A deep dive into oxygen sensing

Oxygen is fundamental for life in most organisms. As such, most animals have developed sophisticated ways to control the amount of oxygen present in circulating blood and tissues. However, mammals and birds are thought to be unable to sense oxygen directly in the blood but instead use carbon dioxide (CO₂) as a proxy for changes in oxygen concentration. This poses a particular challenge for diving animals that spend most of their lives underwater holding their breath, a condition that can cause a dangerous buildup of CO₂ (hypercapnia) and insufficient oxygen (hypoxia). On page 1276 of this issue, McKnight et al. (1) report that gray seals (Halichoerus grypus) can directly sense the level of blood oxygen and modify their diving behavior accordingly, independent of the amount of CO₂ present. This raises the question of direct sensing of blood oxygen by other animals as well.