Do sharks hold their breath underwater? This species might.

Scalloped hammerhead sharks may be holding their breath when they dive deep into frigid waters. The revelation, published today in Science, suggests that this strategy may allow the warm-water dwellers to regulate their temperature while they hunt.  

This technique is “completely unexpected,” says Mark Royer, a shark biologist at the University of Hawaii who led the research. “This kind of behavior has never been observed in any kind of deep-diving fish,” he says, and it now raises questions about how widespread breath-holding may be among other species. 

These critically endangered hammerhead sharks typically rely on forward movement to force water across their gills, which enables them to extract needed oxygen to breathe. Yet when the predators swim half a mile down to catch squid and other prey, the colder water could impact their metabolism, cardiac function, and eyesight—all factors that would reduce their hunting prowess.

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By closing their gills and mouths—holding their breath—the animals could limit their exposure to cold water.

Some species, like bluefin tuna and mako sharks, have specialized anatomy that allows them to conserve their body heat in cold waters, but scalloped hammerheads don’t have that advantage. 

That’s why some scientists have theorized that scalloped hammerheads maintain their body heat using simple thermal inertia—essentially relying on their large body size to help maintain their temperature and carry heat down as they dive into cold depths.

“Imagine you are trying to defrost your 15-pound Thanksgiving turkey: you bring it out of your freezer, and it takes a long time to thaw—that’s like thermal inertia,” says Marianne Porter, a biologist who studies shark movement at Florida Atlantic University and wasn’t involved in the study.

Yet tiny sensors that the team placed on adult scalloped hammerhead sharks—basically a “Fitbit for sharks” as Royer says—suggest that thermal inertia isn’t the reason they remain warm during their hunting trips at depth.

Hunting in frigid waters 

For the study, the team analyzed detailed information about the swimming behavior, depth, and location of a small group of six tagged male sharks that collectively completed more than a hundred dives around Hawaii over several weeks.

Tiny probes also recorded the animals’ muscle temperatures during these repeated nighttime dives. That data, alongside further modeling, indicated that the sharks managed to largely maintain their body temperatures both when they were at surface waters around 80 degrees and when they plunged to more than 2,500 feet below where temperatures dropped by roughly half, to around 41 degrees.

Surprisingly the animals’ body temperatures did eventually drop when they had ascended and reached slightly warmer waters about halfway back to the surface—apparently when the sharks had opened their gills to obtain needed oxygen. That’s not what you would expect with thermal inertia, since there’d be a more constant warming and cooling to reach equilibrium, Royer says.

Though they didn’t observe the sharks actually closing their gills, his team suspects that’s what’s happening. To fully confirm their breath-holding hypothesis, Royer says, they will next need to affix cameras to the pectoral fins of these hammerheads to observe the gills opening and closing as sharks dive.

It’s still unclear how the 12-foot-long species may have acquired this breath-holding skill. “It’s possible they learn it from social interactions from other hammerheads that are diving,” says Royer, adding “another possibility is they’re following the example of other animals that dive and eat prey at the same depth.” 

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Compelling evidence

Further backing the team’s breath-holding conclusion: prior footage from a remotely operated vehicle that shows an adult scalloped hammerhead shark swimming in Tanzania at more than 3,000 feet below the surface with its gills closed, and footage of the species in its typical surface waters with open gill slits. What’s more, another experiment from Royer’s team with deceased hammerheads in warm and cold water baths also supported their results.

“It’s a compelling paper—the way they have all these lines of evidence suggesting this might be happening,” says Porter. “I’m convinced” there’s breath-holding, she says. 

Female scalloped hammerhead sharks also conduct deep dives, she says, so though this study only included males she suspects this applies to non-pregnant females as well. 

(Learn more: Explore a rare hammerhead shark nursery.)

The implications of this breath-holding work are “remarkable,” write the University of Western Australia’s Mark Meekan and Murdoch University’s Adrian Gleiss in an accompanying scientific commentary also published in Science.

If these rapidly declining species can pause respiration while foraging at depth they may cope with the increasingly common low-oxygen environments in our changing world better than expected, they write. Already, scalloped hammerheads occupy low-oxygen waters in the Gulf of California, and this newly discovered breath-holding skill may explain their presence, they note.

It's true that these animals may survive in low-oxygen areas for short periods, Porter says, but that’s likely not sustainable long-term. “I would wonder what happens if that becomes their reality all the time.”