Women's subsistence strategies predict fertility across cultures, but context matters.

While it is commonly assumed that farmers have higher, and foragers lower, fertility compared to populations practicing other forms of subsistence, robust supportive evidence is lacking. We tested whether subsistence activities-incorporating market integration-are associated with fertility in 10,250 women from 27 small-scale societies and found considerable variation in fertility. This variation did not align with group-level subsistence typologies. Societies labeled as "farmers" did not have higher fertility than others, while "foragers" did not have lower fertility. However, at the individual level, we found strong evidence that fertility was positively associated with farming and moderate evidence of a negative relationship between foraging and fertility. Markers of market integration were strongly negatively correlated with fertility. Despite strong cross-cultural evidence, these relationships were not consistent in all populations, highlighting the importance of the socioecological context, which likely influences the diverse mechanisms driving the relationship between fertility and subsistence.

Child and adolescent foraging: New directions in evolutionary research.

Young children and adolescents in subsistence societies forage for a wide range of resources. They often target child-specific foods, they can be very successful foragers, and they share their produce widely within and outside of their nuclear family. At the same time, while foraging, they face risky situations and are exposed to diseases that can influence their immune development. However, children's foraging has largely been explained in light of their future (adult) behavior. Here, we reinterpret findings from human behavioral ecology, evolutionary medicine and cultural evolution to center foraging children's contributions to life history evolution, community resilience and immune development. We highlight the need to foreground immediate alongside delayed benefits and costs of foraging, including inclusive fitness benefits, when discussing children's food production from an evolutionary perspective. We conclude by recommending that researchers carefully consider children's social and ecological context, develop cross-cultural perspectives, and incorporate children's foraging into Indigenous sovereignty discourse.

A game of raids: Expanding on a game theoretical approach utilising the prisoner's dilemma and ethnography in situ.

In this commentary, we set out the specifics of how Glowacki's game theoretical framework for the evolution of peace could be incorporated within broader cultural evolutionary approaches. We outline a formal proposal for prisoner's dilemma games investigating raid-based conflict. We also centre an ethnographic lens to understand the norms surrounding war and peace in intergroup interactions in small-scale communities.

Hormone physiology and sleep dynamics among BaYaka foragers of the Congo Basin: Gendered associations between nighttime activity, testosterone, and cortisol.

Sleep quality is an important contributor to health disparities and affects the physiological function of the immune and endocrine systems, shaping how resources are allocated to life history demands. Past work in industrial and post-industrial societies has shown that lower total sleep time (TST) or more disrupted nighttime sleep are linked to flatter diurnal slopes for cortisol and lower testosterone production. There has been little focus on these physiological links in other socio-ecological settings where routine sleep conditions and nighttime activity demands differ. We collected salivary hormone (testosterone, cortisol) and actigraphy-based sleep data from Congolese BaYaka foragers (N = 39), who have relatively short and fragmented nighttime sleep, on average, in part due to their typical social sleep conditions and nighttime activity. The hormone and sleep data collections were separated by an average of 11.23 days (testosterone) and 2.84 days (cortisol). We found gendered links between nighttime activity and adults' hormone profiles. Contrary to past findings in Euro-American contexts, BaYaka men who were more active at night, on average, had higher evening testosterone than those with lower nighttime activity, with a relatively flat slope relating nighttime activity and evening testosterone in women. Women had steeper diurnal cortisol curves with less disrupted sleep. Men had steeper cortisol diurnal curves if they were more active at night. BaYaka men often hunt and socialize when active at night, which may help explain these patterns. Overall, our findings indicate that the nature of nighttime activities, including their possible social and subsistence contexts, are potentially important modifiers of sleep quality-physiology links, meriting further research across contexts.

Quantifying quality: The impact of measures of school quality on children's academic achievement across diverse societies.

Recent decades have seen a rapid acceleration in global participation in formal education, due to worldwide initiatives aimed to provide school access to all children. Research in high income countries has shown that school quality indicators have a significant, positive impact on numeracy and literacy-skills required to participate in the increasingly globalized economy. Schools vary enormously in kind, resources, and teacher training around the world, however, and the validity of using diverse school quality measures in populations with diverse educational profiles remains unclear. First, we assessed whether children's numeracy and literacy performance across populations improves with age, as evidence of general school-related learning effects. Next, we examined whether several school quality measures related to classroom experience and composition, and to educational resources, were correlated with one another. Finally, we examined whether they were associated with children's (4-12-year-olds, N = 889) numeracy and literacy performance in 10 culturally and geographically diverse populations which vary in historical engagement with formal schooling. Across populations, age was a strong positive predictor of academic achievement. Measures related to classroom experience and composition were correlated with one another, as were measures of access to educational resources and classroom experience and composition. The number of teachers per class and access to writing materials were key predictors of numeracy and literacy, while the number of students per classroom, often linked to academic achievement, was not. We discuss these results in the context of maximising children's learning environments and highlight study limitations to motivate future research. RESEARCH HIGHLIGHTS: We examined the extent to which four measures of school quality were associated with one another, and whether they predicted children's academic achievement in 10 culturally and geographically diverse societies. Across populations, measures related to classroom experience and composition were correlated with one another as were measures of access to educational resources to classroom experience and composition. Age, the number of teachers per class, and access to writing materials were key predictors of academic achievement across populations. Our data have implications for designing efficacious educational initiatives to improve school quality globally.

Costly teaching contributes to the acquisition of spear hunting skill among BaYaka forager adolescents.

Teaching likely evolved in humans to facilitate the faithful transmission of complex tasks. As the oldest evidenced hunting technology, spear hunting requires acquiring several complex physical and cognitive competencies. In this study, we used observational and interview data collected among BaYaka foragers (Republic of the Congo) to test the predictions that costlier teaching types would be observed at a greater frequency than less costly teaching in the domain of spear hunting and that teachers would calibrate their teaching to pupil skill level. To observe naturalistic teaching during spear hunting, we invited teacher-pupil groupings to spear hunt while wearing GoPro cameras. We analysed 68 h of footage totalling 519 teaching episodes. Most observed teaching events were costly. Direct instruction was the most frequently observed teaching type. Older pupils received less teaching and more opportunities to lead the spear hunt than their younger counterparts. Teachers did not appear to adjust their teaching to pupil experience, potentially because age was a more easily accessible heuristic for pupil skill than experience. Our study shows that costly teaching is frequently used to transmit complex tasks and that instruction may play a privileged role in the transmission of spear hunting knowledge.

There and back again: The biosocial dynamics of returning from the field.

BACKGROUND: Leaving "home" to pursue fieldwork is a necessity but also a rite of passage for many biological anthropology/human biology scholars. Field-based scientists prepare for the potential changes to activity patterns, sleep schedules, social interactions, and more that come with going to the field. However, returning from extended fieldwork and the reverse-culture shock, discomforts, and mental shifts that are part of the return process can be jarring, sometimes traumatic experiences. A failure to acknowledge and address such experiences can compromise the health and wellbeing of those returning. AIMS: We argue for an engaged awareness of the difficult nature of returning from the field and offer suggestions for individuals and programs to better train and prepare PhD students pursuing fieldwork. MATERIALS & METHODS: Here, we offer personal stories of "coming back" and give professional insights on how to best ready students and scholars for returning from fieldwork. DISCUSSION/CONCLUSION: By bringing forward and normalizing the difficulty of the fieldwork-return process, we hope that this reflection acts as a tool for future scholars to prepare to come home as successfully and consciously as possible.

Cultural Components of Sex Differences in Color Preference.

Preferences for pink and blue were tested in children aged 4-11 years in three small-scale societies: Shipibo villages in the Peruvian Amazon, kastom villages in the highlands of Tanna Island, Vanuatu, and BaYaka foragers in the northern Republic of Congo; and compared to children from an Australian global city (total N = 232). No sex differences were found in preference for pink in any of the three societies not influenced by global culture (ds - 0.31-0.23), in contrast to a female preference for pink in the global city (d = 1.24). Results suggest that the pairing of female and pink is a cultural phenomenon and is not driven by an essential preference for pink in girls.

Out of the empirical box: A mixed-methods study of tool innovation among Congolese BaYaka forager and Bondongo fisher-farmer children.

Tool innovation has played a crucial role in human adaptation. Yet, this capacity seems to arise late in development. Before 8 years of age, many children struggle to solve the hook task, a common measure of tool innovation that requires modification of a straight pipe cleaner into a hook to extract a prize. Whether these findings are generalizable beyond postindustrialized Western children remains unclear. In many small-scale subsistence societies, children engage in daily tool use and modification, experiences that theoretically could enhance innovative capabilities. Although two previous studies found no differences in innovative ability between children from Western and small-scale subsistence societies, these did not account for the latter's inexperience with pipe cleaners. Thus, the current study investigated how familiarity with pipe cleaners affected hook task success in 132 Congolese BaYaka foragers (57 girls) and 59 Bondongo fisher-farmers (23 girls) aged 4-12 years. We contextualized these findings within children's interview responses and naturalistic observations of how pipe cleaners were incorporated into daily activities. Counter to our expectation, prior exposure did not improve children's performance during the hook task. Bondongo children innovated significantly more hooks than BaYaka children, possibly because they participate in hook-and-line fishing. Observations and interviews showed that children imagined and innovated novel uses for pipe cleaners outside the experimental context, including headbands, bracelets, and suspenders. We relate our findings to ongoing debates regarding systematic versus unsystematic tool innovation, the importance of prior experience for the ontogeny of tool innovation, and the external validity of experimental paradigms.

Gender-Typed and Gender-Segregated Play Among Tanzanian Hadza and Congolese BaYaka Hunter-Gatherer Children and Adolescents.

Few data exist on gender-typed and gender-segregated play in hunter-gatherer societies, despite their unique demographic and cultural features which may influence children's gendered play. Using naturalistic observations of Hadza (N = 46, 41% female) and BaYaka (N = 65, 48% female) hunter-gatherer 3- to 18-year-olds from Tanzania and the Republic of Congo, we showed that access to playmates was negatively associated with playing in mixed-gender groups. Young boys did not engage in more rough-and-tumble play than girls, but adolescent boys participated in this type of play more than adolescent girls. Children were also more likely to participate in work-themed play which conformed to gender norms within their society. Findings are discussed within the context of gendered division of labor, child autonomy, and demography.

Sex differences in daily activity intensity and energy expenditure and their relationship to cortisol among BaYaka foragers from the Congo Basin.

OBJECTIVES: The pooling of energetic resources and food sharing have been widely documented among hunter-gatherer societies. Much less is known about how the energetic costs of daily activities are distributed across individuals in such groups, including between women and men. Moreover, the metabolic physiological correlates of those activities and costs are relatively understudied. MATERIALS AND METHODS: Here, we tracked physical activity, energy expenditure (EE), and cortisol production among Congo Basin BaYaka foragers engaged in a variety of daily subsistence activities (n = 37). Given its role in energy mobilization, we measured overall daily cortisol production and short-term cortisol reactivity through saliva sampling; we measured physical activity levels and total EE via the wGT3X-bt actigraph and heart rate monitor. RESULTS: We found that there were no sex differences in likelihood of working in common activity locations (forest, garden, house). Across the day, women spent greater percentage time in moderate-to-vigorous physical activity (%MVPA) and had lower total EE than men. Females with higher EE (kCal/hr) produced greater cortisol throughout the day. Though not statistically significant, we also found that individuals with greater %MVPA had larger decreases in cortisol reactivity. DISCUSSION: BaYaka women sustained higher levels of physical activity but incurred lower energetic costs than men, even after factoring in sex differences in body composition. Our findings suggest that the distribution of physical activity demands and costs are relevant to discussions regarding how labor is divided and community energy budgets take shape in such settings.

Fatherhood, egalitarianism, and child health in two small-scale societies in the Republic of the Congo.

OBJECTIVES: The study goals were to (a) characterize the cultural model of fatherhood among the BaYaka, a community of egalitarian foragers in the Republic of the Congo; (b) test if BaYaka fathers' quality in relation to the cultural model predicts their children's energetic status; and (c) compare the variance in BaYaka children's energetic status to that of children of neighboring Bondongo fisher-farmers, among whom there is less cooperative caregiving, less resource sharing, and greater social inequality. METHODS: We used informal interviews to establish the cultural model of fatherhood, which we used to build a peer ranking task to quantify father quality. Children's energetic status was assessed by measuring height, weight, and triceps skinfold thickness. We then tested for associations between father quality scores derived from the ranking task and children's energetic status using ordinary least squares regression. Equality of variance tests were used to compare BaYaka and Bondongo children's energetic statuses. RESULTS: The BaYaka described fathers as responsible for acquiring resources and maintaining marital harmony, welcoming others to the community and sharing well with them, and teaching their children about the forest. Agreement on men's quality in these domains was high, but father quality did not significantly predict children's energetic status. BaYaka children had lower variance in energetic status overall compared to Bondongo children. CONCLUSIONS: We suggest that the core BaYaka values and practices that maintain egalitarian social relations and distribution of resources help buffer children's health and well-being from variation in their fathers' qualities in culturally valued domains.

Epigenetic aging in children from a small-scale farming society in The Congo Basin: Associations with child growth and family conflict.

Developmental environments influence individuals' long-term health trajectories, and there is increasing emphasis on understanding the biological pathways through which this occurs. Epigenetic aging evaluates DNA methylation at a suite of distinct CpG sites in the genome, and epigenetic age acceleration (EAA) is linked to heightened chronic morbidity and mortality risks in adults. Consequently, EAA provides insights on trajectories of biological aging, which early life experiences may help shape. However, few studies have measured correlates of children's epigenetic aging, especially outside of the U.S. and Europe. In particular, little is known about how children's growth and development relate to EAA in ecologies in which energetic and pathogenic stressors are commonplace. We studied EAA from dried blood spots among Bondongo children (n = 54) residing in a small-scale, fisher-farmer society in a remote region of the Republic of the Congo. Here, infectious disease burdens and their resultant energy demands are high. Children who were heavier for height or taller for age, respectively, exhibited greater EAA, including intrinsic EAA, which is considered to measure EAA internal to cells. Furthermore, we found that children in families with more conflict between parents had greater intrinsic EAA. These results suggest that in contexts in which limited energy must be allocated to competing demands, more investment in growth may coincide with greater EAA, which parallels findings in European children who do not face similar energetic constraints. Our findings also indicate that associations between adverse family environments and greater intrinsic EAA were nonetheless observable but only after adjustment for covariates relevant to the energetically and immunologically demanding nature of the local ecology.

Testosterone, fathers as providers and caregivers, and child health: Evidence from fisher-farmers in the Republic of the Congo.

Males in vertebrate species with biparental care commonly face a life history trade-off between investing in mating versus parenting effort. Among these males, testosterone is frequently elevated during mating and competition and reduced when males help raise offspring. These physiological patterns may be adaptive, increasing males' fitness through investments in young. However, for some species, including humans, indirect parenting often benefits young but can also involve male competition and risk-taking behavior and may be facilitated by elevated testosterone. Despite potential adaptive functions of biological responses to invested fatherhood, few if any mammalian studies have linked fathers' testosterone to offspring outcomes; no studies in humans have. Using data from a small-scale society of fisher-farmers from the Republic of the Congo, we find that fathers who were rated as better providers by their peers had higher testosterone, compared to other fathers in their community. However, children whose fathers had middle-range T compared to fathers with higher or lower levels had better energetic status (higher BMI; greater triceps skinfold thickness). Fathers' indirect and direct care helped to account for these associations between paternal T and children's energetic profiles. Given that human paternal direct and, especially, indirect care are thought to have been important evolutionarily and remain so in many contemporary societies, these findings help to shed light on the facultative nature of human biological responses to fatherhood and the relevance of these factors to children's well-being.

Physical activity and time budgets of Hadza forager children: Implications for self-provisioning and the ontogeny of the sexual division of labor.

OBJECTIVES: To determine the effects of age and sex on physical activity and time budgets of Hadza children and juveniles, 5-14 years old, including both in-camp and out-of-camp activities. METHODS: Behavioral data were derived from ~15 000 hourly in-camp scan observations of 76 individuals and 13 out-of-camp focal follows on nine individuals. The data were used to estimate energy expended and percentage of time engaged in a variety of routine activities, including food collection, childcare, making and repairing tools, and household maintenance. RESULTS: Our results suggest that (1) older children spend more time in economic activities; (2) females spend more time engaged in work-related and economic activities in camp, whereas males spend more time engaged in economic activities out of camp; and (3) foraging by both sexes tends to net caloric gains despite being energetically costly. CONCLUSIONS: These results show that, among the Hadza, a sexual division of labor begins to emerge in middle childhood and is well in place by adolescence. Furthermore, foraging tends to provide net caloric gains, suggesting that children are capable of reducing at least some of the energetic burden they place upon their parents or alloparents. The findings are relevant to our understanding of the ways in which young foragers allocate their time, the development of sex-specific behavior patterns, and the capacity of children's work efforts to offset the cost of their own care in a cooperative breeding environment.

Dyadic inter-group cooperation in shotgun hunting activities in a Congo Basin village.

Understanding the dynamics of inter-group cooperation in human adaptation has been the subject of recent empirical and theoretical studies in evolutionary anthropology, beginning to fill gaps in our knowledge of how interactions across political, economic and social domains can - and often do - lead to stable, large-scale cooperation. Here we investigate dyadic intergroup cooperation in shotgun hunting in the Republic of the Congo. In the Congo Basin, inter-group cooperation between foragers and farmers is at the centre of an exchange system maintained by traditional norms and institutions such as fictive kinship. Here, we focused on what factors predict cooperative shotgun hunting exchanges between BaYaka and Yambe. We conducted structured interviews with 48 BaYaka hunters and 18 Yambe men who organise hunts in a village along the Motaba River. We used Bayesian multilevel regression models to investigate the influence of Yambe and BaYaka attributes on probability of dyadic cooperation. We found that BaYaka men's reputations as skilled hunters and their family size each predicted cooperation in shotgun hunting, whereas there was no effect of Yambe attributes (status, wealth, family size). We discuss the results in terms of evolutionary models of men as hunters and inter-group cooperation, as well as biodiversity conservation implications.

Intergroup Cooperation in Shotgun Hunting Among BaYaka Foragers and Yambe Farmers from the Republic of the Congo.

Whereas many evolutionary models emphasize within-group cooperation or between-group competition in explaining human large-scale cooperation, recent work highlights a critical role for intergroup cooperation in human adaptation. Here we investigate intergroup cooperation in the domain of shotgun hunting in northern Republic of the Congo. In the Congo Basin broadly, forest foragers maintain relationships with neighboring farmers based on systems of exchange regulated by norms and institutions such as fictive kinship. In this study, we examine how relationships between Yambe farmers and BaYaka foragers support stable intergroup cooperation in the domain of shotgun hunting. In the study village, shotgun hunting is based on a specialization-based exchange wherein Yambe farmers contribute shotguns and access to markets to buy cartridges and sell meat while BaYaka foragers contribute their specialized forest knowledge and skill. To understand how costs and benefits are distributed, we conducted structured interviews with 77 BaYaka hunters and 15 Yambe gun owners and accompanied hunters on nine hunting trips. We found that hunts are organized in a conventional manner within a fictive kinship structure, consistent with the presence of intercultural mechanisms to stabilize cooperation. However, because bushmeat demand is high, gun owners can gain significant cash profit, while compensating hunters only with cigarettes, alcohol, and a traditional hunter's portion of meat. To level payoffs, hunters strategically hide kills or cartridges from gun owners to feed their own families. Our results illustrate how each group prioritizes different currencies (e.g., cash, meat, family, intergroup relations) and provide insights into how intergroup cooperation is stabilized in this setting. The example of this long-standing intergroup cooperative system is discussed in terms of its contemporary entwinement with logging, the bushmeat trade, and growing market intersection.

Evidence for an emotional adaptive function of dreams: a cross-cultural study.

The function of dreams is a longstanding scientific research question. Simulation theories of dream function, which are based on the premise that dreams represent evolutionary past selective pressures and fitness improvement through modified states of consciousness, have yet to be tested in cross-cultural populations that include small-scale forager societies. Here, we analyze dream content with cross-cultural comparisons between the BaYaka (Rep. of Congo) and Hadza (Tanzania) foraging groups and Global North populations, to test the hypothesis that dreams in forager groups serve a more effective emotion regulation function due to their strong social norms and high interpersonal support. Using a linear mixed effects model we analyzed 896 dreams from 234 individuals across these populations, recorded using dream diaries. Dream texts were processed into four psychosocial constructs using the Linguistic Inquiry and Word Count (LIWC-22) dictionary. The BaYaka displayed greater community-oriented dream content. Both the BaYaka and Hadza exhibited heightened threat dream content, while, at the same time, the Hadza demonstrated low negative emotions in their dreams. The Global North Nightmare Disorder group had increased negative emotion content, and the Canadian student sample during the COVID-19 pandemic displayed the highest anxiety dream content. In conclusion, this study supports the notion that dreams in non-clinical populations can effectively regulate emotions by linking potential threats with non-fearful contexts, reducing anxiety and negative emotions through emotional release or catharsis. Overall, this work contributes to our understanding of the evolutionary significance of this altered state of consciousness.

How small-scale societies achieve large-scale cooperation.

For most of our species' history, humans have lived in relatively small subsistence communities, often called small-scale societies. While these groups lack centralized institutions, they can and often do maintain large-scale cooperation. Here, we explore several mechanisms promoting cooperation in small-scale societies, including (a) the development of social norms that encourage prosocial behavior, (b) reciprocal exchange relationships, (c) reputation that facilitates high-cost cooperation, (d) relational wealth, and (e) risk buffering institutions. We illustrate these with ethnographic and psychological evidence from contemporary small-scale societies. We argue that these mechanisms for cooperation helped past and present small-scale communities adapt to diverse ecological and social niches.

Foraging complexity and the evolution of childhood.

Our species' long childhood is hypothesized to have evolved as a period for learning complex foraging skills. Researchers studying the development of foraging proficiency have focused on assessing this hypothesis, yet studies present inconsistent conclusions regarding the connection between foraging skill development and niche complexity. Here, we leverage published records of child and adolescent foragers from 28 societies to (i) quantify how skill-intensive different resources are and (ii) assess whether children's proficiency increases more slowly for more skill-intensive resources. We find that foraging returns increase slowly for more skill-intensive, difficult-to-extract resources (tubers and game), consistent with peak productivity attained in adulthood. Foraging returns for easier-to-extract resources (fruit and fish/shellfish) increase rapidly during childhood, with adult levels of productivity reached by adolescence. Our findings support the view that long childhoods evolved as an extended period for learning to extract complex resources characteristic of the human foraging niche.