Let’s Stay Together: Investigating the Evolutionary History of Social Monogamy

The primary explanation we have seen so far this year for the prevalence of monogamous relationships in mammals is the need for parental care. A good percentage of mammals are often k-selectors, meaning they are characterized by, among other things, large body size and fewer produced offspring. With a smaller amount of bigger offspring, parents only have perhaps one or two chances to pass their genes along through their progeny, and said progeny are often so large that they require more resources than other animals. Thus, mammalian parents have a lot of resources to acquire, a lot of lessons to teach, and in general a lot to do for their offspring to have the best chance of survival—and therefore, for the parents’ genes survival. These jobs are usually associated with the female parent, but when both maternal and paternal care are involved, as in monogamous parenting, adequate resources and training are more likely to be acquired while keeping the offspring safe (e.g. one can protect the offspring while another searches for food). Monogamous relationships can therefore be explained with this theory in that a stronger, constant relationship where both parents spend energy attending to offspring, not searching for new mates, increases offspring viability and the survivability of parental genes.


The first article, written by Opie, Atkinson, Dunbar, and Shultz, postulates that social monogamy in primates is a result of high male infanticide, which means the killing of one male’s offspring by a separate male to make the mother receptive to mating once again, as females often delay this return when the lactation period is longer than the gestation period and they could potentially have to wean two offspring at once. After testing three possible explanations of social monogamy, Opie et al. did indeed find that for primates, “only male infanticide precedes the initial shift to social monogamy” using ancestral state reconstructions (statistical evaluations of what certain characteristics and traits were like at the beginning of organisms’ evolutionary history) and model rates (projected frequencies of states), which showed heavy correlations between the two. Opie et al.’s article claims that primate social monogamy overwhelmingly tends to succeed polygyny with high male infanticide according to a coevolution analysis (which statistically analyzes if one state—in this case social monogamy—is more likely to be dependent or independent of another factor—in this case male infanticide) and model rates.


The second article, written by Lukas and Clutton-Brock, states that for all mammals, social monogamy derived from the ancestral social system of solitary individuals. A phylogenic reconstruction allowed them to see that initially, “in the common ancestor of all mammalian species, females were solitary and males occupied ranges or territories overlapping several females.” Subsequent parsimonious reconstructions (traces of mammalian evolutionary trees using the smallest—and therefore most likely—amount of evolutionary changes) confirmed that “in all but one case, socially monogamous species in [their] data set appear to have been derived from an ancestor where females were solitary and lived in individual home ranges and males ranged independently.” Their reasoning behind this evolutionary change is that “female competition” (females competing for limited resources), “female intolerance” (multiple females unable to live close together), and “low female density” (a relatively small number of females existing across an area) resulted in males associating closely with one mate. They could only successfully defend one, as the others were too far apart. Lukas and Clutton-Brock provided evidence that this was the most likely explanation using Markov chain Monte Carlo, or MCMC, analyses (probability-based statistical evaluations) of density, body mass, and female home-range size.


Opie et al.’s explanation was more convincing, as they postulated a rationalization for only one subset of mammals—the primates. Their investigation spanned 230 primate species, close to 100% of the total number of primate species recognized by many scientists according to Palomar College; however, the Lukas and Clutton-Brock study investigated about 2500 mammal species, only about 50% of the total number (5416) according to Mammal Species of the World. Therefore, the Opie et al. study’s sample size was more comprehensive and therefore more likely to be widely accurate. Lukas and Clutton-Brock simply seemed to be trying to explain too much. Opie et al. also offered transparent evidence, displaying five concrete figures and tables backing up their work as well as specific references to where other explicit evidence could be found, as in “ancestral state reconstructions (SI Appendix and Figs. S4-S7).” Lukas and Clutton-Brock offered only one graph and a simple explanatory flowchart. The rest of their evidence was not apparent or simply given in inaccessible mathematical figures.


These articles indeed changed my understanding of monogamy in primates, as I had initially not thought of another explanation beyond the need for parental care. However, the research completed by Opie et al. and Lukas and Clutton-Brock helped me see that this was, at least according to their studies, more of a result of social monogamy or an umbrella cause than a primary reason for it. Especially after reading Opie et al.’s article, I realized how important investigating the temporal order of the appearance of characteristics must be when studying evolutionary biology, and that this is part of the reason this question remains in dispute. The most surprising thing to me about these articles was how much statistics were involved. I had been curious to see how one offers proof for evolutionary change, and thought it would be mostly centered around fossils, ancient DNA, and things like that. However, instead, both groups used some form of predictive mathematics or trend-following to discover, time and again, the “most likely” explanation for behaviors. This was a fascinating insight into all the creative ways modern scientists use to give evidence for arguments they have no real other methods to prove.

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