Joint Attention in Human and Chimpanzee Infants in Varied Socio-Ecological Contexts.

Joint attention (JA) is an early manifestation of social cognition, commonly described as interactions in which an infant looks or gestures to an adult female to share attention about an object, within a positive emotional atmosphere. We label this description the JA phenotype. We argue that characterizing JA in this way reflects unexamined assumptions which are, in part, due to past developmental researchers' primary focus on western, middle-class infants and families. We describe a range of cultural variations in caregiving practices, socialization goals, and parenting ethnotheories as an essential initial step in viewing joint attention within inclusive and contextualized perspectives. We begin the process of conducting a decolonized study of JA by considering the core construct of joint attention (i.e., triadic connectedness) and adopting culturally inclusive definitions (labeled joint engagement [JE]). Our JE definitions allow for attention and engagement to be expressed in visual and tactile modalities (e.g., for infants experiencing distal or proximal caregiving), with various social partners (e.g., peers, older siblings, mothers), with a range of shared topics (e.g., representing diverse socialization goals, and socio-ecologies with and without toys), and with a range of emotional tone (e.g., for infants living in cultures valuing calmness and low arousal, and those valuing exuberance). Our definition of JE includes initiations from either partner (to include priorities for adult-led or child-led interactions). Our next foundational step is making an ecological commitment to naturalistic observations (Dahl, 2017, Child Dev Perspect, 11(2), 79-84): We measure JE while infants interact within their own physical and social ecologies. This commitment allows us to describe JE as it occurs in everyday contexts, without constraints imposed by researchers. Next, we sample multiple groups of infants drawn from diverse socio-ecological settings. Moreover, we include diverse samples of chimpanzee infants to compare with diverse samples of human infants, to investigate the extent to which JE is unique to humans, and to document diversity both within and between species. We sampled human infants living in three diverse settings. U.K. infants (n = 8) were from western, middle-class families living near universities in the south of England. Nso infants (n = 12) were from communities of subsistence farmers in Cameroon, Africa. Aka infants (n = 10) were from foraging communities in the tropical rain forests of Central African Republic, Africa. We coded behavioral details of JE from videotaped observations (taken between 2004 and 2010). JE occurred in the majority of coded intervals (Mdn = 68%), supporting a conclusion that JE is normative for human infants. The JA phenotype, in contrast, was infrequent, and significantly more common in the U.K. (Mdn = 10%) than the other groups (Mdn < 3%). We found significant within-species diversity in JE phenotypes (i.e., configurations of predominant forms of JE characteristics). We conclude that triadic connectedness is very common in human infants, but there is significant contextualization of behavioral forms of JE. We also studied chimpanzee infants living in diverse socio-ecologies. The PRI/Zoo chimpanzee infants (n = 7) were from captive, stable groups of mixed ages and sexes, and included 4 infants from the Chester Zoo, U.K. and 3 from the Primate Research Institute, Kyoto University, Japan. The Gombe chimpanzee infants (n = 12) were living in a dynamically changing, wild community in the Gombe National Park, Tanzania, Africa. Additionally, we include two Home chimpanzee infants who were reared from birth by a female scientist, in the combined U.S., middle-class contexts of home and university cognition laboratory. JE was coded from videotaped observations (taken between 1993 and 2006). JE occurred during the majority of coded intervals (Mdn = 64%), consistent with the position that JE is normative for chimpanzee infants. The JA phenotype, in contrast, was rare, but more commonly observed in the two Home chimpanzee infants (in 8% and 2% of intervals) than in other chimpanzee groups (Mdns = 0%). We found within-species diversity in the configurations comprising the JE phenotypes. We conclude that triadic connectedness is very common in chimpanzee infants, but behavioral forms of joint engagement are contextualized. We compared JE across species, and found no species-uniqueness in behavioral forms, JE characteristics, or JE phenotypes. Both human and chimpanzee infants develop contextualized social cognition. Within-species diversity is embraced when triadic connectedness is described with culturally inclusive definitions. In contrast, restricting definitions to the JA phenotype privileges a behavioral form most valued in western, middle-class socio-ecologies, irrespective of whether the interactions involve human or chimpanzee infants. Our study presents a model for how to decolonize an important topic in developmental psychology. Decolonization is accomplished by defining the phenomenon inclusively, embracing diversity in sampling, challenging claims of human-uniqueness, and having an ecological commitment to observe infant social cognition as it occurs within everyday socio-ecological contexts. It is essential that evolutionary and developmental theories of social cognition are re-built on more inclusive and decolonized empirical foundations.

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.

A genome scan for genes underlying adult body size differences between Central African hunter-gatherers and farmers.

The evolutionary and biological bases of the Central African "pygmy" phenotype, a characteristic of rainforest hunter-gatherers defined by reduced body size compared with neighboring farmers, remain largely unknown. Here, we perform a joint investigation in Central African hunter-gatherers and farmers of adult standing height, sitting height, leg length, and body mass index (BMI), considering 358 hunter-gatherers and 169 farmers with genotypes for 153,798 SNPs. In addition to reduced standing heights, hunter-gatherers have shorter sitting heights and leg lengths and higher sitting/standing height ratios than farmers and lower BMI for males. Standing height, sitting height, and leg length are strongly correlated with inferred levels of farmer genetic ancestry, whereas BMI is only weakly correlated, perhaps reflecting greater contributions of non-genetic factors to body weight than to height. Single- and multi-marker association tests identify one region and eight genes associated with hunter-gatherer/farmer status, and 24 genes associated with the height-related traits. Many of these genes have putative functions consistent with roles in determining their associated traits and the pygmy phenotype, and they include three associated with standing height in non-Africans (PRKG1, DSCAM, MAGI2). We find evidence that European height-associated SNPs or variants in linkage disequilibrium with them contribute to standing- and sitting-height determination in Central Africans, but not to the differential status of hunter-gatherers and farmers. These findings provide new insights into the biological basis of the pygmy phenotype, and they highlight the potential of cross-population studies for exploring the genetic basis of phenotypes that vary naturally across populations.

Human preferences for sexually dimorphic faces may be evolutionarily novel.

A large literature proposes that preferences for exaggerated sex typicality in human faces (masculinity/femininity) reflect a long evolutionary history of sexual and social selection. This proposal implies that dimorphism was important to judgments of attractiveness and personality in ancestral environments. It is difficult to evaluate, however, because most available data come from large-scale, industrialized, urban populations. Here, we report the results for 12 populations with very diverse levels of economic development. Surprisingly, preferences for exaggerated sex-specific traits are only found in the novel, highly developed environments. Similarly, perceptions that masculine males look aggressive increase strongly with development and, specifically, urbanization. These data challenge the hypothesis that facial dimorphism was an important ancestral signal of heritable mate value. One possibility is that highly developed environments provide novel opportunities to discern relationships between facial traits and behavior by exposing individuals to large numbers of unfamiliar faces, revealing patterns too subtle to detect with smaller samples.

Ontogeny of prosocial behavior across diverse societies.

Humans are an exceptionally cooperative species, but there is substantial variation in the extent of cooperation across societies. Understanding the sources of this variability may provide insights about the forces that sustain cooperation. We examined the ontogeny of prosocial behavior by studying 326 children 3-14 y of age and 120 adults from six societies (age distributions varied across societies). These six societies span a wide range of extant human variation in culture, geography, and subsistence strategies, including foragers, herders, horticulturalists, and urban dwellers across the Americas, Oceania, and Africa. When delivering benefits to others was personally costly, rates of prosocial behavior dropped across all six societies as children approached middle childhood and then rates of prosociality diverged as children tracked toward the behavior of adults in their own societies. When prosocial acts did not require personal sacrifice, prosocial responses increased steadily as children matured with little variation in behavior across societies. Our results are consistent with theories emphasizing the importance of acquired cultural norms in shaping costly forms of cooperation and creating cross-cultural diversity.

Hunter-gatherer genomes reveal diverse demographic trajectories during the rise of farming in Eastern Africa.

The fate of hunting and gathering populations following the rise of agriculture and pastoralism remains a topic of debate in the study of human prehistory. Studies of ancient and modern genomes have found that autochthonous groups were largely replaced by expanding farmer populations with varying levels of gene flow, a characterization that is influenced by the almost universal focus on the European Neolithic.1-5 We sought to understand the demographic impact of an ongoing cultural transition to farming in Southwest Ethiopia, one of the last regions in Africa to experience such shifts.6 Importantly, Southwest Ethiopia is home to several of the world's remaining hunter-gatherer groups, including the Chabu people, who are currently transitioning away from their traditional mode of subsistence.7 We generated genome-wide data from the Chabu and four neighboring populations, the Majang, Shekkacho, Bench, and Sheko, to characterize their genetic ancestry and estimate their effective population sizes over the last 60 generations. We show that the Chabu are a distinct population closely related to ancient people who occupied Southwest Ethiopia >4,500 years ago. Furthermore, the Chabu are undergoing a severe population bottleneck, which began approximately 1,400 years ago. By analyzing eleven Eastern African populations, we find evidence for divergent demographic trajectories among hunter-gatherer-descendant groups. Our results illustrate that although foragers respond to encroaching agriculture and pastoralism with multiple strategies, including cultural adoption of agropastoralism, gene flow, and economic specialization, they often face population decline.

Opportunities for Interaction : Natural Observations of Children's Social Behavior in Five Societies.

We examine the opportunities children have for interacting with others and the extent to which they are the focus of others' visual attention in five societies where extended family communities are the norm. We compiled six video-recorded datasets (two from one society) collected by a team of anthropologists and psychologists conducting long-term research in each society. The six datasets include video observations of children among the Yasawas (Fiji), Tanna (Vanuatu), Tsimane (Bolivia), Huatasani (Peru), and Aka (infants and children 4-12 years old; Central African Republic). Each dataset consists of a series of videos of children ranging in age from 2 months to 12 years in their everyday contexts. We coded 998 videos and identified with whom children had opportunities to interact (male and female adults and children) as well as the number of individuals and the proportion of observed time that children spent with these individuals. We also examined the proportion of time children received direct visual gaze (indicating attention to the child). Our results indicate that children less than 5 years old spend the majority of their observed time in the presence of one female adult. This is the case across the five societies. In the three societies from which we have older children (Aka, Yasawa, Peru), we find a clear shift around 5 years of age, with children spending the majority of their time with other children. We also coded the presence or absence of a primary caregiver and found that caregivers remained within 2 ft of target children until 7 years of age. When they were in the company of a primary caregiver, children older than seven spent the majority of their time more than 2 ft from the caregiver. We found a consistent trend across societies with decreasing focal attention on the child with increasing child age. These findings show (1) remarkable consistency across these societies in children's interaction opportunities and (2) that a developmental approach is needed to fully understand human development because the social context is dynamic across the lifespan. These data can serve as a springboard for future research examining social development in everyday contexts.

Can life history trade-offs explain the evolution of short stature in human pygmies? A response to Migliano et al. (2007).

Walker et al. ["Growth rates and life histories in twenty-two small-scale societies," Am. J. Hum. Biol. 18:295-311 (2006)] used life history theory to develop an innovative explanation for human diversity in stature. Short stature could have been selected for in some human populations as a result of the advantage of an earlier growth cessation and earlier reproduction in a context of high mortality. Migliano et al. ["Life history trade-offs explain the evolution of human pygmies," Proc. Natl. Acad. Sci. USA 104:20,216-20,219 (2007)] recently published an important article that tested this hypothesis to explain short stature in human pygmy populations. However innovative this work may be, we believe that some of the data and results presented are controversial if not questionable. As problematic points we note (1) the use of an arbitrary threshold of height (155 cm) to categorize populations into pygmies and nonpygmies; (2) the use of demographic data from Philippine pygmy groups that have experienced dramatic cultural and environmental changes in the last 20 years, and (3) the use of demographic data concerning African pygmy groups because good systematic data on these groups are not available. Finally, we report here mathematical errors and loopholes in the optimization model developed by Migliano and colleagues. In this paper we suggest alternative trade-offs that can be used to explain Migliano's results on more reliable bases.

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.

Greater wealth inequality, less polygyny: rethinking the polygyny threshold model.

Monogamy appears to have become the predominant human mating system with the emergence of highly unequal agricultural populations that replaced relatively egalitarian horticultural populations, challenging the conventional idea-based on the polygyny threshold model-that polygyny should be positively associated with wealth inequality. To address this polygyny paradox, we generalize the standard polygyny threshold model to a mutual mate choice model predicting the fraction of women married polygynously. We then demonstrate two conditions that are jointly sufficient to make monogamy the predominant marriage form, even in highly unequal societies. We assess if these conditions are satisfied using individual-level data from 29 human populations. Our analysis shows that with the shift to stratified agricultural economies: (i) the population frequency of relatively poor individuals increased, increasing wealth inequality, but decreasing the frequency of individuals with sufficient wealth to secure polygynous marriage, and (ii) diminishing marginal fitness returns to additional wives prevent extremely wealthy men from obtaining as many wives as their relative wealth would otherwise predict. These conditions jointly lead to a high population-level frequency of monogamy.

Autonomy, Equality, and Teaching among Aka Foragers and Ngandu Farmers of the Congo Basin.

The significance of teaching to the evolution of human culture is under debate. We contribute to the discussion by using a quantitative, cross-cultural comparative approach to investigate the role of teaching in the lives of children in two small-scale societies: Aka foragers and Ngandu farmers of the Central African Republic. Focal follows with behavior coding were used to record social learning experiences of children aged 4 to 16 during daily life. "Teaching" was coded based on a functional definition from evolutionary biology. Frequencies, contexts, and subtypes of teaching as well as the identity of teachers were analyzed. Teaching was rare compared to observational learning, although both forms of social learning were negatively correlated with age. Children received teaching from a variety of individuals, and they also engaged in teaching. Several teaching types were observed, including instruction, negative feedback, and commands. Statistical differences in the distribution of teaching types and the identity of teachers corresponded with contrasting forager vs. farmer foundational cultural schema. For example, Aka children received less instruction, which empirically limits autonomous learning, and were as likely to receive instruction and negative feedback from other children as they were from adults. Commands, however, exhibited a different pattern suggesting a more complex role for this teaching type. Although consistent with claims that teaching is relatively rare in small-scale societies, this evidence supports the conclusion that teaching is a universal, early emerging cognitive ability in humans. However, culture (e.g., values for autonomy and egalitarianism) structures the nature of teaching.

Teaching in hunter-gatherer infancy.

A debate exists as to whether teaching is part of human nature and central to understanding culture or whether it is a recent invention of Western, Educated, Industrial, Rich, Democratic cultures. Some social-cultural anthropologists and cultural psychologists indicate teaching is rare in small-scale cultures while cognitive psychologists and evolutionary biologists indicate it is universal and key to understanding human culture. This study addresses the following questions: Does teaching of infants exist in hunter-gatherers? If teaching occurs in infancy, what skills or knowledge is transmitted by this process, how often does it occur and who is teaching? The study focuses on late infancy because cognitive psychologists indicate that one form of teaching, called natural pedagogy, emerges at this age. Videotapes of Aka hunter-gatherer infants were used to evaluate whether or not teaching exists among Aka hunter-gatherers of central Africa. The study finds evidence of multiple forms of teaching, including natural pedagogy, that are used to enhance learning of a variety of skills and knowledge.

A Biocultural Investigation of Gender Differences in Tobacco Use in an Egalitarian Hunter-Gatherer Population.

In the developing world, the dramatic male bias in tobacco use is usually ascribed to pronounced gender disparities in social, political, or economic power. This bias might also reflect under-reporting by woman and/or over-reporting by men. To test the role of gender inequality on gender differences in tobacco use we investigated tobacco use among the Aka, a Congo Basin foraging population noted for its exceptionally high degree of gender equality. We also tested a sexual selection hypothesis-that Aka men's tobacco use is related to risk taking. Tobacco use, income, tobacco purchases, tobacco sharing, reasons for using tobacco, risk taking, and other variables were measured using structured surveys and peer reports. Tobacco use was verified by testing for salivary cotinine, a nicotine metabolite. Contrary to expectations, we found a very large male bias in tobacco use. Low levels of use among females appeared to be explained by aversions to tobacco, concerns over its negative effects on fetal health, and a desire to attract husbands, who prefer nonsmoking wives. High male use appeared to be related to a desire to enhance hunting abilities and attract and/or retain wives, who prefer husbands that smoke. We conclude that low levels of smoking by Aka women are better explained by the hypothesis that women evolved to avoid plant toxins to protect their fetuses and nursing infants. High male use might be better explained by sexual selection. We also highlight the important role that recreational drugs appear to play in hunter-gatherer sharing relationships.

Male Circumcision and the Epidemic Emergence of HIV-2 in West Africa.

BACKGROUND: Epidemic HIV-2 (groups A and B) emerged in humans circa 1930-40. Its closest ancestors are SIVsmm infecting sooty mangabeys from southwestern Côte d'Ivoire. The earliest large-scale serological surveys of HIV-2 in West Africa (1985-91) show a patchy spread. Côte d'Ivoire and Guinea-Bissau had the highest prevalence rates by then, and phylogeographical analysis suggests they were the earliest epicenters. Wars and parenteral transmission have been hypothesized to have promoted HIV-2 spread. Male circumcision (MC) is known to correlate negatively with HIV-1 prevalence in Africa, but studies examining this issue for HIV-2 are lacking. METHODS: We reviewed published HIV-2 serosurveys for 30 cities of all West African countries and obtained credible estimates of real prevalence through Bayesian estimation. We estimated past MC rates of 218 West African ethnic groups, based on ethnographic literature and fieldwork. We collected demographic tables specifying the ethnic partition in cities. Uncertainty was incorporated by defining plausible ranges of parameters (e.g. timing of introduction, proportion circumcised). We generated 1,000 sets of past MC rates per city using Latin Hypercube Sampling with different parameter combinations, and explored the correlation between HIV-2 prevalence and estimated MC rate (both logit-transformed) in the 1,000 replicates. RESULTS AND CONCLUSIONS: Our survey reveals that, in the early 20th century, MC was far less common and geographically more variable than nowadays. HIV-2 prevalence in 1985-91 and MC rates in 1950 were negatively correlated (Spearman rho = -0.546, IQR: -0.553--0.546, p≤0.0021). Guinea-Bissau and Côte d'Ivoire cities had markedly lower MC rates. In addition, MC was uncommon in rural southwestern Côte d'Ivoire in 1930.The differential HIV-2 spread in West Africa correlates with different historical MC rates. We suggest HIV-2 only formed early substantial foci in cities with substantial uncircumcised populations. Lack of MC in rural areas exposed to bushmeat may have had a role in successful HIV-2 emergence.

Distribution and Numbers of Pygmies in Central African Forests.

Pygmy populations occupy a vast territory extending west-to-east along the central African belt from the Congo Basin to Lake Victoria. However, their numbers and actual distribution is not known precisely. Here, we undertake this task by using locational data and population sizes for an unprecedented number of known Pygmy camps and settlements (n = 654) in five of the nine countries where currently distributed. With these data we develop spatial distribution models based on the favourability function, which distinguish areas with favourable environmental conditions from those less suitable for Pygmy presence. Highly favourable areas were significantly explained by presence of tropical forests, and by lower human pressure variables. For documented Pygmy settlements, we use the relationship between observed population sizes and predicted favourability values to estimate the total Pygmy population throughout Central Africa. We estimate that around 920,000 Pygmies (over 60% in DRC) is possible within favourable forest areas in Central Africa. We argue that fragmentation of the existing Pygmy populations, alongside pressure from extractive industries and sometimes conflict with conservation areas, endanger their future. There is an urgent need to inform policies that can mitigate against future external threats to these indigenous peoples' culture and lifestyles.

Providing care and facing death: nursing during Ebola outbreaks in central Africa.

Few studies have focused on describing the experiences of health care workers during rapid killing epidemics. In this article, the views and experiences of nurses during three outbreaks of Ebola hemorrhagic fever (EHF) in Central Africa are examined. These three outbreaks occurred in Kikwit, Democratic Republic of Congo (DRC, 1995); Gulu, Uganda (2000-2001); and Republic of Congo (ROC, 2003). Open-ended and semistructured interviews with individuals and small groups were conducted during the outbreaks in Uganda and ROC; data from DRC are extracted from published sources. Three key themes emerged from the interviews: (a) lack of protective gear, basic equipment, and other resources necessary to provide care, especially during the early phases of the outbreaks; (b) stigmatization by family, coworkers, and community; and (c) exceptional commitment to the nursing profession in a context where the lives of the health care workers were in jeopardy.

Cultural variation in the use of overimitation by the Aka and Ngandu of the Congo Basin.

Studies in Western cultures have observed that both children and adults tend to overimitate, copying causally irrelevant actions in the presence of clear causal information. Investigation of this feature in non-Western groups has found little difference cross-culturally in the frequency or manner with which individuals overimitate. However, each of the non-Western populations studied thus far has a history of close interaction with Western cultures, such that they are now far removed from life in a hunter-gatherer or other small-scale culture. To investigate overimitation in a context of limited Western cultural influences, we conducted a study with the Aka hunter-gatherers and neighboring Ngandu horticulturalists of the Congo Basin rainforest in the southern Central African Republic. Aka children, Ngandu children, and Aka adults were presented with a reward retrieval task similar to those performed in previous studies, involving a demonstrated sequence of causally relevant and irrelevant actions. Aka children were found not to overimitate as expected, instead displaying one of the lowest rates of overimitation seen under similar conditions. Aka children copied fewer irrelevant actions than Aka adults, used a lower proportion of irrelevant actions than Ngandu children and Aka adults, and had less copying fidelity than Aka adults. Measures from Ngandu children were intermediate between the two Aka groups. Of the participants that succeeded in retrieving the reward, 60% of Aka children used emulation rather than imitation, compared to 15% of Ngandu children, 11% of Aka adults, and 0% of Western children of similar age. From these results, we conclude that cross-cultural variation exists in the use of overimitation during childhood. Further study is needed under a more diverse representation of cultural and socioeconomic groups in order to investigate the cognitive underpinnings of overimitation and its possible influences on social learning and the biological and cultural evolution of our species.

Children's daily activities and knowledge acquisition: A case study among the Baka from southeastern Cameroon.

BACKGROUND: The acquisition of local knowledge occurs through complex interactions between individual and contextual characteristics: as context changes, so it changes the acquisition of knowledge. Contemporary small-scale societies facing rapid social-ecological change provide a unique opportunity to study the relation between social-ecological changes and the process of acquisition of local knowledge. In this work, we study children's involvement in subsistence related activities (i.e., hunting and gathering) in a context of social-ecological change and discuss how such involvement might condition the acquisition of local knowledge during childhood. METHODS: We interviewed 98 children from a hunter-gatherer society, the Baka, living in two different villages in southeastern Cameroon and assessed their involvement in daily activities. Using interviews, we collected self-reported data on the main activities performed during the previous 24 h. We describe the frequency of occurrence of daily activities during middle childhood and adolescence and explore the variation in occurrence according to the sex, the age group, and the village of residency of the child. We also explore variation according to the season in which the activity is conducted and to the predicted potential of the activity for the acquisition of local knowledge. RESULTS: Baka children and adolescents engage in subsistence-related activities (i.e., hunting and gathering) and playing more frequently than in other activities (i.e., traditional tales or schooling). Gender differences in children's subsistence activities emerge at an early age. Engagement in activities also varies with age, with adolescents spending more time in agricultural activities, modern leisure (i.e., going to bars), and socializing than younger children. When conducting similar activities, adolescents use more complex techniques than younger children. CONCLUSION: Subsistence activities, which present a high potential for transmission of local knowledge, continue to be predominant in Baka childhood. However, Baka children also engage in other, non-traditional activities, such as modern forms of leisure, or schooling, with a low potential for the transmission of local knowledge. Baka children's involvement in non-traditional activities might have unforeseen impacts on the acquisition of local knowledge.

Allomaternal nursing in humans.

Few studies exist of allomaternal nursing in humans. It is relatively common among some cultures, such as the Aka and Efé hunter-gatherers of the Congo Basin, but it does not occur in other foragers such as the !Kung and Hadza of Southern and East Africa. This paper utilizes focal follow observations of Aka and Efé infants, interviews with Aka mothers, ethnographic reports from researchers working with hunter-gatherers, and a survey of the eHRAF cultures to try to answer the following questions: how often does allomaternal nursing occur, who provides it, and under what contexts does it take place? The study indicates that it occurs in many cultures (93% of cultures with data) but that it is normative in relatively few cultures; biological kin, especially grandmothers, frequently provide allomaternal nursing and that infant age, mother's condition, and culture (e.g., cultural models about if and when women other than the mother can nurse an infant or colostrum taboos) impact the nature and frequency of allomaternal nursing. The empirical results of this exploratory study are discussed in the context of existing hypotheses used to explain allomaternal nursing.