Cognitive perception of circulating oxygen in seals is the reason they don’t drown

Marine mammals rely on maintaining sufficient blood oxygen levels while diving to prevent drowning. Generally, oxygen is cognitively imperceptible to mammals that instead sense rising carbon dioxide as a proxy for low oxygen. Not perceiving oxygen, however, is risky for diving mammals. We argue that any ability to alter dives based upon direct perception of oxygen should have been strongly selected for. We exposed diving seals to inhaled gas mixes that were experimentally altered to affect circulating levels of oxygen and carbon dioxide. Dive duration was positively correlated with circulating oxygen levels but unaffected by carbon dioxide levels and pH. These results suggest that seals do cognitively perceive circulating oxygen and use this to alter dive behavior. Marine mammals have adapted to an underwater environment and spend long periods of time underwater foraging despite the need for breathing air. As such, they are especially susceptible to low blood oxygen levels, which could lead to drowning while submerged. McKnight et al. exposed diving gray seals to various gas mixes and found that the seals responded to low levels of blood oxygen but not high levels of carbon dioxide (see the Perspective by Hawkes and Kendall-Bar). Nearly all other mammals are insensitive to blood oxygen levels, instead gauging blood oxygen by sensing increasing carbon dioxide. Direct sensing of oxygen levels likely improves diving efficiency. ?Sacha Vignieri