28 May 2012

Caring guys and choosy girls led to monogamy

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A ‘sexual revolution’ among our ancestors, led by subordinate males against alpha males, could have led to stable pair bonds and the evolution of modern families, claims a U.S. theoretical evolutionary biologist.
The evolution of the family unit

The unit family evolved from an earlier society where there was promiscuity, like in chimpanzee groups, or harems, like in gorilla groups. Credit: iStockphoto

BRISBANE: A ‘sexual revolution’ led by the masses of subordinate males and directed against the ‘rule’ of alpha males could have led to the formation of stable pair bonds in our early ancestors and the establishment of modern families, claims a U.S. theoretical evolutionary biologist.

The first ‘sexual revolution’ in the history of our species, leading from polygamy to monogamy, had major implications for the development of human society.

“Pair-bonding provided a foundation for the later emergence of the institution of modern family […and] additional processes, such as wealth accumulation and inheritance,” said Sergey Gavrilets a theoretical evolutionary biologist at the University of Tennessee in Knoxville, USA.

Modelling mate choice

Gavrilets created a theoretical model to simulate the evolution of early hominin mating systems, and his findings are published in the American journal Proceeding of the National Academy of Sciences.

In promiscuous species, the only obvious family relationships are those between mothers and their offspring. But human children are born at an early developmental stage and females need help to raise them. This becomes only possible once stable pair bonds are established and fathers know their offspring, he said.

According to Gavrilets, under a promiscuous mating system males face the decision if they should invest their limited resources into ‘appropriation’ or ‘production’: They can either fight other males to increase their dominance status and therefore the number of matings or they can invest into the survival of their offspring.

The male’s dilemma

“The species as a whole would be much better off if individuals didn’t waste their energy in conflict,” said Gavrilets. “[But] presumably our ancestral social structure was pretty much what chimpanzees have. This was a very hierarchically organised group [in which] alpha males completely dominated everything.”

Since the high-ranked males largely monopolised matings, they weren’t interested in change and the only way for a subordinate male to reproduce was to invest all his resources into fighting his way to the top.

Just like the ‘prisoner’s dilemma’, this is a situation where there is a conflict between the interests of an individual and the group as a whole. In such a situation “selfish behaviour is very difficult to overcome,” said Gavrilets.

As he shows in his model, the transition to a largely monogamous society can happen if females actively choose their mates and subordinate males offer food or paternal care to gain their affection.

“The general approach in game theory is to assume that individuals are equal” and everybody can become an alpha male. In reality, weaker males may need to employ alternative strategies to be able to reproduce.

Gavrilets’ model also explicitly considers the possibility that females become faithful to males.

“My logic was that once males start provisioning, they are motivated to find females who would be faithful to them. So males would impose selection for faithfulness on females. Simultaneously females are always interested to hook up with males who are better providers,” he said. “That creates a coevolutionary process where both provisioning and faithfulness increase in parallel.”

“At the end, except for a very small proportion of the top-ranked individuals, males invest exclusively in provisioning females who have evolved very high fidelity to their mates,” concluded Gavrilets. But the females’ faithfulness is not complete. It is the result of a balance between selection for better genes, which can be supplied by top-ranked males, and better food and care, provided largely by low-ranked males.

This new study not only provides support for one particular scenario of the transition from a promiscuous to a predominantly monogamous mating system, it models this transition long before our ancestors had developed any culture or language.

“This model deals with what animal biologists call social instincts and shows that some of these behaviours can be coded in our genes,” said Gavrilets. “Culture came much later and only augmented things that were already in place.”

Controversy about the details

Commenting on the paper, Jacobus Boomsma, director of the Centre for Social Evolution at the University of Copenhagen in Denmark stresses the importance of monogamy for human evolution. “Pair-bonding is crucial for making males invest in offspring and was a key mating system innovation after our early ancestors split off from a sister clade that was likely as promiscuous as chimpanzees are today.”

The paper makes an important contribution by showing that variation in male strength and female ability to choose and be faithful to provisioning mates can make most individuals of a population live monogamously, he wrote.

Bernard Chapais, on the other hand, said he is “rather sceptical” about this study. The paleoanthropologist at the University of Montréal in Canada said that “the assumption [that offspring needed additional care by their fathers] is based on the present-day structure of the human family […]. That assumption is not only speculative it is extremely unlikely because at that time early hominins were typical primates […] and there is no reason to think that mothers had special needs that called for parental help in the form of male provisioning.”

For Chapais, a stepwise transition to pair-bonding via a polygynous system, where individual males permanently bond with a harem of several females – as this is the case in gorillas – is more realistic.

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