High-altitude champions: birds that live and migrate at altitude

Abstract

High altitude is physiologically challenging for vertebrate life for many reasons, including hypoxia (low environmental oxygen); yet, many birds thrive at altitude. Compared with mammals, birds have additional enhancements to their oxygen transport cascade, the conceptual series of steps responsible for acquiring oxygen from the environment and transporting it to the mitochondria. These adaptations have allowed them to inhabit a number of high-altitude regions. Waterfowl are a taxon prolific at altitude. This minireview explores the physiological responses of high-altitude waterfowl (geese and ducks), comparing the strategies of lifelong high-altitude residents to those of transient high-altitude performers, providing insight into how birds champion high-altitude life. In particular, this review highlights and contrasts the physiological hypoxia responses of bar-headed geese (Anser indicus), birds that migrate biannually through the Himalayas (4,500-6,500 m), and Andean geese (Chloephaga melanoptera), lifelong residents of the Andes (4,000-5,500 m). These two species exhibit markedly different ventilatory and cardiovascular strategies for coping with hypoxia: bar-headed geese robustly increase convective oxygen transport elements (i.e., heart rate and total ventilation) whereas Andean geese rely predominantly on enhancements that are likely morphological in origin (i.e., increases in lung oxygen diffusion and cardiac stroke volume). The minireview compares the short- and long-term cardiovascular and ventilatory trade-offs of these different physiological strategies and offers hypotheses surrounding their origins. It also draws parallels to high-altitude human physiology and research, and identifies a number of areas of further research. The field of high-altitude avian physiology offers a unique and broadly applicable insight into physiological enhancements in hypoxia.

Keywords: altitude; bird; cardiovascular; hypoxia; respiratory.

Fig. 1.

Decreases in barometric pressure (P_B), inspired partial pressure of oxygen ($P{I}_{O_2}$), and inspired fractional composition of oxygen ($F{I}_{O_2}$) equivalent to sea level with increases in altitude. Relationship between x and y variables is from Bouverot (11). The tagged bar-headed goose migration data refer to actual telemetry studies conducted on bar-headed geese (7, 27), whereas altitude denoted by the anecdotal evidence for the bar-headed goose migration refers to a one-time sighting described by Swan (69). Altitude information for Andean geese, Andean ducks, and low-altitude ducks is derived from literature (23, 29, 37, 42, 43, 50). [Adapted from Bouverot (11) with permission from Springer. Copyright © Springer-Verlag Berlin Heidelberg 1985.]

Fig. 2.

The driving partial pressure of oxygen (Po₂) through the vertebrate oxygen transport cascade decreases with increasing altitude. A: a generalized schematic of the main diffusive and convective portions of the vertebrate oxygen transport cascade, with the four chambers of the heart abbreviated (RA, right atrium; RV, right ventricle; LA, left atrium; LV, left ventricle). B: decreases in the Po₂ through the oxygen transport cascade at sea level (0 m) and at high altitude (e.g., Mt. Everest Summit: 8,848 m). The solid lines are based on data collected in Ref. , and the dotted lines are hypothetical. “a”, Arterial Po₂ values; $\overline{v}$, venous Po₂ values. [Adapted from Bouverot (10) with permission from Springer. Copyright © Springer-Verlag Berlin Heidelberg 1985.]