Enlarged spleen key to diving endurance of 'sea nomads'

The new study found that the spleens of the Bajau people are 50 per cent larger than those of their land-dwelling neighbours the Saluan. This image shows a Bajau diver displaying a traditional wooden diving mask

The new study found that the spleens of the Bajau people are 50 per cent larger than those of their land-dwelling neighbours the Saluan. This image shows a Bajau diver displaying a traditional wooden diving mask

But how does the spleen help someone hold their breath under water? In a study published April 19 in the journal Cell, researchers report that the extraordinary diving abilities of the Bajau may be thanks in part to their unusually large spleens.

The spleen holds oxygenated red blood cells, so presumably an enlarged spleen - those of the sea nomads, or Bajau people, are about 50 percent larger than the spleens of unrelated, non-diving neighboring groups - injects more blood cells into the circulation and makes more oxygen available for basic body functions during prolonged dives. This contraction generates an oxygen boost by ejecting red blood cells into circulation - corresponding in as much as a 9 percent increase in oxygen. These examples can be key to understanding human physiology and human genetics.

Prior to her travels, Ilardo spent months learning the Indonesian language, Bahasa Indonesia, which the Bajau people speak in addition to their native language, so that she could communicate with them directly, and convey to them the research findings. Melissa Ilardo images and editing, Linus Mørk filming. "That is something we would like to explore next", says Nielsen. "It's a hypoxia experiment that nature has made for us and allows us to study humans in a way that we can't in a laboratory".

The study also has implications for the world of medical research.

Among the Bajau people, there wasn't a meaningful difference in spleen size between divers and non-divers.

"This study is a wonderful example of the value of studying these small populations living under extreme conditions", said Professor Eske Willerslev of the University of Copenhagen, who carried out the genetic study. Ilardo added, "until now it has been entirely unknown whether Sea Nomad populations genetically adapt to their extreme lifestyle". "The first sign that we were maybe onto something was when we saw that both the Bajau divers and non-divers had larger spleens than the Saluan, a nearby, non-diving population". They have been living this hunter-gatherer way of life for thousands of years. A chronicler of one of Ferdinand Magellan's voyages recorded their unusual lifestyle in 1521.

Ilardo was intrigued by the Bajau (be joe') and proposed a Ph.D. thesis project to find out whether and how they adapted to sustained deep dives. "But I also think natural selection is a lot more powerful than we sometimes give it credit for, and maybe we should be looking for it in more places than we thought".

Ilardo was struck by how the Bajau seemed "so incredibly comfortable in the water". "That is really remarkable, even compared to other professional or traditional divers". "There's nothing like seeing them in the water". The study authors created a phylogenetic tree, calculating that the Bajau and Saluans diverged about 15,000 years ago, suggesting that the Bajau-unique genetic variants evolved some time after this divergence. Hypoxia has been well studied in populations living at high altitudes, where the lack of oxygen is much more chronic. Though both Willerslev and Nielsen were dubious about the success of Ilardo's project, they supported the idea of testing it out.

"The spleen is a weird one", Ilardo said. The village, Jaya Bakti, provides easy access to the ocean for often weeks-long excursions during which the Bajau live off the fruits of the sea.

Using an ultrasound machine, Ilardo found that the Bajau had spleens that were roughly 50% larger than those of their land-dwelling neighbors on the Indonesian island of Sulawesi. That may translate into about a 10 per cent increase in oxygen supply, Nielsen said.

The spleen is directly related to how long a person can dive. For instance, the new study might one day lead to an association between genetics and physiological response to acute hypoxia. "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 RBCs when you really need them". This genotype was previously tied to PDE10A expression in neuronal tissue and in the thyroid and was also linked with thyroid hormone levels. The researchers collaborated with a group in the Netherlands, which confirmed that higher PDE10A levels are associated with an increased release of thyroid hormone.

There is no known link between thyroid hormones and spleen size in humans - but in mice, there is. "Nothing is really known about the genetic basis of spleen size in humans, so it is hard to validate without further research".

The genetic variant they identified wasn't specific to the Bajau, Ilardo said, and there could be many other genes that affect spleen size.

"There's not a lot of information out there about human spleens in terms of physiology and genetics", says Melissa Ilardo, the first author of the paper. "Most of these populations are completely understudied, and I think there is a huge benefit, not only potentially to them, but also to the rest of mankind by actually paying some attention to them".