Monogamous relationships in mammals seem to be the result of high-cost offspring that require more care than what one parent can provide over an extended period of time. Some birds, for example, necessitate a large food supply upon birth that a single parent may not be able to adequately provide. By choosing social monogamy, a male might find it more advantageous to ensure the survival of the offspring from a single mate than to seek other mates without the guarantee of viable offspring.
Mammals are K-selectionists; that is, they allocate more energy to ensuring the growth and survival of a small number of offspring instead of producing a large number. This provides a competitive advantage because although they have a decreased number of offspring, the offspring are more likely to survive because the parents invest more time and energy into ensuring their survival. Social monogamy can yield more biparental care, increasing the likelihood of offspring survival because they are better equipped to compete in an environment with limited resources.
The article by Christopher Opie et al. proposes three separate hypotheses to explain social monogamy: parental care, mate guarding, and infanticide risk. Using ancestral-state reconstructions and model rate parameters, the researchers distinguished between traits that caused social monogamy—infanticide risk—and traits that were a consequence of it—parental care and mate guarding. Male infanticide is the only trait of the three that is seen prior to the shift from polygynandry to monogamy.
According to Opie, “social monogamy facilitates a shorter lactation period compared with gestation, thereby reducing infanticide risk,” which accounts for between 34% (in gorillas) to 64% (in langurs) of all infant deaths. The article explains that by allowing females to allocate more resources to lactation because of the decreased costs of infant care, paternal care in monogamous primate species shortens the lactation period and appears to be a benefit of monogamy, though not necessarily the cause.
As primates, offspring are subjected to long development and lactation periods, and an “extended dependency period increases the time unweaned infants are vulnerable to male infanticide.” In fact, what surprised me quite a bit was if a female’s estrus cycle is delayed, a male who is not the father of the offspring may kill an unweaned infant in order to return the female to her estrus cycle. Social monogamy can provide protection against the vulnerability associated with long periods of development.
The second article by D. Lukas and T.H. Clutton-Brock proposes that social monogamy evolved in mammals because of intolerant breeding females and low female density, which makes it difficult for males to claim multiple females. Social monogamy evolved in places “where females are solitary and males are unable to defend access to more than one female at a time.” Studies of 2545 mammalian species have revealed, through parsimonious phylogenetic reconstructions, that all but one socially monogamous species in the set seem to have an ancestor that exhibited solitary females who lived in large home ranges and males who ranged independently.
These solitary females and roaming males gave way to female competition, female intolerance, and low female densities, rendering males unable to guard more than one breeding female. These females “rely on high-quality, low-density” diets which supports the evidence of more social monogamy in Primates and Carnivore than in herbivorous orders. Lukas and Clutton-Brock suggest “the evolution of low range-overlap in females and social monogamy may be a consequence of a reliance on resources of high nutritional value but low abundance.” Low abundance of food means more competition and, subsequently, a lower density of females.
I believe Lukas and Clutton-Brock make a more convincing argument because, as stated in their article, male infanticide, which is typically found in species where the lactation period is longer than gestation, is not the case in most socially monogamous species: 20/75 species, 27%. This is in comparison to 148/355 species, 44%, where females are solitary. Interestingly, both articles utilize BayesTraits’ models to provide evidence of opposing sides: Opie used a “weaning proportion” to conclude that socially monogamous species reduce their risk of infanticide “in terms of the proportion of the breeding cycle devoted to lactation.” However, Lukas and Clutton-Brock use BayesTraits’ models to conclude that they “provide no evidence of an association between the evolution of social monogamy and lactation durations that exceed gestation.”
Lukas and Clutton-Brock qualify that male infanticide seems to occur less often among socially monogamous species, but that does not seem to be a direct result because analysis of phylogenetic independent contrasts and BayesTraits’ models imply independent evolution. Both articles agree that paternal care does work toward increasing the fitness of the offspring, but that it is most likely an effect, not cause, of social monogamy.
According to Lukas and Clutton-Brock, there is a 44% possibility of a species being socially monogamous at low population densities, but that probability drops to 6% at high population densities. The article by Opie does not take population densities into account, and there is mention of both monogamous pairing and promiscuous pairing as being anti-infanticide strategies—“… females ensure paternity confusion through multiple mating with community males”—which undermines the proposal that male infanticide is the main reason for social monogamy.
Both of these articles have changed my understanding of social monogamy in primates by providing hypotheses for why and how it evolved; from an evolutionary perspective, it is considered disadvantageous as it reduces diversity. However, one must take into consideration pressure from the environment and the benefits social monogamy provides as well; the environment and type of care offspring need are significant factors affecting mating relationships. It is clear that for some species, social monogamy seems to be the more efficient breeding strategy.