Characterization of hunter-gatherer networks and implications for cumulative culture

Testing adaptive hypotheses of alloparenting in Agta foragers

Human children are frequently cared for by non-parental caregivers (alloparents), yet few studies have conducted systematic alternative hypothesis tests of why alloparents help. Here we explore whether predictions from kin selection, reciprocity, learning-to-mother and costly signalling hypotheses explain non-parental childcare among Agta hunter-gatherers from the Philippines. To test these hypotheses, we used high-resolution proximity data from 1,701 child-alloparent dyads. Our results indicated that reciprocity and relatedness were positively associated with the number of interactions with a child (our proxy for childcare). Need appeared more influential in close kin, suggesting indirect benefits, while reciprocity proved to be a stronger influence in non-kin, pointing to direct benefits. However, despite shared genes, close and distant kin interactions were also contingent on reciprocity. Compared with other apes, humans are unique in rapidly producing energetically demanding offspring. Our results suggest that the support that mothers require is met through support based on kinship and reciprocity.

Cultural selection shapes network structure

Cultural evolution relies on the social transmission of cultural traits along a population's social network. Research indicates that network structure affects information spread and thus the capacity for cumulative culture. However, how network structure itself is driven by population-culture co-evolution remains largely unclear. We use a simple model to investigate how populations negotiate the trade-off between acquiring new skills and getting better at existing skills and how this trade-off shapes social networks. We find unexpected eco-evolutionary feedbacks from culture onto social networks and vice versa. We show that selecting for skill generalists results in sparse networks with diverse skill sets, whereas selecting for skill specialists results in dense networks and a population that specializes on the same few skills on which everyone is an expert. Our model advances our understanding of the complex feedbacks in cultural evolution and demonstrates how individual-level behavior can lead to the emergence of population-level structure.

Strong links promote the emergence of cooperative elites

The maintenance of cooperative behavior is fundamental for the prosperity of human societies. Empirical studies show that high cooperation is frequently associated with the presence of strong social ties, but they are silent on whether a causal mechanism exists, how it operates, and what features of the social environment are conducive to its emergence. Here we show experimentally that strong ties increase cooperation and welfare by enabling the emergence of a close-knit and strongly bound cooperative elite. Crucially, this cooperative elite is more prevalent in social environments characterized by a large payoff difference between weak and strong ties, and no gradation in the process of strengthening a tie. These features allow cooperative individuals to adopt an all or nothing strategy to tie strengthening based on the well-known mechanism of direct reciprocity: participants become very selective by forming strong ties only with other cooperative individuals and severing ties with everyone else. Once formed, these strong ties are persistent and enhance cooperation. A dichotomous society emerges with cooperators prospering in a close-knit, strongly bound elite, and defectors earning low payoffs in a weakly connected periphery. Methodologically, our set-up provides a framework to investigate the role of the strength of ties in an experimental setting.

Variability in the organization and size of hunter-gatherer groups: Foragers do not live in small-scale societies

Mobile hunter-gatherers are often characterized as living in small communities where mobility and group size are products of the environmentally determined distribution of resources, and where social organization is multi-scalar: groups of co-residents are nested within small communities that are, in turn, nested within small-scale societies. Such organization is often assumed to be reflective of the human past, emerging as human cognition and communication evolved through earlier fission-fusion social processes, typical of many primate social systems. We review the history of this assumption in light of recent empirical data of co-residence and social networks among contemporary hunter-gatherers. We suggest that while residential and foraging groups are often small, there is little evidence that these groups are drawn from small communities nested within small-scale societies. Most mobile hunter-gatherers live in groups dominated by links between non-relatives, where residential group membership is fluid and supports large-scale social networks of interaction. We investigate these dynamics with fine-grained observational data on Martu foraging groups and social organization in Australia's Western Desert. The composition of Martu foraging groups is distinct from that of residential groups, although both are dominated by ties between individuals who have no close biological relationships. The number of individuals in a foraging group varies with habitat quality, but in a dynamic way, as group size is shaped by ecological legacies of land use. The flexible size and composition of foraging groups link individuals across their "estates": spatially explicit storehouses of ritual and relational wealth, inherited across generations through maintaining expansive networks of social interaction in a large and complex society. We propose that human cognition is tied to development of such expansive social relationships and co-evolved with dynamic socio-ecological interactions expressed in large-scale networks of relational wealth.

Food Income and the Evolution of Forager Mobility

Forager mobility tends to be high, although ethnographic studies indicate ecological factors such as resource abundance and reliability, population density and effective temperature influence the cost-to-benefit assessment of movement decisions. We investigate the evolution of mobility using an agent-based and spatially explicit cultural evolutionary model that considers the feedback between foragers and their environment. We introduce Outcomes Clustering, an approach to categorizing simulated system states arising from complex stochastic processes shaped by multiple interacting parameters. We find that decreased mobility evolves under conditions of high resource replenishment and low resource depletion, with a concomitant trend of increased population density and, counter-intuitively, decreased food incomes. Conversely, increased mobility co-occurs with lower population densities and higher food incomes. We replicate the well-known relationships between mobility, population density, and resource quality, while predicting reduced food income, and consequently the reduction in health status observed in early sedentary populations without the need to invoke factors such as reduced diet quality or increased pathogen loads.

Social Networks and Knowledge Transmission Strategies among Baka Children, Southeastern Cameroon

The dynamics of knowledge transmission and acquisition, or how different aspects of culture are passed from one individual to another and how they are acquired and embodied by individuals, are central to understanding cultural evolution. In small-scale societies, cultural knowledge is largely acquired early in life through observation, imitation, and other forms of social learning embedded in daily experiences. However, little is known about the pathways through which such knowledge is transmitted, especially during middle childhood and adolescence. This study presents new empirical data on cultural knowledge transmission during childhood. Data were collected among the Baka, a forager-farmer society in southeastern Cameroon. We conducted structured interviews with children between 5 and 16 years of age (n = 58 children; 177 interviews, with children being interviewed 1–6 times) about group composition during subsistence activities. Children’s groups were generally diverse, although children tended to perform subsistence activities primarily without adults and with same-sex companions. Group composition varied from one subsistence activity to another, which suggests that the flow of knowledge might also vary according to the activity performed. Analysis of the social composition of children’s subsistence groups shows that vertical and oblique transmission of subsistence-related knowledge might not be predominant during middle childhood and adolescence. Rather, horizontal transmission appears to be the most common knowledge transmission strategy used by Baka children during middle childhood and adolescence, highlighting the importance of other children in the transmission of knowledge.

The effect of dispersal on rates of cumulative cultural evolution

The ability to develop cultural adaptations to local environments is critical to the biological success of humans. Although overall population size and connectedness are thought to play an important role in increasing the rate of cumulative cultural evolution, the independent effect of dispersal rules on rates of cultural evolution has not been examined. Here, a computational model is used to explore the effect of dispersal on the rate of cultural evolution in traits transmitted patrilineally (from father to son), matrilineally (mother to daughter) and bilineally (through both sexes). Two dispersal conditions are modelled: patrilocality (where females disperse and males stay) and bilocality (where either sex may disperse). The results suggest that when only females disperse, the capacity for cumulative cultural evolution in traits shared only among males is severely constrained. This occurs even though overall rates of dispersal and the number of cultural models available to males and females are identical in both dispersal conditions. The constraints on the evolution of patrilineally inherited traits could be considered to represent a process of 'cultural inbreeding', analogous to genetic inbreeding.

The "sociotype" construct: Gauging the structure and dynamics of human sociality

Exploring the pertinence of a "sociotype" construct, established along the conceptual chain genotype-phenotype-sociotype, is the essential purpose of the present paper. Further, by following the sociotype's conceptual guidelines, a new psychometric indicator has been developed in order to gauge the level of social interaction around each individual-the sociotype questionnaire (SOCQ). A first version of this questionnaire has been elaborated by gathering data about the different classes of social bonds (family, friends, acquaintances, and work/study colleagues) in general population and about the dynamic update of these bonds via face-to-face conversation and other modes of interaction. A specific fieldwork was undertaken, involving 1,075 participants, all of them Spanish adults (with diverse social and regional backgrounds). The data obtained were analyzed by means of the correlational method with an analytical cross-sectional design: the number of factors and the consistency and reliability of the resulting scales were evaluated and correlated. The new sociotype indicator resulting from that fieldwork, in spite of its limitations, seems to be valid and reliable, as well as closely associated with widely used metrics of loneliness and psychological distress. It is interesting that the construct noticeably varies throughout the life course and circumstances of individuals, based on their gender and age, and adjusting to the different situations of social networking. This is the first study, to the best of our knowledge, which has tried to reach both a theoretical and an operational formulation of the sociotype construct, by establishing an ad hoc psychometric questionnaire. We think that the information provided by this operational definition opens a new direction of work that could be useful to guide the development and evaluation of programs aimed at improving and strengthening social networking in people at risk, especially for the elderly.

Hunter-Gatherer Social Networks and Reproductive Success

Individuals' centrality in their social network (who they and their social ties are connected to) has been associated with fertility, longevity, disease and information transmission in a range of taxa. Here, we present the first exploration in humans of the relationship between reproductive success and different measures of network centrality of 39 Agta and 38 BaYaka mothers. We collected three-meter contact ('proximity') networks and reproductive histories to test the prediction that individual centrality is positively associated with reproductive fitness (number of living offspring). Rather than direct social ties influencing reproductive success, mothers with greater indirect centrality (i.e. centrality determined by second and third degree ties) produced significantly more living offspring. However, indirect centrality is also correlated with sickness in the Agta, suggesting a trade-off. In complex social species, the optimisation of individuals' network position has important ramifications for fitness, potentially due to easy access to different parts of the network, facilitating cooperation and social influence in unpredictable ecologies.