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Seals spend most of their time underwater but rarely do they drown. A new study in the journal Science reveals the unusual way that these diving mammals can tell when they would run out of oxygen. NPR's Jonathan Lambert has more.
JONATHAN LAMBERT, BYLINE: In 2019, ecologist Christopher McKnight took a break from studying seals to study humans at the Freediving World Championships.
CHRISTOPHER MCKNIGHT: In the first day, I think there were three divers blacked out underwater - so basically ran too low on oxygen, lost consciousness. To me, that was quite jarring to observe. But for the free divers, it was just like, well, it just happens, you know? It's a thing.
LAMBERT: It's a thing in part because free divers can hack their body into thinking it's got more air than it does by hyperventilating. That lowers carbon dioxide, which rapidly increases when we hold our breath, driving the urge to breathe.
MCKNIGHT: Seeing the fallibilities in oxygen management in humans sort of just posed the question that I guess I hadn't really thought about in seals.
LAMBERT: That question is, essentially, why don't these mammals drown? It's true that seals can store tons of oxygen in their bodies, which helps them stay under for up to 30 minutes.
MCKNIGHT: But it doesn't really matter how much you have if you don't know where you are in the fuel gauge. It just takes you to go over that once, and your fitness is zero. You're out of the population.
LAMBERT: There are essentially two gases that could serve as this fuel gauge - carbon dioxide, or CO2, and oxygen, also called O2. While mammals' bodies can respond to both, in most circumstances, mammals only detect changes in CO2. That's because CO2 can build up in harmful ways before we run out of oxygen. But...
MCKNIGHT: I think when we flip to animals that live the majority of their life in an oxygen-limited system, then O2 is the one that makes most sense to be able to perceive or detect.
LAMBERT: But it's never been shown that animals can cognitively perceive oxygen in that way. To find out, McKnight and his team at the University of St. Andrews had to get creative.
MCKNIGHT: Effectively, what we wanted to do was to be able to control the amount of oxygen and CO2 that the animals were inhaling to then affect the amount that is in their blood.
LAMBERT: They used an experimental setup where captured gray seals came up to breathe in a special air chamber before diving for food in an enclosed pool. They then exposed seals to high CO2, high oxygen, low oxygen and ambient air, watching to see how long they stayed underwater.
MCKNIGHT: If we increase the oxygen available to them, dive duration was longer. If we halved oxygen, we find that the dives were shorter.
LAMBERT: But when they increased CO2 to 200 times the amount in ambient air...
MCKNIGHT: Dive duration was exactly the same, which was a real surprise because it has such a strong, perceptible effect on most mammals.
LAMBERT: The results pretty clearly show that these seals can directly perceive their circulating oxygen levels, says Michael Tift, a biologist at the University of North Carolina Wilmington.
MICHAEL TIFT: It's a little bit of a surprising result, but actually, it's also not that surprising of a result.
LAMBERT: Seals have been shown to handle big swings in CO2 during dives, so it makes sense to him that they'd pay more attention to oxygen. How they do that wasn't assessed in this study, but McKnight has some ideas. All mammals have basically the same hardware for measuring oxygen and CO2 . What's changing essentially could be the software.
MCKNIGHT: So what we think is there's a little bit, potentially, a reconfiguration of how that information is processed.
LAMBERT: To McKnight, that raises the possibility that other diving mammals, and maybe even diving reptiles and birds, have evolved similar ways of not drowning.
Jonathan Lambert, NPR News. Transcript provided by NPR, Copyright NPR.
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