Causes of Social Monogamy in Primates

Based on readings in the textbook and our discussions in class, before reading these articles, I believed that the reason for monogamous relationships in mammals had most to do with the needs of the offspring. For example, if the offspring had no needs, there would be no parental care, like in fish or in insects. If the offspring needed more food or resources than one parent can supply, however, a male would increase his reproductive fitness by helping to care for offspring rather than going off in search of more mates. At the end of the ecology unit, that was more or less all I knew about the cause of monogamous relationships in mammals.

Both articles dealt with social monogamy, but one article dealt with social monogamy in primates in particular while the other dealt with the evolution of social monogamy in general. In the first article I read by Opie, Atkinson, Dunbar, and Shultz, the authors analyzed multiple behaviors such as a male-mate guarding strategy or the benefits of parental care as being possible causes for monogamy in primates but conclude that since “high male infanticide alone consistently preceded the appearance of social monogamy”; in their study, high male infanticide in primate populations must be the cause of social monogamy. They explain that this makes sense as the cause not only because both parents are able to defend the infant, but also because “biparental care shortens relative lactation length”, which ensures that no offspring will go unweaned and offspring will be able to reach maturity. In the second article by Lukas and Clutton-Brock, the authors argue that it is not the risk of male infanticide that is the cause of monogamy, but instead that “social monogamy is derived from an ancestral state in which females are solitary and male [geographic] ranges overlap those of several females”.

I found the article by Opie, Atkinson, Dunbar, and Shultz to be more convincing because I felt that they outlined their experiment more clearly than Lukas and Clutton-Brock did in “The Evolution of Social Monogamy” since they outlined their process step by step in their article, and I felt that their sampling method and analysis were enough to claim a clear correlation between high male infanticide rates and social monogamy because of the quantity and quality of data that they studied. In “Male infanticide leads to social monogamy in primates”, the authors used primary sources to find the primate trait data and demonstrated that sampling bias could not have affected their study.

These articles changed my understanding of monogamy in primates. To be fair, I hadn’t thought about it very much before since it always seemed like a sensible social prerogative to only seek out one mate, and I was at first completely satisfied by the parental care explanation. After reading these articles, however, I am convinced that it is high rates of male infanticide that lead to social monogamy. The most surprising thing I found in these articles is that having a period of lactation that is longer than the period of gestation can be such a problem since it keeps the mother from being able to mate, making the offspring a source of competition for a male who is not the father. The article explains that it can pay for these other males “to kill an unweaned infant” so that the female is available for mating sooner. Although these studies and what we learned in behavioral ecology do not apply to humans, I still think it’s interesting that the gestation period in humans is less than the lactation period because that isn’t advantageous in most of the animal kingdom. Obviously the threat of infanticide is less of an evolutionary prerogative in human populations.

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