The evolution of altruism through war is highly sensitive to population structure and to civilian and fighter mortality

Many evolutionary theorists have suggested that the human capacity for altruism was forged in war, with cohesive and altruistic groups outcompeting their selfish neighbors. Assessing this “parochial altruism” hypothesis relies largely on computational modeling. Here, I reexamine a well-known model that explores the coevolution of altruism and war. As well as clarifying the importance of factors such as the lethality of war to fighters and civilians, the results show that the evolution of altruism in this model relies on a degree of genetic differentiation between groups that exceeds that seen among hunter-gatherers. Furthermore, when the model produces a more realistic population structure, altruism does not evolve, casting doubt on the plausibility of the parochial altruism hypothesis

Contrasts in kinship structure in mammalian societies

Comparative studies of mammals confirm Hamilton’s prediction that differences in cooperative and competitive behavior across species will be related to contrasts in kinship between group members. Although theoretical models have explored the factors affecting kinship within social groups, few have analyzed the causes of contrasts in kinship among related species. Here, we describe interspecific differences in average kinship between group members among social mammals and show that a simple mathematical model that includes the number of breeding females, male reproductive skew, and litter size successfully predicts ~95% of observed variation in average kinship between group members across a sample of mammals. Our model shows that a wide range of conditions can generate groups with low average relatedness but only a small and rather specific set of conditions are likely to generate high average levels of relatedness between their members, providing insight into the relative rarity of advanced forms of cooperation in mammalian societies

Limited Open Access in socio-ecological systems: how do communities deal with environmental unpredictability?

Classical theory on the commons holds that rules are fundamental to sustainability. However, open access may be present in many sustainable socioecological systems. Here, we explore the interaction between environmental unpredictability and cooperation in a fishery in the Pantanal wetland, Brazil. We show that a variable annual flood pulse combined with channel blockages results in a high turnover in fishing grounds. To counter this variability, fishers openly share information about fishing areas with all community members, but are highly territorial with neighboring communities. We argue that this open access within communities but common property between communities represents a system of limited open access and, using a mathematical model, suggest that such a system is favored under conditions of moderate competition and high levels of resource unpredictability. Failing to take into account the social norms that underpin limited open access systems may undermine conservation interventions

Accurate age estimation in small-scale societies

Precise estimation of age is essential in evolutionary anthropology, especially to infer population age structures and understand the evolution of human life history diversity. However, in small-scale societies, such as hunter-gatherer populations, time is often not referred to in calendar years, and accurate age estimation remains a challenge. We address this issue by proposing a Bayesian approach that accounts for age uncertainty inherent to fieldwork data. We developed a Gibbs sampling Markov chain Monte Carlo algorithm that produces posterior distributions of ages for each individual, based on a ranking order of individuals from youngest to oldest and age ranges for each individual. We first validate our method on 65 Agta foragers from the Philippines with known ages, and show that our method generates age estimations that are superior to previously published regression-based approaches. We then use data on 587 Agta collected during recent fieldwork to demonstrate how multiple partial age ranks coming from multiple camps of hunter-gatherers can be integrated. Finally, we exemplify how the distributions generated by our method can be used to estimate important demographic parameters in small-scale societies: here, age-specific fertility patterns. Our flexible Bayesian approach will be especially useful to improve cross-cultural life history datasets for small-scale societies for which reliable age records are difficult to acquire

Why so many Agta boys? Explaining ‘extreme’ sex ratios in Philippine foragers

Male-biased sex ratios have been observed in multiple small-scale societies. Although intentional and systematic female-biased mortality has been posited as an explanation, there is often a lack of ethnographic evidence of systematic female neglect and/or infanticide. The Agta, a foraging population from the Philippines, have a skewed sex ratio of 1.29 (129 males per 100 females) aged 15 years or under. We hypothesised that this skew was not caused by greater female deaths, but due to an adaptive response, where more males were produced at birth in reaction to high male-biased extrinsic mortality. To test this hypothesis we utilise census, childcare and mortality data from 915 Agta. The Agta's sex ratio is significantly male-biased in the <1 (n = 48, 2:1) and 1–5 year (n = 170, 1.39:1) age cohorts; however, we find no evidence of systematic female neglect in patterns of childcare. Furthermore, the sex ratio decreases over cohorts, becoming balanced by the end of the juvenile period, owing to significantly higher male mortality. Taken together, these results are not supportive of female infanticide or neglect, and instead suggest an adaptive mechanism, acting in utero as a response to male-biased juvenile mortality, following Fisherian principles of equalising parental investment

Resilience of social-ecological systems: drastic seasonal change is associated with economic but not social flexibility among fishers in the Brazilian Pantanal

In attempting to predict the impact of major ecological or climatic change on livelihoods, insights can be gained by looking at communities who experience extreme seasonal or annual variation. Here, we compare the ecology, economy, and social network of a community of traditional fishers in the Brazilian Pantanal between the dry season and the flood season in which their wetland ecosystem is transformed. Using data derived from satellite imaging we show that during the flood season of 2019 the total amount of open water accessible to fishers more than doubled and led to drastic qualitative changes. We show that although fishers adapted to this extreme seasonality by changing where, how, and what they fish between seasons, the structure of the social network in our study community did not differ. We argue that strong networks are especially important in social-ecological systems with extreme seasonal changes. More generally, we suggest that case studies of seasonal adaptation such as ours can contribute to a broader understanding of how communities may be able to successfully adapt to novel social-ecological changes

Global WEIRDing: Transitions in Wild Plant Knowledge and Treatment Preferences in Congo Hunter-Gatherers

Cultures around the world are converging as populations become more connected. On the one hand this increased connectedness can promote the recombination of existing cultural practices to generate new ones, but on the other it may lead to the replacement of traditional practices and global WEIRDing. Here we examine the process and causes of changes in cultural traits concerning wild plant knowledge in Mbendjele BaYaka hunter–gatherers from Congo. Our results show that the BaYaka who were born in town reported knowing and using fewer plants than the BaYaka who were born in forest camps. Plant uses lost in the town-born BaYaka related to medicine. Unlike the forest-born participants, the town-born BaYaka preferred Western medicine over traditional practices, suggesting that the observed decline of plant knowledge and use is the result of replacement of cultural practices with the new products of cumulative culture

Wealth, health and inequality in Agta foragers

Supplementary data are available online at https://academic.oup.com/emph/article/11/1/149/7162654#supplementary-data .Copyright © The Author(s) 2023. Background and objectives: There is significant evidence from large-scale, industrial and post-industrial societies that greater income and wealth inequality is negatively associated with both population health and increasing health inequalities. However, whether such relationships are inevitable and should be expected to impact the health of small-scale societies as they become more market-integrated is less clear. Methodology: Here, using mixed-effect models, we explore the relationship between health, wealth, wealth inequality and health inequalities in a small-scale foraging population from the Philippines, the Agta. Results: Across 11 camps, we find small to moderate degrees of wealth inequality (maximal Gini Coefficient 0.44) which is highest in the most permanent camps, where individuals engage more heavily in the formal market. However, in both adults (n = 161) and children (n = 215), we find little evidence that either wealth or wealth inequality associates with ill health, except for one measure of nutritional condition—red blood cell count. Conclusions and implications: We interpret these results in the light of high levels of cooperation among the Agta which may buffer against the detrimental effects of wealth inequality documented in industrial and post-industrial societies. We observe little intergenerational wealth transmission, highlighting the fluid nature of wealth, and thus wealth inequality, particularly in mobile communities. The deterioration of nutritional status, as indicated by red blood cell counts, requires further investigation before concluding the Agta’s extensive cooperation networks may be beginning to breakdown in the face of increasing inequality.A.E.P. received funding from the MRC & DFID (MR/P014216/1). A.B.M. received funding from the Leverhulme Trust (RP2011-R 045). D.S. was supported by the John Templeton Foundation (grant ID: 61917)

Oral microbiomes from hunter-gatherers and traditional farmers reveal shifts in commensal balance and pathogen load linked to diet

Maladaptation to modern diets has been implicated in several chronic disorders. Given the higher prevalence of disease such as dental caries and chronic gum diseases in industrialized societies, we sought to investigate the impact of different subsistence strategies on oral health and physiology, as documented by the oral microbiome. To control for confounding variables such as environment and host genetics, we sampled saliva from three pairs of populations of hunter‐gatherers and traditional farmers living in close proximity in the Philippines. Deep shotgun sequencing of salivary DNA generated high‐coverage microbiomes along with human genomes. Comparing these microbiomes with publicly available data from individuals living on a Western diet revealed that abundance ratios of core species were significantly correlated with subsistence strategy, with hunter‐gatherers and Westerners occupying either end of a gradient of Neisseria against Haemophilus, and traditional farmers falling in between. Species found preferentially in hunter‐gatherers included microbes often considered as oral pathogens, despite their hosts' apparent good oral health. Discriminant analysis of gene functions revealed vitamin B5 autotrophy and urease‐mediated pH regulation as candidate adaptations of the microbiome to the hunter‐gatherer and Western diets, respectively. These results suggest that major transitions in diet selected for different communities of commensals and likely played a role in the emergence of modern oral pathogens

Characterization of hunter-gatherer networks and implications for cumulative culture

Social networks in modern societies are highly structured, usually involving frequent contact with a small number of unrelated friends' 1. However, contact network structures in traditional small-scale societies, especially hunter-gatherers, are poorly characterized. We developed a portable wireless sensing technology (motes) to study within-camp proximity networks among Agta and BaYaka hunter-gatherers in fine detail. We show that hunter-gatherer social networks exhibit signs of increased efficiency 2 for potential information exchange. Increased network efficiency is achieved through investment in a few strong links among non-kin friends' connecting unrelated families. We show that interactions with non-kin appear in childhood, creating opportunities for collaboration and cultural exchange beyond family at early ages. We also show that strong friendships are more important than family ties in predicting levels of shared knowledge among individuals. We hypothesize that efficient transmission of cumulative culture 3-6 may have shaped human social networks and contributed to our tendency to extend networks beyond kin and form strong non-kin ties