Social Monogamy in Primates

While monogamy may not be the most advantageous method of spreading one’s genes, certain situations demand it. Most times, only the female knows for sure that the offspring is her own, thus the male usually engages in multiple reproductive opportunities in order to ensure that he has passed on his genes. In cases that the male does know that the offspring is his own, he may engage in the care for the offspring. Evolution may cause trade-offs in animals that reflect the need to increase survivability within offspring. Monogamy offers advantages such as the extra hand needed to gather resources and or to fend off predators. Females often have their hands full as they have less mating opportunities than males and thus focus more time and energy on the care of their offspring.  This trade-off between gathering resources and teaching the offspring may cause an environment where the offspring is more vulnerable and weak. In cases where the female is unable to gather enough resources for both her and her offspring, the male’s presence greatly increases the survivability of the offspring. Monogamy ensures that the male’s genes are passed on at a much higher success rate.

In Opie et al.’s article, the cause of social monogamy within primates is determined to be due to high infanticide rates. While paternal care and discrete female ranges are acknowledged to be possible causes of social monogamy, Opie et al. state that they are instead what follow the shift to monogamy. In figure 1 of Opie et al.’s article, it is shown that parental care evolves after switching to monogamy, supported by the fact that after the switch to monogamy is made, paternal care is unlikely to be lost. Discrete female ranges is acknowledged as a factor that contributes to the maintaining of social monogamy, but was not a defining factor in the development of social monogamy. Male infanticide however, was found to be a factor that preceded social monogamy. The studies indicate that once social monogamy evolves, there is a high probability that male infanticides will drop and a small probability of the primates transitioning back into polygyny with infanticides remaining at a high rate. This is because of the advantages that social monogamy provides. It is a counter-strategy to infanticides as now there will always be at least one member able to defend the offspring. Social monogamy is also shown to decrease the lactation periods, decreasing infanticide risks. This then hastens the resumption of the female’s oestrus.

In Lukas and Clutton-Brock’s article, social monogamy is due to a male mating strategy that arose due to feeding competition between females, females being intolerant of each other, and their low population density. This meant that it would be much more efficient for a male to engage in the guarding of only one female as a breeding strategy. Paternal care is not a cause of social monogamy and is instead a consequence, supported by the study of the transitions to social monogamy. “Approximately half of all independent transitions to paternal care have occurred in instances where social monogamy was already established.”(Lukas and Clutton-Brock, 2013) Male infanticide is also shown to not be a primary cause of of social monogamy as analyses of phylogenetic independent contrasts and Bayestraits models suggest that the two traits are independent of each other.

I would have to say that Lukas and Clutton-Brock make a much more convincing argument. It may possibly be due to the fact that their article addresses Opie et al.’s article, but also because of their sheer amount of data presented. Each point they arrive at is supported clearly by their data, which gives us the sample size and a number of other points that support their statements. They also clarify how they classify their terms, addressing how the difference in classification of breeding systems may alter the way species differences are interpreted. The large sample size of their study (61 independent evolutionary transitions) also enabled them to determine the link between population density and the arising of social monogamy. Lukas and Clutton-Brock are able to come up with a cause that is able to address more groups of primates, instead of Opie et al.’s reason of infanticide that is restricted to only primates that have group breeding females.

Yes, these articles have changed my understanding of monogamy in primates. Originally I only had one reason for monogamy, increased offspring survival due to the benefits conferred from paternal care. With two parents, there would be a well developed offspring that had access to sufficient resources and enough time to learn essential behaviors necessary for survival. I was surprised to see that not only was paternal care not a cause of social monogamy at all, but also the surprising amount of factors that played a part in the evolution of social monogamy. From Lukas and Clutton-Brock I was surprised to learn that the only reason primates would switch to monogamy was due to the inability to guard more than one female at a time. I had no idea that population density was a defining factor in the development of monogamy. The rates of infanticides also shocked me. It disturbed me to learn that a rival male would resort to killing the infant of another male’s in order for to increase their own chances to mate.

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