Behind the Scenes of Social Monogamy

In my own perspective, the reason for monogamous relationships in mammals is that males want to protect their females and maintain their breeding rights from other males (as we learned in class, specifically about the moose), and so they stay near their females for pair-living. I also believe that in order for many offspring to develop successfully and safely, they need a male to protect them and assist the female in raising her young. In class we learned that in many animals, an advantage to pair living is the ability to raise young together. This is a complex yet believable theory as to how and why monogamy might evolve.

Christopher Opie, Quentin D. Atkinson, Robin I. M. Dunbar, and Susanne Shultz claim that the cause of social monogamy in primates is due to risk of male infanticide. They hypothesize that “it is only infanticide that reliably increases the probability of a shift to social monogamy”. When the ability of a female to mate is delayed, a male who is not the father will “kill an unweaned infant so that the female returns to oestrus sooner.” D. Lukas and T. H. Clutton-Brock claim that social monogamy in primates occurs in places where solitary females are intolerant of each other and female density is low. They hypothesize that social monogamy evolves in communities where “males are unable to defend access to multiple females”. Lukas and Clutton-Brock concluded that “social monogamy is derived from an ancestral condition” in which females and males lived on their own. When competition between females is high and population density is low, males needed to develop a more efficient breeding strategy and it is more efficient to stay with one female for a long period of time.

I believe that the article, “The Evolution of Social Monogamy in Mammals”, written by Lukas and Clutton-Brock, made a more convincing scientific argument. This article defined three different hypotheses that directly correlated to the topic at hand, identified which one they believed to be true, analyzed the evolution of that trait with social monogamy, and ‘disproved’ the other hypotheses that carried alternative explanations. With the use of phylogenetic methods, the other article’s authors, Opie, Atkinson, Dunbar, and Shultz were able to test for correlated evolution between mating systems and paternal care, infanticide risk, and mate guarding. The results of their data conclude that “high male infanticide alone consistently preceded the appearance of social monogamy across primates”, but did not utilize real world examples and statistics from a variety of animals, as the other article did quite well. The data tables and graphs in the Lukas and Clutton-Brock article very clearly portrayed the data they were trying to express. As population density decreased, the probability of social monogamy increased. This article was more convincing because it accounted for all types of extraneous details that could bias the results. They “classified all social systems” of non-human mammals (2545 species) as foragers, socially monogamous, or group living individuals and came up with results for each type. The data showed that 9% of all species were living monogamously, giving the researchers a better idea of what amount of the population they were dealing with. Lukas and Clutton-Brock also were able to provide statistics for the other hypotheses, disproving them with strong examples. For male infanticide, since it was the hypothesis the other article claimed was true, this article stated that “male infanticide was typically found in species where the duration of lactation exceeds the duration of gestation”, something not typically found in socially monogamous species.

These articles did not change my understanding of monogamy in general, because I could understand/had thought about the different hypotheses of monogamy that both articles claimed before reading the actual articles. I can now see how a high risk of male infanticide would prove monogamy useful in primates as the males would protect the female and their offspring from other males, but had never thought of that a root cause of  monogamy in primates. It was surprising to see how phylogenetic diagrams and statistics could prove the two hypotheses correct (rendering which one developed and evolved before and after social monogamy would occur), providing strong data for both sides of the comparison articles to portray in their results.

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