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Why hammerhead sharks 'hold their breath' in deeper, colder waters

MARY LOUISE KELLY, HOST:

Sharks are among the best swimmers on the planet, but a new study in the journal Science shows that one species may be diving deep using a trick common to humans. NPR's Geoff Brumfiel has more.

GEOFF BRUMFIEL, BYLINE: The scalloped hammerhead shark lives in oceans all over the planet.

MARK ROYER: It's one of the larger but not the largest hammerhead species.

BRUMFIEL: That's Mark Royer, a shark researcher at the University of Hawaii at Manoa. Scalloped hammerheads have a really unusual skill. They can dive to over 2,500 feet below the surface. At those depths, even the most sunny, tropical oceans become dark and frigid.

ROYER: Imagine you're on a warm, sunny beach, and you hop out of the warm water and then immediately plunge into an ice bath.

BRUMFIEL: It'd be unpleasant for a human, but it's potentially deadly for a shark. A shark can't generate its own body heat. If it gets too cold, it can't swim. And if it stops swimming, water doesn't flow across its gills. It can't breathe. It dies. So here's the question.

ROYER: How is it that a coastal, warm, tropical species is able to go down into these deep depths and survive?

BRUMFIEL: To find out, Royer and his colleagues went to a bay where the hammerheads swim.

ROYER: We do this all in a small 17-foot Boston whaler, so it's almost, like, the size of a dinghy.

BRUMFIEL: You don't think you need a bigger boat?

ROYER: We don't, no. It's like the smaller, the better because we want to be able to lean over and get as close as possible.

BRUMFIEL: In order to attach a bunch of electronics to each shark's fin.

ROYER: This is essentially like putting a Fitbit on the shark.

BRUMFIEL: When Royer and his colleagues later analyzed that sharky Fitbit data, what they found amazed them. The sharks dive, spend just a few minutes at the bottom, probably hunting squid.

ROYER: And then they pitch themselves at an 80-degree angle and then shoot towards the surface.

BRUMFIEL: But what's really wild is their body temperature doesn't drop. It stays steady until they start coming back from the deep. Royer quickly realized what was going on.

ROYER: They were closing their gill slits and preventing that water from flowing across their gills that would cool their body down.

BRUMFIEL: They're holding their breath. The sharks are holding their breath.

ROYER: Yes, they're holding their breath.

BRUMFIEL: Remember; unlike humans, sharks use gills to breathe underwater. This is all about temperature. Passing cold water over the gills would cool the shark's blood, putting it in danger. It makes sense, Royer says, but he still can't quite believe it.

ROYER: After doing this study, it still shocks and baffles me.

BRUMFIEL: That a shark would need to hold its breath underwater. Geoff Brumfiel, NPR News. Transcript provided by NPR, Copyright NPR.

Geoff Brumfiel works as a senior editor and correspondent on NPR's science desk. His editing duties include science and space, while his reporting focuses on the intersection of science and national security.