Tibetan women’s adaptations to high altitude lead to greater reproductive success

Researchers are discovering how specific genetic and cardiovascular traits give Tibetan women an advantage in surviving and thriving at high altitudes, linking better oxygen delivery to more successful pregnancies.

Study: Higher oxygen content and transport characterize high-altitude Tibetan women with the highest lifetime reproductive success. Image credit: sanyanwuji /Shutterstock

In a recent study published in the journal PNASResearchers explored how characteristics related to oxygen intake in women of Tibetan origin aged 46 to 86 years living at high altitude in Nepal are linked to their number of live births. Their results indicate that women with greater oxygen supply to their tissues had the greatest reproductive success over their lifetime, particularly those who gave birth early and had a long marriage.

Background

Studying how natural selection affects adaptive traits requires combining data on genetics, physiology, sociocultural factors, and reproductive history. For this reason, human communities exposed to stress over long periods of time, such as those experiencing hypoxic conditions at high altitudes, are ideal for researchers to study natural selection. Tibetans have been exposed to high altitude conditions for around 10,000 years and have distinctive biological characteristics related to oxygen supply, which help them adapt to hypoxia. These include low hemoglobin concentrations, high ventilation, high blood flow, and low pressure in the pulmonary arteries, which distinguishes them from other groups that also live in high altitude regions, such as the Andes.

While pregnancies in high-altitude regions are associated with increased risks of low birth weight and preeclampsia, thereby increasing infant and maternal mortality, Tibetan women often have heavier infants and have higher levels oxygen saturation, lower hemoglobin concentration and improved uterine blood compared to migrants. to high altitude regions. These traits are associated with a greater number of live births, suggesting that natural selection is acting on oxygen supply. Several genes show signs of selection in Tibetans, which are crucial for the body’s response to hypoxia.

About the study

In this study, researchers examined how the cardiovascular, hematological, and respiratory characteristics of 417 high-altitude Tibetan women, aged 46 to 86, were associated with their reproductive success, as measured by the number of live births they had . They also assessed the genetic mechanisms underlying these characteristics. The study population consisted of women of Tibetan origin, Nepalese citizens, living in villages located between 3,500 and 4,100 meters above sea level and included only those who had been married or pregnant and had lived their entire lives in the district of Upper Mustang. About 94% of participants were not in school and more than 40% had used contraception to avoid pregnancy.

Researchers collected measurements using heart rate tests, echocardiography and anthropometric tools. During data analysis, models were reduced to 50 potential predictors, including altitude, contraceptive use, and age. Specific cardiovascular measurements, such as left ventricular outflow tract diameter (LVOTDiamIndex), have been identified as critical factors in predicting reproductive success, highlighting the importance of efficient oxygen transport .

Results

The 417 women reported 2,193 pregnancies, resulting in 2,076 live births, 46 stillbirths and 71 miscarriages. Women had an average of 5.3 pregnancies and 5.2 live births, ranging from 0 to 14 live births. Pregnancies and live births showed a high positive correlation of 0.97.

Women over 63 had the highest number of live births, likely because they had children before family planning technology was accessible. The main predictors of more live births included longer marital durations, earlier ages at first birth, low heart rate recovery rate, intermediate hemoglobin concentration around 13.4 g/dL and high oxygen saturation. This intermediate hemoglobin level was critical because lower and higher concentrations were linked to a decrease in the number of live births, either due to insufficient oxygen supply or increased blood viscosity which could impair the traffic. Larger diameters in the left ventricular outflow tract, particularly in those with twins, were also significant predictors of reproductive success. These results highlight the role of improved oxygen transport in the number of live births.

Parametric analysis showed that heart size, oxygen transport characteristics and age are significant predictors, while tree-based analysis also identified variables such as age at first pregnancy and duration of marriage. However, lung and respiratory characteristics do not predict reproductive success, potentially due to the higher age range of study participants or differences in measurement protocols compared to younger Tibetan samples. Genetic analysis also linked the number of live births to heart rate recovery, hemoglobin concentration and oxygen saturation, highlighting the role of specific genes like EPAS1, which is crucial for maintaining health. oxygen homeostasis at high altitude.

Conclusions

The researchers hypothesized that traits related to oxygen supply would predict reproductive success and show genetic associations. Their results support this hypothesis, with cardiovascular and hematological characteristics predicting more live births. However, lung and respiratory characteristics did not have good predictive power, which might be due to measurement methods or age differences. For example, although younger Tibetan populations showed distinctive lung traits, these did not translate into higher reproductive success in this older cohort, suggesting that other cardiovascular factors may take precedence with age.

Intermediate hemoglobin concentration was found to be an important predictor. Low hemoglobin levels reduce live births due to a lack of oxygen, while high levels can cause health problems by increasing blood viscosity. The optimal hemoglobin concentration of approximately 13.4 g/dL matches the mode observed in the sample, suggesting a balance that favors oxygen delivery without increasing blood viscosity, a crucial factor in maintaining blood cardiovascular health in cases of hypoxic stress. Although the researchers used multiple analyzes to increase confidence in their main findings, they were unable to infer causality, and the relatively small sample size limits the ability to detect subtle genetic effects. The study’s limited genetic analysis suggests the need for larger-scale studies to validate these findings and uncover more nuanced genetic relationships with reproductive success.

Overall, efficient oxygen transport systems were essential for reproductive success and survival in high altitude regions, with this research providing insight into human adaptation to hypoxia and selection natural. This study highlights the complex interplay between genetics, physiology, and environmental adaptation, demonstrating that maintaining efficient oxygen delivery is a multifaceted challenge influenced by a range of cardiovascular characteristics.