The current understanding of the extent of human biological variation is largely limited in scope. Up until now, most of the research has been conducted in largely sedentary, industrialized populations. Recently however, several foundational studies have demonstrated that these populations only represent one notch on the complete spectrum of human variation. As chronic diseases continue to rise worldwide, it is necessary for research to pivot its lens towards understanding the complete extent of human biological variation and the ecological, cultural, and psychological factors that influence its expression. This dissertation expands knowledge on human variation by examining the relationships between two metabolic hormones (leptin and adiponectin) and several health conditions among the Tsimane, a physically active population of forager-horticulturalists in lowland Bolivia that also face significant parasitic and pathogenic stressors. Leptin and adiponectin are key indicators of energy availability and have well-documented associations with certain health conditions in the literature. However, they have been virtually unstudied outside of the typical urban center research contexts. First, I examined the relationship between leptin and adiponectin and their association with both food insecurity and depression. Secondly, I examined the associations between leptin and adiponectin and several indicators of cardiovascular disease. Lastly, I performed laboratory validations to assess the potential limitations of using a relatively new and considerably cheaper option for biomarker analysis (dried blood spot sampling). These studies found associations between the metabolic hormones and food insecurity, depression, and several cardiovascular health indicators. However, several associations deviated from what had been reported in urban settings, demonstrating the value of exploring human variation outside of typical research contexts. Additionally, this study found that dried blood spot sampling is a very stable alternative to the more expensive and more cumbersome methods of blood collection, even despite location or equipment accessibility for non-local researchers. This opens up an avenue for future researchers to conduct studies that appreciate the extent of human variation without being hindered by cost, travel, and infrastructure.

For most of human history hunting has been the primary economic activity of men. Hunted animals are valued for their food energy and nutrients, however, hunting is associated with a high risk of failure. Additionally, large animals cannot be consumed entirely by the nuclear family, so much of the harvest may be shared to others. This has led some researchers to ask why men hunt large and difficult game. The “costly signaling” and “show-off” hypotheses propose that large prey are hunted because the difficulty of finding and killing them is a reliable costly signal of the phenotypic quality of the hunter.

These hypotheses were tested using original interview data from Aché (hunter gatherer; n=52, age range 50-76, 46% female) and Tsimané (horticulturalist; n=40, age range 15-77, 45% female) informants. Ranking tasks and paired comparison tasks were used to determine the association between the costs of killing an animal and its value as a signal of hunter phenotypic quality for attracting mates and allies. Additional tasks compared individual large animals to groups of smaller animals to determine whether assessments of hunters’ phenotypes and preferred status were more impacted by the signal value of the species or by the weight and number of animals killed.

Aché informants perceived hunters who killed larger or harder to kill animals as having greater provisioning ability, strength, fighting ability, and disease susceptibility, and preferred them as mates and allies. Tsimané informants held a similar preference for hunters who killed large game, but not for hunters targeting hard to kill species. When total biomass harvested was controlled, both populations considered harvesting more animals in a given time period to be a better signal of preferred phenotypes than killing a single large and impressive species. Male and female informants both preferred hunters who consistently brought back small game over hunters who sometimes killed large animals and sometimes killed nothing. No evidence was found that hunters should forgo overall food return rates in order to signal phenotypic qualities by specializing on large game. Nutrient provisioning rather than costly phenotypic signaling was the strategy preferred by potential mates and allies.

This study examined whether cortisol changes caused by examination stress are more associated with acute psychological state or physical symptoms of stress. Participants’ salivary cortisol was assayed before and after taking a final examination, and a survey was administered to assess their psychological state for depression, tension, and fatigue, as well as the degree to which they experienced a variety of physical symptoms. Physical symptoms, tension, and depression were found to positively correlate with changes in cortisol across the examination period with depression showing the strongest correlation. No correlation was observed between fatigue and changes in cortisol during the examination period. Additionally, physical symptoms were found to positively correlate with average cortisol across the examination period while depression and fatigue were found to negatively correlate with average cortisol across the examination period. No correlation was observed between tension and average cortisol during the examination period. None of these findings were statistically significant, which suggests that no relationship exists between cortisol and acute psychological state or physical symptoms of stress; however, the study was limited by its small sample size and several potentially confounding variables, making it difficult to draw any firm conclusions.

This study examines associations between clean water, sanitation, mosquito net usage, and immune biomarkers among the Tsimane, a remote subsistence population of forager-horticulturalists with a high pathogen load. Interviews with heads of household (n=710, aged 18-92, median age 40 years) were conducted to ascertain household water sources, ownership and usage of mosquito nets, and latrine use. In this sample, 21% of households used latrines, 20% always boiled their water, and 85% used mosquito nets. Regression models estimate their associations biomarkers of pathogen exposure, including white blood cell count (WBC), hemoglobin (Hb), eosinophils, and sedimentation rate (ESR). Controlling for age, sex, and distance from the closest market town, latrine use (Std. β = -0.11, p= 0.017) and boiling water (Std. β = -0.08, p= 0.059) are associated with lower WBCs. Latrine use is marginally associated with higher hemoglobin (Std. β = 0.09, p= 0.048), but not boiling water (p= 0.447). ESR trends toward lower levels for households that always boil water (Std. β= -0.09, p= 0.131), but is not associated with latrine use (p=0.803). Latrine use was significantly associated with lower eosinophil counts (Std. β= -0.14, p=0.013), but not boiling water (p=0.240). Mosquito nets are not associated with any of these biomarkers. Both boiling water and latrine use are associated with better health outcomes in this sample. These results suggest that scarce public health resources in rural subsistence populations without malarial risk may wish to prioritize boiling water and latrine use to improve health outcomes.

For the past couple decades, there has been a continuous rise in obesity and Type II Diabetes which has been attributed to the rise in calorically dense diets, especially those heavy in fats. Because of its rising prevalence, accompanied health concerns, and high healthcare costs, detection and therapies for these metabolic diseases are in high demand. Insulin resistance is a typical hallmark of Type II Diabetes and the metabolic deficiencies in obesity and is the main focus of this project. The primary purpose of this study is (1) detect the presence of two types of insulin resistance (peripheral and hepatic) as a function of age, (2) distinguish if diet impacted the presence of insulin resistance, and (3) determine both the short-term and long-term effects of caloric restriction on metabolic health. The following study longitudinally observed the changes in insulin resistance in high-fat fed and low-fat fed rodents under ad libitum and caloric restriction conditions over the course of 23 weeks. Fasting blood glucose, fasting insulin, body weight, and sensitivity of insulin on tissue were monitored in order to determine peripheral and hepatic insulin resistance. A high fat diet resulted in higher body weights and higher hepatic insulin resistance with no notable effect on peripheral insulin resistance. Caloric restriction was found to alleviate insulin resistance both during caloric restriction and four weeks after caloric restriction ended. Due to sample size, the generalizability of the findings in this study are limited. However, the current study did provide considerable results and can be viewed as a pilot study for a larger-scale study.