Tsimane Forager-horticulturalists Research Articles

Leadership solves collective action problems in small-scale societies.

Observation of leadership in small-scale societies offers unique insights into the evolution of human collective action and the origins of sociopolitical complexity. Using behavioural data from the Tsimane forager-horticulturalists of Bolivia and Nyangatom nomadic pastoralists of Ethiopia, we evaluate the traits of leaders and the contexts in which leadership becomes more institutional. We find that leaders tend to have more capital, in the form of age-related knowledge, body size or social connections. These attributes can reduce the costs leaders incur and increase the efficacy of leadership. Leadership becomes more institutional in domains of collective action, such as resolution of intragroup conflict, where collective action failure threatens group integrity. Together these data support the hypothesis that leadership is an important means by which collective action problems are overcome in small-scale societies.

No Sex or Age Difference in Dead-Reckoning Ability among Tsimane Forager-Horticulturalists.

Sex differences in reproductive strategy and the sexual division of labor resulted in selection for and maintenance of sexual dimorphism across a wide range of characteristics, including body size, hormonal physiology, behavior, and perhaps spatial abilities. In laboratory tasks among undergraduates there is a general male advantage for navigational and mental-rotation tasks, whereas studies find female advantage for remembering item locations in complex arrays and the locations of plant foods. Adaptive explanations of sex differences in these spatial abilities have focused on patterns of differential mate search and routine participation in distinct subsistence behaviors. The few studies to date of spatial ability in nonindustrial populations practicing subsistence lifestyles, or across a wider age range, find inconsistent results. Here we examine sex- and age-based variation in one kind of spatial ability related to navigation, dead-reckoning, among Tsimane forager horticulturalists living in lowland Bolivia. Seventy-three participants (38 male) aged 6-82years pointed a handheld global positioning system (GPS) unit toward the two nearest communities and the more distant market town. We find no evidence of sex differences in dead reckoning (p = 0.47), nor do we find any evidence of age-related decline in dead-reckoning accuracy (p = 0.28). Participants were significantly more accurate at pointing toward the market town than toward the two nearest villages despite its being significantly farther away than the two nearest communities. Although Tsimane do show sexual dimorphism in foraging tasks, Tsimane women have extensive daily and lifetime travel, and the local environment lacks directional cues that typically enhance male navigation. This study raises the possibility that greater similarity in mobility patterns because of overlapping subsistence strategies and activities may result in convergence of some male and female navigation abilities.

Calcaneal Quantitative Ultrasound Indicates Reduced Bone Status Among Physically Active Adult Forager-Horticulturalists

Sedentary lifestyle contributes to osteoporosis and fragility fracture risks among modern humans, but whether such risks are prevalent in physically active preindustrial societies with lower life expectancies is unclear. Osteoporosis should be readily observable in preindustrial societies if it was regularly experienced over human history. In this study of 142 older adult Tsimane forager-horticulturalists (mean age ± SD, 62.1 ± 8.6 years; range, 50 to 85 years; 51% female) we used calcaneal quantitative ultrasonography (qUS) to assess bone status, document prevalence of adults with reduced bone status, and identify factors (demographic, anthropometric, immunological, kinesthetic) associated with reduced bone status. Men (23%) are as likely as women (25%) to have reduced bone status, although age-related decline in qUS parameters is attenuated for men. Adiposity and fat-free mass positively co-vary with qUS parameters for women but not men. Leukocyte count is inversely associated with qUS parameters controlling for potential confounders; leukocyte count is positively correlated within adults over time, and adults with persistently low counts have higher adjusted qUS parameters (6% to 8%) than adults with a high count. Reduced bone status characteristic of osteoporosis is common among active Tsimane with minimal exposure to osteoporosis risk factors found in industrialized societies, but with energetic constraints and high pathogen burden.

Skill ontogeny among Tsimane forager-horticulturalists.

We investigate whether age profiles of Tsimane forager-horticulturalists' reported skill development are consistent with predictions derived from life history theory about the timing of productivity and reproduction. Previous studies of forager skill development have often focused on a few abilities (e.g. hunting), and neglected the broad range of skills and services typical of forager economies (e.g. childcare, craft production, music performance, story-telling). By systematically examining age patterns in reported acquisition, proficiency, and expertise across a broad range of activities including food production, childcare, and other services, we provide the most complete skill development study of a traditional subsistence society to date. Our results show that: (1) most essential skills are acquired prior to first reproduction, then developed further so that their productive returns meet the increasing demands of dependent offspring during adulthood; (2) as postreproductive adults age beyond earlier years of peak performance, they report developing additional conceptual and procedural proficiency, and despite greater physical frailty than younger adults, are consensually regarded as the most expert (especially in music and storytelling), consistent with their roles as providers and educators. We find that adults have accurate understandings of their skillsets and skill levels -an important awareness for social exchange, comparison, learning, and pedagogy. These findings extend our understanding of the evolved human life history by illustrating how changes in embodied capital and the needs of dependent offspring predict the development of complementary skills and services in a forager-horticulturalist economy.

Challenging the Inevitability of Prostate Enlargement: Low Levels of Benign Prostatic Hyperplasia Among Tsimane Forager-Horticulturalists.

Often considered an inevitable part of male aging, benign prostatic hyperplasia (BPH) is the most common non-life threatening disease to affect men in Western populations. We examine age-related change in prostate size and BPH risk and related serum biomarkers among the Tsimane Amerindians of the Bolivian Amazon who live a traditional lifestyle of hunting and small-scale horticulture. The Tsimane are a critical case study for understanding the etiology of BPH as they have low levels of obesity and metabolic syndrome, as well as lower levels of testosterone than age matched U.S. males, factors associated with BPH in previous research. Ultrasounds were conducted on 348 men aged 28-89 years (median age 56 years). Testosterone, prostate specific antigen, sex hormone binding globulin, and glycosylated hemoglobin were examined in relationship to prostate size and BPH. Tsimane have less than half of the BPH prevalence experienced by U.S. men, and prostate volumes 62.6% smaller. While Tsimane have low levels of testosterone and subclinical levels of metabolic syndrome compared to U.S. men, Tsimane with high testosterone were more likely to experience BPH, as were those with higher glycosylated hemoglobin, suggesting targets for clinical interventions to reduce BPH. These results have clinical significance for the growing number of men taking testosterone supplementation; even at low levels the additional testosterone exposure could be placing these men at higher risk of BPH. Overall, these data suggest that BPH may not have been an inevitable part of male aging throughout human evolutionary history.

Sex difference in travel is concentrated in adolescence and tracks reproductive interests.

Sexual selection theory suggests that the sex with a higher potential reproductive rate will compete more strongly for access to mates. Stronger intra-sexual competition for mates may explain why males travel more extensively than females in many terrestrial vertebrates. A male-bias in lifetime distance travelled is a purported human universal, although this claim is based primarily on anecdotes. Following sexual maturity, motivation to travel outside the natal territory may vary over the life course for both sexes. Here, we test whether travel behaviour among Tsimane forager-horticulturalists is associated with shifting reproductive priorities across the lifespan. Using structured interviews, we find that sex differences in travel peak during adolescence when men and women are most intensively searching for mates. Among married adults, we find that greater offspring dependency load is associated with reduced travel among women, but not men. Married men are more likely to travel alone than women, but only to the nearest market town and not to other Tsimane villages. We conclude that men's and women's travel behaviour reflects differential gains from mate search and parenting across the life course.

Testosterone and male cognitive performance in Tsimane forager-horticulturalists.

Testosterone plays a vital role in brain function and behavior. Among humans, age-related decline in testosterone is associated with declining cognitive functioning, and aging men with higher testosterone maintain better cognitive performance. However, most research focuses on industrialized populations with widespread access to formal schooling, high testosterone, and low parasite and pathogen load. We examine whether men's testosterone is associated with cognitive performance among Tsimane forager-horticulturalists of Bolivia despite relatively lower levels of testosterone and higher immune burden. Ninety-four Tsimane men aged 36-86 (median = 49) participated in a cognitive battery (assessing short- and long-term recall, digit span, semantic memory, and visual scan) and provided urine and blood samples to measure testosterone and markers of immune activation. Linear mixed effects regressions were used to model associations between cognitive performance and testosterone, controlling for age, years of schooling, Spanish fluency, and village residence. For a subset (n = 66) we included immune activation markers to examine mediator effects. Testosterone is positively associated with short- and long-term verbal memory (β = 0.267, P = 0.018; β = 0.326, P = 0.005 respectively) and visual scanning (β = 0.306, P = 0.008) after controlling for potential confounders. Markers of immune activation were negatively associated with cognitive function, but did not change the associations between testosterone and cognitive performance. Tsimane men show positive associations between testosterone and cognitive performance, particularly for recall and visual scanning, despite higher immune burden. Testosterone may help motivate both physical and cognitive capacities that were essential for extracting the difficult-to-acquire, high-quality resources upon which humans relied over evolutionary history.

The evolutionary fitness of personality traits in a small-scale subsistence society

The maintenance of personality variation remains an unexplained puzzle in evolutionary biology. Despite evidence among non-humans that personality variation affects fitness, few data exist to assess the personality–fitness relationship in humans. Among Tsimane forager–horticulturalists (n=632), we test whether personality traits (assessed using a 43-item Big Five Inventory administered orally in native language) predict fertility, offspring survivorship, age of first reproduction, and other fitness correlates (extramarital affairs, conflicts, social visitation, food production, and several health measures). Among men, several personality factors associate with higher fertility, more time spent producing food and social visitation. Among women, the relationship between personality and fitness varies across regions of Tsimane territory. The only case of an intermediate personality level associated with highest fitness was found for Industriousness in men. We find that personality factors positively associated with fitness do not associate with greater health costs, although greater Extraversion and Openness may lead to more conflicts among men. Factor heritability ranges from 60% for Prosociality and Extraversion to 8% for Neuroticism. We interpret our results in light of evolutionary models that explain maintenance of personality variation, including incomplete directional selection, mutation–selection balance, condition-dependent reaction norms and fluctuating selection based on sex or spatial variability in selection pressures.

Age-independent increases in male salivary testosterone during horticultural activity among Tsimane forager-farmers

Testosterone plays an important role in mediating male reproductive trade-offs in many vertebrate species, augmenting muscle and influencing behavior necessary for male–male competition and mating-effort. Among humans, testosterone may also play a key role in facilitating male provisioning of offspring as muscular and neuromuscular performance is deeply influenced by acute changes in testosterone. This study examines acute changes in salivary testosterone among 63 Tsimane men ranging in age from 16 to 80 (mean 38.2) years during one-hour bouts of tree-chopping while clearing horticultural plots. The Tsimane forager-horticulturalists living in the Bolivian Amazon experience high energy expenditure associated with food production, have high levels of parasites and pathogens, and display significantly lower baseline salivary testosterone than age-matched US males. Mixed-effects models controlling for BMI and time of specimen collection reveal increased salivary testosterone (p<0.001) equivalent to a 48.6% rise, after one hour of tree chopping. Age had no effect on baseline (p=0.656) or change in testosterone (p=0.530); self-reported illness did not modify testosterone change (p=0.488). A comparison of these results to the relative change in testosterone during a competitive soccer tournament in the same population reveals larger relative changes in testosterone following resource production (tree chopping), compared to competition (soccer). These findings highlight the importance of moving beyond a unidimensional focus on changes in testosterone and male–male aggression to investigate the importance of testosterone–behavior interactions across additional male fitness-related activities. Acutely increased testosterone during muscularly intensive horticultural food production may facilitate male productivity and provisioning.

From the womb to the tomb: The role of transfers in shaping the evolved human life history

Humans are the longest living and slowest growing of all primates. Although most primates are social, humans are highly cooperative and social in ways that likely co-evolved with the slow human life history. In this paper we highlight the role of resource transfers and non-material assistance within and across generations in shaping low human mortality rates. The use of complex cooperative strategies to minimize risk is a necessary precursor for selecting further reductions in mortality rate in late adulthood. In conjunction with changes in the age-profile of production, the impacts of resource transfers and other forms of cooperation on reducing mortality likely played an important role in selection on post-reproductive lifespan throughout human evolution. Using medical data and ethnographic interviews, we explore several types of common risks experienced by Tsimane forager–horticulturalists, and quantify the types and targets of aid. Our results illustrate the importance of transfers in several key domains and suggest that the absence of transfers would greatly increase human mortality rates throughout the life course.

A bioeconomic approach to marriage and the sexual division of labor.

Children may be viewed as public goods whereby both parents receive equal genetic benefits yet one parent often invests more heavily than the other. We introduce a microeconomic framework for understanding household investment decisions to address questions concerning conflicts of interest over types and amount of work effort among married men and women. Although gains and costs of marriage may not be spread equally among marriage partners, marriage is still a favorable, efficient outcome under a wide range of conditions. This bioeconomic framework subsumes both cooperative and conflictive views on the sexual division of labor. We test hypotheses concerning marriage markets, assortative mating, and men’s labor motivations among Tsimane forager-horticulturalists of Bolivia and find that: (1) men and women both value work effort in marital partners, (2) marital labor contributions are complementary, (3) work effort is correlated between spouses, (4) total production is correlated with total reproduction, and (5) better hunters have higher fitness gains within marital unions.

Collective Action in Action: Prosocial Behavior in and out of the Laboratory

ABSTRACT Experiments have become a popular method to study altruism and cooperation in laboratory and, more recently, in field settings. However, few studies have examined whether behavior in experiments tells us anything about behavior in the “real world.” To investigate the external validity of several common experimental economics games, we compare game behavior with prosocial behavior among Tsimane forager‐horticulturalists of lowland Bolivia. We find that food‐sharing patterns, social visitation, beer production and consumption, labor participation, and contributions to a feast are not robustly correlated with levels of giving in the economics games. Payoff structure and socioecological context may be more important in predicting prosocial behavior in a wide variety of domains than stable personality traits. We argue that future experimental methods should be tailored to specific research questions, show reduced anonymity, and incorporate repeat measures under a variety of conditions to inform and redirect ethnographic study and build scientific theory. [Keywords: altruism, cooperation, experimental economics, Tsimane]