Are Humans Monogamous?

Humans, like most other animals, are sexual beings. However, unlike other animals, we are an intensely cultural species. This makes understanding our sexual nature incredible difficult. As biological anthropologist Jonathan Marks remarked: “Culture is inseparable from being human [and therefore] cannot be scraped off, like the icing on a cake, to reveal the human nature below.” (Marks, 2009).

As a primatologist, if I study the sexual behaviour of a ring-tailed lemurhamadryas baboon, or white-handed gibbon, it is relatively easy to characterize their socio-sexual system, and test for the selection pressures that may have led to its development. However, the evolutionary history of these species was dominated by biological evolution. In contrast, the human condition was produced via the interaction of biology and culture, a uniquely co-evolutionary process (Wilson, 2012).

As a result, is it possible to understand our sexual nature? Do we have a sexual nature at all? Or is our sexual behaviour simply a product of complex social systems contingent upon cultural evolution? Hopefully our understanding of the evolution of primate behaviour, as well as sexual selection theory, will help us answer these questions.

Primatologists have been researching primate socio-sexual systems and developing sexual selection theory for over four decades. Over this time period we have come to realize that pairbonding in one-male/one-female sexually exclusive units is rare. In fact, only 3% of primates are known to have evolved monogamous social systems. And phylogenetic studies have shown that all monogamous systems are derived states that have convergently evolved 7-10 times (Fuentes, 1998). Behavioural studies have also shown that monogamous behaviour tends to be flexible and conditional on numerous ecological variables. As a result, ancestral non-monogamous sexual states are often used as alternatives to monogamy in different circumstances.

This means that although monogamy is expressed within our order, it is not common, and it is certainly not a stable evolutionary strategy. Furthermore, no primate exhibits exclusive monogamous behaviour over an entire lifetime. Take for example, the gibbon, a lesser ape classically used as an example of primate monogamy. Researchers originally believed that the gibbon was both socially and sexually monogamous. They lived in one-male/one-female adult pairs, and appeared to remain exclusively pairbonded with one individual for decades. However, long-term studies have revealed that 12% of gibbon copulations are “extra-pair” copulations that their pairbonded partner is unaware of (Reichard, 1995). Essentially, gibbons get married and then cheat.

But why even form these systems in the first place? Even though monogamy is rare, and very rarely exclusive over the course of an individual lifespan, it has evolved 7-10 times independently within our order. There must be some important benefits to being exclusive.

Surprisingly, there may be as many as four different selection pressures for the behaviour: male defense of resources, infanticide reduction, direct male care, and male mate guarding. However, none of these pressures is universally necessary, and it is likely that a different combination of these pressures has caused different “types of monogamy” to evolve in different primate species currently defined as monogamous or pairbonded.

This information can all be a little overwhelming and hard to make sense of. Monogamy is rare, never completely explicit, and the evolutionary benefits are highly variable. What can this mess tell us about human sexuality?

It is evident that pairbonds between adult males and females has been a massive component of human socio-sexual systems both throughout history and in contemporary times. Did we evolve to organize ourselves in this way? Were early modern humans in the Middle Paleolithic living in systems we would classify as monogamous? Or did our ancestors organize themselves in different sexual systems? And if monogamy was uncommon in the past, why does it appear to be the system that has been culturally promoted throughout most of written history by various culture groups?

There are some important evolutionary clues regarding the composition of our ancestral socio-sexual system. The biggest comes from our pronounced sexually dimorphic traits. Human males are on average taller, heavier, and stronger. Our level of dimorphism is moderate when compared to say, gorillas, which are highly sexually dimorphic. However, our level of dimorphism is characteristic of a species with a moderately polygynous mating system with higher levels of male-male competition for mates, than female-female competition for mates. Past behaviours do not fossilize, but our dimorphism indicates a combination of moderate male harem building and strong female mate choice for large body size.

During the formation of early city-states between 5,000-10,000 years ago, several human populations made a transition from a traditionally hunter-gatherer lifestyle to a settled agricultural existence. Many evolutionary theorists contend that during this transition, the demands of agricultural life distorted human mate choice patterns. Humans were increasingly sedentary and had to prepare and maintain a plot of land for an entire lifetime. This new system required long-term investment from two adult individuals. Also, for the first time in human history, individuals could accumulate substantial wealth and surplus resources. As a result, males could use a lifetimes worth of resources as leverage against other males in competition for access to mates. In order to level the playing field, it is likely that many early city-states promoted monogamy, to avoid male harem building and avoid the collapse of early agricultural networks (Sanderson, 2001).

So does this mean that we are all naturally polygynous? Are cultures that promote monogamy simply the product of the early agricultural city-states attempt to promote equality of mating opportunity between disproportionately wealthy males?

You may have guessed that it isn’t that simple. From a neurological perspective humans really are designed to pairbond. Chemicals like oxytocin and vasopressin are released in our brains when we establish long-term sexual relationships with one individual. Studies have also shown that humans that form long-term pairbonds live longer and are psychologically healthier. Pairbonds also serve important evolutionary functions.

Quinlan & Quinlan (2007) conducted a massive cross-cultural study on human pairbonds in order to understand what specific pressures may have selected for pairbond formation. They discovered that human pairbonds form and are most stable cross-culturally when paternal investment and male-male competition is high. Their results indicated that a pairbond with little paternal investment is nearly worthless to women. As a result, the bond quickly disintegrates. Interestingly, pairbond stability was also unstable when male contribution was disproportionately higher than female contribution. The most stable pairbonds formed with equal contribution rates (Figure below). Pairbonds were also the most prevalent and stable when male-male competition for mates was high. Combined, this indicates that monogamous human socio-sexual systems are most likely when subsistence requires reciprocal cooperation, and when females have more control over mate choice than males.

Disparity in mate choice may be important because males, by nature, are less choosy. Females invest more matter and energy into producing eggs than males invest in sperm. Consequently, potential male fecundity increases with increase in mating partners, whereas female fecundity does not (Trivers, 1972). Ergo, it shouldn’t surprise us that reducing male mate choice is key to establishing stable pairbonds.

This all matters regarding our evolutionary interpretation of monogamy. The Quinlan & Quinlan study provides solid data that there are important adaptive functions of monogamy actually being played out among human societies today, regardless of culture. It shows that there are important ecological and environmental mechanisms that can increase (or decrease) the probability that humans will exhibit monogamous behaviour.

But are we getting closer to our answer? Of course humans can be monogamous, but are they monogamous?

After analyzing the data and theory evolutionary studies has to offer, it seems evident that we are a sexual hybrid. Within certain socio-cultural and environmental settings, humans are biologically capable of engaging in the most intensely monogamous behaviour within the Order Primates, and perhaps the entire animal kingdom. Pairbonding has really strong neurological effects that have been selected for, and offer us some really important long-term benefits.

However, as with other “monogamous” primates, polygyny is almost certainly our ancestral state. And like other “monogamous” primates, in certain circumstances we can use our ancestral state as a viable alternative to monogamy.

In conclusion, it may be a general rule among primates that species with derived monogamous socio-sexual systems are by nature highly flexible sexually and exist as sexual beings conditionally upon important ecological variables.

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References:

Fuentes, A.  1998.  Re-Evaluating Primate Monogamy.  American Anthropologist, 100: 890-907.

Marks, J.  2009.  Nature/Culture.  pp. 260-279.  In: Why I Am NOT A Scientist.  Berkely: University of California Press.

Quinlan, R.J. & Quinlan, M.B.  2007.  Evolutionary Ecology of Human Pair-Bonds: Cross-Cultural Tests of Alternative Hypotheses.  Cross-Cultural Research, 41: 149-169.

Reichard, U.  1995.  Extra-pair copulations in a Monogamous Gibbon (Hylobates lar).  Ethology, 100: 99-112.

Sanderson, S.K.  2001.  Explaining Monogamy and Polygyny in Human Societies: Comment on Kanazawa and Still.  Social Forces, 80: 329-335.

Trivers, R.  1972.  Parental investment and sexual selection.

Wilson, E.O.  2012.  The Social Conquest of Earth.  New York: W.W. Norton.

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About Cadell Last
Hello. I'm probably drinking coffee and reading.

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