They often work as entertainers or activists, promoting the preservation of natural underwater environments.
They are also, more often than not, professional divers who recognize the many difficulties that come with the passion for being under water — such as holding their breath for as long as possible.
Holding your breath can be dangerous, because you become deprived of oxygen that — carried by the blood flow to all extremities of your body — "feeds" your organs and helps them to stay alive and functional.
Normally, we can't hold our breath for more than a few seconds, although professional freedivers — who spend years training their bodies to get used to staying under water for longer — can hold their breath for around 3 minutes.
Across the world, there are small populations that, for many generations, have made their living out of freediving. In Japan, for instance, ama divers are women who dive in search of pearl oysters and seafood.
Their tradition is slowly dying out. However, in the islands of Southeast Asia, some populations — specifically, the Bajau people — are known as "sea nomads." Many of them still practice their freediving lifestyle, which also provides them with their livelihood on a day-to-day basis.
The Bajau freedive for food every single day at dizzying depths of more than 70 meters — hunting fish and octopi, or gathering sea cucumbers — and they spend 60 percent of their working days under water.
So, how can these people have persisted in this pursuit for generations? And has their freediving in any way influenced how their bodies function?
Melissa Ilardo — a former doctoral student at the University of Copenhagen in Denmark and now a postdoctoral researcher at the University of Utah in Salt Lake City — has been fascinated with the Bajau people's sea nomad lifestyle, and she had a theory.
Perhaps the bodies of the Bajau had evolved over generations to accommodate their freediving needs.
'Closest thing to the Bajau — sea otters'
Adaptive bodily evolutions among people who have lived for generations in unusual conditions are certainly not unheard of. For example, a 2014 study concluded that Tibetans have adapted to living at oxygen-sparse high altitudes thanks to a particular genetic mutation.
However, Ilardo considered the Bajau's potential adaptations through a slightly different lens. She thought about deep-diving mammals, such as seals and otters, which have larger spleens that allow them to store a higher number of blood cells than other mammals.
Through a reflexive contraction of the spleen, these deep-diving animals increase their red blood cell count while under water, thus also boosting their blood oxygen levels.
And, the comparison between the Bajau people and seals or otters was not drawn at random.
"The closest thing to the Bajau in terms of underwater working time is sea otters; they are also spending about 60 percent of their time in the water. "
"That is really remarkable, even compared to other professional or traditional divers," Ilardo notes. "They are just spending an extraordinarily long time underwater compared to their recovery time."
Sea nomads have larger spleens
To prove her theory, Ilardo traveled to Indonesia in 2015 and contacted a Bajau community to see if they would be happy to help her in her research. As it turned out, the Bajau were interested in learning more about their own bodies and their unique skills.
So, during two different expeditions, she used a portable ultrasound device to ascertain the spleen size of 59 Bajau individuals and compare it with that of 34 non-Bajau participants, hailing from a nearby village whose inhabitants did not practice freediving.
Her findings, published yesterday in the journal Cell, indicated that the Bajau had spleens that were approximately 50 percent larger than those of their landlubbing neighbors.
No difference in spleen size was ascertained between freediving Bajau and Bajau who chose not to take up this practice.
This may mean that these people could increase their red blood cell count by about 10 percent while diving, compared with individuals with regular-sized spleens.
"While it is unhealthy to have high concentrations of red blood cells all the time, it is really good for you if you have high [red blood cells] when you really need them," explains senior study author Rasmus Nielsen.
He adds that the Bajau "have increased the storage capacity in the spleen for when they need it, but they don't have any negative effects of constantly having too high red blood cells."
Additionally, saliva samples collected by Ilardo from the participants revealed that individuals in the Bajau community expressed certain gene variants that were uncommon in neighboring populations.
A particular gene variant — PDE10A — encodes the enzyme phosphodiesterase, which plays a role in the upregulation of thyroid hormones. This finding led to another theory, which the researchers are now eager to put to the test.
"We think the way it works is that the expression of this variant gene changes thyroid hormone release, which then has an effect on spleen size," says Nielsen.
Yet he remains cautious, noting, "Nothing is really known about the genetic basis of spleen size in humans, so it is hard to validate without further research."