Abstract
High-altitude life poses physiological challenges to all animals due to decreased environmental oxygen (O(2)) availability (hypoxia) and cold. Supporting high metabolic rates and body temperatures with limited O(2) is challenging. Many birds, however, thrive at high altitudes. The O(2)-transport cascade describes the pathway involved in moving O(2) from the environment to the tissues encompassing: (i) ventilation, (ii) pulmonary O(2) diffusion, (iii) circulation, (iv) tissue O(2) diffusion, and (v) mitochondrial O(2) use for ATP production. Shared avian traits such as rigid lungs with cross-current gas exchange and unidirectional airflow aid in O(2) acquisition and transport in all birds. Many high-altitude birds, however, have evolved enhancements to some or all steps in the cascade. In this review, we summarize the current literature on gas exchange and O(2) transport in high-altitude birds, providing an overview of the O(2)-transport cascade that principally draws on the literature from high-altitude waterfowl, the most well-studied group of high-altitude birds. We close by discussing two important avenues for future research: distinguishing between the influences of plasticity and evolution and investigating whether the morphological and physiological differences discussed contribute to enhanced locomotor or thermogenic performance, a potential critical link to fitness.This article is part of the theme issue 'The biology of the avian respiratory system'.