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.     

Altruistic cooperation during foraging by the Ache, and the evolved human predisposition to cooperate

This paper presents quantitative data on altruistic cooperation during food acquisition by Ache foragers. Cooperative activities are defined as those that entail a cost of time and energy to the donor but primarily lead to an increase in the foraging success of the recipient. Data show that Ache men and women spend about 10% of all foraging time engaged in altruistic cooperation on average, and that on some days they may spend more than 50% of their foraging time in such activities. The most time-consuming cooperative activity for both sexes is helping during the pursuit of game animals, a pattern that is probably linked to the widespread sharing of game by Ache foragers. Cooperative food acquisition and subsequent food redistribution in hunter-gatherer societies are critical behaviors that probably helped shape universal, evolved, cooperative tendencies that are well illustrated in modern experimental economics. This paper was originally presented at a Max Planck symposium on cooperation in Leipzig in June 1999. This work was partially funded by a grant from the L.S.B. Leakey Foundation and NSF grant BNS 9727656. Kim Hill is a professor of anthropology in the Human Evolutionary Ecology (HEE) program at the University of New Mexico. His primary research interests include hunter-gatherer behavioral ecology, life history theory, food acquisition strategies, food sharing, cooperation, and biodiversity conservation in lowland South America. He has done fieldwork with Nahautl, Ache, Guarani, Hiwi, Mashco Piro, Matsiguenga, and Yora indigenous peoples of Central and South America.     

Foraging decisions among Aché hunter-gatherers: New data and implications for optimal foraging models

This article summarizes 5 years of research on resource choice and foraging strategy among Ache foragers in eastern Paraguay. Successes and failures of simple models from optimal foraging theory (OFT) are discussed and revisions are suggested in order to bring the models in line with empirical evidence from the Ache. The following conclusions emerge: (1) Energetic returns from various alternative resources and foraging strategies is probably the best single predictor of foraging patterns. (2) Nutrient constraints should be added only when they significantly improve the predictive power of the model. Importance of meat versus vegetable resources may be one important modification based on nutrients that enhances the ability of OFT models to account for empirical reality in human foragers. (3) Men's and women's abilities and foraging patterns differ enough that they should be treated separately in all OFT analyses. (4) Opportunity costs associated with resources that are processed when foraging is not possible may be sufficiently low to predict that high processing time resources will be included in the optimal diet even when their associated return rates (including processing) are lower than mean foraging returns. (5) When food sharing is extensive and foraging bands include several adult males and females, foragers may not need to modify foraging strategies in other ways in order to reduce the risk of not eating on some days.     

Reservation food sharing among the Ache of Paraguay

We describe food transfer patterns among Ache Indians living on a permanent reservation. The social atmosphere at the reservation is characterized by a larger group size, a more predictable diet, and more privacy than the Ache typically experience in the forest while on temporary foraging treks. Although sharing patterns vary by resource type and package size, much of the food available at the reservation is given to members of just a few other families. We find significant positive correlations between amounts transferred among pairs of families, a measure of the "contingency" component required of reciprocal altruism models. These preferred sharing partners are usually close kin. We explore implications of these results in light of predictions from current sharing models. This research was supported by an L.S.B. Leakey Foundation grant and an NSF Graduate Fellowship to M. Gurven, and NSF Grant #9617692 to K. Hill and A. M. Hurtado. Michael Gurven recently obtained his Ph.D. from the University of New Mexico and is now an assistant professor at UC-Santa Barbara. His current interests include exploring ways in which socioecology influences variation in cooperation within and across human groups, and how cultural norms of fairness co-evolve with systems of resource production and distribution. Wesley Allen-Arave is pursuing his Ph.D. in anthropology at the University of New Mexico. His primary research interests focus on exploring variations across time and space in nonreciprocated altruistic acts, cooperation within social networks, and concerns over social approval. Kim Hill is a professor of anthropology in the Human Evolutionary Ecology (HEE) program at the University of New Mexico. His primary research interests include hunter-gatherer behavioral ecology, life history theory, food acquisition strategies, food sharing, cooperation, and biodiversity conservation in lowland South America. He has done fieldwork with Nahautl, Ache, Guarani, Hiwi, Mashco Piro, Matsiguenga, and Yora indigenous peoples of Central and South America. A. Magdalena Hurtado is associate professor of anthropology at the University of New Mexico. Her research interests include the evolution of cooperation between the sexes, infectious disease and immune system adaptations, the epidemiology of hunter-gatherer societies in transition, and the effects of health on economic productivity. During the past 20 years she has conducted fieldwork among several South American native populations but now works primarily among the Ache of eastern Paraguay.     

“It's a Wonderful Life”: signaling generosity among the Ache of Paraguay

Intensive food sharing among foragers and horticulturists is commonly explained as a means of reducing the risk of daily shortfalls, ensuring adequate daily consumption for all group members who actively pool resources. Consistently high food producers who give more than they receive, however, gain the least risk-reduction benefit from this daily pooling because they are the least likely to go without food on any given day. Why then do some high producers consistently share food, and why do some average producers share proportionally more food than others? We propose that although these individuals may not receive the same amounts they give (i.e., strict Tit-for-Tat), one explanation for their generosity is that they receive additional food during hard times. These include brief episodes of sickness, disease, injury, or accidents—fairly common events in traditional societies that can render individuals incapable of producing food, thereby having long-term effects on morbidity and fecundity and ultimately on lifetime reproductive success. Data collected among the Ache, a group of South American forager-horticulturists, indicate that those who shared and produced more than average (signaling cooperative intent and/or ability to produce) were rewarded with more food from more people when injured or sick than those who shared and produced below average. These results, framed within the context of tradeoffs between short-term and long-term fitness, may provide insight into motivations behind costly expenditures for establishing and reinforcing status and reputation.     

Why hunt? Why gather? Why share? Hadza assessments of foraging and food-sharing motive

Over the last half century, anthropologists have vigorously debated the adaptive motivations underlying food acquisition choices and food-sharing among hunter-gatherer groups. Numerous explanations have been proposed to account for high-levels of generosity in food-sharing, including self- and family-provisioning, reciprocity, tolerated theft and pro-social- or skill-signaling. However, few studies have asked foragers directly and systematically about the motivations underlying their foraging and sharing decisions. We recruited 110 Hadza participants and employed a combination of free-response, yes/no, ranking and forced-choice questions to do just this. In free-response answers, respondents typically gave outcome-oriented accounts of foraging motive (e.g., to get food) and moralistic accounts of sharing motive (e.g., I have a good heart). In ranking tasks, participants gave precedence to reciprocity as a motive for sharing food beyond the household. We found small but clear gender differences in foraging motive, in line with previous predictions: women were more likely than men to rank family-provisioning highly whereas men were more likely than women to rank skill-signaling highly. However, despite these gender differences, the relative importance of different motivations was similar across genders and skill-signaling, sharing and family-provisioning were the most important motivators of foraging activity for both men and women. Contrary to the expectations of tolerated theft, peer complaints and requests for food ranked very low. There are several compelling reasons that evolutionary thinkers, typically interested in ultimate-level adaptive processes, have traditionally eschewed direct and explicit investigations of motive. However, these data may yet provide important insights.     

Risk and reciprocity in Meriam food sharing

Foragers who do not practice food storage might adapt to fluctuating food supplies by sharing surplus resources in times of plenty with the expectation of receiving in times of shortfall. In this paper, we derive a number of predictions from this perspective, which we term the risk reduction reciprocity (RRR) model, and test these with ethnographic data on foraging (fishing, shellfish collecting, and turtle hunting) among the Meriam (Torres Strait, Australia). While the size of a harvest strongly predicts that a portion will be shared beyond the household of the acquirer, the effects of key measures of foraging risk (e.g., failure rate) are comparatively weak: Harvests from high-risk hunt types are usually shared more often than those from low-risk hunt types in the same macropatch, but increases in risk overall do not accurately predict increases in the probability of sharing. In addition, free-riders (those who take shares but do not reciprocate) are not discriminated against, those who share more often and more generously do not predictably receive more, and most sharing relationships between households (over 80%) involve one-way flows.     

Trade-Offs between female food acquisition and child care among hiwi and ache foragers

Even though female food acquisition is an area of considerable interest in hunter-gatherer research, the ecological determinants of women’s economic decisions in these populations are still poorly understood. The literature on female foraging behavior indicates that there is considerable variation within and across foraging societies in the amount of time that women spend foraging and in the amount and types of food that they acquire. It is possible that this heterogeneity reflects variation in the trade-offs between time spent in food acquisition and child care activities that women face in different groups of hunter-gatherers. In this paper we discuss the fitness trade-offs between food acquisition and child care that Hiwi and Ache women foragers might face. Multiple regression analyses show that in both populations the daily food acquisition of a woman’s spouse is negatively related to female foraging effort. In addition, nursing mothers spend less time foraging and acquire less food than do nonnursing women. As the number of dependents that a woman has increases, however, women also increase foraging time and the amount of food they acquire. Some interesting exceptions to these general trends are as follows: (a) differences in foraging effort between nursing and nonnursing women are less pronounced when fruits and roots are in season than in other seasons of the year; (b) foraging return rates decrease for Ache women as their numbers of dependents increase; and (c) among Ache women, the positive effect of number of dependents on foraging behavior is less pronounced when fruits are in season than at other times of the year. Lastly, in the Hiwi sample we found that postreproductive women work considerably harder than women of reproductive age in the root season but not in other seasons of the year. We discuss how ecological variation in constraints, the number of health insults to children that Hiwi and Ache mothers can avoid, and the fitness benefits they can gain from spending time in food acquisition and child care might account for differences and similarities in the foraging behaviors of subgroups of Hiwi and Ache mothers across different seasons of the year. Valid tests of the explanations we propose will require considerable effort to measure the relationship between maternal food acquisition, child care, and adverse health outcomes in offspring. This paper is dedicated to Nutsiya, the hardest-working grandmother we ever observed Kristen Hawkes contributed useful information on female foraging among the Hadza. The research was funded by the National Science Foundation (BNS-8613215, BNS-538228, BNS-8309834, BNS-8121209) and the L. S. B. Leakey Foundation. The senior author was supported by fellowships from the Fundacion Gran Mariscal de Ayacucho of Venezuela and the National Institute of Health (Grant No. 1 RO1 HD16221-01A2). A. Magdalena Hurtado, Kim Hill, and Hillard Kaplan collaborate in research on the evolutionary ecology of the division of labor by sex. Ines Hurtado is Senior Research Scientist at the Instituto Venezolano de Investigaciones Científicas, Venezuela. She studies the immunology of parasite load and atopic illness in Hiwi foragers.     

Interhousehold Meat Sharing among Mayangna and Miskito Horticulturalists in Nicaragua

Recent analyses of food sharing in small-scale societies indicate that reciprocal altruism maintains interhousehold food transfers, even among close kin. In this study, matrix-based regression methods are used to test the explanatory power of reciprocal altruism, kin selection, and tolerated scrounging. In a network of 35 households in Nicaragua’s Bosawas Reserve, the significant predictors of food sharing include kinship, interhousehold distance, and reciprocity. In particular, resources tend to flow from households with relatively more meat to closely related households with little, as predicted by kin selection. This generalization is especially true of household dyads with mother-offspring relationships, which suggests that studies of food sharing may benefit from distinctions between lineal and collateral kin. Overall, this analysis suggests that exchanges among kin are primarily associated with differences in need, not reciprocity. Finally, although large game is distributed widely, qualitative observations indicate that hunters typically do not relinquish control of the distribution in ways predicted by costly signaling theory.     

Early dry season subsistence ecology of Cuiva (Hiwi) foragers of Venezuela

The subsistence ecology of Venezuelan Cuiva foragers during the early dry season is described. Data on diet, time allocation, demography, and physical measurements are presented. Analyses show that the Cuiva depend primarily on game and wild roots during the early dry season for their subsistence. Sex differentials in productive efficiency, total contribution to the diet, and time allocation to food acquisition and other activities are also examined. As in most other foraging societies, men specialize in hunting while women specialize in gathering. During the early dry season, men provide more calories than women and are the more efficient food producers. However, men spend slightly less time than women in food acquisition. Demographic data show that child mortality rates, female infertility rates, female infanticide rates,and the sex ratio among juveniles are high in the Cuiva population. Comparisons between the patterns found among the Cuiva and other foraging populations are made.     

How does the presence of a conspecific individual change the behavioral game that a predator plays with its prey?

Behavioral games predators play among themselves may have profound effects on behavioral games predators play with their prey. We studied the behavioral game between predators and prey within the framework of social foraging among predators. We tested how conspecific interactions among predators (little egret) change the predator–prey behavioral game and foraging success. To do so, we examined foraging behavior of egrets alone and in pairs (male and female) in a specially designed aviary consisting of three equally spaced pools with identical initial prey (comet goldfish) densities. Each pool was comprised of a risky microhabitat, rich with food, and a safe microhabitat with no food, forcing the fish to trade off food and safety. When faced with two versus one egret, we found that fish significantly reduced activity in the risky habitat. Egrets in pairs suffered reduced foraging success (negative intraspecific density dependence) and responded to fish behavior and to their conspecific by changing their visiting regime at the different pools—having shorter, more frequent visits. The time egret spent on each visit allowed them to match their long-term capture success rate across the environment to their capture success rate in the pool, which satisfies one aspect of optimality. Overall, egrets in pairs allocated more time for foraging and changed their foraging tactics to focus more on fish under cover and fish ‘peeping’ out from their shelter. These results suggest that both prey and predator show behavioral flexibility and can adjust to changing conditions as needed in this foraging game.     

Patterns of food sharing among mother and infant chimpanzees at Gombe National Park, Tanzania

The patterns of food sharing among mother and infant chimpanzees at the Gombe National Park, Tanzania, are primarily influenced by the infants' ability to procure and process foods independently. Foods infants could find and process on their own were not shared by mothers as frequently as foods infants could not obtain independently. Temporal changes were observed in the patterns of interactions as infants matured. It is suggested that food sharing may be considered a behavioral strategy which facilitates the infant's transition from dependent suckling to independent foraging.     

Juvenile foraging among the Hadza: Implications for human life history

The longstanding view that children among foraging populations are largely dependent on the food collection efforts of others is an assumed and implicit characteristic of several models of human life history and family formation. The evolution of protracted juvenility in humans is often explained using the “embodied capital model” which argues that prolonged investment in growth and delayed reproduction evolved because a long training period is required to learn difficult foraging tasks and become a self-sufficient forager. The model suggests that if juvenile investment in growth and learning yields an increase in adult productivity, then selection will favor delayed maturity, long life span, and increased brain size. Here, we test the embodied capital model with naturalistic foraging and consumption data among juvenile Hadza hunter–gatherers of Tanzania to determine the extent to which children self-provision. We found that sex had a significant effect on both the type and the amount of food brought back to camp and consumed while out foraging; compared to their male counterparts, young female foragers consumed less while foraging and returned to camp with more food. A wide variation in caloric returns was seen across all foragers in the sample. When analyzing only food brought back to camp, age was not a significant predictor. When combining the amount of food back to camp and the amount consumed while out foraging, however, older children consistently collected more food. The data presented here suggest that although older children do appear to have greater overall foraging success, even very young children are capable of collecting a considerable amount of food. Our data, although lending support to the embodied capital model, suggest that although foraging efficiency increases with age, it remains difficult to determine if this efficiency is a byproduct of learning, strength, or perhaps individual motivation. In addition, our results indicate that juvenile self-provisioning may have played a key role in the evolution of food sharing and cooperation during hominin evolution.     

Learning to Choose Among Social Foraging Strategies in Adult House Sparrows (Passer domesticus)

Social foragers may be regarded as being engaged in a producer–scrounger game in which they can search for food independently or join others who have discovered food. Research on the producer–scrounger game has focused mainly on the different factors influencing its evolutionarily stable strategy (ESS) solution, but very little is known about the actual mechanisms that shape players' decisions. Recent work has shown that early experience can affect producer–scrounger foraging tendencies in young house sparrows and that in nutmeg mannikins learning is involved in reaching the ESS. Here, we show that direct manipulation of the success rate experienced by adult sparrows when following others can change their strategy choice on the following day. We presented to live sparrows an experimental regime, where stuffed adult house sparrows in a feeding position were positioned on a foraging grid that included two reward regimes: a positive one, in which the stuffed models were placed near food, and a negative one, in which the models were placed away from food. There was a significant increase in joining behavior after the positive treatment (exhibited by 84% of the birds), but no change after the negative treatment. Further analysis demonstrated that sparrows more frequently used the strategy with which they were more successful (usually joining) and that differences in strategy use were correlated with differences in success. These results suggest that adult birds can monitor their success and learn to choose among social foraging strategies in the producer–scrounger game.     

Risk and the evolution of human exchange

Compared with other species, exchange among non-kin is a hallmark of human sociality in both the breadth of individuals and total resources involved. One hypothesis is that extensive exchange evolved to buffer the risks associated with hominid dietary specialization on calorie dense, large packages, especially from hunting. 'Lucky' individuals share food with 'unlucky' individuals with the expectation of reciprocity when roles are reversed. Cross-cultural data provide prima facie evidence of pair-wise reciprocity and an almost universal association of high-variance (HV) resources with greater exchange. However, such evidence is not definitive; an alternative hypothesis is that food sharing is really 'tolerated theft', in which individuals possessing more food allow others to steal from them, owing to the threat of violence from hungry individuals. Pair-wise correlations may reflect proximity providing greater opportunities for mutual theft of food. We report a laboratory experiment of foraging and food consumption in a virtual world, designed to test the risk-reduction hypothesis by determining whether people form reciprocal relationships in response to variance of resource acquisition, even when there is no external enforcement of any transfer agreements that might emerge. Individuals can forage in a high-mean, HV patch or a low-mean, low-variance (LV) patch. The key feature of the experimental design is that individuals can transfer resources to others. We find that sharing hardly occurs after LV foraging, but among HV foragers sharing increases dramatically over time. The results provide strong support for the hypothesis that people are pre-disposed to evaluate gains from exchange and respond to unsynchronized variance in resource availability through endogenous reciprocal trading relationships.     

Grandmothering and the evolution of homo erectus

Despite recent, compelling challenge, the evolution of Homo erectus is still commonly attributed to big game hunting and/or scavenging and family provisioning by men. Here we use a version of the "grandmother" hypothesis to develop an alternative scenario, that climate-driven adjustments in female foraging and food sharing practices, possibly involving tubers, favored significant changes in ancestral life history, morphology, and ecology leading to the appearance, spread and persistence of H. erectus. Available paleoclimatic, environmental, fossil and archaeological data are consistent with this proposition; avenues for further critical research are readily identified. This argument has important implications for widely-held ideas about the recent evolution of long human lifespans, the prevalence of male philopatry among ancestral hominids, and the catalytic role of big game hunting and scavenging in early human evolution.     

Interpopulation resource partitioning of Lesser Frigatebirds and the influence of environmental context

Conspecific individuals inhabiting nearby breeding colonies are expected to compete strongly for food resources owing to the constraints imposed by shared morphology, physiology, and behavior on foraging strategy. Consequently, colony‐specific foraging patterns that effectively partition the available resources may be displayed. This study aimed to determine whether intraspecific resource partitioning occurs in two nearby colonies of Lesser Frigatebirds (Fregata ariel). A combination of stable isotope analysis and GPS tracking was used to assess dietary and spatial partitioning of foraging resources during the 2013 and 2014 breeding seasons. These results were compared to vessel‐derived estimates of prey availability, local primary productivity, and estimates of reproductive output to suggest potential drivers and implications of any observed partitioning. Isotopic data indicated a more neritic source of provisioned resources for near‐fledged chicks at an inshore colony, whereas their offshore counterparts were provisioned with resources with a more pelagic signal. Deep pelagic waters (>200 m) had higher availability of a preferred prey type despite a trend for lower primary productivity. Differences in foraging ecology between the two populations may have contributed to markedly different reproductive outputs. These findings suggest environmental context influences dietary and spatial aspects of foraging ecology. Furthermore, the effect of colony‐specific foraging patterns on population demography warrants further research.     

Female Bechstein's Bats Share Foraging Sites with Maternal Kin but do not Forage Together with them – Results from a Long‐Term Study

In many social animals, group members exchange information about where to feed. Thereby, they may gain direct benefits, for example, if social hunting enhances individual foraging success. Alternatively, individuals may receive indirect fitness benefits by preferentially sharing information about suitable feeding sites with kin. Indeed, in some species, a positive correlation between the degree of relatedness among individuals and the overlap among their foraging areas was found. However, sharing foraging sites with kin can also have costs if it increases food competition, which is not compensated by direct benefits. The goal of this study was to investigate whether sharing of individual foraging areas in female Bechstein's bats is best explained by kin selection or by direct benefits through social foraging. To assess their individual foraging behaviour, we analysed radio‐tracking data of 22 members of one maternity colony, including nine mother–daughter pairs, seven pairs of less closely related individuals and six pairs of unrelated bats. We examined the bats' fidelity to specific foraging areas during several years and quantified the influence of kinship on the overlap among individual foraging areas. By measuring how close to each other the bats foraged, we assessed whether individuals with overlapping areas are likely to forage together. Our study confirms previous findings that Bechstein's bats show high fidelity to foraging areas across years. Moreover, we found that relatives share foraging areas significantly more often compared with unrelated colony members. Finally, our data reveal for the first time that most colony members that share foraging areas are unlikely to forage together. This suggests that female Bechstein's bats gain no direct benefits from sharing foraging areas with members of the same maternal lineage. Our findings also have implications for conservation as habitat loss within a colony's home range might expose entire matrilines to high risks.