Life at high altitude presents unique physiological challenges for organisms that were explored in a recent review published in the American Journal of Physiology – Regulatory, Integrative and Comparative Physiology. For placental mammals, offspring born at high altitude (>2500 m above sea level) typically weigh less at birth compared to lowland animals. Fetal growth restriction is problematic as it is associated with long-term health risks and reduced survival. It is thought to result from reduced atmospheric oxygen as well as limits in delivering oxygen to the developing fetus.
Oxygen is undoubtedly essential for metabolism and life. As such, some of the physiological mechanisms that kick in to help maintain oxygen delivery throughout the body at high altitude are also seen in response to other conditions that limit oxygen availability in the body, like anemia. With anemia, the body tries to maintain homeostasis by increasing the production of red blood cells, which contain the oxygen carrier hemoglobin. At altitude, this presents a potential problem as animals may have a normal red blood cell count to begin with. If that happens, such increases in red blood cells can thicken the blood and force the heart to work harder to pump it. This is an example of a maladaptive response to high altitude that may decrease survival of an organism. Other short term physiological adjustments animals make when acclimating to high altitude include increases in breathing rate as well as heart rate to help maximize oxygen uptake and delivery throughout the body. Although such increases can also be maladaptive if they lead to changes in blood chemistry or pH.
Even though it presents physiological challenges, there are certainly populations of humans and animals that have successfully adapted to life at high altitude. These populations have changed in ways that increase their survival and reproduction, factors that increase an animal’s fitness. For example, indigenous Andean as well as Tibetan populations of humans have less severe growth restriction of offspring compared to populations adapted to low altitude that travel to high elevations. Specific adaptations include lower heart rates during pregnancy as well as a larger uterine artery to deliver nutrients and oxygen to the developing fetus. In addition, while Andean women have lower breathing rates compared to lowland populations, Tibetan women maintain higher breathing rates at altitude. Studies have also shown that high altitude Andean fetuses have adaptations that allow for greater oxygen binding in the blood (higher hematocrit and hemoglobin) whereas Tibetan offspring have lower hematocrit compared to lowland Han Chinese populations. The low level of hematocrit found in Tibetans may be due to their inheritance of a gene called EPAS1 which helps prevent increases in hematocrit as well as altitude sickness. Their copy of the gene is in fact quite similar to that found in Denisovans, an extinct population of humans that were related to Neanderthals, suggesting that modern Tibetans have some Denisovan ancestry.
Wilsterman K, Cheviron ZA. Fetal growth, high altitude, and evolutionary adaptation: A new perspective. American Journal of Physiology – Regulatory, Integrative and Comparative Physiology. 321(3): R279-R294, 2021.