Monogamy, big brains and resilience

Meadowcreek is in a socially conservative region.  No alcohol can be sold in our county. Nearly everyone believes that people should find a mate, get married and stay together.  They believe in monogamy.  This is tough for some younger Meadowcreek residents to accept.  Some of them say monogamy is not “natural.”

Three_raccoons_in_a_treeEvolutionists mostly agree.  Since mating with only one female at a time tends to lower a male’s chances of producing as many offspring as possible, why would any any species be monogamous? Biologists consider monogamy an evolutionary puzzle.

Yet wolves, beavers, pileated woodpeckers, and many other species mate for life.  In fact brain size is correlated with monogamy.  Monogamous species have bigger brains.

Large brains are humans’ most distinctive anatomical feature. Our brains are about four times bigger than chimpanzees’ and gorillas’ brains.  Brains use twenty times the calories of muscles at rest. Brains require maintaining a constant temperature. Large brains are easily injured, and make childbirth difficult. Intelligence has many costs, yet doesn’t directly help an animal survive (e.g., a big brain doesn’t make you run faster or survive colder weather).

Many small brained species are very resilient.  The prefrontal cortex is the part of the brain responsible for abstract thinking. Possums have one of the smallest prefrontal cortexes of all mammals, yet their abundance on the rural roads indicates they are doing pretty well.

So if monogamy doesn’t make evolutionary sense and big brains aren’t needed for resilience, then why do we have either?  And why would they be associated?

The answer begins with just being social.  In 1992 British anthropologist Robin Dunbar published an article showing that, in primates, the ratio of the size of the neo-cortex to that of the rest of the brain consistently increases with increasing social group size. For example, the Tamarin monkey has a brain size ratio of about 2.3 and an average social group of size of about 5 members. On the other hand, a Macaque monkey has a brain size ratio of around 3.8 but a very large average group size of about 40 members. From this work Dunbar put forward what is now known as the “social brain hypothesis.” The relative size of the neo-cortex rose as social groups became larger in order to maintain the complex set of relationships necessary for stable co-existence. Most famously, Dunbar suggested that given the human brain ratio we have an expected social group size of around 150 people, about the size of what Dunbar called a “clan.”

Now, in the journal Proceedings of the Royal Society B, Dunbar and his colleagues have shown that the size of each individual’s social network is linearly related to the neural volume in a frontal region of each individual’s brain, the orbital prefrontal cortex. This provides strong support for Dunbar’s original conjecture at the individual level for what was previously proposed based on species-level data: Our brains are not as large as they are in order to provide each of us with the raw computational power to think our way out of a sticky situation, instead our brain size helps each of us to deal with the large and complex network of relationships we rely on to thrive.

However, many animals live in social groups but are not monogamous.  Humans are certainly not strictly monogamous. But even in polygamous relationships, individual men and women form long-term bonds — a far cry from the arrangement in chimpanzees and bonobos. They live in large groups where the females mate with lots of males when they’re ovulating. Male chimpanzees will fight with each other for the chance to mate, and they’ve evolved to produce extra sperm to increase their chances that they get to father a female’s young.

Why do males stick around and stay with one female?  Once monogamy has evolved, then male care is far more likely. Once a monogamous primate father starts to stick around, he has the opportunity to raise the odds that his offspring will survive. He can carry them, groom their fur and protect them from attacks.  One theory is that the threat of infanticide leads males to stick with only one female, protecting her and their child from other males.

In our own lineage, however, fathers went further. They had evolved the ability to hunt and scavenge meat, and they were supplying some of that food to their children. They went beyond what is normal for monogamous primates.

The extra supply of protein and calories that human children started to receive is widely considered a watershed moment in our evolution. It could explain why we have brains far bigger than other mammals.

Brains are hungry organs, demanding 20 times more calories than a similar piece of muscle. Only with a steady supply of energy-rich meat were we able to evolve big brains — and all the mental capacities that come with it.

This enabled humans to push through a ceiling of brain size.

Monogamy has many other benefits.  The hormone levels that course through monogamous primates are different from those of other species, possibly because the males aren’t in constant battle for females.  Being monogamous is less stressful.

So for a peaceful, longer life, monogamy seems to be the way to go.  Maybe the folks who live around Meadowcreek are onto something.

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