The Haenyeo women who dive deep into the East China Sea to harvest sea urchins and shellfish spend the most time underwater of any humans ever studied — one to five hours a day, researchers report August 18 in Current Biology.
“It’s as close as you get to studying a mermaid,” says Chris McKnight, a marine mammal biologist at the University of St. Andrews in Scotland.
McKnight and colleagues worked with seven Haenyeo divers who live on Jeju Island in South Korea. Over 1,786 dives, the women wore devices that measured how long they stayed underwater and how deep they dove. The devices also tracked oxygen levels in the women’s brains and muscles.
The women dove repeatedly for two to 10 hours each day and spent an average of 56 percent of the time beneath the surface — more underwater time than many aquatic mammals, including beavers, polar bears and sea otters, the researchers say.
Chronicles of female divers in this part of Asia date back 3,000 years. Researchers have studied the divers in the lab, but the team wanted to monitor them in a real-life setting to put their abilities in context with other water-dwelling mammals. Studying the Haenyeo women’s physiological adaptations may shed light on how the ancestors of whales and other marine mammals successfully transitioned from a terrestrial to an aquatic environment, McKnight says.
The team expected to see low oxygen levels and slow heart rates in the Haenyeo, characteristic of other mammals’ physiological dive response. Instead, it was the opposite: The women had elevated heart rates and very little oxygen reduction in their brain or muscles while diving, a surprise that might be explained by the women’s frequent, shallow dives. The Haenyeo dives range from three to 15 feet deep, averaging 11 seconds in length with just nine seconds between dives to recover on the surface.
What makes the feats even more impressive is the women’s average age: 70. “This speaks to an incredible health span, which is really exciting, because we could really dig into what’s allowing that to be possible in this population,” says study coauthor Melissa Ilardo, an evolutionary geneticist at the University of Utah in Salt Lake City. Ilardo led a lab-based study that compared the Haenyeo women with non-diving Koreans, and uncovered genetic variations in the divers from Jeju Island associated with their blood pressure as well as their ability to withstand cold water, which may contribute to skillful diving, she and colleagues reported May 27 in Cell Reports.
McKnight says that the Haenyeo may also have adapted to tolerate high levels of carbon dioxide, which builds up in the body after repeated breath holds. Elevated carbon dioxide levels are “incredibly uncomfortable” for most humans, McKnight notes, inciting anxiety and panic. He hopes to monitor the women’s CO2 levels in the future.
The foraging practices of the Haenyeo are similar to “a surface-oriented, shallow-diving marine mammal like a sea otter,” says marine biologist Ted Cheeseman, who heads Happywhale, an online photo database of marine mammals, and was not affiliated with this study. The adaptations that allow these women to spend more than half their time underwater “really speaks to culture driving evolution,” Cheeseman says.