Gas exchange, oxygen transport and metabolism in high-altitude waterfowl

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’.