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.

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.

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.

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.

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.