Scalloped hammerheads shut their gills to avoid freezing in cold waters (Credit: Cory Fults/ University of Hawaii at Manoa/ CC-BY-SA-2.0)

Like most fish, sharks are largely ectothermic, or cold-blooded. This means their body temperature depends on the water around them. To avoid freezing, most shark species remain close to the ocean's surface, where the water temperatures are warmer. However, scalloped hammerheads have long been known to dive thousands of feet into the ocean's cold depths in search of prey. Researchers from the University of Hawaii at MńĀnoa have now uncovered the shark's long-kept secret. The smart fish shut their gills and "hold their breath" to stay warm.

For their study, Mark Royer and his team fitted special sensors on the backs of some scalloped hammerhead sharks off the coast of Oahu, Hawaii. The devices allowed the team to observe their internal body temperature and that of the surrounding water. It also tracked how deep the animal was diving.

"It was kind of like attaching a Fitbit to a shark," Royer told NPR. "It allowed me to get precise details on what the shark was doing."

Over the next 23 days, the tagged sharks conducted several deep-sea dives. The fish went down 2,600 feet (792 meters) to waters as frigid as 41 degrees Fahrenheit (5¬į C). They remained there to feed for about four minutes before returning to the surface. The scientists observed that the shark's body temperature remained elevated throughout the dive. It only cooled when the fish came closer to the surface toward the end of each dive.

Computer modeling indicated the hammerhead sharks prevented heat loss from the gills by closing them. The cooling at the surface was likely the result of the sharks opening the gills to resume breathing during the ascent while still in relatively cold water.

Video footage from a 2015 study confirmed the scientists' theory. It showed the sharks swimming at a depth of more than 3,400 ft. (1,036 meters) with their gills tightly shut. However, the images of them at the water's surface had their gills open.

Scalloped hammerheads can be found in tropical waters worldwide (Credit: CC-BY-SA-2.0)

"Holding their breath keeps scalloped hammerhead sharks warm but also shuts off their oxygen supply," said Royer. "So, although these sharks hold their breath for an average of 17 minutes, they only spend an average of four minutes at the bottom of their dives at extreme depths before quickly returning to warmer, well-oxygenated surface waters where breathing resumes."

The researchers, who published their findings in the journal Science on May 11, 2023, say the shark's strategy offers them an evolutionary advantage. But it also makes them vulnerable to human activities like deep-sea mining and large-scale deep-sea fishing.

As the name indicates, hammerheads have thick, broad heads that resemble a double-headed hammer. Scalloped hammerheads are named for the notches found along the front edge of their heads. The sharks can be found in coastal warm temperate and tropical seas worldwide. They are critically endangered in parts of the Atlantic. Their population has declined by over 95 percent in the past three decades due to overfishing and nursery habitat loss.