Are humans monogamous?

Cian White

New insights into the evolution of monogamy and some surprising social phenomenon

Weeelll, maybe? As with everything it depends on your definition and scientists have helpfully come up with two for monogamy. Firstly; socially monogamous. Or as I call it: not monogamous. A  wife that spends the day making love to the fireman next door but comes home to tuck the kids in and kiss her husband good night is socially monogamous. Secondly; sexually monogamous. The none the wiser husband would be sexually monogamous in this case, faithful to the last and a devout follower of Moses.

So which are we? Well some of us are sexually monogamous and some of us plainly aren’t! In a paper published in 2000, it said that is estimated that 50 to 60% of married men and 45 to 55% of married women would engage in extra marital sex1.

Damn!

But hang on, let’s just take these figures with a grain of salt! There’s no reference in the paper to the studies that actually found this so I can’t check it myself, meaning  we have to put our trust in the peer review process. Also included in extra marital sex was something called cyber sex, which of course I didn’t google and so can’t tell you anything about. However, I’d agree that looking at certain content online is an act of cheating that would class you as socially monogamous! Besides, we all know someone who has physically been less than faithful to a partner right?

But let’s bring in the animals, I’m a zoologist after all, it’s what I do. Looking at this in a wider context, Homo sapiens seems to be the quite devoted partner! Only around three percent of mammals are monogamous, either socially or sexually, with beavers, wolves and the lesser known prairie vole2. The reason for this is that monogamy only occurs under a strict set of circumstances. It comes down to basically two variables3.

(The whole point of life is to propagate your genes into the next generation! It’s the big unifying theory of biology. See Sin, Sex and Animal Diversity for an outline of this.)

Firstly, are the young precocious or altrical? Which in plain English means are the young able to move about and feed for themselves straight away or do they need to be looked after.  Think of a blind, mewling Cheetah kit in comparison to a Wildebeest calf which looks like fresh and sprightly, if a bit rickety. That Cheetah kit needs much more mother love than the Wildebeest. So it may pay the male to stay and help raise the kit to ensure that his genes survive. On the other hand he could just go looking for more mates. It’s a trade off between investing time and effort in a few offspring to increase their chance of survival or to just play the numbers game.

Secondly, is the female able to be defended by a male. If a female can be defended by a male he will do so to increase the amount of genes that he will get into the next generation. If the females needs a territory size similar to a males territory size, to ensure she gets enough food, water and essential nutrients like salt, it’s another step on the road to a monogamous interaction.

territory-variable
How territory size affects sexual interactions

If the female is defended, known as mate guarding, the male can be surer of the paternity of the young, he knows he’s the father, and so it’s worth his time investing in the young.

Seems simple right? So why do only three percent of mammals exhibit monogamy. Passing on your genetic material is really a battle of the sexes. Monogamy is a midway ground where both male and female get a reasonable amount of genetic material into the next generation but not the most they could get. Females would prefer if they had multiple partners who would look after the young when born so they can get back down to the business of making babies. The males would prefer to leave all the raising to the mothers so they can continue wooing the girl next door. It’s just a fact of life that the females have to go through the pregnancy, during which they can’t have any more children, leaving plenty of time for the male to swan away, and at the end of which they are left with young. If they don’t look after them (in mammals there is nearly always at least some parental care) then their genes don’t make it and they’ve evolved to take on that responsibility.

Now after all that, where are we left with humans?

Polygamy, which in 99.9% of cases really mean polygyny, is actually quite widely practiced in the human world.

legality_of_polygamy
The non black areas are where polygyny is legal. Outside Africa only Muslims are allowed to practice polygyny.

Polygamy is where either a man or woman has multiple partners at the same time. Polygyny is where one man has multiple wives at the same time and polyandry is where a women has multiple husbands at the same time. Polyandry is a much rarer case than polygyny most notably practiced in the Tibetan mountains where land is given to each son with a wife. However, where the land is too small to be divided up all the sons marry the same wife and so polyandry. In early Christian Ireland the custom was for the younger sons to become priests or monks to ensure celibacy so that the land didn’t need to be divided up.

Africa and the Islamic countries are where polygyny is now most widely practiced but polygamy was allowed in Pre-Christian Ireland by the Brehon Laws, especially among the nobles. In fact in the Ethnographic Atlas of 1967, 1231 societies that were studied. Of these 453 practiced occasional polygamy, 588 practiced more frequent polygyny, while only 186 practiced monogamy.

Again let’s take let’s look at these numbers critically. There is  huge sample bias towards African societies and the definition of a discrete society is watery. For example there are 331 Niger- Congo societies included in that 1231, of which only 2 do not practice polygamy. Still there is a very valid argument that polygamy is practiced by at least 25% of the world’s population today, going with the map above, and was probably acceptable to a greater extent of the world’s population in the past.

On the whole however humans tend to be monogamous, or at least socially monogamous, with 75% of the world’s population living in a country where polygamy is illegal.

Going on this basis, scientists would class humans as socially monogamous. Now the question is how did the this evolve? It has always been pointed out that humans have an insane task in caring for a baby. And we do, they are completely helpless until about 5 or 6 years old! Imagine trying to look after a baby in a jungle! So it has been the conventional wisdom that men would have to help look after the child to ensure some reproductive success.

Yet there is a growing body of evidence which lends support to the theory that men only started parental care after they had become monogamous, it was a by product rather than a cause. A paper published last month in Nature says that there is a spectrum of male mating preferences based on the availability of females4. If there are lots of females, then the males will mate with lots of them and not exhibit any parental care. If it’s the other way round with lots of males and not many females it makes sense for the male to guard the female at all times to ensure paternity by stopping any sneaky matings.

Basically, the reason humans are monogamous is that back in our distant past, the sex ratio was skewed towards men! Seems unlikely that a sex ratio skew would be sustained long enough to lead to mate guarding and subsequently monogamy right? Well let’s take menopause and our long lives into account. It’s likely that, as men can reproduce at any age, men experienced a skew in the operational sex ratio (sexually active males to females) leading to a scarcity of receptive females and creating the conditions under which monogamy developed!

Then why do we see polygyny still today? Shouldn’t we all be monogamous? Well as I said before, the mating system of a species is a spectrum, which will change under different conditions. Perhaps in Africa and the Middle East, multiple male matings was again favoured by natural selection due to a change in the operational sex ratio. War, a common human pastime could lead to a scarcity of males, inverting the sex ratio leading to a situation where polygyny is favoured. This is just conjecture though, I’ve no real answer to that question. We have to keep in mind culture too, as this can propagate life styles that wouldn’t be selected for naturally. Just look at a priest!

stop-and-think
Source

References:

1 : Atwood J.D. & Schwartz L., (2002) Cyber-Sex, The New Affair Treatment Considerations. Journal of Couple & Relationship Therapy, 1:3, pp. 37-56.

2 : Kleiman D.G., (1977) Monogamy in Mammals, The Quarterly Review of Biology 52: 1, pp. 39-69.

3: Clutton-Brock, T.H. (1989) Mammalian Mating Systems, Proceedings to the Royal Society of London, 238:1285, pp. 339-372.

4: Schacht, R. and Bell, A. V., (2016) The evolution of monogamy in response to partner scarcity, Science Reports, 6, 32472; doi: 10.1038/srep32472 

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