Monogamous relationships are determined by the needs of the offspring. There is no parental care for offspring with little to no need for it, especially organisms that practice semelparity. For organisms whose offspring require more care, a single parent – often a female – may not have the capabilities to provide for it. This is true for organisms that exhibit a longer generational period. Instead of searching for another mate, males may have to help care for the offspring. Mammals tend to favor this behavior of iteroparity in hopes of increasing the survival and fitness of their offspring.
Opie et al. argues in Male Infanticide Leads to Social Monogamy in Primates that social monogamy evolved as a direct result of the high rates of male infanticide. They observed the traits of parental care and female ranges along with the phenomenon of male infanticide and concluded that only male infanticide precedes the initial shift to social monogamy. This suggested that the evolution of discrete female ranges and parental care evolved following the shift to social monogamy and are therefore not the cause of it. In The Evolution of Social Monogamy in Mammals, Lukas and Clutton-Brock refuted Opie et. al’s conclusions. Lukas and Clutton-Brock recognized a decrease in male infanticide in socially monogamous species, but the difference is not a direct association between the two. In fact, their analysis suggests that the two traits evolved independently from each other. Social monogamy, then, came from an ancestral state in which females are solitary and male ranges are greater than that of females. Because feeding competition between females was intense, breeding females were intolerant of each other, causing a low population density. As a result, the investment of energy for the males to select multiple mates proved to be too great; conditions called for the protection of an individual female.
Both studies looked at the phylogenetic traits linked to social monogamy such as parental care, discrete female ranges, and male infanticide. However, Lukas and Clutton-Brock’s argument proved more convincing. They referenced the study stating that social monogamy may be associated with the risk of male infanticide in primates, but made it clear that there should not be a generalization with primates to mammals: “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” (Lukas & Clutton-Brock). They used data from more than 2,500 mammals and identified 61 independent evolutionary transitions to social monogamy while Opie et al. combined data from solely 230 primate species. With every conclusion that they draw, Lukas and Clutton-Brock follows with small inserts of relevant data to support it.
These articles have changed my understanding of monogamy. Before reading these studies, I only thought that monogamy is needed for organisms that produce offspring with high needs of parental care and as a result a greater investment of energy. Some benefits of monogamy include the higher fitness of the offspring as well as the guarantee that both the parents’ genes are perpetuated. What I did not know was the impact that male and female ranges and male infanticide played in monogamy. I believed that parental care allowed for the perpetuation of genes through the increased viability and fitness of the offspring, causing a development in social monogamy. I was surprised when both Opie et al. and Lukas and Clutton-Brock confirmed that parental care followed social monogamy rather than preceded it.