Dr Kit Opie and the evolution of primate monogamy

One of the biggest mysteries about human sexuality is the question of whether or not monogamy is ‘natural’.  I spoke to Dr. Kit (Christopher) Opie, an evolutionary anthropologist, and one of the authors of a paper that argues monogamy in humans evolved as an adaptive response to male infanticide.  That is; boys killing other boys babies.

About 90% of bird species are considered monogamous compared to only 3% of mammal species.  So amongst mammals like ourselves monogamy is very rare, although it is relatively more common in primates (about 25%) than in other mammalian orders.  But unusually, social monogamy appears to have evolved independently in each of the major primate clades.

Dr. Opie is of the belief that humans are indeed a naturally monogamous species.  However, what is absolutely crucial to this understanding is that an academic concept of primate social monogamy is not the same thing as our typical mainstream understanding of monogamy in humans.  But I'll get to that later.

There are 3 main hypotheses as for why humans evolved to be monogamous:


1.  Male Parental Care
2. Female Spacing
3. Male Infanticide

Dr. Opie outlined each of the three for me.  The first hypothesis, male parental care, is fairly straightforward.  The theory is that in some primate species, infants require more care than a mother can provide alone, and often specifically require a present male. 

A male can increase his reproductive success by sticking around and caring for his offspring because it increases their chances of survival.  In order to cater to this arrangement a mating system of monogamy is adopted.

A gibbon learning to walk a tightrope instead of caring for his infant.   Image via Flickr

A gibbon learning to walk a tightrope instead of caring for his infant.   Image via Flickr

But the problem with this hypothesis is that there are some primate species that are monogamous but the males do nothing in terms of looking after the infants.  Dr. Opie used Gibbons, apes fairly closely related to us, as an example. The male gibbons don’t care for the infant at all, groom them, or carry them, or anything like that. And they don’t provide any kind of predator protection, because they don’t really have any predators. 

“Monogamous male primates would love to mate with more than one female at a time, but they simply can’t because they are spaced too far apart.”

 

So why are some primate species monogamous even though the males don't need to provide any assistance to the female?  And as Dr. Opie pointed out, “some might question exactly how much care human males provide”. 

Well the second hypothesis, female spacing, provides an explanation. This hypothesis is a little weirder to think about in terms of human behaviour.  It basically says that monogamous male primates would prefer to mate with more than one female at a time, but they simply can’t because they are too far apart.

Female gibbons, for example, are sparsely placed among their habitat due to their feeding habits.  Male gibbons would absolutely love to be banging multiple lady gibbons, but they just can’t get to them because they’re too spread out! So the male gibbons have to make do with just the one female within easy reaching distance.

“It’s rare to ever find an isolated human female.”

Some researchers, such as Dr. Opie and Dr. Robin Dunbar, are not in to this hypothesis AT ALL.  Modelling shows that male gibbons are perfectly capable of monopolising more than one female at a time, and in fact, could easily cover the range of up to about 5 females gibbons.

Typical human females in a group

Typical human females in a group

Also, how could this hypothesis possibly apply to humans?  In the present, and throughout our evolutionary past, female humans have been extremely social animals.  Sparsely placed females can’t be an explanation for monogamy evolving in humans simply because it’s rare to ever find an isolated human female.

So the third hypothesis is the one that Dr. Opie finds strong support for in his research. This is that monogamous mating systems in primates function as protection against males killing the infants of other males.

“If an unrelated male kills an infant, it speeds up the mothers return to fertility.”

It used to be thought that infanticide amongst primates was a result of human encroachment on their habitat.  But Dr. Opie told me that a number of researchers have shown that actually it’s got nothing to do with that, and he explained how instead primate infanticide is seemingly related to brain size.

A few primate skulls with brain masses indicated

As primate brain size has grown throughout evolution, females have needed to lactate for longer as their infants require an increasing amount of milk (it takes longer for a larger brain to develop).  Natural human populations will breastfeed their infants for about 4 years, and some other unfortunate ape species lactate for even longer. 

Larger brains and longer lactation times = a longer delay in females returning to oestrus.  Females have a reduced chance of conceiving if they are lactating, which is a little evolutionary trick that helps to ensure two infants of different ages aren’t being breastfed at the same time.  Dr. Opie explained that this is useful because breast milk has different constituents depending on the age of the feeding infant.

So here’s where the baby killing comes in.  If an unrelated male kills an infant, it speeds up the mothers return to fertility, giving him a chance to knock her up and so up his reproductive success.

An example: gorillas have high rates of infanticide.  One silverback will take over from another silverback, and kill every single un-weaned infant in the group.  This sends all the females back in to oestrus and allows him to mate with the whole bunch and father all the subsequent offspring.

After he explained this, I told Dr. Opie that I definitely would not mate with that murderous Silverback after he killed mine AND all my friends babies.  But Dr. Opie surprised me by explaining that female gorillas have quite a lot of choice in determining the likely outcome of a fight between two silverbacks, and will often sacrifice an infant if they think the new silverback is worth it.  So... they're in to it.

So, as primates with large brain sizes, and consequently long lactation periods, rather than adopting a similar mating strategy to gorillas, we may have evolved monogamy in order to protect against male infanticide.  Keeping the father of your offspring around while you raise them provides defence from other males.  The father is invested in their own offspring’s survival and so will protect them. 

Further more, if a species is monogamous, then it’s likely that most males will be paired up with a female.  This lessens the likelihood of there being a band of marauding males providing any kind of threat to infants.  If a monogamous male goes off to threaten another male’s infant, chances are he’s left his own infant unprotected.

Dr. Opie says monogamy isn’t the only tactic that a species could opt for in order to protect offspring from infanticide.  In our closest relatives, chimpanzees and bonobos, infanticide rates are relatively low even though the risks are extremely high.

Slutty bonobos

Slutty bonobos

Instead of adopting a monogamous mating system and mating only with one male, female bonobos and chimps employ the opposite tactic, mating with ALL the males.  This leads to ‘paternity uncertainty’ in the males and prevents them from becoming infanticidal - they won’t kill any of the babies because they might have fathered them.  This is another effective strategy for dealing with infanticide.

“Why am I not mating with ALL the males?”

So why aren’t we doing it the same why as our closest ape relatives?  Why aren't human females mating with ALL the males, and instead (typically) just mating with one at a time?  More evolution stuff.

About 2 million years ago, Homo erectus, the first species belonging to our genus, evolved, and their brain sizes increased to about double that of a chimpanzee.  That would have increased lactation length to even longer than that of the chimpanzee’s 5 years, which is just a ridiculous burden on females.  If Homo erectus had used the same mating tactics as chimps, their reproductive rate would be extremely low, because females would only be able to have an infant about once every 7 or 8 years. 

Actually, Dr. Opie says chimps are at risk of going extinct with their current rate of reproduction.  And knowing that somehow Homo erectus spread throughout Africa, Asia and Europe, suggests a very rapidly expanding population. 

So it’s likely at this point in our evolutionary history, that Homo erectus adopted some kind of monogamy, and the males began providing some kind of infant care or protection. This then reduced the load on females, in turn reducing the lactation period and freeing them up to have more babies.  In all other ape species we find that the males do fuck all in terms of infant care, and we have the shortest lactation period of all (in natural human populations the inter-birth interval is about 3 ½ to 4 years).

Dr. Opie’s research used Bayesian phylogenetic methods to find the likeliest factors that led to primate monogamy and found it was very likely to have evolved as an adaptive response to control against male infanticide.  And it appears to work, as we do find that monogamous species have lower rates of infanticide.

Now before leaving you with this, I should really clarify the concept of monogamy like I said I would at the start. 

We are NOT talking about a sort of religious ideal of lifelong monogamy and faithfulness.  We are NOT talking about 'mating for life'.  In fact Dr. Opie’s paper makes specific reference to social monogamy, or pair living, which is not the same as sexual or ‘genetic’ monogamy. 

Social monogamy technically refers only to the pattern of pair living arrangements (eg. sharing territory, resources, child rearing).  Due to this arrangement, a pair is likely to also be sexually monogamous, but sexual behaviour is not strictly relevant to social monogamy.

Sexual or genetic monogamy refers to only mating or reproducing with the one same partner.  Most species that we consider to be monogamous don't tend to be strictly sexually monogamous, and actually engage in a fair amount of extra-pair copulation (aka. infidelity).  And social monogamy isn't a life contract, the pair living arrangement is more often than not, sequential.

So monogamy in these terms, as applied to humans, is simply that at least for the time of infant growth, the father would be around providing protection, and nutritional input. 

So that’s what?  5 to 10 years TOPS.  So when we say ‘humans are naturally monogamous’ that doesn’t mean humans are built to only have sex with one person for life.

It seems that it means we are most likely evolved to form strong pair bonds and socially monogamous relationships for long-term periods, and that (perceived) sexual monogamy is a little add in for paternity certainty.

And as Dr. Opie mentioned to me, the old ‘7 year itch’ fits in pretty well with the timeline of a couple meeting, falling in love, and staying together just long enough to care for an infant until the point of finishing lactation.

So it seems we've had this hunch about our natural state of social monogamy since at least the 50s.

 


Opie, Christopher, et al. "Male infanticide leads to social monogamy in primates." Proceedings of the National Academy of Sciences 110.33 (2013): 13328-13332.