamy in terms of what children get produced. That’s not the same thing as sexual monogamy. I mean, for most animals it is, but we should obviously realize that modern humans with access to birth control can separate the two, and studies suggest even ancient humans had a sense of family planning. And of course, a “serially monogamous” species — one that mates for an extended period but takes another partner when one dies for example — would create half-siblings.

It's also interesting that the percentage of non-monogamy and the degree of “half-siblingness” are not linearly related. Sure, at 100% monogamy you get 100% full siblings, and at 0% monogamy something like 0%, but the in-between levels look like this.

The way the math works out, small amounts of non-monogamy in a population lead to comparatively larger rates of half-siblings. We can use this to back-calculate from humans’ 70% full-sibling rate to suggest that humans have a non-monogamy rate of around 12%. Pretty darn low compared to our mammalian brethren.

It’s worth recognizing how unusual our degree of monogamy is. I already mentioned that the other great apes are quite non-monogamous. In fact, 91% of mammalian species are non-monogamous, so we’re very much in the minority to begin with. What’s odder, when you look at that small subset of monogamous mammalian species, you find a pattern — they almost all live in groups with only a single breeding pair, or where only one female out of many reproduces. There is only one other mammalian species that lives in a large group of males and females that breed in monogamous pairs — the Patagonian mara.

But basically, we are the outlier. All the other community-living mammalian species with multiple adults who breed tend to switch breeding partners quite a bit. This may increase cohesiveness within the group. Or it may be that uncertain paternity actually leads to more shared responsibility for the various young. Or at least less infanticide.

But humans have flipped that. The authors suggest that our development of monogamy — sometime after we split off from the other great apes — is because of the immense amount of effort it takes to rear a human child. With their huge brains that are just massive calorie sinks for years, kids may need two parents. What’s more, from a genetic evolutionary perspective, a full sibling has more to gain from helping to raise their little brother or sister than a half sibling. It really does take a village.

I want to clarify that this says nothing about what types of families are ethical or moral. It doesn’t say that single parents can’t raise wonderful children. What it does say is that raising children is really hard because the offspring of our species require immense amounts of care. So much care, in fact, that evolution pushed us into a very unusual mating structure to provide that care. It would seem that our responsibility now, if we are to continue this very special evolutionary path we are on, is to remember that the more care we can provide to Our offspring (and I’m using a capital O there), the better off we will all be.

Just something to think about when you spend time with family or friends, but especially children, this holiday season.