Biology subjects

Hawkes, K.

Publications and source records attributed to Hawkes, K..

3 recordsLinked to original sources

bioRxiv2025

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

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.

evolutionary biology↗

bioRxiv2025

Half as high for twice as long: male bias in the fertile-age sex ratio

Humans are distinguished from our closest living relatives, other great apes, by our extended postmenopausal longevity, later first births, and shorter birth intervals. Those features likely evolved as ancestral grandmothers foraging subsidized dependants in habitats lacking foods that youngsters could manage for themselves. If so, as female post-fertile years increased, older years increased in males too. Those still-fertile old males in the paternity competition pushed the average male fertility rate below the female average. As R. A. Fisher explained, Mendelian inheritance requires equal contributions from both sexes to descendant gene pools. Higher fertility in one sex makes tendencies to overproduce it pass to more grandchildren, equalising averages in descendant generations. Yet lower average number of offspring per year in men persists, their fertility lasting decades longer than womens. Here, we present a simple mathematical model to investigate this phenomenon. We show that a male-biased sex ratio in the fertile ages is consistent with an offspring sex ratio of 1:1. We show that if male fertile careers are twice as long as those of females, the extended male fertility soon balances the higher female average, maintaining Fishers equilibrium offspring sex ratio under human life history conditions. Significance StatementWhy are offspring sex ratios usually near even in humans when the fertile careers of women and men differ so much? Fishers principle explains why parents are usually expected to produce equal offspring sex ratios. Since every sexually produced grandchild has both a mother and father, the average number of grandchildren expected through offspring of either sex depends on the sex ratio of the fertile pool. If that sex ratio is biased, the average must be higher for the rarer sex. Then any tendency to overproduce the sex with higher average fertility will spread, reducing its rarity until expected averages through sons and daughters are equal. Human life history complicates this simple logic. Female fertility ends in mid-life whereas male fertility does not, creating a male-biased fertile-age population in which average number of offspring per year is lower in males than in females. We show that sex differences in fertility duration can maintain Fishers equilibrium offspring sex ratio by altering Reproductive Value in overlapping generations, providing a foundation for persistent patriarchy.

evolutionary biology↗

bioRxiv2024

Evolution of human pair bonds as a consequence of male-biased mating sex ratios?

Compared to our closest primate relatives, human life history involves greater longevity, which includes a distinctive postmenopausal life stage. The extension of the human lifes-pan (and continued fertility in old males) without lengthening female fertility directly changes the ratio of fertile males to fertile females, called the adult sex ratio (ASR). Additionally, this affects a more fine-grained ratio, the operational sex ratio (OSR), defined as the ratio of males to females currently able to conceive. Here, we construct an ODE model with minimal age structure, in which males compete for paternities using either a multiple-mating or mate-guarding strategy. Our focus is on investigating the differences of strategy choice between populations with chimpanzee-like and human-like life histories. By simulating the system, we determine the dominant strategy and its dependence on various parameter combinations. We introduce a new measure we call the lifetime paternity opportunities (LPO) of a given male strategy. The LPO directly calculates the payoffs of different male strategies and hence enables us to predict when strategies may shift. Our results show that an increase in OSR and ASR correlates well with a change in the dominant strategy from multiple mating to guarding.

evolutionary biology↗