Why don’t birds get Chronic Mountain Sickness?

Jessie Williamson is a PhD student in the Department of Biology at UNM studying evolutionary adaptation of birds to high altitude in the Andes.

This research was made possible in part by funding from the Latin American & Iberian Institute and a Tinker Foundation Field Research Grant (FRG). For more information about the FRG, please visit the LAII website.

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Our first field site, located at 3,700 meters in elevation in central Peru. Arid montane scrub is a common habitat type on the dry west slope of the Andes, and flowering cacti and red chinchilcoma flowers are favorites of the high-altitude bird community.

Over >140 million people worldwide live permanently at high altitude (1), where risk of Chronic Mountain Sickness (CMS) is serious. CMS is a disease in which red blood cell volume increases dangerously and oxygen levels in the blood decrease abnormally, which can be fatal. Rates of CMS in the Andes are among the highest in the world. In Peru, CMS is considered a major public health problem, where it is estimated to affect 14.8–18.2% of the population in certain regions (2, 3). Despite the parallels in high-altitude adaptation between humans and birds, birds do not suffer from CMS. This is intriguing, given that some bird species have lived in the Andes far longer than humans (~6–10 million versus ~11,000 years; 4, 5) In my research, I aim to reveal the genes and physiological responses that birds use to protect themselves against low oxygen pressures. This will enable me to understand how birds differ from humans, and what contributes to their ability to live at high-altitude without suffering from CMS.

Life in Thin Air

The Band-winged Nightjar (Systellura longirostrus) complex is a good example of both an elevational generalist and specialist, and it was common at many of our field sites in the central Peruvian Andes.

At altitudes >2,000 meters in elevation (m), each breath of air contains >20% less oxygen than at sea level. To cope, the body must make immense alterations to its normal state, including increased breathing, blood flow, heart rate, and hemoglobin concentration, to maintain oxygen supply to the cells. This ability to change physical characteristics depending on the environment (6, 7) is partly what allows organisms to survive at high altitudes. Flexibility to express different physical characteristics is thought to benefit individuals by increasing the range of conditions necessary for survival and reproduction. It also expands the range of variation that can be favored by natural selection, opening up the possibility of new evolutionary adaptations (8).

However, emerging evidence suggests that a high degree of flexibility in physical characteristics can actually hinder adaptation. Andeans are an example of high-altitude colonists whose physiological responses compensate for low oxygen-availability but also damage health and longevity (4). During acclimatization to high-altitude, Andeans overproduce red blood cells via erythropoiesis that can lead to CMS (9). Recent studies of high-altitude humans have revealed that natural selection has favored inhibition of the gene pathway that causes erythropoiesis (10), effectively shutting down negative health consequences associated with altitude acclimatization. As in humans, a high-altitude specialist bird has shown reduced flexibility of cardiac and blood characteristics compared to a related altitude generalist (10),  as well as other blood structural characteristics that reduce flexible responses (12). This evidence suggests that some populations living permanently at high altitude have evolved ways to prevent harmful health outcomes of life in the mountains – I hope to identify these in my research.

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Field Research in the Andes

LAII Field Research Grantee, Jessie Williamson, scouts field sites at 3,000 meters above sea level in the arid montane scrub habitat of the central Peruvian Andes.

During the summer of 2018, with funding from the Latin American and Iberian Institute and Tinker Foundation Field Research Grant program, I traveled to Peru to conduct field research for my dissertation. This phase of my research was focused on obtaining data on the physiological responses and blood characteristics of bird populations at different altitudes in the Andes, which will enable me to address how low- and high-elevation birds differ in their whole-body response to altitude. These results, in turn, will enable me to compare how patterns I observe in birds differ from humans with CMS.

I carried out my field research in valleys on the dry west slope of the central Peruvian Andes, where many high-altitude bird species are found in large numbers. With two field assistants from the Centro de Ornitología y Biodiversidad (CORBIDI), I traveled >2,500 kilometers to scout and verify study sites based on safety, adequate habitat observed in situ, and presence avian focal species. Habitat at our field sites ranged from arid montane scrub (low shrubs, thorny bushes, and cacti), to greener semi-humid montane scrub, and humid Polylepis forest. The fact that field sites encompassed a wide range of habitat and latitudes is important for my research, as it will enable me to identify whether responses to altitude are related to environmental variables. The sites where I conducted fieldwork are key locations where elevational generalist birds (species that span wide elevational ranges, from <1,000 meters to >3,000 meters) and specialist birds (species that are restricted to high elevation habitats only, typically >2,500 meters) are known to co-occur.

The Giant Hummingbird (Patagona gigas) was of particular interest on my trip. It is the largest hummingbird species in the world, and one of the most exceptional displays of both an elevational generalist and specialist. The Giant Hummingbird ranges from sea level to ~4,500 meters in the Andes. The high-altitude resident subspecies (P. g. peruviana) breeds and winters at high altitudes in Peru (~2,500-4,500 meters), while the altitudinal migrant subspecies (P. g. gigas) breeds at sea level during the austral summer in central Chile, migrating to higher latitudes and altitudes in Peru during the winter. At present, differences in physiology and blood characteristics between these subspecies are unknown and have long been of interest for their importance to understanding acclimatization and adaptation to high altitude.
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The genetic samples I collected in the field, combined with existing specimen and genomic data from the Museum of Southwestern Biology at UNM, will allow me to compare biochemical and physiological responses between specialist and generalist birds. My findings will elucidate the combination of molecular and physical conditions that are key to survival at high-altitude, shedding light on how wild birds protect themselves against the detrimental health consequences of Chronic Mountain Sickness.

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Note About Permits & Approval: This research was conducted with UNM Institutional Animal Care and Use Committee (IACUC) approval and Peru national research permits.

Works Cited

1. S. Sahota, N. S. Panwar, Prevalence of Chronic Mountain Sickness in high altitude districts of Himachal Pradesh. Indian J. Occup. Environ. Med. 17, 94–100 (2013).
2. A. Arregui et al., Migraine, Polycythemia and Chronic Mountain Sickness. Cephalalgia. 14, 339–341 (1994).
3. C. Monge, F. León-Velarde, A. Arregui, Increasing prevalence of excessive erythrocytosis with age among healthy high-altitude miners. N. Engl. J. Med. 321, 1271 (1989).
4. C. M. Beall, Two routes to functional adaptation: Tibetan and Andean high-altitude natives. Proc. Natl. Acad. Sci. 104, 8655–8660 (2007).
5. C. Hoorn et al., Amazonia through time: Andean uplift, climate change, landscape evolution, and biodiversity. Science. 330, 927–931 (2010).
6. T. Piersma, J. Drent, Phenotypic flexibility and the evolution of organismal design. TRENDS Ecol. Evol. 18, 228–233 (2003).
7. P. J. Yeh, T. D. Price, Adaptive Phenotypic Plasticity and the Successful Colonization of a Novel Environment. Am. Nat. 164 (1997).
8. C. K. Ghalambor, J. K. McKay, S. P. Carroll, D. N. Reznick, Adaptive versus non-adaptive phenotypic plasticity and the potential for contemporary adaptation in new environments. Funct. Ecol. 21, 394–407 (2007).
9. C. M. Beall, Andean, Tibetan, and Ethiopian patterns of adaptation to high-altitude hypoxia. Integr. Comp. Biol. 46, 18–24 (2006).
10. T. S. Simonson et al., Genetic Evidence for High-Altitude Adaptation in Tibet. Science (80-. ). 329, 72–75 (2010).
11. S. G. Dubay, C. C. Witt, Differential high-altitude adaptation and restricted gene flow across a mid-elevation hybrid zone in Andean tit-tyrant flycatchers. Mol. Ecol. 23, 3551–3565 (2014).
12. J. F. Storz, Genes for High Altitudes. Science. 329, 40–41 (2010).