Monogamy Evolved As A Mating Strategy
July 30, 2013

Male Monogamy Evolved To Thwart Infanticide

redOrbit Staff & Wire Reports - Your Universe Online

The evolution of monogamy among male mammals appears to have been triggered by the threat of infants being killed by unrelated males, researchers reported on Monday in the journal PNAS.

The study is the first to reveal this evolutionary pathway for the emergence of pair living, and contradicts the theory monogamy resulted from a need for extra parental care by the father.

The researchers also found males were more likely to care for their offspring following the emergence of monogamy.

"This is the first time that the theories for the evolution of monogamy have been systematically tested, conclusively showing that infanticide is the driver of monogamy. This brings to a close the long running debate about the origin of monogamy in primates," said Dr. Kit Opie at the University College London, lead author of the study published in the journal PNAS.

Infants are most vulnerable when they are fully dependent on their mother because females delay further conception while nursing slowly developing young. This leads to the threat from unrelated males, who can bring the next conception forward by killing the infant.

Sharing the costs of raising young both shortens the period of infant dependency and can allow females to reproduce more quickly, the researchers said.

An additional benefit of sharing the burden of care is females can then have more young. The considerable cognitive requirements of living in complex societies have resulted in many primate species having large - and costly - brains. However, growing a big brain is expensive and requires offspring mature slowly.

Caring fathers can help alleviate the burden of looking after young with long childhoods, and may explain how large brains could evolve in humans, the researchers said.

Humans, unique among primates, have both very long childhoods and mothers that can reproduce quickly relative to other great apes. Until now, a number of hypotheses have been proposed to explain the evolution of monogamy among mammals. These include:

- Paternal care, when the cost of raising offspring is high

- Guarding solitary females from rival males

- Infanticide risk, where males can provide protection against rival males

To uncover the evolutionary pathway, the researchers gathered data across 230 primate species, and plotted them on a family tree of the relationships between those species. Bayesian methods were used to re-run evolution millions of times across the family tree to discover whether different behaviors evolved together across time, and if so, which evolved first.

This allowed the researchers to determine the timing of trait evolution and show male infanticide is the cause of the switch from a multi-male mating system to monogamy in primates, while bi-parental care and solitary ranging by females are a result of monogamy, not the cause.

"What makes this study so exciting is that it allows us to peer back into our evolutionary past to understand the factors that were important in making us human. Once fathers decide to stick around and care for young, mothers can then change their reproductive decisions and have more, brainy offspring," said researcher Dr. Susanne Shultz at the University of Manchester.