Social Monogamy: Male Infanticide or Mate Guarding?

The idea of monogamy is a fascinating subject in behavioral ecology given how it restricts the father’s ability to maximize the number of times he reproduces. Based on our reading, the reason for monogamous relationships in mammals is the need for paternal care for the offspring. Given that development is long and needs are high at birth, two parents are necessary to supply the adequate resources to create viable offspring, even if there are fewer of them.

Two papers in the summer of 2013 seem to challenge these findings, however. In “Male Infanticide leads to Social Monogamy in Primates” it is argued that male infanticide is the primary cause of social monogamy. Opie et al. suggest that although paternal care and female ranging patterns can be correlated to social monogamy, the only causal linkage present is for infanticide. First, the scientists established that social monogamy evolved from polygynandry by citing a 2012 Opie et. al paper and Shultz 2010. Next, they used Bayesian phylogenetic comparison to examine the three hypothetical causes of social monogamy. They found strong correlation for all three, but based on ancestral state reconstruction and model states reasoned only male infanticide drove monogamy, citing that there is “little to support…transition from polygyny to monogamy” in the absence of high infanticide. The second paper—“The Evolution of Social Monogamy in Mammals”—reasons the primary cause of social monogamy is female aggression and low female density. This results in males not being able to defend access to more than one female at a time. First, Lukas and Clutton-Brock determine in “all but one case” in their analysis social monogamy is derived from solitary living in females and only one instance—the Eulemur—has evolved from group living. Next, they diminish the possibility that male care or male infanticide determines social monogamy. Male care is absent in 41% of monogamous relationships and Lukas and Clutton-Brock use an analysis of transitions to determine in roughly half of the transitions, “social monogamy was already established.” Male infanticide—although reduced by social monogamy—is only found in 27% of socially monogamous species, where lactation exceeds gestation and support their findings with a Bayes Traits’ model. Finally, they established there was greater overlap of female ranges in solitary living conditions, necessitating an evolution of social monogamy and cite data for primates.

Lukas and Clutton-Brock provide a more compelling argument than Opie et. al for the causes of social monogamy because they make fewer assumptions, are  more effective in their counter-arguments, and cite more data. For example, Opie et. al’s argument is highly predicated on polygyny and group living preceding social monogamy—they note there is “little support for a transition for polygyny to monogamy with low infanticide” and that social monogamy is “more likely to evolve from polygyny in the presence of infanticide. However,  the prior claim—polygyny preceding infanticide is only supported by the citation of a previous Opie article and a paper by Shultz. Lukas and Clutton-Brock refute this by citing 61 independent transitions. Additionally, the main counter-argument Opie et. al provide against the female range density hypothesis is it might contribute to the maintenance of social monogamy but was “not a causal factor”. However, they do not do a sufficient job establishing male infanticide is causal; their reasons—such as high infanticide during social monogamy creates an “unstable state”—could be explained by correlations as well. Perhaps after social monogamy, male infanticide decreased because it was less advantageous. Finally, Lukas and Clutton-Brock cite more of their methodology and data—immediately in the introduction they note they used data for “more than 2500 mammals to identity 61 independent transitions” whereas hypothesis about infanticide cannot provide a “general explanation” because they only rely on “six transitions to social monogamy in primates”.

These articles have changed my understanding of monogamy in primates. While I was aware of the infanticide and male parental care hypothesis, I was not aware of the female range density hypothesis which intuitively and based off the data makes sense to me. The most surprising thing I read was that in some species, infanticide accounts for up to “64%…of all infant deaths”. This was surprising because the sheer number is very large and is an important statistic that seems to disprove that the infanticide hypothesis because these infanticide deaths have not resulted in transitions to social monogamy. Opie et. al however look over this, simply noting that ecological  conditions may not be sufficient, which is not persuasive given the overwhelming data Lukas and Clutton-Brock provide.

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