Based on our textbook readings and class discussions, monogamous relationships evolved in mammals in order to increase the survival of offspring and increase the male’s certainty of paternity. Monogamous relationships limit the number of opportunities a male has to mate and produce offspring, therefore decreasing his reproductive fitness. However, if an offspring requires more resources than one parent can supply, the male may increase his reproductive fitness by helping to care for the offspring and increasing its survival rather than finding a new mate. In mammals, the male’s certainty that an offspring is his (certainty of paternity) is very low due to internal fertilization and therefore male parental care is uncommon. Because social monogamy involves one male mating with one female, it may have evolved in order to increase the male’s certainty of paternity (Campbell & Reece, 2005).
Lukas and Clutton-Brock suggest that social monogamy in primates and other nonhuman mammals evolved as a result of ancestral conditions of female intolerance and low female density, which decreased male access to breeding females. In their article, they state, “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” (Lukas & Clutton-Brock, 2013). Lukas and Clutton-Brock also suggest that paternal care is a consequence rather than a cause of social monogamy, stating, “about half of all independent transitions to paternal care have occurred in instances where social monogamy was already established” (Lukas & Clutton-Brock, 2013). They also refute the idea that social monogamy evolved to allow males to protect their offspring from infanticidal competitors, stating that there is no evidence of an association between the two and that they evolved independently of each other. They ultimately reach the conclusion that social monogamy evolved in mammals where feeding competition between females was intense, breeding females were intolerant of each other, and a low female density prevented males form guarding more than one breeding female (Lukas & Clutton-Brock, 2013).
In contrast, Opie et al. suggest that social monogamy evolved in mammals as a defense against male infanticide and that the evolution of discrete female ranges and paternal care followed the shift to social monogamy (Opie et al., 2013). Opie et al. suggest that high rates of male infanticide lead to a shift towards social monogamy, which then in turn resulted in an increase in paternal care, which decreased rates of infanticide because one or more of the pair-members could defend the infant. According to Opie et al., “In primates, paternal care is associated with a shortening of interbirth intervals and an increase in reproductive rates” (Opie et al., 2013). This finding supports the idea that social monogamy, which is often accompanied by paternal care, increases reproductive rates and therefore makes monogamy more advantageous. Social monogamy also facilitates a shorter lactation period (because females can increase the resources devoted to lactation when relieved of some infant care), which also decreases infanticide risk. Ultimately, Opie et al. conclude that the cause of social monogamy in primates is male infanticide (Opie et al., 2013).
I think that the argument presented by Opie et al. is more convincing because it clearly addresses each of the three current hypotheses for the evolution of social monogamy (paternal care, female ranging patterns, and male infanticide) and presents compelling rebuttals for why paternal care and female ranging patterns could not have resulted in social monogamy. One of the methods that were used was a model of correlated evolution between mating systems and different traits, which showed correlations between the presence of social monogamy and paternal care, female ranging patterns, and male infanticide (Opie et al., 2013). However, the article acknowledges that correlation does not imply causation and goes on to explain why paternal care and female ranging patterns both evolved independently after the emergence of social monogamy. Opie et al. also present a detailed account of the methodology used to collect data at the end of the article. In this account of methodology, they also address any sources of error and how sampling issues were accounted for. This description of methodology and analysis is very clear and succinct, as opposed to Lukas & Clutton-Brock’s more disorganized description. Lukas & Clutton-Brock’s article often refutes a hypothesis by simply presenting statistical data without elaborating any further on the meaning of it. For example, when addressing social monogamy and male infanticide, Lukas & Clutton-Brock state, “an analysis of independent contrasts (t=-0.402, P=0.69) and BayesTraits’ models suggest an independent evolution of the two traits” (Lukas & Clutton-Brock, 2013). Granted, this article was likely intended for scientists who understand this statistical jargon and could make meaning of it. Nonetheless, I believe that a clear elaboration of this analysis would add to the credibility of the article.
These articles added to my understanding of monogamy in primates, but were not very surprising. With my current understanding, I agree more with Opie et al.’s conclusion that social monogamy evolved to defend against male infanticide. One idea that I found very interesting and surprising was Opie et al.’s discussion about why more primate species are not monogamous if, in fact, monogamy reduces rates of infanticide. Rates of infanticide in species such as gorillas and langurs are very high, accounting for between 34-64% of all infant deaths (Opie et al., 2013). It was surprising to me that there would not be a strong selective pressure for social monogamy in these species, but the article states that other factors such as predation pressures may also be at play. For example, predation pressures cause larger group sizes to be more advantageous and therefore social monogamy may not be as likely to evolve in this environment (Opie et al., 2013).