Why do mammals commit to monogamous relationships?

Sorry it’s been so long…graduate school gets busy sometimes.  But in honor of the rapidly approaching new year, I’ll be blogging regularly again.  To get us all started, here’s an article I wrote a few weeks ago for Science in the News, a student-run organization dedicated to explaining the science behind news articles that pertain to scientific breakthroughs.  Anyway, it occurred to me that I might as well share it here.  Happy reading!

Our culture generally assumes that human beings are a monogamous species, with two people committed to one another for a long-term relationship.  Scientifically, the existence of monogamy seems counter-intuitive.  One of the principles of evolution is that all animals want to maximize their reproductive success.  Parents want their genes to be passed on to the next generation, and having more offspring increases the likelihood that progeny will survive into adulthood, thus increasing the prevalence of the parents’ genes in the next generation.  It is therefore advantageous for males to mate with multiple females in order to increase their chances of siring healthy offspring.  Particularly for mammals, where females have long gestation and lactation periods during which they are unavailable to mate again, it would to be to the male’s advantage to move on to a new mate, leaving the female to raise the offspring alone.  How, then, could any species have developed monogamy, if bonding with a single female counteracts the advantages of mating with multiple females?

It’s easier to explain this phenomenon for some species than for others.  Birds, for example, are overwhelmingly monogamous, and scientists reason that this is because both males and females can participate in caring for the eggs and feeding the hatchlings – the equitable division of labor gives males a reason to stick around.  Male mammals can’t help with the gestation of a fetus or the feeding of a newborn; nevertheless, around nine percent of mammalian species are monogamous.  Our closest relatives on the evolutionary chain are even more likely to be monogamous: up to a quarter of primate species live in male-female breeding pairs.

Overriding the desire to spread genes around with multiple mates

Many factors may have contributed to the development of monogamy in mammals, but there are three prevailing theories (see the figure below).  One theory postulates that males stick around after mating in order to protect their progeny from being killed by rival males.  These rivals kill the existing progeny, which don’t have their genes, so that the females will be ready to mate again more quickly.  This “infanticide” practice allows the new male to pass on his own genes.  An alternative theory, called the “discrete range” theory, is that males were forced into monogamy because competition and food availability forced females to live far apart, so that it was impossible for males to control more than one female.  A third theory predicts that the advantages of paternal care select for monogamy because the extra care and protection provided by a second parent increases the survival rate of their offspring.  Unfortunately, it has proven difficult to determine which theory was the primary force in developing monogamy.

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There are three main theories for how evolution evolved in mammals. The risk of progeny being killed by rival males may have encouraged males to stay and protect the female/offspring (A). Alternatively, females may have lived too far apart for polygamy to be sustainable, forcing males to stay with a single female (B). The addition of paternal care allows offspring to grow healthier and smarter, which may have selected for monogamy (C).

Two recent studies attempted to lay this debate to rest, but the question will have to remain unanswered for a while longer – they reached different conclusions.  A study led by Dieter Lukas at the University of Cambridge in England focused on 230 primate species (monkeys, gorillas, chimpanzees, etc.).  The lab identified which species practice infanticide, which species have females living in discrete territories, and which species are monogamous.  They then used computer modeling and statistical analysis to reconstruct the most probable evolutionary history of these three traits (more on how this works in the next section).  Based on their findings, they concluded that infanticide most often precedes a switch from polygamy to monogamy, while the distance between females (whether they lived in discrete ranges) did not correlate strongly with monogamy.  These results lead them to conclude that the primary force behind developing monogamy was the need to protect offspring from infanticide, since its practice usually corresponded with a development of monogamy.

The other study, performed by Opie et. al. at University College London, based its analysis on more than 2,500 mammals (nearly half of all mammalian species).  They, too, classified each species as monogamous or not, noted whether females live in discrete or overlapping territories, and whether males practice infanticide.  They concluded that almost every time monogamy evolved, it was in species in which females lived far from each other.  They further analyzed just the primate subsection of their data, again finding that it supported the hypothesis that discrete female territories, and not infanticide, drove the development of monogamy in species that practice it today.

The role of prior knowledge (bias) in affecting these studies’ outcomes

Although both teams used similar experimental approaches, they reached dissimilar conclusions.  The differences may come down to the details in the way they defined their data and set up the statistical analyses.  Both teams used a statistical technique called “Bayesian inference.”  This method is used to determine how true a hypothesis is based on a particular set of data.  This method provides a mathematical way to combine new evidence with prior knowledge, rather than depending on the evidence alone.  For example, imagine that you want to know the weather without checking your phone or going to the window.  You predict that it is rainy, sunny, or snowing.  You are then presented with evidence in the form of a picture of your front yard covered in snow.  Based solely on this evidence, you could decide that it is snowy outside; however, you also know it is July and you live in Boston, so your prior belief in the probability of there being snow outside your window in July is very small to begin with, making it highly unlikely that it is snowing, regardless of the pictorial evidence.  Bayesian inference provides a way for you to include that prior knowledge in the mathematical analysis.  In both of the monogamy studies, the scientists involved framed their hypotheses and defined their data based on prior knowledge.

However, it’s important to recognize that prior knowledge is inherently biased.  For example, the two research groups didn’t classify mating systems exactly the same way: one group strictly defined each species as either monogamous or polygamous, while the other group classified species that practice both living styles into both categories.  These differences may have influenced the conclusions reached in each study.

Evolution of human monogamy: a mystery for another day

Lukas et. al. also included humans in their analysis, and claim that their conclusion about infanticide driving monogamy could apply to the existence of monogamy in human societies.  However, it may be too soon to apply these studies to humans.  It’s important to note that while the majority of humans live in monogamous relationships, it is by no means the only type of relationship practiced.  Some societies allow men to have several wives, and there are examples of cultures where women marry several husbands.  The practice of monogamy with short- or long-term sexual relationships with someone outside the marriage is also relatively common.  Interestingly, the more closely scientists study “monogamous” animal species, the more examples they find of sexual liaisons by both males and females with mates outside of their monogamous pairing – these extramarital relationships may be more evolutionarily likely than previously thought.   Another consideration is that these studies focus on the males’ choices, but the females’ choices, particularly for humans, may also have played a role in the evolution of monogamy.  Future studies will be needed to determine which of these factors were important for human monogamy.  However, scientists who study humans will also have to consider something that is less prevalent in animal species: culture.  The importance of culture in driving the formation of monogamy cannot be overstated – human culture is an enormous force in shaping many aspects of our lives.  Ultimately, the evolution of monogamy in human societies is likely a combination of several or all of these forces.

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