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

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

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

Competition for Cooperation: variability, benefits and heritability of relational wealth in hunter-gatherers

Many defining human characteristics including theory of mind, culture and language relate to our sociality, and facilitate the formation and maintenance of cooperative relationships. Therefore, deciphering the context in which our sociality evolved is invaluable in understanding what makes us unique as a species. Much work has emphasised group-level competition, such as warfare, in moulding human cooperation and sociality. However, competition and cooperation also occur within groups; and inter-individual differences in sociality have reported fitness implications in numerous non-human taxa. Here we investigate whether differential access to cooperation (relational wealth) is likely to lead to variation in fitness at the individual level among BaYaka hunter-gatherers. Using economic gift games we find that relational wealth: a) displays individual-level variation; b) provides advantages in buffering food risk, and is positively associated with body mass index (BMI) and female fertility; c) is partially heritable. These results highlight that individual-level processes may have been fundamental in the extension of human cooperation beyond small units of related individuals, and in shaping our sociality. Additionally, the findings offer insight in to trends related to human sociality found from research in other fields such as psychology and epidemiology

Relatedness within and between Agta residential groups

Social media summary: Agta hunter–gatherers do not experience age- or sex-related differences in relatedness to their campmates.Data availability: The data and code are available as Supplementary Materials.Supplementary material is available online at: https://www.cambridge.org/core/journals/evolutionary-human-sciences/article/relatedness-within-and-between-agta-residential-groups/BA9602DCFAA9A22DD7C8C738ABE01FAF#supplementary-materials-tab .Theoretical models relating to the evolution of human behaviour usually make assumptions about the kinship structure of social groups. Since humans were hunter–gatherers for most of our evolutionary history, data on the composition of contemporary hunter–gatherer groups has long been used to inform these models. Although several papers have taken a broad view of hunter–gatherer social organisation, it is also useful to explore data from single populations in more depth. Here, we describe patterns of relatedness among the Palanan Agta, hunter–gatherers from the northern Philippines. Across 271 adults, mean relatedness to adults across the population is r = 0.01 and to adult campmates is r = 0.074, estimates that are similar to those seen in other hunter–gatherers. We also report the distribution of kin across camps, relatedness and age differences between spouses, and the degree of shared reproductive interest between camp mates, a measure that incorporates affinal kinship. For both this this measure (s) and standard relatedness (r), we see no major age or sex differences in the relatedness of adults to their campmates, conditions that may reduce the potential for conflicts of interest within social groups.A.B.M. received funding from the Leverhulme Trust (grant no. RP2011-R 045)

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

Hunter-Gatherer Children’s Close-Proximity Networks: Similarities and differences with cooperative and communal breeding systems

Among vertebrates, allomothering (non-maternal care) is classified as cooperative breeding (help from sexually mature non-breeders, usually close relatives) or communal breeding (shared care between multiple breeders who are not necessarily related). Humans have been described with both labels, most frequently as cooperative breeders. However, few studies have quantified the relative contributions of allomothers according to whether they are a) sexually mature and reproductively active, and b) related or unrelated. We constructed close-proximity networks of Agta and BaYaka hunter-gatherers. We used portable remote-sensing devices to quantify the proportion of time children under the age of four spent in close proximity to different categories of potential allomother. Both related and unrelated, and reproductively active and inactive campmates had substantial involvement in children’s close - proximity networks. Unrelated campmates, siblings and subadults were the most involved in both populations; whereas, the involvement of fathers and grandmothers was the most variable between the two populations. Finally, the involvement of sexually mature, reproductively inactive adults was low. Where possible, we compared our findings to studies of other hunter-gatherer societies, and observed numerous consistent trends. Based on our results we discuss why hunter-gatherer allomothering cannot be fully characterised as cooperative or communal breeding

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