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.

Prevalence and Predictors of Postpartum Cosleeping Practices after Midwife-Led Births in the US.

OBJECTIVE: To evaluate patterns of mother-infant sleeping behaviors among US-based mothers who received care from midwives and breastfed their infants the majority of time at 6 weeks postpartum. STUDY DESIGN: Infant sleep locations were reported for 24 915 mother-infant dyads followed through 6 weeks postpartum, following midwife-led singleton births. Using data derived from medical records, we used multinomial logistic regression to identify predictors of sleep location. RESULTS: The median maternal age was 31 years (IQR, 27-34 years). The majority were White (84.5%), reported having a partner or spouse (95%), had a community birth (87%), and reported bedsharing with their infant for part (13.2%) or most of the night (43.8%). In the adjusted analysis, positive predictors of always bedsharing included increasing maternal age (OR, 1.17; 95% CI, 1.13-1.21; per 5 years), cesarean birth (OR, 1.49; 95% CI, 1.18-1.86), Medicaid eligibility (OR, 1.76; 95% CI, 1.62-1.91), and maternal race/ethnicity (Black OR, 1.40 [95% CI, 1.09-1.79]; Latinx OR, 1.53 [95% CI, 1.35-1.74]; multiracial OR, 1.69 [95% CI, 1.39-2.07]). Negative predictors of bedsharing included having a partner/spouse (OR, 0.66; 95% CI, 0.56-0.77) and birth location in hospitals (OR, 0.56; 95% CI, 0.49-0.64) or birthing centers (OR, 0.48; 95% CI, 0.44-0.51). Partial breastfeeding dyads were less likely to bedshare than those who were exclusively breastfeeding (always bedsharing OR, 0.48 [95% CI, 0.41-0.56]; sometimes bedsharing OR 0.69 [95% CI, 0.56-0.83]). CONCLUSIONS: These data suggest that cosleeping is common among US families who choose community births, most of whom exclusively breastfeed through at least 6 weeks.

Effects of a mixed-subsistence diet on the growth of Hadza children.

INTRODUCTION: We investigated the preliminary effects of dietary changes on the anthropometric measurements of child and adolescent Hadza foragers. METHODS: We conducted a cross-sectional study comparing height and weight of participants (aged 0-17 years) at two time points, 2005 (n = 195) and 2017 (n = 52), from two locations: semi-nomadic "bush camps" and sedentary "village camps". World Health Organization (WHO) calculators were used to generate standardized z-scores for weight-for-height (WHZ), weight-for-age (WAZ), height-for-age (HAZ), and BMI-for-age (BMIFAZ). Cross tabulations were constructed for each measurement variable as a function of z-score categories and the variables year, location, and sex. RESULTS: Residency in a village, and associated mixed-subsistence diet, was associated with favorable growth, including greater WAZ (P < .001), HAZ (P < .001), and BMIFAZ (P = .004), but not WHZ (P = .717). Regardless of residency location, participants showed an improved WAZ (P = .021) and HAZ (P < .001) in the 2017 study year. We found no sex differences. DISCUSSION AND CONCLUSION: These preliminary findings suggest that a mixed-subsistence diet may confer advantages over an exclusive wild food diet, a trend also reported among other transitioning foragers.

An evolutionary theory of music needs to care about developmental timing.

Both target papers cite evidence from infancy and early childhood to support the notion of human musicality as a somewhat static suite of capacities; however, in our view they do not adequately acknowledge the critical role of developmental timing, the acquisition process, or the dynamics of social learning, especially during later periods of development such as middle childhood.

Navigating cross-cultural research: methodological and ethical considerations.

The intensifying pace of research based on cross-cultural studies in the social sciences necessitates a discussion of the unique challenges of multi-sited research. Given an increasing demand for social scientists to expand their data collection beyond WEIRD (Western, educated, industrialized, rich and democratic) populations, there is an urgent need for transdisciplinary conversations on the logistical, scientific and ethical considerations inherent to this type of scholarship. As a group of social scientists engaged in cross-cultural research in psychology and anthropology, we hope to guide prospective cross-cultural researchers through some of the complex scientific and ethical challenges involved in such work: (a) study site selection, (b) community involvement and (c) culturally appropriate research methods. We aim to shed light on some of the difficult ethical quandaries of this type of research. Our recommendation emphasizes a community-centred approach, in which the desires of the community regarding research approach and methodology, community involvement, results communication and distribution, and data sharing are held in the highest regard by the researchers. We argue that such considerations are central to scientific rigour and the foundation of the study of human behaviour.

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.

Postpartum Maternal Mood Among Hadza Foragers of Tanzania: A Mixed Methods Approach.

Infant and maternal mortality rates are among the highest in the world in low and middle-income countries where postpartum depression impacts at least one in five women. Currently, there is a dearth of data on maternal mood and infant health outcomes in small-scale non-industrial populations from such countries, particularly during the postnatal period. Here, we present the first investigation of postpartum maternal mood among a foraging population, the Hadza of Tanzania. We administered the Edinburgh Postnatal Depression Scale (EPDS) to twenty-three women, all with infants under the age of 12 months. Semi-structured interviews on happiness and unhappiness during the post-partum period served as a validity cross-check for the EPDS. The combined results of the EPDS surveys and the interview responses suggest that a high proportion of Hadza women experience significant mood disturbances following birth and that postpartum unhappiness is associated with self-reports of pain, anxiety, and disturbed sleep patterns. These findings suggest that many of the mothers in our sample are experiencing post-partum unhappiness at levels similar to or higher than those reported for low to middle income countries in general, including Tanzania. These data are critical for improving our understanding of the etiologies of postpartum mood disturbances cross-culturally.

Dental microwear of living Hadza foragers.

OBJECTIVES: Studies of dental microwear of bioarchaeological assemblages and extant mammals from museum collections show that surface texture can provide a valuable proxy for reconstructing diets of past peoples and extinct species. However, no study to date has focused on occlusal surface microwear textures of living hunter-gatherers. Here we present the first such study of the Hadza foragers of Tanzania. METHODS: We took high-resolution dental impressions of occlusal surfaces for a total of 43 molds representing 25 men and women, 1-3 samples each, at different times during the rainy and dry seasons. Dental replicas were prepared and scanned by confocal profilometry and standard microwear texture parameters were calculated. Central tendencies and dispersions of variable scores were compared by season and by sex. RESULTS: We found no differences between sexes or seasons in texture attribute central tendency, but some for dispersion. Females had notably low microwear texture dispersion in the dry season while males had higher dispersion in some attributes, particularly in the dry season. These differences seem to be driven primarily by low variance among females in the dry season. CONCLUSIONS: This study demonstrates microwear texture data can be generated for living foragers. Given caveats of small samples available and consideration of foraging groups in transition, this study hints at variation in microwear texture dispersion between sexes and seasons for the Hadza, suggesting that such analyses might be of value for assessing hunter-gatherer diet.

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.

Small-scale societies exhibit fundamental variation in the role of intentions in moral judgment.

Intent and mitigating circumstances play a central role in moral and legal assessments in large-scale industrialized societies. Although these features of moral assessment are widely assumed to be universal, to date, they have only been studied in a narrow range of societies. We show that there is substantial cross-cultural variation among eight traditional small-scale societies (ranging from hunter-gatherer to pastoralist to horticulturalist) and two Western societies (one urban, one rural) in the extent to which intent and mitigating circumstances influence moral judgments. Although participants in all societies took such factors into account to some degree, they did so to very different extents, varying in both the types of considerations taken into account and the types of violations to which such considerations were applied. The particular patterns of assessment characteristic of large-scale industrialized societies may thus reflect relatively recently culturally evolved norms rather than inherent features of human moral judgment.

A red meat-derived glycan promotes inflammation and cancer progression.

A well known, epidemiologically reproducible risk factor for human carcinomas is the long-term consumption of "red meat" of mammalian origin. Although multiple theories have attempted to explain this human-specific association, none have been conclusively proven. We used an improved method to survey common foods for free and glycosidically bound forms of the nonhuman sialic acid N-glycolylneuraminic acid (Neu5Gc), showing that it is highly and selectively enriched in red meat. The bound form of Neu5Gc is bioavailable, undergoing metabolic incorporation into human tissues, despite being a foreign antigen. Interactions of this antigen with circulating anti-Neu5Gc antibodies could potentially incite inflammation. Indeed, when human-like Neu5Gc-deficient mice were fed bioavailable Neu5Gc and challenged with anti-Neu5Gc antibodies, they developed evidence of systemic inflammation. Such mice are already prone to develop occasional tumors of the liver, an organ that can incorporate dietary Neu5Gc. Neu5Gc-deficient mice immunized against Neu5Gc and fed bioavailable Neu5Gc developed a much higher incidence of hepatocellular carcinomas, with evidence of Neu5Gc accumulation. Taken together, our data provide an unusual mechanistic explanation for the epidemiological association between red meat consumption and carcinoma risk. This mechanism might also contribute to other chronic inflammatory processes epidemiologically associated with red meat consumption.

Chronotype variation drives night-time sentinel-like behaviour in hunter-gatherers.

Sleep is essential for survival, yet it also represents a time of extreme vulnerability to predation, hostile conspecifics and environmental dangers. To reduce the risks of sleeping, the sentinel hypothesis proposes that group-living animals share the task of vigilance during sleep, with some individuals sleeping while others are awake. To investigate sentinel-like behaviour in sleeping humans, we investigated activity patterns at night among Hadza hunter-gatherers of Tanzania. Using actigraphy, we discovered that all subjects were simultaneously scored as asleep for only 18 min in total over 20 days of observation, with a median of eight individuals awake throughout the night-time period; thus, one or more individuals was awake (or in light stages of sleep) during 99.8% of sampled epochs between when the first person went to sleep and the last person awoke. We show that this asynchrony in activity levels is produced by chronotype variation, and that chronotype covaries with age. Thus, asynchronous periods of wakefulness provide an opportunity for vigilance when sleeping in groups. We propose that throughout human evolution, sleeping groups composed of mixed age classes provided a form of vigilance. Chronotype variation and human sleep architecture (including nocturnal awakenings) in modern populations may therefore represent a legacy of natural selection acting in the past to reduce the dangers of sleep.

The evolution of human sleep: Technological and cultural innovation associated with sleep-wake regulation among Hadza hunter-gatherers.

Sleep is necessary for the survival of all mammalian life. In humans, recent investigations have generated critical data on the relationship between sleep and ecology in small-scale societies. Here, we report the technological and social strategies used to alter sleep environments and influence sleep duration and quality among a population of hunter-gatherers, the Hadza of Tanzania. Specifically, we investigated the effects that grass huts, sound levels, and fire had on sleep. We quantitatively compared thermal stress in outdoor environments to that found inside grass hut domiciles to test whether the huts function as thermoregulated microhabitats during the rainy season. Using physiological equivalent temperature (PET), we found that the grass huts provide sleep sites with less overall variation in thermal stress relative to outside baseline environments. We also investigated ambient acoustic measures of nighttime environments and found that sound significantly covaried with sleep-wake activity, with greater sound levels associating with less sleep. Finally, after controlling for ecological variables previously shown to influence sleep in this population, fire was shown to neither facilitate nor discourage sleep expression. Insofar as data among contemporary sub-tropical foragers can inform our understanding of past lifeways, we interpret our findings as suggesting that after the transition to full time terrestriality, it is likely that early Homo would have had novel opportunities to manipulate its environments in ways that could have significantly improved sleep quality. We further conclude that control over sleep environment would have been essential for migration to higher latitudes away from equatorial Africa.

Current views on hunter-gatherer nutrition and the evolution of the human diet.

Diet composition and food choice are not only central to the daily lives of all living people, but are consistently linked with turning points in human evolutionary history. As such, scholars from a wide range of fields have taken great interest in the role that subsistence has played in both human cultural and biological evolution. Central to this discussion is the diet composition and nutrition of contemporary hunters and gatherers, who are frequently conscripted as model populations for ancestral human nutrition. Research among the world's few remaining foraging populations is experiencing a resurgence, as they are making the final transition away from diets composed of wild foods, to those dominated by domesticated cultigens and/or processed foods. In an effort to glean as much information as possible, before such populations are no longer hunting and gathering, researchers interested in the evolution of human nutrition are rapidly collecting and accessing new and more data. Methods of scientific inquiry are in the midst of rapid change and scholars are able to revisit long-standing questions using state of the art analyses. Here, using the most relevant findings from studies in ethnography, nutrition, human physiology, and microbiomes, we provide a brief summary of the study of the evolution of human nutrition as it has specifically pertained to data coming from living hunter-gatherers. In doing so, we hope to bridge the disciplines that are currently invested in research on nutrition and health among foraging populations.

Hadza sleep biology: Evidence for flexible sleep-wake patterns in hunter-gatherers.

OBJECTIVES: Cross-cultural sleep research is critical to deciphering whether modern sleep expression is the product of recent selective pressures, or an example of evolutionary mismatch to ancestral sleep ecology. We worked with the Hadza, an equatorial, hunter-gatherer community in Tanzania, to better understand ancestral sleep patterns and to test hypotheses related to sleep segmentation. METHODS: We used actigraphy to analyze sleep-wake patterns in thirty-three volunteers for a total of 393 days. Linear mixed effects modeling was performed to assess ecological predictors of sleep duration and quality. Additionally, functional linear modeling (FLM) was used to characterize 24-hr time averaged circadian patterns. RESULTS: Compared with post-industrialized western populations, the Hadza were characterized by shorter (6.25 hr), poorer quality sleep (sleep efficiency = 68.9%), yet had stronger circadian rhythms. Sleep duration time was negatively influenced by greater activity, age, light (lux) exposure, and moon phase, and positively influenced by increased day length and mean nighttime temperature. The average daily nap ratio (i.e., the proportion of days where a nap was present) was 0.54 (SE = 0.05), with an average nap duration of 47.5 min (SE = 2.71; n = 139). DISCUSSION: This study showed that circadian rhythms in small-scale foraging populations are more entrained to their ecological environments than Western populations. Additionally, Hadza sleep is characterized as flexible, with a consistent early morning sleep period yet reliance upon opportunistic daytime napping. We propose that plasticity in sleep-wake patterns has been a target of natural selection in human evolution.

Assessing digestibility of Hadza tubers using a dynamic in-vitro model.

OBJECTIVES: Bioaccessibility is a useful measure for assessing the biological value of a particular nutrient from food, especially foods such as tubers. The wild tubers exploited by Hadza foragers in Tanzania are of interest because they are nontoxic, consumed raw or briefly roasted, and entail substantial physical barriers to consumers. In this study, we attempted to elucidate the biological value of Hadza tubers by measuring the absorption of glucose through in-vitro digestion. METHODS: We quantified digestibility using data from 24 experimental trials on four species of Hadza tuber using a dynamic in-vitro model that replicates digestion in the stomach and small intestine. Analysis of glucose in the input meal and output dialysate revealed the accessible glucose fraction. We also conducted assays for protein, vitamin, and mineral content on whole tubers and meal fractions. RESULTS: Bioaccessibility of glucose varies depending on tuber species. Holding effects of chewing constant, brief roasting had negligible effects, but high intraspecific variation precludes interpretive power. Overall, Hadza tubers are very resistant to digestion, with between one- and two-thirds of glucose absorbed on average. Glucose absorption negatively correlated with glucose concentration of the tubers. CONCLUSIONS: Roasting may provide other benefits such as ease of peeling and chewing to extract edible parenchymatous tissue. A powerful factor in glucose acquisition is tuber quality, placing emphasis on the skill of the forager. Other nutrient assays yielded unexpectedly high values for protein, iron, and iodine, making tubers potentially valuable resources beyond caloric content.

Old stones' song: use-wear experiments and analysis of the Oldowan quartz and quartzite assemblage from Kanjera South (Kenya).

Evidence of Oldowan tools by ∼2.6 million years ago (Ma) may signal a major adaptive shift in hominin evolution. While tool-dependent butchery of large mammals was important by at least 2.0 Ma, the use of artifacts for tasks other than faunal processing has been difficult to diagnose. Here we report on use-wear analysis of ∼2.0 Ma quartz and quartzite artifacts from Kanjera South, Kenya. A use-wear framework that links processing of specific materials and tool motions to their resultant use-wear patterns was developed. A blind test was then carried out to assess and improve the efficacy of this experimental use-wear framework, which was then applied to the analysis of 62 Oldowan artifacts from Kanjera South. Use-wear on a total of 23 artifact edges was attributed to the processing of specific materials. Use-wear on seven edges (30%) was attributed to animal tissue processing, corroborating zooarchaeological evidence for butchery at the site. Use-wear on 16 edges (70%) was attributed to the processing of plant tissues, including wood, grit-covered plant tissues that we interpret as underground storage organs (USOs), and stems of grass or sedges. These results expand our knowledge of the suite of behaviours carried out in the vicinity of Kanjera South to include the processing of materials that would be 'invisible' using standard archaeological methods. Wood cutting and scraping may represent the production and/or maintenance of wooden tools. Use-wear related to USO processing extends the archaeological evidence for hominin acquisition and consumption of this resource by over 1.5 Ma. Cutting of grasses, sedges or reeds may be related to a subsistence task (e.g., grass seed harvesting, cutting out papyrus culm for consumption) and/or a non-subsistence related task (e.g., production of 'twine,' simple carrying devices, or bedding). These results highlight the adaptive significance of lithic technology for hominins at Kanjera.

What We Can Learn About Emotion by Talking With the Hadza.

Emotions are often thought of as internal mental states centering on individuals' subjective feelings and evaluations. This understanding is consistent with studies of emotion narratives, or the descriptions people give for experienced events that they regard as emotions. Yet these studies, and contemporary psychology more generally, often rely on observations of educated Europeans and European Americans, constraining psychological theory and methods. In this article, we present observations from an inductive, qualitative analysis of interviews conducted with the Hadza, a community of small-scale hunter-gatherers in Tanzania, and juxtapose them with a set of interviews conducted with Americans from North Carolina. Although North Carolina event descriptions largely conformed to the assumptions of eurocentric psychological theory, Hadza descriptions foregrounded action and bodily sensations, the physical environment, immediate needs, and the experiences of social others. These observations suggest that subjective feelings and internal mental states may not be the organizing principle of emotion the world around. Qualitative analysis of emotion narratives from outside of a U.S. (and western) cultural context has the potential to uncover additional diversity in meaning-making, offering a descriptive foundation on which to build a more robust and inclusive science of emotion.

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.

Ethical microbiome research with Indigenous communities.

Human-microbiome interactions have been associated with evolutionary, cultural and environmental processes. With clinical applications of microbiome research now feasible, it is crucial that the science conducted, particularly among Indigenous communities, adheres to principles of inclusion. This necessitates a transdisciplinary dialogue to decide how biological samples are collected and who benefits from the research and any derived products. As a group of scholars working at the interface of biological and social science, we offer a candid discussion of the lessons learned from our own research and introduce one approach to carry out ethical microbiome research with Indigenous communities.