Gray seals may sense their own blood oxygen levels

Gray seals may possess a secret sense that helps them survive at sea.

The marine mammals adjusted their time spent underwater based on the amount of oxygen in the air they breathed before diving, researchers report in the March 21 Science. The finding suggests gray seals (Halichoerus grypus) perceive their own blood oxygen levels, an ability that prevents them from drowning on dives that can last up to an hour.

Mammals need oxygen to live. But they usually can’t detect its presence in their blood. Instead, most rely on the effects of elevated carbon dioxide as a proxy for low oxygen. In humans, an increase in circulating carbon dioxide, monitored via sensory organs in the carotid arteries, leads to shortness of breath, hunger for air and panic. A person will eventually pass out.

But marine mammals, who spend most of their lives submerged, can’t afford to let their oxygen levels drop to the point where they lose consciousness, says ecologist Chris McKnight of the University of St. Andrews in Scotland. There “would have been such a strong evolutionary pressure to come up with an adaptation or a trait that stops that, or certainly really minimizes the risk of it.”

To figure out what that adaptation might be, McKnight and colleagues gathered six juvenile gray seals from a nearby wild population. One by one, the animals swam in a pool, traveling 60 meters back and forth between an underwater feeding station and a breathing chamber with controlled concentrations of gases. Four gas combinations were used: One mimicking ambient air (21 percent oxygen and 0.04 percent carbon dioxide, among other standard gases), one with an oxygen concentration at roughly twice that of ambient air, one with an oxygen concentration at roughly half that of ambient air and a mix with a standard amount of oxygen but a carbon dioxide concentration at 200 times greater than in ambient air. The researchers recorded 510 individual dives across all animals.

The seals spent, on average, about four minutes underwater after breathing ambient air. The more oxygen available, the longer the animals stayed underwater during each dive, and vice versa. Elevating carbon dioxide, however, had no effect on dive time compared with ambient air.

Because the seals were making their own decisions — and were never dangerously low on oxygen due to the short dives — they were probably aware of their blood oxygen levels, and varying their time underwater accordingly, McKnight says. The seals probably evolved to have a blunted response to carbon dioxide because it builds up in their bodies over successive dives.

McKnight suspects the same “hardware” that tracks carbon dioxide in humans and other mammals allows gray seals to detect their oxygen levels. The difference may lie in how their brains process the information about the blood’s contents.