Social Monogamy and its Evolution

Currently, the belief is that since mammals are larger organisms and mature later on in life, they need access to more parental care. Having monogamous relationships in the parental generation allows for better survival of their offspring and thus more time to mature and go on to reproduce. Mammals are known as a K strategist, meaning that they are often larger in size, care for their offspring, and have long life spans. For this reason, sharing a relationship with only one mate would ensure care for offspring.

In the article “Male Infanticide leads to social monogamy in primates,” the article proposes three hypotheses to the evolution of social monogamy in primates, among other mammals: parental care, mate guarding, and infanticide risk. The authors, including Christopher Opie, Quentin D. Atkinson, Robin Dunbar, and Susanne Shutlz conclude that only one of their hypotheses could have occurred before monogamy took place – infanticide risk. They conclude that male infanticide can only precede social monogamy as determined by the log Bayes factor. Their research suggests that socially monogamous species are less likely to have higher rates of male infanticide because social monogamy provides a “counter strategy.” By having two parents that can defend the male infant, it is more likely to survive and thus counter its predicted mortality.

They also suggest in their research that species with social monogamy and high infanticide is unstable while social monogamy with low infanticide is stable – indicating that social monogamy with low rates of infanticide is evolutionarily beneficial. They suggest that socially monogamous species are at a lower risk of infanticide than those who are not. They proposed that this is because socially monogamous species have a shorter lactation period than gestation period, which reduces the risk of infanticide. As seen in the MCMC test, the weaning population without care shows higher risk of infanticide than those with care. This may suggest that when species are socially monogamous, they offer more parental care, and therefore are at less of a risk of losing infant males that are an important part of their species.

The other article, “The Evolution of Social Monogamy in Mammals,” suggests a hypothesis that is completely different than the one found in the previous article.  Lukas and Clutton-Brock suggest that social monogamy developed as a mating strategy in cases where female density is low, breeding females are intolerant of one another, and in the case that a single male is not able to defend his access to multiple female partners. They suggest through their findings from over 2500 mammals that the ancestry of almost all of these animals, with the exception of one, suggest that the females were in solitude while the men were in overlapping ranges to the females. The shift to social monogamy, they predicted was due to the fact that the female members of the species lived in isolation and that the male members were less likely to defend more than one female.

In my opinion, “The Evolution of Social Monogamy in Mammals,” by D.Lukas and T.H. Clutton-Brock makes a more convincing scientific argument than the other article on social monogamy. This is because Lukas and Clutton-Brock support their claims with more data and more specific data. They also compare more variables, which makes their results more reliable as they are reducing the number of factors that can affect results. They also are more convincing because they support their claim that the hypothesis of social monogamy coming from male infanticide rates is not accurate. They say in response to evidence that supports the male infanticide theory; “However, this seems unlikely to provide a general explanation for the evolution of social monogamy in mammals because breeding females occur much less likely in other taxonomic groups.” This strengthens their argument because it refutes the other hypothesis.

Lukas and Clutton-Brock also test a large population size of over 2500 mammals and identify 61 independent evolutionary transitions. They also use several techniques to retrieve their data, which include regression models and parsimonious approaches. Their techniques allow them to share their results with more accuracy and confirm their results. Their research suggests that there is a 0.99 chance that the ancestral female species lived in solitude. The graph they share in figure 1 also shows a prediction based on population density of the likelihood of social monogamy to take place.

This article’s authors in contrast to the other article’s on social monogamy go to the extent to discuss all possible claims that may refute their hypothesis. They suggest, through a series of numerical findings that male care is a consequence of social monogamy. They state that according to previous research, about half of time when a shift in care occurred, it had occurred after monogamy had been established. They also discuss male infanticide as a possible hypothesis but support their claim that it is not a direct cause of social monogamy because analysis of phylogenetic models suggests that there is no correlation between the two.

I believe that reading and interpreting these articles have given me a better understanding of monogamy in primates. I now know that there are several potential factors that can influence whether a species is monogamous and why. Reading these articles makes clear the advantages of being monogamous and how that correlates specifically to primates because of their environment and genetic makeup. I think the most surprising thing that I read was in The Evolution of Social Monogamy in Mammals,” in which the authors claim that social monogamy may be a result of female being intolerable to other females. I think that was surprising to me because I did not  imagine animals to have traits that were similar to humans. The females in the population prefer to have one mate and do not appreciate if anyone else tries to hinder that.  And although the articles essentially limit themselves to social monogamy in primates, they suggest that there may be some type of correlation between parental investment and female density in populations, as seen in other species like primates.

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