Parental care is currently the primary hypothesis for the evolution of mammalian monogamy. Mammals exhibit k-selection breeding, meaning that they invest in the viability of offspring rather than in the frequency of birth. Parental care, therefore, supports k-selection patterns in that paternal contribution to post-birth parental care increases offspring viability. The articles “Male Infanticide Leads to Social Monogamy in Primates” (Opie, Atkinson, Dunbar, Shultz) and “The Evolution of Social Monogamy in Mammals” (Lukas, Brock) provide alternative hypotheses for mammalian monogamy.
Lukas & Brock claim that the evolution of social monogamy in mammals was caused by the decreased feasibility of male promiscuity due to feeding competition between females, female intolerance, and low population density. Together, these factors necessitated social monogamy, as guarding an individual female became the most efficient breeding pattern for males. Opie et al., however, state that mammalian monogamy evolved as a consequence of high infanticide rates. In their phylogenetic comparative analysis of several primate species, infanticide rates were more consistently correlated with the development of social monogamy than paternal care and discrete female ranges. Lukas & Brock challenge these findings by pointing out their relatively low applicability to the entire mammalian domain; the infanticide findings primarily relate to species where the duration of lactation exceeds the duration of gestation, which account for only 27% of socially monogamous species, whereas species in which females are solitary account for 44%.
While I find the article by Opie et al. more clear-spoken than Lukas & Brock’s—whose writing is rife with inadequately explained mathematics and an over-reliance on “BayesTraits models”—it seems that Lukas & Brock’s hypothesis for the evolution of mammalian monogamy is more plausible. As stated above, the Opie et al. findings apply only to a small portion of socially monogamous mammals. Additionally, Lukas & Brock’s findings propose an altogether more likely-seeming course of events leading to the evolution of social monogamy: increased female competition, increased female intolerance, and decreased population density. All three of these factors are subjectively easy to imagine taking place in a mammalian habitat. Granted, the imagination is not a credible scientific source.
These articles changed my understanding of monogamy in primates. Before, I was only aware of one hypothesis for the evolution of social monogamy: paternal care. I was most interested by (in other words, the most surprising thing I read) the research methods of Opie et al. because it changed the way I think about evolutionary biology. Whereas in the past I thought only of obvious correlations between Darwinian pillars (i.e. competition) and a behavior, now I know that the antecedents of evolutionary events are oftentimes more complex, interweaved into the behaviors of a species.