Currently, we believe that monogamous relationships exist in mammals because they allow for greater survival of offspring. It is more advantageous for the father to invest time in one female instead of many because it can increase the survival of his offspring with that female. If the offspring have needs that cannot be met by one parent, a mate may increase his reproductive fitness by helping to care for the offspring instead of going off in search of more mates.
According to “Male Infanticide leads to social monogamy in primates”, by Opie et al., there can only be three reasons for social monogamy: biparental care, mate guarding, and infanticide risk. From the results of the study, the authors have concluded that the social monogamy in primates is due to high risks of infanticide. In these situations, males can provide protection against other infanticidal males. Supporting this is evidence that of the three possible reasons for social monogamy in primates, only male infanticide precedes the initial shift to monogamy. The study suggests that parental care arose as a result of social monogamy because parental care shortens the lactation period by relieving the female of some of the costs of infant care and allowing her to increase resources devoted to lactation, which shortens the interbirth periods, and can therefore increase reproductive rates in primates. Biparental care is also not present in all socially monogamous mammals. Opie et al. also argue that discrete female ranges arose independently of social monogamy, and while they might have facilitated the evolution of social monogamy by making it difficult for males to monopolize more than one female, they are not the cause for social monogamy. However, Lukas and Clutton-Brock argue in “The Evolution of Social Monogamy in Mammals” that social monogamy is a result of breeding females being intolerant of each other, a low female density, and males are unable to defend access to multiple females, which created conditions where guarding females was the most efficient breeding strategy for males. Both Opie et al. and Lukas & Clutton-Brock agree that biparental care is a result of social monogamy and evolved after social monogamy because it allowed greater reproductive success for both males and females. Lukas & Clutton-Brock however, point out that male infanticide is unlikely to be the cause of social monogamy because it only occurs in a small number of monogamous species.
I think Lukas & Clutton-Brock made a more compelling argument in “The Evolution of Social Monogamy in Mammals”. Although both studies used a Bayesian approach, it seemed as if Lukas & Clutton-Brock had a larger sample size than Opie et al., and that they classified nonhuman mammalian species more specifically than Opie et al. did. They also provided exact numbers for their findings; for example, when talking about females, of the 2545 mammalian species’ social systems they used, they identified the percentage of breeding females that were classified as solitary, socially monogamous, and living in groups. Both studies also used phylogenetic reconstruction, but Lukas & Clutton-Brock do a better job at using their phylogenetic reconstruction to support their hypothesis, while Opie et al. do not use it as much to explain their results. Lukas and Clutton-Brock also talk about field studies that were done for research, something Opie et al. do not mention.
These articles have changed my understanding of monogamy in primates because I did not know that monogamy was a trait that evolved, I just assumed that primates had always been monogamous and that scientists were trying to understand why. Lukas & Clutton-Brock mentioned that in the common ancestor of all mammalian species, the females were solitary and the males occupied territories that overlapped several females, which I found very interesting because I did not think that it would be females that were solitary. I assumed that males would have territories and females would roam into their territories and mate with them. I think the most surprising aspect of these articles were the infanticidal males which are common in many primate societies. Infanticide would allow these males to get rid of infants who were dependent upon the female, therefore increasing their own reproductive fitness by increasing the chances of the female having their offspring.