All animals take in oxygen from the air they breathe in, and release CO₂ from their blood when breathing out. Most mammals (哺乳动物) can’t directly detect oxygen levels in the blood supplied to their tissues. Instead, they rely on the rising level of CO₂ in their blood to signal that they might need to take a breath. But a recent study published in Science suggests seals (海豹) can sense the amount of oxygen in the blood, and change their diving behavior in response.
To find out if oxygen levels affected seal behavior, Professor McKnight at the University of St. Andrews and his colleagues created a special section in a pool where young seals were held. In one corner, there was a breathing chamber (呼吸室), where they were sheltered from the rain and the wind.
The breathing chamber was surrounded by panels that prevented surface swimming, yet swimming below the surface for about 200 feet would give the seals access to a feeder where they could eat as much fish as they liked. Once the seals got familiar with the setup, the researchers started to gradually change the composition of the air in the breathing chamber, increasing or reducing the levels of oxygen and CO₂ to see an effect on their behavior. Sure enough: the higher the level of oxygen, the longer the seals stayed at the feeder.
The finding suggests that seals don’t just physically respond to oxygen levels by changing their heart rate or breathing, but that they are sufficiently aware of them to change their behavior. This ability would put seals in a class beyond any land mammals that have been tested. Since oxygen levels on land remain stable, humans don’t seem to have evolved (演化) to notice low blood oxygen levels, sometimes not even when they’re about to pass out in free-diving.
Therefore, in free-diving without oxygen tanks, accidents are quite common. Our reliance on sensing CO₂ levels in our blood instead of oxygen may be to blame. Actually, this is a perfectly reasonable strategy on land, where growing CO₂ tends to signal breathing issues. But when holding our breath during diving, relying on CO₂ levels is risky, especially on repeated dives. Because every time we surface and breathe in, our sensitivity to CO₂ is decreased, even if its levels are already high, and this increases the chance that a person will, without awareness, pass out before they get to the surface.

What results in humans’ inability to notice low oxygen levels in their blood?