In order for monogamous relationships to become prevalent among mammals, they must provide some advantage over other mating patterns. The problem, then, is to determine what exactly the advantage is. According to both articles, there are three big possible explanations for the evolution of monogamy in mammals:
Additional parental care increases both offspring survival rates and the number of times a female can reproduce;
In situations where females occupy small, defined ranges, monogamy allows males to monopolize a female and guard her from rival males;
A present father can protect offspring against infanticidal males who want to kill the infants for the purpose of more quickly returning the female to a fertile state.
(Opie et al., Male infanticide leads to social monogamy in primates; Lukas et al., The evolution of social monogamy in mammals)
The two articles concur that while paternal care is correlated with monogamy, it likely comes after the appearance of monogamy, not before. As Lukas et al. write, “Although paternal care and social monogamy are associated, an analysis of transitions suggests that male care is probably a consequence rather than a cause of the evolution of social monogamy.” Opie et al. agree: “It may be that rather than high-cost offspring giving rise to monogamy, monogamy enables the production of high-cost offspring.”
Where the articles disagree is on what, if not paternal care, is the driving force for monogamy. Opie et al. argue that it is the third reason, male infanticide, that has caused monogamy to evolve. The authors claim that, “Of the traits we tested, high male infanticide alone consistently preceded the appearance of social monogamy across primates.” Because infanticide preceded the appearance of monogamy while the other possibilities followed it, infanticide is more likely to be a factor in the evolution of monogamy while the rest are results of the shift.
Lukas et al., on the other hand, argue that monogamy developed primarily as a mate guarding strategy. The say that social monogamy “has evolved where females are solitary and males are unable to defend access to more than one female at a time. Evidence that socially monogamous species are derived from ancestors where females are solitary supports this suggestion.”
Although it was difficult to work through these articles, I had even more trouble understanding why only one explanation can be right. The article by Lukas et al. states that
“a recent comparative analysis of primate social systems … concluded that social monogamy is derived from an ancestral condition where both sexes are social and live in unstable groups, supporting the suggestion that its evolution may be associated with the risk of male infanticide. However, this seems unlikely to provide a general explanation for the evolution of social monogamy in mammals because groups of breeding females occur much less frequently in other taxonomic groups.”
The authors acknowledge that male infanticide is a possible cause of monogamy, but they dismiss it because these conditions rarely occur in non-primate mammals. Why isn’t it possible that infanticide was a major pressure for the evolution of monogamy in primates, but mate guarding was the primary pressure for other mammals?
If I were to choose the one most convincing article, I would go with Lukas et al. Their research investigated both a greater number and a greater variety of species (2288 species of mammals, versus 230 species of primates). They also provided more numerical evidence to support their conclusions:
“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. … In all but one case, socially monogamous species in our data set appear to have been derived from an ancestor where females were solitary and lived in individual home ranges and males ranged independently.”
Opie et al., on the other hand, repeatedly claim that “high male infanticide alone consistently preceded the appearance of social monogamy across primates,” but never provide data in the text to support this assertion. The authors also acknowledge that their findings “[raise] the question of why more primate species are monogamous;” the fact that the authors had to come up with additional explanations for why many primate species with extremely high infanticide rates are not monogamous makes their main conclusion less convincing.
It’s easy to forget that behaviors like monogamy were not always present. These articles have changed my understanding of monogamy mostly by reminding me that it evolved, and by forcing me to consider why that might have been.