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.     

Showing off: Tests of an hypothesis about men's foraging goals

It is widely assumed that among hunter-gatherers, men work to provision their families. However, men may have more to gain by giving food to a wide range of companions who treat them favorably in return. If so, and if some resources better serve this end, men's foraging behavior should vary accordingly. Aspects of this hypothesis are tested on observations of food acquisition and sharing among Ache foragers of Eastern Paraguay. Previous analysis showed that different Ache food types were differently shared. Resources shared most widely were game animals. Further analysis and additional data presented here suggest a causal association between the wide sharing of game and the fact that men hunt and women do not. Data show that men preferentially target resources in both hunting and gathering which are more widely shared, resources more likely to be consumed outside their own nuclear families. These results have implications for 1) the identification of male reproductive trade-offs in human societies, 2) the view that families are units of common interest integrated by the sexual division of labor, 3) current reconstructions of the evolution of foraging and food sharing among early hominids, and 4) assessments of the role of risk and reciprocity in hunter-gatherer foraging strategies.     

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.     

Are kubo hunters ‘show offs’?

The ‘show off’ hypothesis proposed by K. Hawkes, and tested using data on Ache foragers, makes important connections between food resource choice, reproductive strategies, and food sharing by human foragers. We test predictions derived from that hypothesis concerning contexts of meat acquisition, association between individuals, mobility, and reproductive success among Kubo hunter-horticulturalists of the interior lowlands of Papua New Guinea. Application of the hypothesis to both the Kubo and Ache cases is questioned. Differences between Kubo males in means and variances of returns from hunting arise as a consequence of differential target specialization; they do not map onto variation in reproductive success.     

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.     

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.     

A fine-scale time budget of Cape gannets provides insights into the foraging strategies of coastal seabirds

Central-place foragers organize their feeding trips both to feed themselves and to provide their offspring with food. In seabirds, several long-range foragers have been shown to alternate long and short trips to balance these dual needs. However, the strategies of short-range foragers remain poorly understood. We used a precise, miniaturized motion sensor to examine the time budget of 20 breeding Cape gannets, Morus capensis, foraging off the coast of South Africa. Birds stayed at sea for 5.5-25.3 h, occasionally spending the night at sea. The large number of isolated dives and extended flight time observed during these overnight trips suggested that birds either experienced poor foraging conditions or exploited more distant, yet more profitable prey patches. Conversely, birds that stayed at sea for less than 1 day had relatively consistent activity patterns. Most of these birds (88%) foraged actively at the beginning and at the end of the foraging trip. These feeding bouts were separated by protracted periods of sitting on the sea surface. Such resting periods probably allow birds to digest the food ingested during the first part of the foraging trip, so they initially feed themselves, and then obtain food for their chick on the way back to the breeding site.     

Forager abundance and its relationship with colony activity level in three species of South American Pogonomyrmex harvester ants

The proportion of foragers in ant colonies is a fairly constant species-specific characteristic that could be determined by intrinsic or extrinsic factors. If intrinsic factors are relevant, species with similar life history characteristics (e.g., colony size and foraging strategies) would be expected to have a similar proportion of foragers in their colonies. Within the genus Pogonomyrmex, North American species can vary largely in their colony size, whereas only species with small colonies are known in South America. We studied the characteristics of the foraging subcaste in three sympatric South American species of Pogonomyrmex harvester ants, and compared it with the available information on other species of the same genus. We used two mark-recapture methods and colony excavations to estimate the number and proportion of foragers in the colonies of P. mendozanus, P. inermis, and P. rastratus, and to test the relationship between forager external activity levels and abundance per colony. Forager abundance in the three studied species was lower than in most North American species. The percentage of foragers in their colonies ranged 7–15 %, more similar to North American species with large colonies than to those with small colony size. Foraging activity was positively correlated with forager abundance in all three species, implying that colony allocation to number of foragers allows for higher food acquisition. Further comparative studies involving a wider range of traits in South and North American species would allow to unveil the role of environmental factors in shaping each species’ particular traits.     

Learning rules for social foragers: implications for the producer-scrounger game and ideal free distribution theory

In population games, the optimal behaviour of a forager depends partly on courses of action selected by other individuals in the population. How individuals learn to allocate effort in foraging games involving frequency-dependent payoffs has been little examined. The performance of three different learning rules was investigated in several types of habitats in each of two population games. Learning rules allow individuals to weigh information about the past and the present and to choose among alternative patterns of behaviour. In the producer-scrounger game, foragers use producer to locate food patches and scrounger to exploit the food discoveries of others. In the ideal free distribution game, foragers that experience feeding interference from companions distribute themselves among heterogeneous food patches. In simulations of each population game, the use of different learning rules induced large variation in foraging behaviour, thus providing a tool to assess the relevance of each learning rule in experimental systems. Rare mutants using alternative learning rules often successfully invaded populations of foragers using other rules indicating that some learning rules are not stable when pitted against each other. Learning rules often closely approximated optimal behaviour in each population game suggesting that stimulus-response learning of contingencies created by foraging companions could be sufficient to perform at near-optimal level in two population games.     

Cooperative sentinel calling? Foragers gain increased biomass intake

Many foraging animals face a fundamental tradeoff between predation and starvation. In a range of social species, this tradeoff has probably driven the evolution of sentinel behavior, where individuals adopt prominent positions to watch for predators while groupmates forage. Although there has been much debate about whether acting as a sentinel is a selfish or cooperative behavior, far less attention has focused on why sentinels often produce quiet vocalizations (hereafter known as "sentinel calls") to announce their presence. We use observational and experimental data to provide the first evidence that group members gain an increase in foraging success by responding to these vocal cues given by sentinels. Foraging pied babblers (Turdoides bicolor) spread out more, use more exposed patches, look up less often, and spend less time vigilant in response to sentinel calling. Crucially, we demonstrate that these behavioral alterations lead to an increase in biomass intake by foragers, which is likely to enhance survival. We argue that this benefit may be the reason for sentinel calling, making it a truly cooperative behavior.     

“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.     

A friend in need is a friend indeed: Need-based sharing,rather than cooperative assortment,predicts experimental resource transfers among Agta hunter-gatherers

Despite much theorizing, the evolutionary reasons why humans cooperate extensively with unrelated individuals are still largely unknown. While reciprocity explains many instances of non-kin cooperation, much remains to be understood. A recent suite of models based upon ‘cooperative assortativity’ suggest that non-kin cooperation can evolve if individuals preferentially assort with certain cooperative phenotypes, such as helping those who help others. Here, we test these assortative hypotheses among the Agta, a population of Filipino hunter-gatherers, using an experimental resource allocation game in which individuals divide resources between themselves and camp-mates. Individuals preferentially shared with less cooperative individuals, arguing against cooperative assortativity as a mechanism sustaining resource transfers in this population. Rather, sharing was often based on the recipient's level of need, in addition to kin-based transfers and reciprocal sharing. Contrary to several recent theoretical accounts, in this real-world setting we find no evidence for cooperative assortativity influencing patterns of cooperation. These results may reflect the demands of living in a foraging ecology characterized by high resource stochasticity, necessitating need-based sharing as a system of long-term reciprocity to mitigate repeated subsistence shortfalls.     

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.     

Altruistic self‐removal of health‐compromised honey bee workers from their hive

Social insect colonies represent distinct units of selection. Most individuals evolve by kin selection and forgo individual reproduction. Instead, they display altruistic food sharing, nest maintenance and self‐sacrificial colony defence. Recently, altruistic self‐removal of diseased worker ants from their colony was described as another important kin‐selected behaviour. Here, we report corroborating experimental evidence from honey bee foragers and theoretical analyses. We challenged honey bee foragers with prolonged CO₂ narcosis or by feeding with the cytostatic drug hydroxyurea. Both treatments resulted in increased mortality but also caused the surviving foragers to abandon their social function and remove themselves from their colony, resulting in altruistic suicide. A simple model suggests that altruistic self‐removal by sick social insect workers to prevent disease transmission is expected under most biologically plausible conditions. The combined theoretical and empirical support for altruistic self‐removal suggests that it may be another important kin‐selected behaviour and a potentially widespread mechanism of social immunity.     

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.     

Macronutrient modifications of optimal foraging theory: An approach using indifference curves applied to some modern foragers

The use of energy (calories) as the currency to be maximized per unit time in Optimal Foraging Models is considered in light of data on several foraging groups. Observations on the Ache, Cuiva, and Yora foragers suggest men do notattempt to maximize energetic return rates, but instead often concentrate on acquiring meat resources which provide lower energetic returns. The possibility that this preference is due to the macronutrient composition of hunted and gathered foods is explored. Indifference curves are introduced as a means of modeling the tradeoff between two desirable commodities, meat (protein-lipid) and carbohydrate, and a specific indifference curve is derived using observed choices in five foraging situatiuons. This curve is used to predict the amount o meat that Mbuti foragers will trade for carbohydrate, in an attempt to test the utility of the approach.     

Influence of environmental and colony factors on the initial commodity choice of foragers of the stingless bee Plebeia tobagoensis (Hymenoptera, Meliponini)

Novice foragers of social bees have to decide what food commodity to collect when they start foraging for the first time. In this decision making process two types of factors are involved: internal factors (the response threshold) and external factors (environmental and colony conditions). In this study we will focus on the importance of two external factors, pollen storage level and information from experienced foragers about food availability in the field, on the initial commodity choice of foragers of the stingless bee species Plebeia tobagoensis. We also studied the effect of the initial choice of individuals on their subsequent foraging career. This study was performed in a closed greenhouse compartment, where food availability and colony condition could be controlled. Information on food availability in the field from experienced foragers and pollen storage level both greatly influenced the initial commodity choice of individuals, with more choices for the commodity communicated by experienced foragers or lacking in storage. The initial choice of foragers is of importance for their future foraging career, although a substantial proportion of foragers did switch between food commodities. Because of the ability of novice foragers to become flexibly distributed over foraging tasks, social bees are able to react to changes in their environment without directly having to decrease foraging effort devoted to other foraging tasks. This, in combination with individual flexibility during foraging careers makes it possible for colonies of P. tobagoensis to forage efficiently in an ever-changing environment. Received 7 November 2005; revised 12 January 2006; accepted 16 February 2006.     

华南地区桑园和荒地两种生境中红火蚁觅食工蚁的食物组成

【目的】红火蚁Solenopsis invicta Buren是世界最危险的有害入侵生物之一,2004年入侵我国华南地区,并给农林业安全、生态安全带来极大危害。调查并明确田间红火蚁觅食工蚁的食物种类及数量,不仅为评估红火蚁猎食对生态环境中节肢动物群落的影响,而且为红火蚁防治饵剂的改进提供科学依据。【方法】本研究采用蚁道剖析法,对华南地区桑园和荒地两种生境中红火蚁觅食工蚁搬运的固体残片取样和鉴定;依搬运工蚁及其固体残片尺寸进行测量分级并称重,以分析工蚁多态性与固体食物大小的关系;采用薄层色谱法和氨基酸分析法测定嗉囊液体样品的营养成分及含量。【结果】在华南地区桑园和荒地中,红火蚁觅食工蚁搬运回巢的固体食物包括固体的动物残片和植物种子,其中动物残片属于3门7纲21个类群(包括14个昆虫目),昆虫纲动物所占比例为总固体食物的45.53%~46.10%。工蚁偏好搬运长1.400±0.043 mm~2.306±0.063 mm和宽0.723±0.028 mm~1.261±0.051 mm的固体残片,其重量在0.203~0.413 mg之间。红火蚁觅食工蚁嗉囊液体由多种氨基酸、果糖和葡萄糖组成,在桑园和荒地采集到的红火蚁嗉囊液体样本中的氨基酸总含量分别为1 544.31 mg/L和861.48 mg/L,氨基酸种类分别为33种和32种,其中31种氨基酸为共有的。【结论】华南地区桑园和荒地中,红火蚁固体食物组成均以昆虫纲动物为主;参与固体残片搬运觅食蚁的80%属于中型工蚁,搬运较大型固体残片的大型工蚁仅占5%;红火蚁工蚁嗉囊液体含有丰富的氨基酸和单糖。     

Behavioral role differentiation in the primitively eusocial waspBelonogaster juncea juncea (Hymenoptera: Vespidae)

With a view toward describing behavioral variability among individuals of the primitively eusocial speciesBelonogaster juncea juncea, we recorded the time-activity budget spent on five behavioral categories (foraging, building, feeding, inactivity, and reproduction) by 52 individuals belonging to four postemergence colonies. A principal-components analysis coupled with a hierarchical cluster analysis enabled us to discern four behavioral roles. The reproductive role is reserved to one individual (functional monogyny) and the workers’ role is differentiated into foragers, builders, and guards. The foragers are females that spend the most time (82.6% of their time) foraging, supplying the nest with prey load and liquid matter. The builders are individuals (with 41.5% of their time off the nest) that tend to bring pulp into the nest and then undertake building activities. The guards are those females that spend the most time (79.7% of their time) being inactive on the nest.