The reason for monogamy is an enigma. No one knows for sure why the species that do it actually do it, and how/why it evolved. Our current idea for why mammals have monogamous relationships is that it ensures that the males can provide protection for their mating partners and offspring. In general, mammals are K-selecting species. They are generally large, live longer, and have just a few offspring which they care for extensively. Because they have so few offspring, in order to insure that their genes continue on into future generations, it can be beneficial for a male to stick around and care for his offspring and one mate.
In Male infanticide leads to social monogamy in primates, C. Opie et al. determines that the cause of social monogamy is male infanticide. C. Opie et al. provide evidence and analysis that suggests that the reason that social monogamy evolved is that primates have particularly long lactation periods which makes the infants especially vulnerable to infanticide by other males in the population. C. Opie et al. provide evidence that says that monogamy seldom evolves in populations with low infanticide rates, and that once social monogamy evolves, there is usually a reduction in infanticide. D. Lukas et al. in The Evolution of Social Monogamy in Mammals, on the other hand, suggests that the cause of social monogamy is a social system of solitary individuals. D. Lukas et al. provide evidence that social monogamy evolved in mammal populations where females do not live in close proximity to one another, making it difficult for males to have access to more than one female. D. Lukas et al. determines that monogamy comes from populations where females have a mutual intolerance of one another or where female home ranges are especially large.
D. Lukas et al. in The Evolution of Social Monogamy in Mammals make for a more convincing scientific argument. To start, D. Lukas et al. refers to more sources than C. Opie et al., 44 compared to 31. Moreover, many of the sources in the references of C. Opie et al. are written by C. Opie et al. Maybe I don’t understand how scientific journals work, but that seems like cheating to me. A bigger difference between the two articles/studies is in the sample size taken. C. Opie et al. only used 230 primate species in their research. D. Lukas et al. on the other hand, used 2545 mammalian species, more than 10x as many species representing more diversity (just primates vs. mammals in general). D. Lukas et al. was also very good at pointing out the flaws in other competing studies and explaining how and why they were wrong. They especially pointed out work done by C. Opie et al. D. Lukas et al. not only explained why they were right, but also explained why the other guys are wrong, which leads to a much more convincing argument.
Both articles kind of mentioned human monogamy in passing at the end of their articles, but I think it’s a completely different story. At this point, I’m not sure that either explanation can describe why humans are monogamous and why we have the concept of marriage. C. Opie et al. says that human monogamy comes from females choosing to stay faithful to even low-quality males for sake of securing protection and provisioning. D. Lukas et al. says that human monogamy comes from a need for extended paternal investment. I’m not one to really be speaking on the subject for any reason, but I think it might be better to talk about human monogamy in the context of sociology/anthropology, because it seems like more of a cultural thing.