The Cause of the Evolution of Social Monogamy in Primates

Our current belief is that monogamous relationships in mammals evolved because it increases the survival rate of the offspring, which grants greater fitness to the parents. Mammals practice Iteroparity, in which they reproduce in smaller amounts multiple times throughout life. This is favored in environments that have higher competition for resources, as fewer, better-provisioned offspring have a better chance to survive to reproduce. This same reasoning can be applied to why some, though few, mammals have monogamous relationships. When there is high competition for resources and when offspring need lots of parental care to survive, it is in the best interest of both parents to invest parental care in the offspring. Once the first offspring is born, it will also be in the best interest of the male to stick with the female, as he will not have to waste energy in attempting to attract a mate when the next mating season comes around.

Each of the two articles determine the cause of social monogamy in primates to be different. While Lukas and Clutton-Brock pinpoint mate-guarding as the cause, Opie, Atkinson, Dunbar, and Shultz claim that male infanticide is what led to social monogamy in primates. 

Lukas and Clutton-Brock found that there are a few conditions common to all stable monogamous primate societies: the feeding competition between females must be intense, the breeding females must be intolerant of each other, and the population density must be low. Lukas and Clutton-Brock then argue that it is because of these conditions that male primates found the most efficient breeding strategy to be guarding individual females.  The evidence that socially monogamous species derived from ancestors in which the females are solitary supports this suggestion of mate guarding, as well.

Opie et al., on the other hand, conclude that male infanticide is the cause of the evolution of social monogamy in primates. Their studies show that socially monogamous species have a lower risk of infanticide than polygynous species do, which they attribute to two main factors. First, social monogamy allows for a shorter lactation period in comparison to the gestation period, which quickens the return to oestrus and thereby increases the reproductive rate of both parents – which is advantageous per Darwinian standards. Secondly, the structure of social monogamy allows both parents to defend against infanticide, which increases the survival rate of the offspring and thus the overall fitness of the parents.

While the two articles disagreed about the primary cause of the evolution of social monogamy, they did both agree that biparental care comes as a result of social monogamy, not as a cause. This agreement strengthens both of their arguments. However, I am more convinced by Lukas and Clutton-Brock. Opie et al. clearly recognize the possibility that their conclusion could be incorrect, stating that male infanticide is “the most compelling explanation,” but this is the extent of their conversation on the possibility that they are wrong. However, Lukas and Clutton-Brock, in their discussion, take each contrast that their conclusion has with other scientists’ conclusions on the same topic, and explain why they still believe that their explanation is correct. Additionally, Lukas and Clutton-Brock highlight the subjectivity of social classification  and how that impacts their results. Though Lukas and Clutton-Brock cite 13 more sources than Opie et al., I believe that both groups used sound research methods, and they both cite each other in their sources, which heightens both of their credibility by a lot. Finally, Lukas and Clutton-Brock published their article after the release of Opie et al.’s, which gave Lukas and Clutton-Brock the opportunity to show how Opie et al. is incorrect. Though this may be an unfair advantage to Lukas and Clutton-Brock, I still feel more compelled to agree with them.

The articles did not so much change my understanding of monogamy in primates as deepened it. Before I read the articles, I did not question why primates have monogamous relationships; I just assumed it was a social structure that consistently worked. Now, though, I feel that I have a much more complete view of mammalian monogamy, as I can understand the possible causes for the evolution of this counter-intuitive mating relationship. To be completely honest, the most surprising thing that I read was the rate of infanticide in primates. I had no idea that infanticide was so prevalent! 

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