Allomaternal Care among the Hadza of Tanzania

Hunter-Gatherers and Human Evolution: New Light on Old Debates

One of the most persistent debates in anthropology and related disciplines has been over the relative weight of aggression and competition versus nonaggression and cooperation as drivers of human behavioral evolution. The literature on hunting and gathering societies—past and present—has played a prominent role in these debates. This review compares recent literature from both sides of the argument and evaluates how accurately various authors use or misuse the ethnographic and archaeological research on hunters and gatherers. Whereas some theories provide a very poor fit with the hunter-gatherer evidence, others build their arguments around a much fuller range of the available data. The latter make a convincing case for models of human evolution that place at their center cooperative breeding and child-rearing, as well as management of conflict, flexible land tenure, and balanced gender relations.

Hunter-Gatherers Maintain Assortativity in Cooperation despite High Levels of Residential Change and Mixing

Widespread cooperation is a defining feature of human societies from hunter-gatherer bands to nation states [1, 2], but explaining its evolution remains a challenge. Although positive assortment of cooperators is recognized as a basic requirement for the evolution of cooperation, the mechanisms governing assortment are debated. Moreover, the social structure of modern hunter-gatherers, characterized by high mobility, residential mixing, and low genetic relatedness [3], undermines assortment and adds to the puzzle of how cooperation evolved. Here, we analyze four years of data (2010, 2013, 2014, 2016) tracking residence and levels of cooperation elicited from a public goods game in Hadza hunter-gatherers of Tanzania. Data were collected from 56 camps, comprising 383 unique individuals, 137 of whom we have data for two or more years. Despite significant residential mixing, we observe a robust pattern of assortment that is necessary for cooperation to evolve; in every year, Hadza camps exhibit high between-camp and low within-camp variation in cooperation. We find little evidence that cooperative behavior within individuals is stable over time or that similarity in cooperation between dyads predicts their future cohabitation. Both sets of findings are inconsistent with models that assume stable cooperative and selfish types, including partner choice models. Consistent with social norms, culture, and reciprocity theories, the strongest predictor of an individual's level of cooperation is the mean cooperation of their current campmates. These findings underscore the adaptive nature of human cooperation-particularly its responsiveness to social contexts-as a feature that is important in generating the assortment necessary for cooperation to evolve.

Grandmotherhood across the demographic transition

Grandmothers provide key care to their grandchildren in both contemporary and historic human populations. The length of the grandmother-grandchild relationship provides a basis for such interactions, but its variation and determinants have rarely been studied in different contexts, despite changes in age-specific mortality and fertility rates likely having affected grandmotherhood patterns across the demographic transition. Understanding how often and long grandmothers have been available for their grandchildren in different conditions may help explain the large differences between grandmaternal effects found in different societies, and is vital for developing theories concerning the evolution of menopause, post-reproductive longevity, and family living. Using an extensive genealogical dataset from Finland spanning the demographic transition, we quantify the length of grandmotherhood and its determinants from 1790–1959. We found that shared time between grandmothers and grandchildren was consistently low before the demographic transition, only increasing greatly during the 20^th century. Whilst reduced childhood mortality and increasing adult longevity had a role in this change, grandmaternal age at birth remained consistent across the study period. Our findings further understanding of the temporal context of grandmother-grandchild relationships, and emphasise the need to consider the demography of grandmotherhood in a number of disciplines, including biology (e.g. evolution of the family), sociology (e.g. changing family structures), population health (e.g. changing age structures), and economics (e.g. workforce retention).

Infant allocare in traditional societies

Across human societies infants receive care from both their mothers and others. Reproductive cooperation raises two important questions: how does allocare benefit mothers and infants, and why do caretakers help mothers when they could spend their time in other, perhaps more valuable ways? We use behavioral and biological data from three small-scale societies to evaluate 1) how allocare affects a nursing mother's time, 2) whether a mother's birth interval length, surviving fertility and infant weight vary as a function of the childcare help that she receives, and 3) the opportunity cost for helpers to spend time caring for children. Across our hunter-gatherer and agricultural samples we find that on average mothers provide 57% of the direct care that an infant receives and allocaretakers 43% (±20%). Model results show that for every 10% increase in allocare the probability that a mother engages in direct care diminishes by 25%, a potential savings of an estimated 165 kcals per day. While allocare has a significant immediate impact on mother's time, no detectable effect on delayed fitness measures (birth interval and surviving fertility) or on infant weight status was evident. Cross culturally we find that other than mothers, siblings spend the most time caretaking infants, and they do so without compromising the time that they might otherwise spend in play, economic activities or education. The low opportunity cost for children to help offers an alternative explanation why juveniles are common caretakers in many societies, even in the absence of delayed indirect fitness benefits. While we expect specific patterns to vary cross culturally, these results point to the importance of infant allocare and its immediate time benefits for mothers to maintain flexibility in balancing the competing demands to support both older and younger children.

A model explaining the matrilateral bias in alloparental investment

Maternal grandmothers invest more in childcare than paternal grandmothers. This bias is large where the expression of preferences is unconstrained by residential and lineage norms, and is detectable even where marriage removes women from their natal families. We maintain that the standard evolutionary explanation, paternity uncertainty, is incomplete, and present an expanded model incorporating effects of alloparents on the mother as well as on her children. Alloparenting lightens a mother's load and increases her residual nepotistic value: her expected fitness from later investments in personal reproduction and in her natal relatives. The mother's mother derives fitness from all such investments, whereas her mother-in-law gains only from further investment in children sired by her son, and thus has less incentive to assist the mother even if paternity is certain. This logic extends to kin other than grandmothers. We generate several hypotheses for future research.

Do Step- and Biological Grandparents Show Differences in Investment and Emotional Closeness With Their Grandchildren?

Human children are raised by a variety of caregivers including grandparents. A few studies have assessed potential differences in direct caregiving, financial expenditures, and emotional closeness between biological and step-grandparents. Drawing upon kin selection theory, we hypothesized that step-grandparents would provide less care and be less emotionally close to grandchildren than would biological grandparents. A sample of 341 heterosexual U.S. adults 25-35 years of age in a long-term partnership and with a biological child 5 years of age or younger were recruited via Amazon Mechanical Turk. Subjects provided sociodemographic information and answered questions about the dynamics between their own parent/stepparent and their own youngest biological child (hence, biological/step-grandparenting dynamics). Main analyses were restricted to within-subject comparisons. Results showed that biological grandmothers provided more direct childcare, financial expenditures, and had more emotionally close relationships with grandchildren than did step-grandmothers. Biological grandfathers provided less direct care and had less emotionally close relationships than step-grandfathers but did not exhibit differences in financial expenditures. Biological grandmothers provided more direct care, financial investment, and were more emotionally close to the referential grandchild than were biological grandfathers. Step-grandfathers were more emotionally close and more often played with grandchildren than step-grandmothers. These findings partially support kin selection theory. We discuss the relevance of factors such as competing demands on grandmothers' investment in biological and step-grandchildren and grandfathering serving in part as mating effort. Sex differences in biological grandparenting also mirror those in parenting. We suggest directions for future research, including on grandfathers, particularly in patrilineal societies.

Maternal Competition in Women

We examined maternal competition, an unexplored form of competition between women. Given women's high investment in offspring and mothers' key role in shaping their reproductive, social, and cultural success as adults, we might expect to see maternal competition between women as well as mate competition. Predictions about the effect of maternal characteristics (age, relationship status, educational background, number of children, investment in the mothering role) and child variables (age, sex) were drawn from evolutionary theory and sociological research. Mothers of primary school children (in two samples: N = 210 and 169) completed a series of questionnaires. A novel nine-item measure of maternal competitive behavior (MCQ) and two subscales assessing Covert (MCQ-C) and Face-to-Face (MCQ-FF) forms of competition were developed using confirmatory factor analysis. Competitiveness (MCQ score) was predicted by maternal investment, single motherhood, fewer children, and (marginally) child's older age. The effect of single motherhood (but not other predictors) was partially mediated by greater maternal investment. In response to a scenario of their child underperforming relative to their peers, a mother's competitive distress was a positive function of the importance she ascribed to their success and her estimation of her child's ability. Her competitive distress was highly correlated with the distress she attributed to a female friend, hinting at bidirectional dyadic effects. Qualitative responses indicated that nonspecific bragging and boasting about academic achievements were the most common irritants. Although 40% of women were angered or annoyed by such comments, less than 5% endorsed a direct hostile response. Instead, competitive mothers were conversationally shunned and rejected as friends. We suggest that the interdependence of mothers based on reciprocal childcare has supported a culture of egalitarianism that is violated by explicit competitiveness.

Sibling Competition & Growth Tradeoffs. Biological vs. Statistical Significance

Early childhood growth has many downstream effects on future health and reproduction and is an important measure of offspring quality. While a tradeoff between family size and child growth outcomes is theoretically predicted in high-fertility societies, empirical evidence is mixed. This is often attributed to phenotypic variation in parental condition. However, inconsistent study results may also arise because family size confounds the potentially differential effects that older and younger siblings can have on young children's growth. Additionally, inconsistent results might reflect that the biological significance associated with different growth trajectories is poorly understood. This paper addresses these concerns by tracking children's monthly gains in height and weight from weaning to age five in a high fertility Maya community. We predict that: 1) as an aggregate measure family size will not have a major impact on child growth during the post weaning period; 2) competition from young siblings will negatively impact child growth during the post weaning period; 3) however because of their economic value, older siblings will have a negligible effect on young children's growth. Accounting for parental condition, we use linear mixed models to evaluate the effects that family size, younger and older siblings have on children's growth. Congruent with our expectations, it is younger siblings who have the most detrimental effect on children's growth. While we find statistical evidence of a quantity/quality tradeoff effect, the biological significance of these results is negligible in early childhood. Our findings help to resolve why quantity/quality studies have had inconsistent results by showing that sibling competition varies with sibling age composition, not just family size, and that biological significance is distinct from statistical significance.

BaAka women's health and subsistence practices in transitional conservation economies: Variation with age, household size, and food security

OBJECTIVES

Using ethnographic interviews and biological measures, this article investigates changing health and nutrition of a hunter-gatherer population transitioning from a forest-based subsistence system to a horticultural and market-driven lifestyle.

METHODS

This study represents biological and dietary recall data for adult female foragers (18+; n = 60) across two villages, Mossapoula (MS) and Yandoumbé (YDBE), in the Dzanga Sangha Protected Areas (APDS), Central African Republic (CAR). Standard anthropometric measurements (height, weight, skinfolds) and hemoglobin values were collected to assess short-term nutritional status.

RESULTS

BMI was similar across all three age classes in YDBE, but differed amongst women of MS (ANOVA; F = 6.34, df = 30, P = 0.005).Values were lowest among the older women in older age class 3 who also had the greatest number of dependents. Overall SS values were significantly negatively correlated with the number of biological children (r = -0.33, P = 0.01) in both villages.

CONCLUSIONS

Here, we identify older BaAka women, caring for their own children and grandchildren, as particularly vulnerable to economic changes and food insecurity. We found older women, especially those in a community with greater restrictions on access to forest resources to have more dependents, reduced market integration, and low BMI relative to younger women in the population. Am. J. Hum. Biol. 28:453-460, 2016. © 2015 Wiley Periodicals, Inc.

Was monogamy a key step on the hominin road? Reevaluating the monogamy hypothesis in the evolution of cooperative breeding

Because human mothers routinely rely on others to help raise their young, humans have been characterized as cooperative breeders.(1-9) Several large-scale phylogenetic analyses have presented compelling evidence that monogamy preceded the evolution of cooperative breeding in a wide variety of nonhuman animals.(10-14) These studies have suggested that monogamy provides a general rule (the monogamy hypothesis) for explaining evolutionary transitions to cooperative breeding.(15) Given the prevalence of cooperative breeding in contemporary human societies, we evaluate whether this suggests a monogamous hominin past.

Hadza hunter-gatherer men do not have more masculine digit ratios (2D:4D)

OBJECTIVES

The ratio between the length of the second and the length of the fourth digit (2D:4D) is sexually dimorphic such that males of many species possess a lower ratio than females, particularly in the right hand. Still, men and women often exhibit overlapping 2D:4D ranges and the ratio is highly variable between populations. In order to further explore populational variability, we chose to analyze 2D:4D in the Hadza, a population of hunter-gatherers living in Tanzania.

MATERIALS AND METHODS

Data were collected separately by two researchers over the course of three years (1998, 2001, 2006) from 152 adult participants (male: n = 76, female: n = 76). Independent samples t-tests were used to explore sex differences, paired samples t-tests were used to explore directional effects within each sex, and linear regression and one-way ANOVA were used to test possible age effects.

RESULTS

In none of the years, or pooled (n = 152), did we find evidence that adult men have a lower 2D:4D than adult women. If anything, the data suggest that women in this population have a significantly lower right hand 2D:4D than men (P < 0.001, d = 0.57). In contrast, left hand 2D:4D did not exhibit a sex difference (P = 0.862, d = 0.03).

DISCUSSION

These findings challenge the current view that lower 2D:4D in men is a uniform characteristic of our species. Cross-populational variance in 2D:4D may be related to known patterns of hormonal variation resulting from both genetic and environmental mechanisms, though this relationship merits further investigation.

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.

Food Sharing among Hadza Hunter-Gatherer Children

Human prosociality is one of the defining characteristics of our species, yet the ontogeny of altruistic behavior remains poorly understood. The evolution of widespread food sharing in humans helped shape cooperation, family formation, life history, language, and the development of economies of scale. While the behavioral and ecological correlates of food sharing among adults are widely studied, very little is known about food sharing among children. Here, in the first study to analyze the food sharing patterns of hunter-gatherer children, we show that while sharing may be biased towards kin, reciprocity characterizes the majority of all sharing dyads, both related and unrelated. These data lend support to the recent claim that discrimination among kin might be linked with reciprocal altruism theory. Furthermore, we show that age positively correlates with an increase in sharing, both in frequency and amount, supporting recent suggestions that prosocial behaviors and egalitarianism develop strongly in middle childhood when children acquire the normative rules of their society.

Household and kin provisioning by Hadza men

We use data collected among Hadza hunter-gatherers between 2005 and 2009 to examine hypotheses about the causes and consequences of men's foraging and food sharing. We find that Hadza men foraged for a range of food types, including fruit, honey, small animals, and large game. Large game were shared not like common goods, but in ways that significantly advantaged producers' households. Food sharing and consumption data show that men channeled the foods they produced to their wives, children, and their consanguineal and affinal kin living in other households. On average, single men brought food to camp on 28% of days, married men without children at home on 31% of days, and married men with children at home on 42% of days. Married men brought fruit, the least widely shared resource, to camp significantly more often than single men. A model of the relationship between hunting success and household food consumption indicates that the best hunters provided 3-4 times the amount of food to their families than median or poor hunters. These new data fill important gaps in our knowledge of the subsistence economy of the Hadza and uphold predictions derived from the household and kin provisioning hypotheses. Key evidence and assumptions backing prior claims that Hadza hunting is largely a form of status competition were not replicated in our study. In light of this, family provisioning is a more viable explanation for why good hunters are preferred as husbands and have higher fertility than others.

Cooperative breeding and Aka children's nutritional status: is flexibility key?

Research suggests that nonmaternal caregivers (allomothers) offer essential assistance through caregiving and provisioning, helping to support lengthy child development. Here, we examine the role of allomothers and the broader social and sharing network on Aka forager children's anthropometrics. We hypothesize that nonmaternal investors strategically target their assistance when it is most needed and when it will have the greatest effect. We evaluate children's nutritional status using WHO standards [weight-for-age (WAZ), height-for-age (HAZ), and weight-for-height (WHZ)] during four periods of child development [early infancy (birth to <9 months), mobility to weaning (9 to <36 months), early childhood (36 to <72 months), and middle childhood (72 to <120 months); N = 127]. We explore the effects of allomothers and the social network across these different risk periods and examine whether the broader social network buffers the loss of a primary allomother. ANOVA results suggest that girls may be experiencing some growth faltering, while boys start smaller and remain small across these stages. We used OLS multiple regression models to evaluate the effects of sex, camp composition, risk periods, and allomothers' presence on WAZ, HAZ, and WHZ. Grandmothers are the most influential allomother, with their effect most evident during the 9 to <36 month period. Camp size was also associated with greater WAZ, suggesting that children residing in small camps may be disadvantaged. Our findings also indicate that, under specific residence patterns, cooperative child rearing networks buffer the loss of a grandmother. Overall, our results suggest the importance of social networks to children's nutritional status and that individuals target investment to critical phases.

The development of human female competition: allies and adversaries

Throughout their lives, women provide for their own and their children's and grandchildren's needs and thus must minimize their risk of incurring physical harm. Alliances with individuals who will assist them in attaining these goals increase their probability of survival and reproductive success. High status in the community enhances access to physical resources and valuable allies. Kin, a mate, and affines share a mother's genetic interests, whereas unrelated women constitute primary competitors. From early childhood onwards, girls compete using strategies that minimize the risk of retaliation and reduce the strength of other girls. Girls' competitive strategies include avoiding direct interference with another girl's goals, disguising competition, competing overtly only from a position of high status in the community, enforcing equality within the female community and socially excluding other girls.

Cooperative breeding and maternal energy expenditure among Aka foragers

OBJECTIVES

Previous research among foragers and theory suggests that nonmaternal caregivers offer essential assistance, which supports female reproduction and the costs associated with lengthy child development. Mothers' face trade-offs in energy allocation between work and childcare, particularly when mothers have an infant. These trade-offs likely have crucial impacts on the pace of reproduction and child health. Caregivers can help mothers with childcare or they can reduce a mother's nonchildcare workload. If caregivers assist mothers by substituting childcare, then maternal energy expenditure (EE) in other work activities should increase. If caregivers assist mothers by substituting labor, then maternal EE in work activities should decrease when caregivers are present.

METHODS

Utilizing detailed, quantitative behavioral observations and EE data, we test these propositions with data from 28 Aka forager mothers with children <35 months old. We isolate paternal, grandmaternal, and other caregiver effects on maternal EE and childcare in multivariate analyses.

RESULTS

Our results show that caregivers (largely grandmothers) significantly reduce mothers' work EE by as much 216 kcal across a 9-hour observation period, while fathers and juveniles appear to increase maternal EE. Direct childcare from grandmothers decreases maternal direct care by about one-to-one indicating a labor substitution. Direct childcare from fathers decreases maternal care by almost 4 to 1, resulting in a net reduction of total direct care from all caregivers.

CONCLUSIONS

Our results indicate that there are multiple pathways by which helpers offset maternal work/childcare trade-offs.

The place of proximity: social support in mother-adult daughter relationships

The mother-adult daughter relationship has been highlighted in both the social sciences and the public health literature as an important facet of social support networks, particularly as they pertain to maternal and child health. Evolutionary anthropologists also have shown positive associations between support from maternal grandmothers and various outcomes related to reproductive success; however, many of these studies rely on proximity as a surrogate measure of support. Here I present data from the Puerto Rican Maternal and Infant Health Survey (PRMIHS) comparing geographic proximity of mother and daughter with a self-reported measure of mother-to-daughter support. These two measures were used to predict infant health outcomes as well as various measures of instrumental and emotional aid provided during pregnancy and after birth. Primary support was shown to have a positive effect across the analyses, whereas geographic proximity was associated with an increased risk of infant mortality and low birth weight as well as reduced odds of receiving support. This paradox was then examined using a combination variable that teased out the interactions of maternal support and proximity. Women who were geographically close to their mothers but who did not consider them a primary source of support had increased odds of infant death and low birth weight, and were less likely to receive either tangible or intangible forms of aid, while women whose mothers were both close and primary showed uniformly positive outcomes. These results place the role of propinquity within the larger context of social support and highlight the need for more detailed studies of social support within evolutionary anthropology.

The foundation of kinship: households

Men's hunting has dominated the discourse on energy capture and flow in the past decade or so. We turn to women's roles as critical to household formation, pair-bonding, and intergenerational bonds. Their pivotal contributions in food processing and distribution likely promoted kinship, both genetic and affinal, and appear to be the foundation from which households evolved. With conscious recognition of household social units, variable cultural constructions of human kinship systems that were sensitive to environmental and technological conditions could emerge. Kinship dramatically altered the organization of resource access for our species, creating what we term "kinship ecologies." We present simple mathematical models to show how hunting leads to dependence on women's contributions, bonds men to women, and bonds generations together. Kinship, as it organized transfers of food and labor energy centered on women, also became integrated with the biological evolution of human reproduction and life history.

Diferenças no investimento materno em função de variáveis socioambientais

A partir da Teoria do Investimento Parental prevê-se diminuição no investimento com a diminuição da relação benefício/custos. Investigou-se a variação no investimento materno em função de variáveis socioambientais que podem apresentar impacto na relação benefício/custo do investimento materno. Os fatores investigados apresentaram, de forma geral, efeito sobre o investimento materno no sentido esperado de acordo com a teoria: quanto maiores os custos representados pela ausência de coabitação com pai, pouca disponibilidade de renda e início precoce da vida reprodutiva, menor o investimento materno. Conclui-se que os fatores socioambientais podem promover ou inibir o investimento materno indicando a importância de políticas públicas no sentido da promoção da estabilidade destes fatores.

How Grandparents Matter: Support for the Cooperative Breeding Hypothesis in a Contemporary Dutch Population

Low birth rates in developed societies reflect women's difficulties in combining work and motherhood. While demographic research has focused on the role of formal childcare in easing this dilemma, evolutionary theory points to the importance of kin. The cooperative breeding hypothesis states that the wider kin group has facilitated women's reproduction during our evolutionary history. This mechanism has been demonstrated in pre-industrial societies, but there is no direct evidence of beneficial effects of kin's support on parents' reproduction in modern societies. Using three-generation longitudinal data anchored in a sample of grandparents aged 55 and over in 1992 in the Netherlands, we show that childcare support from grandparents increases the probability that parents have additional children in the next 8 to 10 years. Grandparental childcare provided to a nephew or niece of childless children did not significantly increase the probability that those children started a family. These results suggest that childcare support by grandparents can enhance their children's reproductive success in modern societies and is an important factor in people's fertility decisions, along with the availability of formal childcare.