The male-biased sex ratio in humans and its role in the transition from promiscuity to pair bonding

Abstract Despite decades of interest and multiple hypotheses formulated to explain the evolution of monogamy, there remains considerable disagreement about its frequency among our primate relatives and its origins in the human lineage. Two recent reviews point out inconsistencies in both the terminology used to categorise observed behaviour and the robustness of data sets commonly used to justify these hypotheses, underscoring the importance of careful analysis. Agent-based mathematical models can help mitigate some of these difficulties by providing simplified models that allow us to focus on important details and add precision to the processes involved. Here, we introduce such a model that permits us to include key life history differences between chimpanzees and humans including our unique longevity, length of post-menopausal lifestage, shorter birth intervals, and the length of our preferential relationships. To do this, we include explicit age structure, controlling for the length of the interbirth interval and mean adult longevity. By adjusting for these two parameters, we determine the dominant strategy by which males compete for paternities. We show that this change aligns with a shift in the Operational Sex Ratio (OSR), which measures the degree of competition required for each available female. We further investigate the dynamics resulting from both paternity uncertainly– caused by multiple mating males obtaining a paternity from a guarded female–and pair-bond breakup. Although several factors likely played a role in the transition to monogamy in humans, our results indicate that a male-biased sex ratio and the subsequent increase in competition for additional paternities was among them.