The origin of monogamy

Pair-bonding in mammals is quite rare. Image: Shutterstock

Monogamy evolved in humans when low-ranking males changed tack from competing with the higher-ranked rivals to revealing their more caring side to potential suitors.

At some point in early human history our ancestors stopped mating in a promiscuous manner (well, most of them) and adopted the new, more orderly, mating system of monogamy. It is still not fully understood why this transition would have occurred.

A US scientist has used simple mathematical modelling to predict how the evolutionary quirk came about — and why it has been influential in shaping some unique features of our species.

Primate groups are generally structured in dominance-driven hierarchies, with mating privileges restricted to the few highest-ranking males. It would not have been a smooth transition for early humans to develop pair-bonding within larger groups, explained Sergey Gavrilets from the University of Tennesee in his paper published in the Proceedings of the National Academy of Sciences (PNAS) journal recently. Males would have been locked in a “social dilemma” where shifting ones effort from competition to caring for their mate would have given an advantage to free riders and male cheaters.

The researchers’ models predict that the process began when lower-ranked males started using the alternative strategy of provisioning to woo potential suitors. It developed further by the evolution of female choice and high fidelity. As the system grew more popular all the males, except for those few highest ranked in the hierarchy, would have been using the new behaviour to court their future companions. The females would have become more responsive to those highly caring, protective males and less taken by the most aggressive and dominating ones.

It is still debated whether monogamy is an evolutionarily successful mating strategy. First, it is the low-ranked (and therefore “less evolutionary fit”) males that benefit most from the type of provisioning tactics described in Gavrilets’ paper. Top-ranked males can easily intimidate and chase away those males lower in the hierarchy, which would make the “investment of low-ranked males in production wasteful”. But the scientist explained once females started to develop preferences for being provisioned, and having mating choices, the investments in low-ranked males paid off.

The development of monogamy gave rise to a number of subsequent evolutionary transitions that have been beneficial for human development and contributed to making us a relatively unique species.

“It [pair-bonding] allows for fathers to know their offspring (and vice versa). This creates conditions for the evolution of parental care, which was necessary to offset the disproportionally high costs of raising human children (because of their large brain and delayed maturity),” explained Gavrilets. “Recognition of kinship networks simplified the evolution of within-group cooperative behaviour, including alloparental care.”

Gavrilets is currently involved in several projects related to the origin of humans and the evolution of human social complexity. He plans to continue working on models aiming to describe how transitions from strongly hierarchical groups of chimpanzees to an egalitarian society of hunter-gatherers could have occurred.

Source: Eurekalert

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