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.     

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.     

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.     

High adult mortality among Hiwi hunter-gatherers: implications for human evolution

Extant apes experience early sexual maturity and short life spans relative to modern humans. Both of these traits and others are linked by life-history theory to mortality rates experienced at different ages by our hominin ancestors. However, currently there is a great deal of debate concerning hominin mortality profiles at different periods of evolutionary history. Observed rates and causes of mortality in modern hunter-gatherers may provide information about Upper Paleolithic mortality that can be compared to indirect evidence from the fossil record, yet little is published about causes and rates of mortality in foraging societies around the world. To our knowledge, interview-based life tables for recent hunter-gatherers are published for only four societies (Ache, Agta, Hadza, and Ju/'hoansi). Here, we present mortality data for a fifth group, the Hiwi hunter-gatherers of Venezuela. The results show comparatively high death rates among the Hiwi and highlight differences in mortality rates among hunter-gatherer societies. The high levels of conspecific violence and adult mortality in the Hiwi may better represent Paleolithic human demographics than do the lower, disease-based death rates reported in the most frequently cited forager studies.     

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.     

Female subsistence strategies among Ache hunter-gatherers of Eastern Paraguay

Anthropologists have frequently proposed that sexual division of labor is produced by childcare constraints on women's subsistence work. We present data on the forest activities of Ache women that show that differences in parental investment partially account for variation in food acquisition among individual women. Data also suggest that childcare constraints are important in understanding the sexual division of labor.     

Energetic demand of multiple dependents and the evolution of slow human growth

This study investigates the consequences of the human foraging niche and multiple dependent offspring on the optimal growth trajectory of humans. We test the hypothesis that the human pattern of slow human growth between age at weaning and puberty helps defer the compound energetic demand on parents with multiple dependents, by using growth and demographic data from two foraging societies, the Ache of eastern Paraguay and the Dobe Ju/'hoansi of Botswana and Namibia. We run simulations of observed and potential growth trajectories among sub-adults and their consequent energetic demands on parents given profiles of fertility, mortality, consumption and production. We find that either sub-adult production or food subsidies from other people must substantially increase in order to compensate for the dramatic increase in energetic demand on parents if offspring were to grow faster at younger ages. Our conclusion is that slow human growth followed by a rapid adolescent growth spurt may have facilitated rising human fertility rates and greater investments in neural capital.     

Pulsed resource availability changes dietary niche breadth and partitioning between generalist rodent consumers

Identifying the mechanisms that structure niche breadth and overlap between species is important for determining how species interact and assessing their functional role in an ecosystem. Without manipulative experiments, assessing the role of foraging ecology and interspecific competition in structuring diet is challenging. Systems with regular pulses of resources act as a natural experiment to investigate the factors that influence the dietary niches of consumers. We used natural pulses of mast‐fruiting of American beech (Fagus grandifolia) to test whether optimal foraging or competition structure the dietary niche breadth and overlap between two congener rodent species (Peromyscus leucopus and P. maniculatus), both of which are generalist consumers. We reconstructed diets seasonally over a 2‐year period using stable isotope analysis (δ¹³C, δ¹⁵N) of hair and of potential dietary items and measured niche dynamics using standard ellipse area calculated within a Bayesian framework. Changes in niche breadth were generally consistent with predictions of optimal foraging theory, with both species consuming more beechnuts (a high‐quality food resource) and having a narrower niche breadth during masting seasons compared to nonmasting seasons when dietary niches expanded and more fungi (a low‐quality food source) were consumed. In contrast, changes in dietary niche overlap were consistent with competition theory, with higher diet overlap during masting seasons than during nonmasting seasons. Overall, dietary niche dynamics were closely tied to beech masting, underscoring that food availability influences competition. Diet plasticity and niche partitioning between the two Peromyscus species may reflect differences in foraging strategies, thereby reducing competition when food availability is low. Such dietary shifts may have important implications for changes in ecosystem function, including the dispersal of fungal spores.     

Seasonal patterns of diet and ranging in two species of tamarin monkeys: Stability versus variability

From June through December, data were collected on the diet and ranging patterns of moustached (Saguinus mystax) and saddle-back (Saguinus fuscicollis) tamarin monkeys in the Amazon Basin of northeastern Peru. During this 7-month period, insects and nonleguminous fruits accounted for 83% of tamarin feeding and foraging time. Despite marked seasonal variation in rainfall and forest productivity, patterns of habitat utilization, day range, dietary diversity, resource exploitation, and activity budget remained relatively stable throughout the year. Moustached and saddle-back tamarins appear to solve problems of food acquisition and exploit patchily distributed feeding sites using a relatively limited set of foraging patterns. In general, these primates concentrate their daily feeding efforts on several trees from a small number of target plant species. These feeding sites are uncommon, produce only a small amount of ripe fruit each day, and are characterized by a high degree of intraspecific fruiting and flowering synchrony. Trees of the same species are frequently visited in succession, and individual feeding sites are revisited several times over the course of 1–2 weeks. This type of foraging pattern occurred during both dry and wet seasons and when exploiting fruit, nectar, legume, and exudate resources. Seasonal variation in the percentage of feeding and foraging time devoted to insectivory was also limited. In this investigation, there was no consistent evidence that temporal changes in overall forest fruit production had a major impact on the feeding, foraging, or ranging behavior of either tamarin species.     

Influence of habitat and season on foraging flock composition in the European Starling (Sturnus vulgaris)

Summary This study examines the relative importance of habitat and season in determining size and composition of foraging flocks of European Starlings (Sturnus vulgaris). We hypothesized that if season was the principal determinant of flock size, variation across seasons (within habitat) would be greater than variation across habitats (within season). The calendar year was divided into 6 seasons corresponding to major stages in the annual cycle of starlings. After measuring the availibility of 15 foraging habitats occurring within the study area, we drove a 184 km route weekly (over a 17 month period) to census starling flock size, species composition, habitat use, and activity.Although flock size was influenced by both season and habitat, it varied relatively less within seasons (across habitats). Only five of nine seasons showed significant habitat effects, and two (of the five) were significant mainly because of large aggregations in habitats with unusually abundant food (e.g. feedlots, cornfields). In contrast, eight of nine habitats showed significant seasonal effects. The only exception (hay) was little used (<1% of all birds observed). For starlings the activities associated with seasonal events of the annual cycle were a major influence on patterns of social foraging. Habitat factors were less important because of constraints imposed by seasonal factors, but under some conditions (e.g. drought, clumped food supply) they became dominant.     

Foraging Success,Agonism, and Predator Alarms: Behavioral Predictors of Cortisol in <Emphasis Type="Italic">Lemur catta</Emphasis>

Primate social grouping is understood as an adaptive strategy for mitigating environmental selection pressures, but the relative importance of various pressures may vary. Physiological measures of well-being can show their short-term impacts and suggest their relative importance and capacity to provide ultimate or proximate control of group size. I examined correlations between pressures commonly proposed as causes of social grouping (foraging success, intergroup and intragroup agonism, and predation risk) and individual levels of fecal cortisol, a hormonal stress measure, in a free-ranging population of Lemur catta. I collected behavioral data on 45 female Lemur catta at Berenty Reserve, Madagascar, over 3 seasons (August 1999-July 2000) and determined individual cortisol levels from 474 fecal samples. Neither predator alarm rates nor intragroup agonism rates correlated with cortisol levels in any season. However, females with low daily food intake and high rates of escalated intergroup defense exhibited higher cortisol levels. The data suggest that acquisition and defense of food resources are principal challenges in Lemur catta, and may be important factors determining social grouping and other behavioral or life history adaptations.     

Male foraging avoidance in female feeding territories in a harem polygynous cichlid in Lake Tanganyika

We studied foraging site partitioning between the sexes in Neolamprologus tetracanthus, a shrimp-eating Tanganyikan cichlid with harem-polygyny. Females maintained small territories against heterospecific food competitors within large territories of males, foraging exclusively at the inner side of their own territories (foraging areas). Males fed as frequently as females in their own territories, but mostly outside female foraging areas, although they frequently entered female territories and repelled food competitors from the territories. Soon after removal of the resident females, however, harem males, as well as many food competitors, invaded the vacant territories and intensively devoured prey of female foraging areas. This indicates that although female foraging areas appear to contain more food than outside the areas, harem males refrained from foraging there when the resident females were present. We suggest that harem males will attempt to keep female foraging areas in good condition, whereby they may get females to reside in male territories and/or promote female gonadal maturation.     

Incubation pattern and foraging effort in the female Water Pipit Anthus spinoletta

Incubation and foraging patterns of female Water Pipits Anthus spinoletta were studied in two breeding seasons in an Alpine valley of Switzerland. Decreased temperature reduced the length of periods spent off the nest (inattentive period), while decreased food availability led to a reduction in time spent incubating (attentiveness), shorter periods spent on the nest (attentive period), more frequent inattentive periods and higher foraging effort, measured as the product of frequency of inattentive periods and twice the flight distance to foraging sites. The negative relation between food availability and foraging effort resulted from more frequent foraging bouts and longer flight distances under poor food conditions. Feeding of the incubating female by the male did not affect foraging effort and attentiveness but did change the temporal pattern of inattentive periods from a few long to several short inattentive periods.     

Feeding ecology and foraging behaviour of impala Aepyceros melampus in Lake Mburo National Park, Uganda

Intersexual and seasonal variation in foraging behaviour of impala (Aepyceros melampus), was studied in the Lake Mburo National Park, Uganda. There was a moderate seasonal difference in foraging efficiency (as measured by ‘acceptable food abundance’), with a minimum in dry season and a maximum in Rainy season. The variation between sexes was more distinct with a pronounced minimum in time spent browsing of males in early wet season. By distinguishing between feeding time spent grazing and feeding time spent browsing the seasonal variation was confirmed. The proportion of foraging time spent feeding (expressed as ‘food ingestion rate’) showed an inverse pattern with a maximum in the late dry season (75.5%), decreasing values throughout the Rainy season and a minimum in early dry season (57.8%). Differences between sexes were explained in terms of reproductive demands and seasonal balance in terms of moderate climate throughout the year. Impala foraging patterns in the bimodal tropics (two Rainy seasons) is discussed and compared with unimodal tropics. The findings are matched against current ideas on optimal foraging.     

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.     

Seasonal variance in the diet of Ache hunter-gatherers in Eastern Paraguay

Seasonal variance in the diet of Ache hunter-gatherers is examined. Fluctuation in the number of calories of honey consumed daily contributed most to the differences in total calories consumed daily during different seasons of the year. Meat, the most important resource in the diet, provided the greatest number of calories daily, and varied little across seasons. The vegetable component of the diet is characterized by low variance in absolute numbers of calories, but high variance in species composition. The mean number of calories consumed daily per capita is high (3827 calories) compared to that reported for other hunter-gathers. Differences in energy expenditure and consumption among modern hunter-gatherers is discussed.     

Effects of habitat use on food acquisition and food caching during a nonbreeding season in a winter-breeding,food-storing passerine

ABSTRACT

Capsule: Wintering male Bull-headed Shrikes Lanius buchepalus preferred vegetable fields with perch sites to search for and detect terrestrial prey, and males occupying territories with large areas of vegetable fields acquired more prey and cached more food.

Aims: To better understand effects of habitat use on food acquisition and food caching of the Bull-headed Shrike, we investigated relationships between habitat quality (measured through foraging-site selection and foraging success) and food caching during the non-breeding season.

Methods: We monitored 66 territorial male shrikes during the non-breeding season from 2014 to 2016, and collected data on foraging-site selection, foraging success, and food caching.

Results: Our field observations showed that male shrikes preferred to forage over vegetable fields and that males occupying territories incorporating large areas of that habitat were able to acquire more food items and store more food caches in their territories during the nonbreeding season.

Conclusion: We suggest that for male Bull-headed Shrikes, a winter-breeding food-storing passerine, the quality of habitat in the nonbreeding season has the potential to affect their subsequent fitness.     

Men's time allocation to subsistence work among the Ache of Eastern Paraguay

Quantitative data on men's time allocation among the Ache of Paraguay are presented. The data indicate that Ache men work almost 7 hours daily in direct food acquisition, which is the major daily activity. The amount of time Ache men work is compared with the amount reported for other modern hunter-gatherers and tribal horticulturalists. The characterization of hunter-gatherers as the original affluent society does not agree with currently available data. The results show high variance across societies, both hunting and horticultural, and suggest that time spent in subsistence work is not simply a function of food needs. We propose that the value of time spent in potential alternative activities must be considered in order to predict time spent in subsistence tasks.     

Age-dependency in hunting ability among the Ache of eastern Paraguay

This paper examines changes in hunting ability across the lifespan for the Ache of eastern Paraguay. Hunting ability is decomposed into two components-finding prey and probability of kill upon encounter- and analyzed for important prey species. Results support the argument that skill acquisition is an important aspect of the human foraging niche with hunting outcome variables reaching peaks surprisingly late in life, significantly after peaks in strength. The implications of this study are important for modeling the role of the human foraging niche in the co-evolution of various outstanding human life history characteristics such as large brains, long lifespans, and extended juvenile periods.