The Bajau can hunt hundreds of feet below the surface because their enlarged spleens raise their oxygen levels, a study shows. New research suggests these impressive feats aren't the result of training, but rather, an example of natural selection at work - which, in this case, has endowed Bajau individuals with abnormally large spleens.
Ilardo was originally warned against undertaking this study for her PhD at the University of Copenhagen by her supervisors - Professor Eske Willerslev who holds dual positions at St John's College, Cambridge and the University of Copenhagen, and Professor Rasmus Nielsen who also holds dual positions, at the University of Copenhagen and the University of California, Berkeley.
'I thought that if selection acted on the seals to give them larger spleens, it could do the same in humans'.
It's possible that the Bajau's way of life has been ongoing for more than 1,000 years, researchers said, citing linguistic analysis.
You can find the Bajau in Indonesia, Malaysia, and the Philippines, who until recent times, they lived on houseboats. Instead they rely on weights, handmade wooden goggles - and a single breath of air. By echographing 59 Bajau and 34 Saluan, she showed that the Bajau's spleen, whether they were divers or not, was about 50% larger than that of the Saluan!
It helps their diving because the spleen reacts to submersion in water by injecting oxygenated red blood cells into the circulation. This genetic variant upregulates thyroid hormone, which in mice has been linked to larger spleen size. This allows them to use oxygen more efficiently so they can stay underwater for longer.
The findings, which are being published in the research journal Cell, could also have medical implications in relation to the condition known as Acute Hypoxia, which can cause complications in emergency medical care.
It's genes like these that may have given some Bajau divers the best shot at surviving deep free dives and collecting food for their families over many generations, Ilardo said.
They're the envy of modern freedivers, sportspeople who swim in deep water without breathing apparatuses, but how the Bajau have become so good at holding their breath has remained a mystery.
The team spent some time in Jaya Bakti, Indonesia to examine the people of this tribe.
Melissa Ilardo from the University of Copenhagen is the lead author of the study. Often referred to as "sea nomads", the Bajau are formidable free divers who have engaged in breath-hold diving for thousands of years. "I basically just showed up at the house of the chief of the village, this weird, foreign girl with an ultrasound machine asking about spleens", she says. They discovered that members of the Bajau have a gene called PDE10A which the Saluan do not. By comparing the Bajau genome to two different populations (Saluan and Chinese Han), scientists found 25 genomic sites with significant differences.
"I think the most surprising part was the genetic component that underlies that physiological adaptation and that seems to be connected to thyroid hormone levels". Because of this, their spleens have developed to allow them to dive for longer. By making their data freely available to other researchers, she and her co-authors hope that some of what they've learned from the Bajau can be applied in medical contexts.
The Bajau having larger spleens than their non-diving neighbors suggested that their diving culture had shaped their physiology.
"We can't really. expose people to new conditions and have controlled genetic experiments in the same way we can do in fruit flies and mice", Nielsen said. However, the researchers did not measure the thyroid hormone levels of the Bajau participants.