About

Richard G. Bribiescas is the J. Clayton Stephenson/Class of 1954 Professor of Anthropology at Yale University, where he also holds a secondary appointment in the Department of Ecology and Evolutionary Biology. He is the Director of Undergraduate Studies in Anthropology and a Primary Investigator in the Yale Reproductive Ecology Laboratory. His research primarily involves the evolutionary biology and endocrinology of human and comparative life histories, reproduction, aging, and metabolism. Bribiescas has conducted field research among the Ache people of Paraguay, as well as populations in Venezuela, Japan, Ecuador, and the United States, along with various species of non-human primates. He earned his B.A. in Anthropology and Psychology from UCLA and his A.M. and Ph.D. in Anthropology from Harvard University. Prior to Yale, he was a Post-Doctoral Research Fellow at Massachusetts General Hospital. He is the author of two books: 'Men: Evolutionary and Life History' (2006) and 'How Men Age: What Evolution Reveals about Male Health and Mortality' (2016). His work has been recognized with awards including the Bronze Medal from the Independent Book Publishers Association and the W.W. Howells Prize from the American Anthropological Association. Bribiescas is a fellow of the Human Biology Association, the American Association for the Advancement of Science, and the American Academy of Arts and Sciences. He serves on the editorial board of the American Journal of Human Biology and is Associate Editor for Evolution, Medicine, and Public Health.

Research topics

• Biology
• Ecology
• Sociology
• Genetics
• Evolutionary biology
• Psychology
• Medicine
• Environmental ethics
• Geography
• Anthropology
• Zoology
• Gender studies
• Endocrinology
• Social psychology

Selected publications

• Market Integration and Reproductive Transitions Among Indigenous Shuar and Neighboring Nonindigenous Ecuadorians in Amazonian Ecuador

  American Journal of Human Biology · 2026-03-01 · 1 citations

  article

  OBJECTIVE: Global reproductive transitions are well documented, yet less is known about how neighboring populations with divergent histories and exposure to market integration (MI) experience them. This study examines how sociocultural and economic proxies of MI predict variation in reproductive outcomes among indigenous Shuar and nonindigenous Ecuadorian mestizos (Colonos) from Amazonian Ecuador, focusing on menarcheal age, age at first birth, parity, breastfeeding duration, and contraceptive use. METHODS: Reproductive histories were collected from 360 Shuar and 205 Colono women (ages 15-90) between 2008 and 2014. General linear and Poisson regression models were used to assess effects of ethnicity, birth cohort, and MI indices (household-, market-, traditional-style-of-life) on reproductive outcomes; logistic regression determined predictors of contraceptive use. RESULTS: Shuar women experience earlier age at first birth (p ≤ 0.05) and higher parity (p ≤ 0.001) with little secular change. However, within more market-integrated Shuar households, earlier menarche (p = 0.04), and lower parity (p ≤ 0.001) were documented. By contrast, Colonos show secular increases in age at first birth and declines in parity (both p ≤ 0.05). In both populations, breastfeeding duration decreases over time (p = 0.006) and higher contraceptive use is associated with more market-integrated households (p = 0.03). CONCLUSIONS: Reproductive transitions in Amazonian Ecuador unfold at differing rates, reflecting different histories and degrees of MI. Colonos show clear secular shifts characteristic of reproductive transitions, whereas Shuar show emerging transitions only among more market-integrated households. Results support the context-specific nature of reproductive transitions and highlight value in disaggregated analyses to understand their dynamics.

  PublisherDOI

• Industrialization increases the estrogen-recycling capacity of the gut microbiome

  Proceedings of the National Academy of Sciences · 2026-04-13

  articleOpen access

  Estrogens influence many aspects of human physiology and health, including fertility, growth, metabolic function, and susceptibility to disease. Up to 65% of circulating estrogens are excreted into the gut via bile, but only 10-15% are eliminated in feces, indicating substantial estrogen reabsorption [H. Adlercreutz and P. Järvenpää, J. Steroid Biochem. 17 , 639–645 (1982); A. A. Sandberg and W. R. Slaunwhite, J. Clin. Investig. 36 , 1266–1278 (1957)]. This estrogen recycling is enabled by the gut estrobolome, a subset of microbes that deconjugate conjugated estrogens in the gastrointestinal tract, facilitating their reabsorption into systemic circulation [C. S. Plottel and M. J. Blaser, Cell Host Microbe 10 , 324–335 (2011)]. To date, it is not known if populations differ in this microbial function, and little is known about its determinants. Here we analyze estrobolomes using publicly available gut microbiome data from 24 populations spanning four continents and subsistence modes ranging from hunting and gathering to pastoralism, rural farming, and industrialized agriculture. We show that industrialized populations exhibit up to seven times greater estrogen-recycling capacity and nearly twofold higher estrobolome diversity compared with nonindustrial groups. We further find that formula-fed infants display two- to threefold higher recycling capacity and as much as eleven times greater estrobolome diversity than breastfed counterparts, revealing early-life divergence in microbial estrogen metabolism. By contrast, sex, age, and BMI are not associated with estrobolome characteristics. These findings demonstrate the crucial impact of industrialized lifestyles, including formula feeding, on the microbial capacity to influence systemic estrogen levels, with implications for life history, reproductive biology, and estrogen-associated diseases, including cancer.

  PublisherOA PDFDOI

• Navigating Practical, Political, and Ethical Challenges of Long‐Term Human Biology Field Research: The Shuar Health and Life History Project

  American Journal of Human Biology · 2026-04-01

  articleOpen access

  Major advances in understanding human biology and health have been made possible by long-term field research projects with Indigenous peoples, whose ecological settings and lifeways are distinct from majority populations in high-income settings. Long-term fieldwork is central to human biology, yet much about this research approach remains opaque. This paper considers a single long-term study-the Shuar Health and Life History Project in Amazonian Ecuador-and discusses how we have navigated practical, political, and ethical challenges of initiating and maintaining a two-decade long collaborative project with integrative field and laboratory components. By enhancing transparency and offering logistical and ethical insights, our goal is to provide guidance to researchers. We offer three sets of conclusions and recommendations. First, our highly integrative approach allows us to ask important scientific questions and answer them in innovative ways. We recommend investment in broad theoretical and methodological training and study design, as well as a project structure that nurtures students' ideas. Second, a project like ours is extremely challenging to operate, requiring major sustained commitment of time and funding to maintain the fieldsite and laboratories. We recommend that scientists initiating major field projects be cognizant of these enormous costs and challenges, develop supportive relationships among collaborating researchers, and incorporate flexibility and excess capacity. Finally, sustainable long-term investment in research communities must be prioritized, and the science and outreach efforts require tailoring to local conditions and realities. We recommend that researchers invest deeply in communities, building trust and establishing long-term relationships that are mutually beneficial.

  PublisherDOI

• Reproductive and metabolic hormone associations in adult Samoan males with and without obesity

  Evolution Medicine and Public Health · 2026-01-01 · 1 citations

  articleOpen access1st authorCorresponding

  Abstract Background Obesity is a global health challenge prevalent in Samoa. However, the influence of obesity on adult male reproductive health in Samoa is poorly understood. Objectives To determine if reproductive hormone levels differed between adult Samoan males with and without obesity. Methodology Reproductive hormones (follicle stimulating hormone [FSH], luteinizing hormone [LH], inhibin b) and sex hormone binding globulin (SHBG) were compared between non-diabetic adult males in Samoa with and without obesity to test the hypothesis that obesity is associated with compromised reproductive function in this population. Metabolic hormones (insulin, leptin, adiponectin), fasting glucose, age, and anthropometrics were assessed and included in multivariable models. Results Males with obesity exhibited higher FSH (P = 0.002), lower inhibin b (P = 0.004) and lower SHBG (P < 0.0001). LH levels were similar (P = 0.43). Significant associations were evident between LH and FSH (obesity: r2 = 0.19, P = 0.003; without obesity: r2 = 0.24, P = 0.001), inhibin b and FSH (obesity: r2 = 0.21, P = 0.002; without obesity: r2 = 0.02, P = 0.41), and LH and SHBG (obesity: r2 = 0.25, P = 0.0005; without obesity: r2 0.01, P = 0.49). Multivariable models revealed insulin as an important contributor to inhibin b levels in all males. Conclusions Obesity status is an important factor in variation in male reproductive hormone profiles in adult Samoan males. Implications Obesity has potentially negative effects on male reproductive hormone function in Samoa. While the effects on male fertility remain unclear, further research is merited.

  PublisherDOI

• Salivary Testosterone, Age, and Adiposity Associations Among Shuar Males in Amazonian Ecuador Challenge Assumptions of “Normal” Testosterone Patterns

  American Journal of Human Biology · 2025-11-01 · 2 citations

  articleOpen access

  OBJECTIVES: Adult male testosterone concentrations in high income countries often decrease with age and adiposity, a pattern typically viewed as "normal." However, testosterone is expected to be adaptively regulated within the range of resource constrained, high pathogen, natural fertility conditions across which it evolved to function. We therefore examine associations among testosterone diurnal variation, age, and adiposity among Indigenous Shuar males of Amazonian Ecuador. METHODS: Morning and evening saliva was sampled over three consecutive days to capture diurnal testosterone variation (n = 104, ages 12-67), with one-time measures of adiposity (body fat, BMI). Multilevel models tested predicted associations. Average morning and evening testosterone ratio was calculated to assess diurnal variation, and regression analyses tested the association between this ratio and age. RESULTS: Variation in testosterone concentrations at waking was apparent by age, with young males exhibiting the highest concentrations. Diurnal testosterone variation decreased with age (β = -0.006, p = 0.001). Significant age-by-BMI or percent body fat interactions were documented (p < 0.05). At lower adiposity levels, mean testosterone concentrations across the day were lowest at younger ages, highest in middle-aged participants, and slightly lower at older ages. At higher adiposity levels this pattern was reversed (for BMI) or attenuated (for percent body fat). CONCLUSIONS: "Normal" testosterone levels are largely based on studies from high-income populations that do not account for diverse ecological conditions known to influence human physiology. This study complements others highlighting the complex relationships that exist among age, adiposity, and diurnal testosterone patterns in subsistence populations, suggesting socio-ecological regulation of testosterone.

  PublisherOA PDFDOI

• Impact of high temperatures on enzyme-linked immunoassay (ELISA) performance for leptin measurements in human milk stored under varied freeze/thaw conditions

  PLoS ONE · 2025-03-19 · 1 citations

  articleOpen accessSenior authorCorresponding

  Ambient temperature conditions are a common concern during laboratory analysis. Due to unexpected shipping conditions, leptin ELISA kits (Leptin Ultrasensitive, ALPCO USA; Catalog #22-LEPHUU-E01) arrived from the manufacturer at our laboratory at a temperature (76.3°F/24.6°C) well above the 2-8°C conditions recommended by the manufacturer. Since no data are available on the effects of high ambient temperature exposure on the performance of this commercial assay, we opportunistically assessed assay performance using human milk samples. Leptin measurement of recently collected and frozen human milk samples was compared between the warm temperature exposed assay kits and Normal kits that arrived and were stored at recommended temperatures (2-8 °C). We found that assay kit exposure to warm temperature during shipping resulted in sample results that were significantly different from Normal kits despite similar standard curve performance. Measurement variability from human milk samples increased with warmed kits in association with greater freeze/thaw times. This suggests that even under high temperature transportation conditions, this leptin assay performance is robust with kit reagents but compromised with human milk samples. We conclude that kits exposed to high temperature during shipment and/or storage should not be used to run human milk samples and that our concerns may extend to other biological media (i.e., serum, urine, or saliva). This study fills a critical gap in the literature on assay performance validation under non-ideal conditions, such as high temperatures. As global temperatures continue to rise, this question will become more pertinent to research integrity if left unaddressed. In light of our findings, we propose that industry standards for ELISA kit shipping and handling should be evaluated to ensure that all kits are being received in an optimal condition.

  PublisherOA PDFDOI

• Soil‐Transmitted Helminths and the Intricacies of Immunoregulation: Evidence From Amazonian Ecuador for the Importance of Considering Species‐Specific Effects Within the Old Friends Hypothesis

  American Journal of Human Biology · 2025-05-30 · 5 citations

  articleOpen access

  OBJECTIVES: The old friends hypothesis (OFH) examines connections between the global increase in immunoregulatory diseases (e.g., allergy and autoimmunity) and reduced exposure to immune-priming symbionts like soil-transmitted helminths. Helminth species, however, vary in their effects on hosts and should be considered separately. We examined relationships between species-specific helminth infection and circulating biomarkers of adaptive immune antibodies (total immunoglobulin E [IgE]), systemic inflammation (C-reactive protein [CRP]), and immune regulation (interleukin-6 [IL-6]), among Indigenous Shuar adults. We predicted that STH infection would be (1) associated with higher levels of IgE and (2) lower levels of CRP, with (3) IL-6 driving these associations based on species-specific relationships. METHODS: One hundred and seventeen Shuar adults provided stool and finger-prick blood samples. BCa bootstrap ANCOVA and partial correlation tests examined relationships among infection status (uninfected, Ascaris infected, Trichuris infected, coinfected), control variables (region, sex, age, body mass), and immune biomarkers. RESULTS: On average, coinfected participants had the highest IgE compared to all other groups. Ascaris-infected individuals had, on average, the lowest CRP levels compared to any other group; this was only significant compared to uninfected participants (p < 0.05). Notably, IL-6 was positively correlated with IgE in Ascaris-infected individuals (p < 0.05) and with CRP in Trichuris-infected individuals (p < 0.05), highlighting its role in differentiating between immunoregulation and inflammation based on species-specific infections. CONCLUSIONS: Immune biomarkers varied by infection status: Ascaris infection may downregulate and Trichuris infection may exacerbate systemic inflammation. These preliminary findings suggest that STH species must be considered separately within the OFH.

  PublisherOA PDFDOI

• Associations between maternal and infant body composition, feeding practices, and perceived infant eating behaviour in Samoan mother–infant dyads aged 1–4 months

  Annals of Human Biology · 2025-08-05 · 4 citations

  articleOpen access

  BACKGROUND: Breastfeeding has been shown to reduce risks for the development of obesity. While initiation of breastfeeding is almost universal in Samoa, there is an opportunity to improve the proportion of infants exclusively breastfed to 6 months (51.7%). AIM: To investigate how Samoan mothers perceive infant appetite in breastfed infants, which has implications for early cessation of exclusive breastfeeding. SUBJECTS AND METHODS: We administered the Baby Eating Behaviour Questionnaire (BEBQ) to 100 mothers of exclusively breastfed infants aged 1-4 months alongside anthropometric and body composition measurements, information on breastfeeding practices, and maternal/household demographic characteristics. Associations of maternal and infant characteristics with the five BEBQ constructs - Food Responsiveness (FR), Enjoyment of Food (EF), Satiety Responsiveness (SR), Slowness in Eating (SE), and General Appetite (GA) - were examined using Pearson correlations and multiple linear regression. RESULTS: We found that BEBQ constructs were correlated with infant age, infant sex, maternal BMI, maternal educational status, infant BMI-for-age, feeding routine, average feeding bout length, and maternal confidence in milk. CONCLUSION: Maternal perception of infant appetite appears to be influenced partially by maternal and infant body composition and breastfeeding patterns in our sample, highlighting potential points of intervention to promote recommended breastfeeding practices.

  PublisherDOI

• Testosterone, 8‐Oxo‐2′‐Deoxyguanosine (8‐OHdG) and Cu/Zn Superoxide Dismutase (SOD) in Adult Shuar Males of Amazonian Ecuador: A Test for Evidence of Trade‐Offs Between Reproductive Effort and Oxidative Stress

  American Journal of Human Biology · 2025-04-01 · 6 citations

  articleOpen access1st authorCorresponding

  ABSTRACT Objectives Reproductive effort incurs the cost of biological aging and morbidity by compromising somatic maintenance when key resources are limited. Oxidative stress is positively correlated with reproductive effort in adult human females and non‐human male animal models, but human males are understudied. We hypothesized that due to its anabolic and metabolic promotion of reproductive effort in human males, testosterone would be positively associated with biomarkers of oxidative stress. Methods Urinary testosterone in adult Shuar males of Amazonia Ecuador, a foraging/horticultural population, was measured with urinary 8‐hydroxy‐2′ ‐deoxyguanosine (8‐OHdG), a biomarker of oxidative stress, and Cu/Zn superoxide dismutase (Cu/Zn SOD), a protective antioxidant against oxidative stress. Age and anthropometric measures were included in multivariate models. Results No significant correlation was observed between testosterone and 8‐OHdG, r 2 = 0.01, p = 0.61, n = 29, or Cu/Zn SOD, r 2 = 0.0005, p = 0.93, n = 17. Multiple linear regression models including testosterone, Cu/Zn SOD, anthropometrics, and age, with 8‐OHdG as the dependent variable, were modestly supportive of an association. The most parsimonious 8‐OHdG model included age, Cu/Zn SOD, and testosterone ( R 2 adjusted = 0.38, p = 0.04, AICc = 141.95). All multivariate models for Cu/Zn SOD were not significant ( p &gt; 0.05). Conclusions Oxidative stress may not be a cost of reproductive effort in this population of adult males; perhaps due to consistently low testosterone levels in non‐industrialized populations, differences in the metabolic cost of reproductive effort between males and females (i.e., aerobic metabolism), and/or study limitations based on cross‐sectional measures of oxidative stress and testosterone.

  PublisherOA PDFDOI

• Variation in diurnal cortisol patterns among the Indigenous Shuar of Amazonian Ecuador

  American Journal of Human Biology · 2024-03-22 · 10 citations

  article

  OBJECTIVES: The hypothalamic-pituitary-adrenal (HPA) axis and its primary end product, the glucocorticoid cortisol, are major components of the evolved human stress response. However, most studies have examined these systems among populations in high-income settings, which differ from the high pathogen and limited resource contexts in which the HPA axis functioned for most of human evolution. METHODS: We investigated variability in diurnal salivary cortisol patterns among 298 Indigenous Shuar from Amazonian Ecuador (147 males, 151 females; age 2-86 years), focusing on the effects of age, biological sex, and body mass index (BMI) in shaping differences in diurnal cortisol production. Saliva samples were collected three times daily (waking, 30 minutes post-waking, evening) for three consecutive days to measure key cortisol parameters: levels at waking, the cortisol awakening response, the diurnal slope, and total daily output. RESULTS: Age was positively associated with waking levels and total daily output, with Shuar juveniles and adolescents displaying significantly lower levels than adults (p < .05). Sex was not a significant predictor of cortisol levels (p > .05), as Shuar males and females displayed similar patterns of diurnal cortisol production across the life course. Moreover, age, sex, and BMI significantly interacted to moderate the rate of diurnal cortisol decline (p = .027). Overall, Shuar demonstrated relatively lower cortisol concentrations than high-income populations. CONCLUSIONS: This study expands the documented range of global variation in HPA axis activity and diurnal cortisol production and provides important insights into the plasticity of human stress physiology across diverse developmental and socioecological settings.

  PublisherDOI

Frequent coauthors

• Felicia C. Madimenos

  Queens College, CUNY

  13 shared

• Samuel S. Urlacher

  Canadian Institute for Advanced Research

  12 shared

• Lawrence S. Sugiyama

  Northern Arizona University

  12 shared

• Anna Ziomkiewicz

  Jagiellonian University

  11 shared

• J. Josh Snodgrass

  University of Oregon

  10 shared

• Dorsa Amir

  University of California, Berkeley

  8 shared

• Tara J. Cepon‐Robins

  University of Colorado Colorado Springs

  8 shared

• Melissa A. Liebert

  Northern Arizona University

  8 shared

Labs

• Yale Reproductive Ecology LaboratoryPI

Awards & honors

• Bronze Medal in the category of science by the Independent B…
• W.W. Howells Prize for best book in biological anthropology…
• Medal of 600 Years Anniversary of the Restoration of the Kra…
• Elected fellow of the Human Biology Association
• Elected fellow of the American Association for the Advanceme…