Our explanation for the persistence of monogamous relationships in mammalian species is the necessity of parental care. Mammalian offspring often require more resources than other animals because of their relatively large size. Parental care primarily describes feeding, but can also include teaching of life skills. Females are most often associated with these roles, but mammalian males may also take them up in order to best provide for their offspring and therefore ensure the perpetuation of their genes. For instance, when the foraging of a sole parent, usually the mother, does not fulfill the needs of her offspring, the father will invest his energy into providing the needed nourishment to their offspring. With the investment of both parents into parental care, the costs of parental care decrease. Females sacrifice their reproductive capacity, energy, and safety to provide parental care, but when joined by male partners, all of these aspects are mitigated by sharing the roles. For instance, a female can watch over her offspring while her mate acquires food.
Opie, Atkinson, Dunbar, and Shultz propose that social monogamy in primates results from the tendency of males to kill the offspring of competing males, or male infanticide. They reasoned that when lactation exceeds the duration of gestation, females delay oestrus, or sexual receptivity, after birth to avoid having to wean multiple infants (likely because decreases the female’s survival odds). Males therefore have the tendency to kill the offspring of other males, stopping the mother from weaning, returning her to oestrus, and increasing their likelihood of mating and perpetuating genes. Male infanticide, as argued by Opie, Atkinson, Dunbar, and Shultz, decreases with and determines social monogamy. Monogamous pairing allows one or both parents to defend their offspring, decrease infanticide risk, and increase their fitness.
On the other hand, Lukas and Clutton-Brock argue that social monogamy results from certain mammal characteristics that only allow for a monogamous mating strategy. The ancestral condition that preceded social monogamy, according to Lukas and Clutton-Brock, was solitary females with individual home ranges and roaming males with territories that overlapped the ranges of multiple females. The evolved characteristics that led to social monogamy were increased female competition for highly nutritious, yet low in abundance resources, subsequent female intolerance due to limited resources, and eventually low female densities. All of these factors promoted monogamous relationships because males could only defend a single female, compared to during ancestral conditions. The females were simply spaced too far apart.
While the paper by Lukas and Clutton-Brock more generally evaluates social monogamy in mammals instead of specifically in primates, their arguments hold more water. Both articles address the three hypotheses for the evolution of social monogamy, but the article by Opie, Atkinson, Dunbar, and Shultz inadequately dismisses the ranging hypothesis. To discount the hypothesis, they write, “Within a few lineages, discrete female ranges arose independently of social monogamy.” In the Lukas and Clutton-Brock article, discrete female ranges are explained to have preceded social monogamy in ancestral conditions, therefore, it would make sense if these species had not shown monogamy without pressures to promote social monogamy.
In addition, the methodology for both articles are similar and involve Bayesian approaches to develop transition models. However, the Lukas and Clutton-Brock article cites more sources and synthesizes data for over 2500 mammals. The Opie, Atkinson, Dunbar, and Shultz article only considers 230 primate species. While it could be argued that primates differ enough from other mammals to support different hypotheses for social monogamy, it would be more sensible that monogamy evolved for the same reason in different mammals—a premise of convergent evolution (convergent because social monogamy would otherwise be present in the common ancestor of mammals and passed down to descendants).
Furthermore, Lukas and Clutton-Brock refutes an earlier analysis by Shultz, Opie, and Atkinson published by Nature in 2011: “It is likely to be a consequence of a contrast in the classification of social systems…. Some species that we classify as socially monogamous were classified by Shultz et al. as group living.” Likely, Opie, Atkinson, Dunbar, and Shultz continued to classify species as such.
Overall, Lukas and Clutton-Brock sufficiently argued their hypothesis by incorporating counterarguments and in-depth analyses each hypothesis. They included statistical data throughout, presenting well balanced and grounded data; there was no question of how a conclusion was arrived at with regard to data. In addition, their second figure was an extremely useful summation of their entire hypothesis.
Both articles considered parental care as a consequence of social monogamy, not a cause of it. Reading the logic behind this conclusion gave an example of reverse causality, which was interesting because I imagine it happens a lot in considering the traits of animals. Before, I would have seen all three hypotheses (biparental care, female ranging, and male infanticide) as all related under the umbrella of parental care. Now I recognize the difference between each due to the parsing of transitions within both articles. While initially skeptical about female ranging, it now makes great deal of sense to me because the ranging was linked to resource competition, and therefore basic survival and fitness.
A surprising statistic was that pairs who participate in monogamous relationships do not always impart biparental care according to the Opie, Atkinson, Dunbar, and Shultz article. This dynamic is interesting because it supports that the female ranging and male infanticide hypotheses can occur without biparental care—males acting solely as guards and protectors.