In many cases, mammals that engage in monogamous paring resort to it from necessity. Offspring that require more parental care due to longer periods of development cannot be taken care of by only one parent. As discussed in our textbook, fathers are able to recognize their offspring through monogamous pairing, which aids in protecting them. Both parents are needed to protect their young from predators, as they are very vulnerable during their growth. This cannot be done as efficiently in a polygamous relationship. The reason for monogamous relationships is currently undetermined, but it is believed to have occurred from the necessity of protecting offspring with longer periods of development and need for parental care.
According to the article Male infanticide leads to social monogamy in primates, “the origin of social monogamy in primates is best explained by long lactation periods caused by altriciality, making primate infants particularly vulnerable to infanticidal males.” It is believed that males kill the offspring of a female who they did not mate with to make her return to her estrus cycle sooner, therefore increasing their potential to mate with her. The offspring are most prone to this when they have long periods of development – the mother lactates for a longer amount of time than that of gestation. A monogamous relationship would prevent this because the father would know which is his offspring and would therefore protect it against infanticidal males. The reason for this system evolving in primates is because “complex sociality in primates is associated with large brains, which in turn is associated with altricial young, and long development and lactation periods.” In Figure 1, the Log10 Bayes Factor indicates that “only male infanticide precedes the initial shift to social monogamy.” (Opie et al.)
On the other hand, in The Evolution of Social Monogamy in Mammals, the authors argue that social monogamy did not evolve from the risk of male infanticide and that paternal care evolved much later. Instead they argue that social monogamy evolved in mammals from the fact that breeding females are intolerant of one another and require separation or a low density. The result of females being solitary except during mating is the inability of one male to control multiple females. “All approaches to reconstructing evolutionary sequences support this inference for the 2288 species included in the updated mammalian supertree, and the likelihood that the common ancestor was solitary is 0.99 for all approaches.” Since almost all mammalian female ancestors appear to have been solitary at some point, the development of social monogamy seems to be derived from female solitude. The authors also argue that male infanticide cannot be the the cause of the evolution of social monogamy as “an analysis of phylogenetic independent contrasts and BayesTraits’ models suggest an independent evolution of the two traits.” (Lukas et al.)
The most convincing argument was made by the article titled The Evolution of Social Monogamy in Mammals (D. Lukas, T. H. Clutton-Brock). Researchers do not know exactly whether the practice of monogamous relationships came before male infanticide and vice versa, therefore they cannot assume that one is the result of the other. Figure 1 suggests that the lower the population density, the higher the probability of a species being socially monogamous. Since the hypotheses of protection against male infanticide and the necessity of paternal care being the causes of social monogamy have been disproven in this article, as stated previously, the best explanation is the inability of males to defend more than one female. To prove this, the data in this article show that socially monogamous mammals live at much lower densities than solitary species, 15 individuals per square kilometer compared to 156 individuals per square kilometer. The fact that the authors used statistics and probability to prove the significance of the relationships between male infanticide, paternal care and female intolerance and social monogamy provides ample evidence to prove their hypothesis.
Both of these articles have changed my understanding of monogamy in primates, especially because I was still unclear as to where it came from. This occurrence is rare in primates, so I wondered what made it evolve in only some of them. I was most surprised by the fact that as this practice is not genetic but behavioral, it is not changed by natural selection. Evidence suggests that once a species develops it, it does not return to the practice of polygamy.