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Why mammals prefer to have just one sexual partner

While people cheating on their partners is frowned upon in modern society, monogamy among mammals is something of an evolutionary puzzle. Some stick to one sexual partner for a lifetime. That is why the…

You are the one, lemur. Stay with me. Tambako The Jaguar

While people cheating on their partners is frowned upon in modern society, monogamy among mammals is something of an evolutionary puzzle. Some stick to one sexual partner for a lifetime. That is why the evolution of monogamy among mammals is hotly debated. Two studies published this week, including one I worked on, weigh in on the debate.

Evolution dictates that genes have the final say. And if there is one thing genes want, it is to spread as far and wide as possible. That is why monogamy is rare among mammals.

Females have to wait for a long gestation period to have a child, where as males could go and inseminate many other females in that time. Most male mammals behave in this manner, but some don’t, and are monogamous.

To understand why, three hypotheses have been proposed. First, it may be that as part of a monogamous pair, the father could help to care for infants, which increases their chances of survival and thus the chances of spreading the father’s genes through grandchildren. Second, monogamy could be selected for because females are sometimes solitary, spread out to maximise resources in their environment, making it hard for males to have more than one partner. Third, it may be that monogamy means that the father can stick around to protect the young from being killed by other males. Other males of the same species can seek to stop the father’s genes from spreading.

Despite years of debates regarding these theories, no consensus has yet emerged. The results of the two studies published this week take science forward, but agreement remains out of reach.

Save the children

In a study just published in the Proceedings of the National Academy of Sciences my colleagues and I tested the three hypotheses among primates. Our analysis finds that the most probable reason for monogamy is the need to protect the children.

Offspring are most vulnerable to infanticidal males when they are still being breastfed. During breastfeeding a mother delays her return to ovulating, and it is this delay that leads to the threat of infanticide by unrelated males. If the infant is killed, the female starts ovulating sooner than she would have done if the infant had lived. This is advantageous to an infanticidal male, because he can then mate with her, and have a greater chance of passing his own genes into the next generation. When infant development is slow and mothers breastfeed for a long time, having the father around provides protection for the nursing offspring.

For this analysis we gathered behavioural data across 230 primate species. These behaviours were then plotted onto a phylogenetic tree, which, like a family tree, shows the relationships between those species. We then used the same Bayesian methods that Nate Silver used to predict the outcome of the last US presidential election. Except the methods outlined what happened in the past, rather than predict the future.

We simulated evolution millions of times across the family tree to discover whether the different behaviours evolved together with mating systems across time or separately. If they co-evolved we could then discover which behaviour evolved first. Crucially it is the ability to determine the timing of the evolution of a trait that allows us to explain how the emergence of one behaviour reliably predicts another. In this way we were able to show that while paternal care and exclusive female ranges co-evolved with mating systems, they appeared following the emergence of monogamy, while infanticide preceded it.

You, me and the kids, that’s all I care about. Trey Ratcliff

Find the female

In contrast to our study, Dieter Lukas and Tim Clutton-Brock at the University of Cambridge looking for the cause of monogamy across all mammalian species, including primates, come to a different conclusion. Their analysis published in the journal Science finds that monogamy in mammals may have evolved because of solitary females.

Their study looked at data from 2500 species of mammals from all orders. And ran an analysis using different methods to ours, but still plotting behaviour on phylogenetic trees and simulating evolution. But the fact their conclusions are different than ours could also be because having a large dataset hides the trigger that led to monogamy among primates.

It is possible that different species of mammals evolved monogamy because of different reasons. Monogamy among mammals is unevenly distributed. For example, the evolution of large primate brains and long childhoods make infants far more vulnerable to infanticidal males, and may have contributed to this peculiarity among primates.

As far as the implications for human evolution of monogamy are concerned, the suggestion by Lukas and Clutton-Brock that the shift to monogamy in humans was triggered by solitary females does not stack up for a species as social as we are. Nevertheless, the debate continues, which is surely the best outcome for science.