Preliminary Results
Ongoing research has been conducted as part of the Tsimane Health and Life History Project since July 2002. To date, the research has had three broad aims. The first is to conduct an integrated study of physical growth, development, aging, health and mortality in one population, the Tsimane', a foragerhorticultural society with little market involvement and access to modern health care. The second aim is to advance theory in the biodemography of the human life course, with a specific focus on aging and lifespan. The third is to lay the groundwork for collaborative, comparative research in human aging across a diverse array of ecological and social settings. The long-term goal of the research has been to explain the age profile of human mortality and the rate at which humans develop and senesce in terms of economic productivity, muscular strength, endurance, body composition, disease resistance, and cognitive function.
The Tsimane population is a high fertility (Total Fertility Rate=9) and high mortality population, where 55% are younger than age 15, 29% between 15 and 35, 12% between 35 and 55, and 5% are 65 and older. The sex ratio is biased towards males for most age classes, except for that of teenagers, which is biased towards females. An analysis based on reproductive history interviews of over 800 adults yielding a database of over 50,000 person-years reveals a mean age at death of 43 years. Roughly 22% of Tsimane die before the age of 15, and 17% die before the age of 5. Tsimane mortality data were added to a dataset we compiled consisting of mortality profiles of ten other subsistence living populations. From this we have demonstrated the robustness of the age-profile of mortality levels in hunter-gatherers and subsistence forager-farmers despite the differences in diet, recent history and disease ecology. These results have appeared in Population and Development Review.
More specific to the Tsimane population, we also found the peculiar result that mortality levels among infants and children have not changed substantially in the last fifty years, but mortality among older individuals has declined considerably. We believe that this is largely due to the fact that greater access to medical care is having a more substantial effect on those who more frequently have medical conditions (e.g. the elderly). These results appear in a paper in American Journal of Human Biology.
Since 2002, six rounds of medical checkups on over 3,000 Tsimane have uncovered a wealth of information concerning health status and disease epidemiology. Human specimen and medical data show that the Tsimane suffer from very high rates of infectious disease and parasitosis. Over three fourths of all Tsimane are infected with at least one species of intestinal parasite. Over half are anemic, and over four-fifths present some complaint or symptom of morbidity during medical examinations. Gastrointestinal illness and respiratory infections are the most frequent diagnosis, with some 30-40% of infants and young children suffering from each. Their prevalence drops significantly among 5-19 year olds, and rises again with age, so that prevalence among people over age 50 is also high (~30% of individual exams). The frequency of all other illness is also high among 0-4 year olds and dips precipitously among 5-19 year olds and then climbs much more steeply with age. Interestingly, however, there were no suspected cases of diabetes (if they existed, it is likely that individuals suffering from Type I diabetes would have died before receiving treatment). The probability of having no diagnosable complaint was low at just over 35% for 0-4 year olds, being greatest among 5-19 year olds (almost 65%) and then dropping to 30-40% among adults. In addition to the high levels of infectious morbidity, health conditions among Tsimane differ substantially from those found in modern, developed nations. For example, less than 2% of Tsimane are obese and less than 4% are hypertensive. The low rates of hypertension among the Tsimane' are consistent with their low body mass indices and high work effort. Low hypertension, coupled with low serum cholesterol levels, and high levels of physical exertion are probably responsible for the low levels of arteriosclerosis and cardiovascular disease observed in traditional foraging and horticultural populations (80). However, we suspect that chronic illness may be underdiagnosed in small-scale foraging populations and are currently investigating cardiac and renal function. Serum creatinine levels indicate that Tsimane renal function declines at a rate comparable to that found in U.S. samples, although different techniques of estimating renal function provide slightly different results. The Tsimane show high levels of C-reactive protein (CRP), an acute-phase inflammatory response, presumably reacting to the high rates of infection. The pattern by age is u-shaped, with a high incidence of elevated CRP in childhood, decreasing in adolescence, and then increasing monotonically in adulthood and old age. Compared with U.S. samples from the NHANES, Tsimane CRP levels are much higher until age 60, when the two populations converge. These data are consistent with the assertion by Finch and Crimmons that chronic exposure to inflammatory processes can accelerate the aging process. This analysis has resulted in a paper that has been accepted to Journal of Gerontological Sciences.
Finally, analyses of individual activity budgets show a slow, steady increase of time investment in more difficult production tasks with age, with juveniles spending a substantial portion of their time in play and leisure activities. Grandparents spend substantial time in both food production and direct care of children's offspring. The amount of calories that juveniles and adolescents produce is relatively small and does not increase dramatically until the early to mid-twenties, peaking in the late thirties, when the cumulative dependency of offspring is quite large. These patterns are similar to those reported for the Ache, Machiguenga, and Piro (92). A principal reason for the delay in peak production is due to the skills-intensive nature of the foraging and agricultural niche. Tests based on observational, interview, and experimental data collected among Tsimane suggest that physical capital, such as body size or strength, alone cannot explain the long delay until peak hunting productivity. These results are published in Journal of Human Evolution.
