How Long Can the Bajau Hold Their Breath?

Bajau divers have been documented staying underwater for as long as 13 minutes at a time, reaching depths of roughly 200 feet on a single breath.¹ Those figures represent the extreme end of what Bajau free divers can achieve, and most working dives are shorter. But even routine Bajau dives far exceed what an untrained person could manage. What makes their performance remarkable is not just practice or technique but a suite of genetic adaptations, centered on an enlarged spleen and a fine-tuned diving reflex, that separate them from neighboring populations who share a similar environment but not the same diving tradition.

A Life Built Around the Water

The Bajau, sometimes called “Sea Nomads,” are an ethnic group spread across the waters of Southeast Asia, primarily in the Philippines, Malaysia, and Indonesia. Historically, many Bajau communities lived entirely on houseboats or stilt houses above the sea, spending much of each day diving for fish, shellfish, sea cucumbers, and other marine resources. Some community members have been reported to spend a cumulative five or more hours per day submerged. While modernization and government resettlement programs have shifted some Bajau toward land-based life, many communities still depend heavily on free diving for food and income. This long, deep relationship with the ocean is central to understanding why their bodies look different under an ultrasound machine.

An Unusually Large Spleen

A landmark 2018 study published in Cell provided the first genetic evidence that the Bajau have evolved larger spleens compared to their non-diving neighbors, the Saluan. The researchers used ultrasound to measure spleen size in both populations and found that Bajau spleens were substantially bigger, even among individuals who did not personally dive. That last detail is important: the enlarged spleen is not simply a training effect from years of diving. It is an inherited, population-level trait shaped by natural selection.²

Why does spleen size matter for breath-hold diving? The spleen acts as a reservoir for red blood cells. Under normal conditions it stores a portion of your blood supply, but when your body detects oxygen deprivation, it contracts and squeezes those extra red blood cells into circulation. More red blood cells mean more hemoglobin available to carry oxygen. A bigger spleen holds a bigger reserve, giving the diver a measurable boost in blood oxygen-carrying capacity at the moment they need it most.³

The Gene Behind It

The study traced the Bajau’s enlarged spleen to a specific genetic variant in a gene called PDE10A. This gene encodes an enzyme involved in signal transduction, the chain of chemical messages that tells cells what to do. In the Bajau, the favored version of this gene is associated with elevated levels of thyroid hormone T4 in the blood. Thyroid hormones influence organ growth, and the researchers concluded that the most likely mechanism is that higher T4 levels drive spleen enlargement during development.²

The connection to thyroid function was not immediately intuitive. PDE10A is most active in the brain, so its role in spleen size came as a surprise. But the critical genetic variant turned out to be most strongly linked to gene expression in the thyroid gland, which then influences spleen growth indirectly. It is a roundabout pathway: a genetic change affects thyroid output, and thyroid output affects how large the spleen grows. This kind of indirect genetic effect is a reminder that evolution does not always redesign the obvious thing. Sometimes the most efficient route to a useful trait goes through what looks like an unrelated organ.

A Sharper Diving Reflex

Beyond the spleen, the Bajau also show signs of selection on a gene called BDKRB2, which is involved in the human diving reflex.⁴ The diving reflex is a set of automatic physiological responses that kick in when your face hits cold water. Your heart rate drops, blood vessels in your extremities constrict, and blood flow is redirected toward the organs that are most vulnerable to oxygen deprivation: the brain, heart, and lungs. The spleen also contracts as part of this reflex, releasing its stored red blood cells.⁵

Everyone has a diving reflex. It is an ancient mammalian trait shared with seals, dolphins, and other air-breathing animals that forage underwater. But the strength and efficiency of the reflex vary from person to person. Selection on BDKRB2 in the Bajau suggests their version of this reflex may be tuned more aggressively, constricting peripheral blood vessels more effectively and perhaps slowing the heart more dramatically. The practical result is that they burn through their limited oxygen supply more slowly while underwater, because less of that oxygen is being wasted on muscles and tissues that can tolerate a temporary shortage.

What Training Does on Top of Genetics

Genetics set the stage, but years of daily diving add another layer of adaptation. Research on trained breath-hold divers from various backgrounds shows that the spleen’s contraction response is itself trainable. Repeated bouts of breath-holding, exercise under low-oxygen conditions, and actual diving all appear to increase both the resting volume of the spleen and the magnitude of its contraction during apnea. The relationship seems to be dose-dependent: the more intense or frequent the breath-holding stimulus, the more the spleen adapts.³

For the Bajau, this means genetics and training are likely working together. A Bajau person inherits a larger-than-average spleen, and then a lifetime of diving may make that already-large spleen even more responsive. Separating the two effects is difficult. The fact that non-diving Bajau still have enlarged spleens tells us genetics account for a significant portion of the difference, but the daily diving practice almost certainly amplifies the advantage further.

Another trainable change involves how the brain responds to rising carbon dioxide levels. When you hold your breath, carbon dioxide builds up in your blood. For most people, this triggers an urgent, uncomfortable need to breathe. Studies of elite breath-hold divers have found that their ventilatory response to rising CO2 is blunted compared to non-divers, meaning they can tolerate higher levels of carbon dioxide before their body forces them to gasp.⁶ This tolerance appears to develop through training rather than being purely innate, since studies comparing trained divers with non-divers have produced somewhat mixed results on CO2 sensitivity but consistently show that divers handle hypoxia differently.⁷ For Bajau divers who have been holding their breath since childhood, this trained CO2 tolerance likely compounds the genetic head start.

How Bajau Diving Differs from Competitive Freediving

It is tempting to compare Bajau divers with competitive freedivers, the athletes who chase static apnea records of over 11 minutes or plunge to extreme depths on a single breath. But the two groups operate very differently. Competitive freedivers typically perform one maximal effort followed by a long recovery. They use specialized preparation routines, including a technique called glossopharyngeal insufflation, commonly known as lung packing. This involves using the tongue and throat muscles to force extra air into already-full lungs, increasing the total oxygen available for a single dive.

Lung packing comes with serious trade-offs. Research on elite freedivers has shown that the maneuver dramatically drops blood pressure and causes acute dysfunction of both sides of the heart. In one study, systolic blood pressure fell from a mean of about 112 mmHg to roughly 52 mmHg during lung packing, and the left ventricle’s pumping efficiency was cut nearly in half.⁸ The pressure overload on the right ventricle is consistent with the extreme lung inflation compressing the heart. Fainting and loss of consciousness in the first seconds of a breath-hold after lung packing have also been documented.⁹

The Bajau, by contrast, are not trying to set single-effort records. Their diving is occupational: dozens of shorter dives in a day, with brief surface intervals, focused on gathering food rather than maximizing any single metric. This repeated-dive pattern places different demands on the body and introduces different risks. Where the competitive freediver’s danger comes primarily from a single extreme effort, the Bajau diver’s risk profile is shaped by the cumulative stress of many dives over many hours.

Neurological Risks of Repeated Diving

One serious health consequence associated with intensive breath-hold diving is a condition called Taravana syndrome, a term originally coined to describe neurological problems observed in Polynesian pearl divers. Taravana is characterized by stroke-like symptoms, including confusion, visual disturbances, motor problems, and cognitive impairment, appearing after deep or repeated dives. The condition’s exact mechanism is still debated, but the most widely accepted theory is that it represents a form of decompression sickness specific to breath-hold divers.¹⁰

Decompression sickness is usually associated with scuba diving, where a diver breathes compressed air at depth and nitrogen dissolves into the blood. Breath-hold divers do not breathe at depth, so the amount of nitrogen absorbed per dive is much smaller. But when a diver performs many deep dives in quick succession, the small amounts of nitrogen absorbed on each dive may not fully clear during the short surface intervals. Over the course of a long diving session, nitrogen can accumulate to problematic levels, potentially forming microbubbles that damage blood vessels in the brain. For the Bajau, whose traditional work pattern involves exactly this kind of repetitive deep diving, Taravana represents a genuine occupational hazard that no amount of genetic adaptation can fully prevent.

Sixteen Thousand Years in the Making

How long have the Bajau been evolving these traits? The 2018 Cell study modeled the population history of the Bajau and estimated that they diverged from their closest non-diving neighbors roughly 16,000 years ago.² That is a long time by cultural standards but relatively short by evolutionary ones, and it suggests that the selection pressures on diving-related genes were strong. Communities that could stay underwater longer caught more food, and the advantage of even a modestly larger spleen or a slightly more efficient diving reflex would have compounded over hundreds of generations.

The Bajau are not the only human population with a deep history of diving. Korean and Japanese women divers, known as haenyeo and ama respectively, have practiced breath-hold diving for centuries, though their diving traditions are thought to be younger and their dives generally shallower. No comparable genetic study has found the same spleen enlargement or PDE10A selection in those groups, which raises interesting questions about whether similar pressures in different populations might produce different solutions, or whether the Bajau’s adaptation is partly a product of their unusually long timeline.

What the Bajau Tell Us About Human Adaptability

The Bajau case is one of the clearest examples of recent natural selection acting on a specific human lifestyle. Most documented examples of recent human evolution involve dietary adaptation, like lactose tolerance in pastoral populations, or disease resistance, like sickle cell trait in malaria-endemic regions. The Bajau adaptation is different because it is driven by an occupational behavior rather than a dietary or pathogenic pressure. People who could dive longer and deeper had better access to food, and over thousands of years, that advantage reshaped their physiology at the genetic level.

The spleen finding has also sparked interest in medical research. If a genetic variant can increase spleen size and improve oxygen delivery during hypoxia, understanding that pathway could eventually have implications for treating conditions involving acute oxygen deprivation, such as stroke or cardiac arrest. Researchers studying the PDE10A-thyroid-spleen axis are interested in whether this mechanism could be leveraged pharmacologically. That work is still in its early stages, but the Bajau have given scientists a natural experiment that would be impossible to design in a lab: a population of humans whose bodies have been quietly optimizing for underwater performance for roughly 800 generations.

The trainability of the spleen contraction response is also drawing attention from sports science. If repeated apnea training can increase both resting spleen volume and the magnitude of its contraction, that has potential applications beyond diving. Any endurance sport where oxygen delivery is a limiting factor could theoretically benefit from protocols designed to enhance the spleen response. Early research in this area is preliminary, and the effect sizes in non-Bajau populations are modest compared to what the Bajau achieve with their genetic head start. But the principle that the spleen is not just a passive organ but an active, trainable contributor to oxygen transport is a relatively recent shift in how exercise physiologists think about human performance.