Grandmothering life histories and human pair bonding

Abstract

The evolution of distinctively human life history and social organization is generally attributed to paternal provisioning based on pair bonds. Here we develop an alternative argument that connects the evolution of human pair bonds to the male-biased mating sex ratios that accompanied the evolution of human life history. We simulate an agent-based model of the grandmother hypothesis, compare simulated sex ratios to data on great apes and human hunter-gatherers, and note associations between a preponderance of males and mate guarding across taxa. Then we explore a recent model that highlights the importance of mating sex ratios for differences between birds and mammals and conclude that lessons for human evolution cannot ignore mammalian reproductive constraints. In contradiction to our claim that male-biased sex ratios are characteristically human, female-biased ratios are reported in some populations. We consider the likelihood that fertile men are undercounted and conclude that the mate-guarding hypothesis for human pair bonds gains strength from explicit links with our grandmothering life history.grandmother hypothesis | human life history | human evolution | mate guarding | mating sex ratios W e call attention to evidence that connects the evolution of human pair bonds to the male-biased sex ratios in fertile ages that characterize human populations. As in mammals generally, age-specific mortality is higher in males than in females (e.g., refs. 1-3). However, this difference is overshadowed by a distinctive feature of human life history: Oldest ages at parturition are about the same in humans as in other living hominids, the great apes (4, 5), whereas longevity is substantially greater and male fertility continues to older ages (6). Exceptional longevity with a distinctive postmenopausal life stage (7-9) may have evolved in our lineage when grandmothers' subsidies for weaned dependents allowed mothers to have next babies sooner. According to this grandmother hypothesis (10-16), longevity increased as longer-lived grandmothers could help more and so left more longer-lived descendants of both sexes. Women's postfertile life stage (7) produces a bias in the sex ratio of fertile adults with repercussions for male strategies. As longevity increased, olderaged males expanded the pool of competitors for the still-fertile females. With more competitors for each paternity, males' average success in finding new mates inevitably declined until defending a current mate became the better option. Our distinctive life history thus supplies previously unrecognized support for a mate-guarding hypothesis for the evolution of human pair bonds.Here we simulate hominid mating sex ratios with an agent-based model of the evolution of human longevity via grandmothering (13, 15). We then compare simulated sex ratios to demographic data from both great apes and human hunter-gatherers. Having identified the human bias, we connect it to increased male payoffs for mate guarding, noting some broad patterns in humans, the tradeoffs observ...

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