In turquoise waters off the Indonesian coast, evolutionary geneticist Melissa Ilardo watched as the diver, wearing handmade, wooden goggles, spotted a giant clam meters below and darted down to retrieve it.

The diver was one of the Bajau people of Southeast Asia, known for holding their breath for long periods while spearing fish and gathering other seafood. During a typical day, these “sea nomads” spend up to five hours in total underwater. And Ilardo had heard that some can hold their breath for as long as 13 minutes during a dive. Their comfort with breath-hold diving may be due to having unusually large spleens, which provides a bigger supply of oxygenated red blood cells, Ilardo and colleagues report online April 19 in Cell.

“Many Bajau children learn to swim before they learn to walk,” says Ilardo, who did the research while at the University of Copenhagen. Certain seal species have larger spleens, and Ilardo wondered if the same was true for the Bajau. When a mammal holds its breath and dives, the body responds by slowing the heart rate, constricting blood vessels in the extremities and contracting the spleen to release stored oxygenated red blood cells.

SPLEEN CHECK Evolutionary geneticist Melissa Ilardo performs an ultrasound on a Bajau diver to measure his spleen size. Her time collecting samples in one Indonesian village often drew crowds of onlookers.

PETER DAMGAARD

More than 50 Bajau from the coastal village of Jaya Bakti provided spit samples for DNA testing and allowed Ilardo to take spleen measurements with ultrasound. She and her colleagues compared the data with those from similar samples given by another ethnic group called the Saluan, who live about 25 kilometers away in the coastal village of Koyoan, but don’t spend much time in the water.

The Bajau’s spleens were about 50 percent larger than the Saluan’s, says Ilardo, now at the University of Utah in Salt Lake City. DNA tests showed that the Bajau had genetic variations associated with spleen size.