(I am republishing a lot of my ‘legacy content’ from our PLOS Neuroanthropology weblog, which has been taken down, along with many of the other founding PLOS Blogs. Some of these, I am putting up because I teach with them. If you have any requests, don’t hesitate to email me at: greg (dot) downey @ mq (dot) edu (dot) au. I suspect many of the links in this piece will be broken, but I will endeavour to try to slowly rebuild this content.)

At the beginning of the film clip, Bajau fisherman Sulbin sits on the side of a boat on the coast of Borneo, gulping air, handling his speargun.  And then, he drops into the water.  The footage suddenly changes and becomes arresting: silent, dreamy, slow, and so blue.  Sulbin strokes deliberately and descends until he strides along the bottom of the ocean, holding his breath, and hunts for fish through handmade goggles. [I’ve had to get a new version of the video clip, 2019.]

Finally, after a couple of minutes, he spears a fish and heads for the surface.  The narrator tells us that Sulbin could stay down twice as long and dive deeper if necessary.  Most viewers, unfamiliar with free diving, exceptional if they can hold their breath longer than thirty seconds, are quite likely to be shaking their heads by the end of the clip, wondering at the ability of the human body to adapt to life in water.  Life as an amphibious human can appear so alien that it’s stranger than science fiction, but painfully beautiful to watch.

I stumbled across the video clip in part because of my academic interest in free diving [I have had to embed a new video clip. GD 2019]. Earlier this month, I was supposed to attend a free diving workshop in New Zealand with one of the sport’s world record holder, Will Trubridge (or see the story on the Times Online).  The workshop fell through at almost the same time I was diagnosed with multiple hernias, so my first free diving experience likely wouldn’t have worked out – I’m still hoping to do it as part of my ethnographic research on extraordinary human performance in the near future.

But the clip of Sulbin on the BBC series, Human Planet episode, Oceans, has inspired me to write a little bit about Homo aquaticus (kidding), adaptation, culture, and what this sort of remarkable human adaptation might imply for the idea of ‘human nature.’

Sulbin’s ability is remarkable, but like so many exceptional human skills, it relies not on innate difference from other individuals, but on the steady cultivation of peculiar changes in the body and in how it is experienced.  What I hope to suggest is that amphibious humans point to the most basic fact of human nature: that we seem particularly adept at finding ways to adapt ourselves – biologically, psychologically, behaviourally, technologically – to a host of niches that then rebound back upon us and shape how we develop.  We are a peculiar self-made species.

This piece is probably best seen as one in a series I’ve been crafting on how human adaptation to situations that we place ourselves in map out the envelope of our bodies’ malleability.  Human skills and adaptation show us how our brains and nervous systems can be trained to do amazing things.  Frequent readers will know that I think much of the discussion of ‘human nature,’ carried out by — to put it nicely — exceptionally sedentary theorists, severely underestimates what our bodies are capable of doing.

Too often, in discussions of human adaptation, we allow a flabby distinction between three basic types of adaptation: genetic, phenotypic (or physiological), and cultural (or technology).  What I’ve been playing with, and will return to at the end of this piece, is the inseparability of these, especially the last two: physiology and culture.  The Bajau fisherman Sulbin shows us how biology and culture are inseparable because what he does ends up shaping his body, but only because he grew up around people who knew how to manage becoming human in this distinctive amphibious way and because his adaptations play upon how his nervous system works, including some intriguing quirks.

If you’re mad keen to learn more about human adaptation and my ongoing obsession, you might check out samples of my work on human quadrupedalism (part two), barefoot running, barefoot climbing, and even overhand throwing (the piece is specifically on ‘throwing like a girl’).  I’ll be posting more in the months to come, so if you’re interested in what the human body can be made to do, pay us a return visit periodically.

‘Sea gypsies’ in Southeast Asia

Sulbin is a member of a number of groups who live wholly or partially as oceanic nomads or sea foragers in South-East Asia.  As the BBC website explains:

Few peoples have a deeper connection with the sea than the Bajau Laut of South-East Asia. Sometimes known as “sea gypsies”, they live in house boats or stilt houses built on top of coral reefs and when they do spend the occasional night on solid ground they often report feeling ‘landsick’.

Malaysia’s best Bajau free-divers can dive to depths of over 20 metres and stay there for several minutes on a single breath as they go in search of fish. And as if that weren’t enough, studies on some “sea gypsy” children from Thailand and Burma show that they have unusually good underwater-vision because their eyes have adapted to the liquid environment.

The Bajau Laut’s livelihood is traditionally totally dependent on the resources of the sea so spear-fishing is vitally important to them, but different cultures have very different ways of catching fish.  (BBC website).

I won’t go into the ethnographic material on the Bajau and other groups called ‘sea gypsies’ (such as the Moken, who live along the coasts of Thailand, Burma and increasingly into Malaysia).  If you’re interested, I’ve placed some links to more material on the Moken and other groups at the end of this article.  Some of the groups experienced a recent spike in interest when they apparently avoided serious casualties in the Boxing Day Tsunami of 2004 because they ‘saw signs in the waves’ of the trouble to come.

This post is really about adapting to diving, and that happens a lot more broadly than just in ‘sea gypsy’ populations.  Although SCUBA and other techniques are replacing breath-hold diving, traditional divers cultivated incredible abilities in the production of sponges in Greece, and pearls in places like Polynesia and the Persian Gulf.  The practice is still widespread among recreational divers, competitive divers, and even in some industries, such as among seafood harvesters in Japan and Korea, where an estimated 20,000 professional divers still worked with minimal equipment as late as the 1990s (see Park et al. 1990, cited in Ferretti 2001).

Learning to ‘hold’ your breath

Human ‘adaptation’ to water is both conscious and unconscious, as so many things about human behaviour.  Even the most basic adaptive reflexes have to be shaped and elaborated, although they can often be learned in implicit, indirect ways or found in basic form very early.  For example, one of the most reliable reactions to a startling new sensation is to gasp, a potentially deadly maladaptive approach to dealing with being dunked in the water.  Fortunately, when water hits the pharynx or larynx, glottal spasms clamp the throat shut with a glottal spasm in part of what is referred to as the ‘diving reflex’.

If we’re going to enjoy the whole underwater swimming experience, however, we’ve got to be taught to stop the airway voluntarily and close the glottis muscularly, or to exhale. It’s more pleasant than just plunging into the water and trusting that the ‘diving reflex’ will save you from winding up with a couple of lungs full of the stuff by triggering a glottal spasm.  In other words, the reflex has to be cultivated into a skill.

The online web resource eHow suggests, in How to Teach a Baby to Hold Breath Underwater, that you first condition an infant by essentially an associative learning process where dipping a washcloth in water is followed shortly by dripping water over his or her face.  The writer advocates following this up at a pool with a learned association between ‘1… 2… 3…’ and subsequently being splashed in the face, later substituted by short immersion.  The diving reflex can cue the early learning, but the goal is to build a more robust voluntary behaviour and to do it in a way that doesn’t so traumatize the kid that he or she never wants to go back to swimming lessons.

When I taught swimming lessons last century (it seems like that long ago), this sort of learned association was also the way that we taught infants, but we also, if we had a particularly difficult case of a gulper, tried to teach the infant to hold its breath by blowing into its face.  Either way, though, the early stages of teaching breath holding, at least in my experience, almost always involve a few rough first attempts, with coughing and crying almost inevitable.  With the slightly older kids, you could teach them to exhale as they went under (‘blow bubbles’), but some coughing and inhaled water, again, was likely inevitable at some point.

My point is that holding your breath is not something that humans do naturally, although breath control is actually really important for speaking and other human abilities.  Breath holding in water may be an easy reaction to instill, but it relies upon someone teaching you or some sort of training, building on top of a more primitive, widely held reflex. Since my father is a non-swimmer, I know firsthand that there’s nothing ‘natural’ about being able to survive in water.

In fact, the Alliance for Safe Children (an Asian NGO) points out that, in some parts of the world, drowning is one of the leading causes of child mortality, contributing approximately fifty deaths each dayin Bangladesh (in contrast, that’s Australia’s typical annual total).  According to the WHO, drowning is the third most common cause of unintentional injury death worldwide (WHO ‘Drowning,’ fact sheet #347).

The ‘breakpoint’: involuntary breathing

Your body wants you to breathe as the oxygen you took in with your last breath gradually converts to carbon dioxide.  A powerful involuntary mechanism overrides most intentional attempts by humans to hold their breath, as M. J. Parkes (2006) outlines, long before you are actually in any distress. Try to hold your breath until you pass out, and most people will simply not be able to manage it (I’m not actually recommending this experiment).

As we hold our breath, above or below the water, the body slowly converts the oxygen our bodies store in our lungs and blood into carbon dioxide.  Levels of oxygen decrease (to hypoxia), and carbon dioxide increases (to hypercapnia).  Left long enough, the lack of oxygen will eventually cause brain death from cerebral hypoxia and heart attack, but most of us get nowhere close to dangerous levels when we hold our breath. As Parkes writes (2006: 2-3):

although the simplest clue to the breakpoint mechanism should emerge from identifying any manoeuvre enabling breath-holding to unconsciousness, scientific reports of breath-holding to unconsciousness are rare and inconsistent, despite popular mythology. Schneider (1930) stated that ‘it is practically impossible for a man at sea level to voluntarily hold his breath until he becomes unconscious’, and subsequent scientific literature supports this in adults. [Anecdotal descriptions of losing consciousness describe subjects breath-holding at low barometric pressures, with low oxygen mixtures or with severe voluntary hyperventilation…]

In fact, Fitz-Clarke (ibid: 57) reports that ‘almost all extreme breath-hold divers have experienced loss of consciousness upon emersion in their career’ likely as a result of the carbon dioxide build-up, especially with depressurization on the ascent from deep free dives.  In addition, partial, temporary loss of motor control (called ‘sambas’ by the divers) were relatively common in the competition Fitz-Clarke observed.

(The practice of diving, especially the pursuit of records, is still dangerous, so much so that in 1991, the World Conference of Underwater Activities stopped recognizing absolute records for depth and started to more tightly restrict competitive free diving.  The drive to go deeper and deeper, using more and more elaborated assistance, was putting lives at risk.  In contrast, in the competition Fitz-Clarke observed, safety standards were very high, as they are in sanctioned free-diving events, with assistance divers in the water with competitors and all participants carefully observed.)

The ‘breakpoint’ mechanism, when you feel an almost overwhelming impulse to breath, turns out to be a convergence of a number of reflexes that are quite difficult to study. One key component, however, is chemoreceptors that detect the levels of gas in the blood, especially carbon dioxide surplus.  We know chemoreceptors play a role because boosting oxygen levels and decreasing absorbed carbon dioxide in the bloodstream — for example, by gulping air as Sulbin appears to be doing — can extend the amount of time that you hold your breath, although many physicians will caution against it.

But research into breath holding has found that there’s no fixed threshold, either of oxygen or carbon dioxide, that will lead to involuntary breathing, so researchers like Parkes (2006) have argued that chemorecption alone cannot explain the break point.  Neurological research has shown that the central nervous system begins to try to restart respiration, including through the diaphragm, before the impetus to breathe becomes almost irresistible to an untrained individual.

Parkes argues that the central respiratory rhythm, an autonomic rhythm like the cardiac rhythm, persists during a breath hold, even though voluntary breath holding suppresses it active expression.  This means that breath-holders are not so much stopping their breathing voluntarily as they are holding their chests open and resisting the respiratory rhythm. Parkes (2006: 8-9) points to a range of evidence which suggests that the respiratory rhythm intensifies during the breath hold, even causing more widespread respiratory-anticipating reactions like ‘trachial tugging’ in the lead up to the break point.

Similarly, in a competitive static apnea event, in which divers hold their breaths in stationary positions, Fitz-Clarke (2006: 59) found that most divers experienced involuntary contractions of the diaphragm several minutes into the event. Successful competitors were able to continue to hold their breath even though the nervous system sought to reinitiate breathing.

In other words, as the voice-over says with the video of Sulbin, exceptionally long breath holding requires that a person learn to resist powerful involuntary reflexes, especially spasms in the diaphragm as it attempts to contract in order to re-initiate breathing.  When you breath hold, you are not so much ‘running out of air’ as you are fighting powerful impulses to breath when you don’t really need the oxygen yet. Breath holding like Sulbin is doing is the active over-coming of automatic processes by conscious suppression; it’s ignoring pain and involuntary muscle actions because you know you don’t need to do what they’re screaming at you to do.

Without these impulses, how long could you hold your breath?  Many people may have heard that three to five minutes without oxygen will cause irreparable brain damage, so they might assume that about four to five minutes would be the maximum that a person could hold his or her breath.  They would be wrong.

Breath holding is less dangerous to the brain then cardiac arrest, because during a breath hold, the brain is still connected to oxygen stores in the lungs and the body quickly starts making adjustments to stretch this oxygen store last as long as possible (Fitz-Clarke 2006: 60).  In fact, in static apnea (stationary breath holding) competitive breath holders can last five to nine minutes as the body puts into place a range of survival responses and competitors learn to make the most of the oxygen they’ve got.

Making your last breath last

So how can you make your breath last as long as possible if you’re going to have to fight your body to do it.  Not all people have the same ‘breakpoint,’ of course, and even the same subjects can demonstrate a wide range of breath-hold times, increased by distraction and by repeated trials (Parkes 2006: 2).  But a range of techniques can stretch out your oxygen supply.

First, if you know what to do before entering the water, you can start out ahead. Although physicians warn not to hyperventilate or gulp air prior to diving, Fitz-Clarke (2006: 56-57) found in a study of a competitive free diving event that

almost all athletes employed “lung packing” in the water prior to submersion. This is an inspiratory technique for hyperinflating the lungs using the pharyngeal and glottic muscles in a repetitive manoeuvre resembling gulping or swallowing

‘Lung packing’ or hyper-inflating the lungs to some degree (not to dizziness) before diving is discouraged for a number of reasons, but it can put more air into the lungs to start with, boost the blood oxygen level in the blood slightly, and – most importantly and dangerously – suppress the level of carbon dioxide in the body.  With less carbon dioxide, you’re breathing reflex is going to be delayed, but this may also be the reason that free diving participants pass out with some frequency; they run low on oxygen before carbon dioxide levels get high enough to prompt breathing.

The simplest way to make your breath last is to do as little as possible and to stay calm. The record for stationary breath-holding is much longer than the record for any dynamic activity.  In addition, the calmer you can stay, the more you will suppress your heart rate, decreasing the speed at which your body runs through its oxygen supply.

If you’ve got to move, move slowly and as efficiently as possible, like Sulbin as he lazily swims toward the bottom.  The more you thrash and the harder you work, the faster you’ll convert your oxygen to carbon dioxide.  Free divers seek to improve their performance by finding the most hydrodynamic postures and eliminating every redundant movement. As Ferretti (2001: 256) writes: ‘As a consequence, the improvement in the dive record [depth] took place without significant changes in the duration of the dives, which remained steady at around 3.5 min.’

The good news is, your body is going to help you.  Thanks to evolution, you’ve been born with a mammalian dive reflex that might keep you alive, or help you to stay under water way longer than you might expect.

Dive reflexes, human and mammalian

The remarkable human ability to adapt to free-diving arises, in part, from the body triggering specific nervous system responses, including a reaction to deprivation from oxygen that can be seen in walruses and other aquatic mammals.  Megan Lane explains on the BBC website, quoting freediving instructor Emma Farrell, the author of One Breath, A Reflection on Freediving:

The mammalian dive reflex – seen in aquatic animals such as dolphins and otters, and in humans to a lesser extent – helps, says Farrell.

“It’s a series of automatic adjustments we make when submerged in cold water. It reduces the heart rate and metabolism to slow the rate you use oxygen.”

In fact, the human dive reflex seems to be triggered especially by apnea (the abrupt stop of oxygen intake) and by cold (see Speck and Bruce 1978).  You can make use of the dive reflex, if you ever need to relax, by temporarily putting your face into cold water; your heart rate should drop, which can be a really godsend if you have to do something that’s ramping up your excitability.

The dive response is generally said to be composed of three changes:

1. bradycardia, or a slowing of the overall heart rate;
2. peripheral vasoconstriction, or the shutting of capillaries on the body’s extremities; and
3. shunting of blood into the torso, especially the chest, which helps to resist increased pressure.

In most people, the drop in heart rate from the dive reflex is not as great as in other mammals; while most humans decrease by about 10-25%, some mammals can drop to as little as 10% of their normal heart rate (Speck and Bruce 1978).

The human dive reflex, however, can increase with training (Schagatay et al. 2000).  A trained diver’s heart rate can drop profoundly during a dive.  Some studies have found veteran divers with pulse rates as low as 20-24 beats per minute, especially at the deepest parts of their dives (see Ferretti 2001: 263).

Veteran breath-hold divers, such as pearl divers and marine harvesters, can even demonstrate a more profound cardiac adaptation to diving that can be found in some of the most dedicated competitive free divers, which is not found in other mammals.  Scholander and colleagues (Scholander et al. 1962: 189) found that Australian pearl divers demonstrated cardiac arrhythmias, perhaps a defense against asphyxia developed at birth they theorized. As these divers went deep, the interval between heartbeats sometimes became irregular; in one case, a diver at 30 m depth had one interval of 7.2 seconds.  Extrapolated, this would be the equivalent of a pulse of eight beats per minute!

Scholander and colleagues argue that this arrhythmia is phylogenetically ancient; fish taken from water exhibit similar cardiac responses. The dysrhythmic response, however, together with an increase in blood pressure, make the human dive reflex a bit dissimilar from the mammalian dive reflex found in species like seals and otters.

Vasoconstrition on the periphery of the body and the centralization of blood flow likely increases the efficiency of the body for the duration of the dive (see Ferretti 2001: 262-263).  The less the blood is carrying oxygen out to the peripheral muscles and skin, the more energy (and oxygen) is available to the central organs.

The peripheral body parts end up relying more on anaerobic energy production, leading to a build up of lactic acid.  So even if you’re swimming along dreamily looking to spear a fish, the muscles in your hands and outer extremities will start switching over to the metabolic process you would use during anaerobic exercise, leading to the possibility of cramps and the same kinds of pains you would feel with lactic acid buildup.

Accommodating the pressure

One of the dangers of diving is, of course, the increased pressure.  Every 10 m of depth raises the pressure on the body by one atmosphere, compressing gas to half its previous volume within the body while the body’s tissue largely remains the same volume.

Boyles’ Law holds that gas expands and contracts depending on the pressure, so the descent into high-pressure depths can compress gasses that might later expand dangerously if the body decompresses too quickly.  Since every 10 m halves the volume of a gas, a 30 m dive (to four atmospheres of pressure) would temporarily compress eight liters of air (a ballpark figure for good lung capacity) down to a single liter of volume.

The earliest research on breath-hold diving assumed that the maximum depth of any diving was determined by the residual volume of the lungs. Theorists assumed that, once the pressure crushed the volume of the air in the lungs below the minimum size of the lungs, the lungs would implode.  But as dive records grew deeper and deeper, especially past Bob Croft’s 73m record in 1968, researchers began to suspect that the body was alleviating the danger of the pressure to the thorax through some sort of adaptation.

As Ferretti (2001: 256) reviews, theorists realized that one way the body might respond to the pressure of the dive was to ‘blood shift’ or to shunt blood from the extremeties into the abdomen.  Imagine the arms and legs are like toothpaste tubes and, under pressure, the blood squeezes into the thorax, which helps to keep the chest from collapsing even though the remaining air in the lungs shrinks and shrinks following Doyle’s law.

Subsequent research has found that, in addition to increased blood volume in the chest cavity, the body also responds with an arching of the diaphragmatic dome upwards (so that the abdomen compresses more than the chest), an engorgement of pulmonary blood vessels (those in the lungs), and an increase in the diameter of the heart (Ferretti 2001: 256-257).  In some ways, all of these are not so much ‘adaptations’ as they are simply how the body responds mechanically and hydraulically to the increasing pressure. Fortunately for free divers, their lungs are not in the fingertips or toes, or the increasing pressure would be a much greater problem for anyone trying to dive.

Most people who watch Hollywood thriller also know vaguely about the dangers of ‘the bends,’ when depressurizing overly quick from very deep dives leads compressed gas to bubble up in the joints, causing severe pain.  ‘The bends’ are not so specifically a danger brought about by compression, but rather by the decompression as one ascends from a dive.

In fact, the complications are even more numerous, as decompression can lead to pulmonary barotrauma (burst lung) or tears in the lung tissue that can result in emphysema and air emboli, which can block arteries.  Overly rapid decompression can lead to air bubbles forming within the brain or spinal cord (causing paralysis or sensory impairment) and in other bodily systems.

Some Bajau and other ‘sea gypsies’ do die of decompression side effects; repeated dives to 10 to 20 m depths actually carry a high risk of decompression sickness, according to the Human Planet website. The danger is increased by rudimentary ‘diving equipment’ that allows divers to stay down longer at slightly deeper depths.  For example, compression divers in the Philippines encountered by the Human Planet team were using garden hoses hooked to air compressors to pump down air and extend their time to work underwater.

But even at shallow depths, the pressurization-depressurization of the body can be dangerous.  Already at 10 to 20 m, the compression of the air in the sinuses and then re-expansion can cause damage, especially if the diver can’t successfully equalize the pressure between sinus cavities and ears, for example.  It’s hard to even imagine how the body can withstand the compression and then depressurization on competitive free dives; the world record for a sled-assisted plunge is more than 200 m.

21 atmospheres of pressure on the air in the lungs and sinuses, and then all released on the way back up!

Physical change from pressure: adaptation?

The situation of how human bodies can ‘adapt’ to the pressure of free diving is a more complicated and ambivalent case of human adaptation then just the dive response, however.  ‘Adapting’ to depths can include multiple ways to deal with pressure change; for SCUBA divers, the solution is to develop guidelines for safe diving, use equipment that accurately measures one’s depth, and protocols for decompression, including charts that show safe ascension rates.

For people who forage underwater like Sulbin, ‘adapting’ can instead involve biological and health trade-offs, as Johnny Langenheim reports in The Guardian:

Since diving is an everyday activity, the Bajau deliberately rupture their eardrums at an early age. “You bleed from your ears and nose, and you have to spend a week lying down because of the dizziness,” says Imran Lahassan, of the community of Torosiaje in North Sulawesi, Indonesia. “After that you can dive without pain.” Unsurprisingly, most older Bajau are hard of hearing. When diving, they wear hand-carved wooden goggles with glass lenses, hunting with spear guns fashioned from boat timber, tyre rubber and scrap metal.

In his review of the literature on breath-hold diving, Ferretti (2001: 255) tells a story of a similar adaptation in a European diver:

A remarkable performance was accomplished in 1913 by a Greek fisherman ashore the island of Skarpanthos, in the Aegean Sea: this man was able to rescue the anchor of an Italian ship, which was grounded at a depth of 70 m, by means of three consecutive breath-hold dives with a 15-kg counterweight on his belt. The ship physician reported that this diver suffered of emphysema and had no eardrums (please refer to the medical report in Appendix 1). The fisherman understated his achievement and claimed that he was used diving to 110 m…  [I’ve put the whole medical report below if you’re as fascinated by this as me.]

I keep putting ‘adaptation’ in scare quotes because carrying around ruptured eardrums may not seem like some folks’ idea of ‘adaptation,’ which may imply a less ambivalent form of biological compensation.

The case of ruptured eardrums from diving also highlights the interlacing of phenotypic and cultural adaptation.  The Bajau undergo a phenotypic change (an ‘adaptation’) because they train themselves to go to these depths; without the social support and knowledge, they probably wouldn’t be diving so deep in the first place. Then, when the pain and scary symptoms set in, fellow Bajau like Imran Lahassan tell them how to deal with the damage; lie down, wait, you’ll be fine,… well, a bit hard of hearing.

Of course, ruptured eardrums are damage to the body, like emphysema, but they are also an adaptation, in the sense that they make the body better suited for diving.  From the outside, we may look at the trade-off and say, ‘that’s crazy,’ but we face our own health-related trade-offs as our bodies adapt to the artificial environments we create for ourselves.  A Greek fisherman capable of recovering an anchor 70 m below the surface might look at the trade-offs of sedentary life and say, ‘that’s crazy!  Look at the price their bodies pay for what they do!’

I’ve written about similar adaptations in a book chapter that’s forthcoming for next year, when capoeira practitioners encourage each other to forge on in training through pain until the body adapts, in the case I discuss in the chapter, to putting your head on the ground.  These sorts of biological changes highlight the inseparability of culture and biology, that our bodies are shaped by collective knowledge just as so much of the shared wisdom in practical communities is precisely about how the body functions, its limits, and what sort of adaptive trade-offs are even possible (as well as support structures to encourage and facilitate these changes).

Building a better diver’s body

Over time and repeated dives, the divers’ bodies adapt to diving.  Bavis and colleagues (2007), drawing on research on human adaptation to hypoxia (low oxygen levels, especially at altitude) and on animal models, suggest that the human respiratory system may have ‘plasticity’ at a number of different levels, from autonomic behavioural adaptations (breathing differently), to structural changes that affect lung volume, and even to biochemical shifts, such as changes to red blood cells.  As they put it, adaptation can occur through ‘modifications to the gas exchanger, respiratory pigments, respiratory muscles, and the neural control systems responsible for ventilating the gas exchanger’ (ibid.: 532).

For one thing, in veteran divers, the dive reflex becomes exaggerated: bradycardia increases so that heart rates become abnormally low, and the divers’ responses to hypercapnia (high carbon dioxide levels) become blunted.  Ferretti (2001: 259) reviews findings that trained free divers are able to absorb almost twice as much carbon dioxide into the blood before needing to breathe. According to Ferretti and Costa (2003: 208-209), similar ventilatory responses have been found in synchronized swimmers, underwater hockey players, submarine escape tower trainers, and Royal Navy divers.  Since the increase in carbon dioxide levels is the primary stimulant to breathe, the ability to tolerate higher levels of CO₂ in the blood (hypercapnia) would allow divers to avoid gasping in conditions that would be hard to resist for normal individuals:

the condition of hypercapnia that was maintained during most of the dive, which could even lead to a reversal of pulmonary carbon dioxide transfer, would compel the diver to resist the drive to breathe elicited by the stimulation of central and peripheral chemoreceptors. This opposition would be facilitated by the observed blunted ventilatory response to carbon dioxide. Carbon dioxide sensitivity could be a primary determinant of the breath-hold duration, at least in professional divers. It is noteworthy that also diving mammals, which are frequently exposed to high arterial PO2 and PCO2 values…, are characterised by blunted ventilatory responses to carbon dioxide compared with non-diving mammals of similar size. (Ferretti 2001: 260)

To my knowledge, no studies have been done on the ‘sea gypsies’ of cardio-vascular adaptations or ventilatory response to hypercapnia.  All research on human hypoxia adaptation of which I’m aware focuses on high altitude populations, where the pressure to adapt would be constant and thus — possibly — more pronounced than in breath-hold diving populations, who only dive when they are foraging or working.

But — and this is the caveat  — the actual accomplishments of breath-hold divers like Sulbin, Greek fishermen, pearl diving Polynesians, and the Ama of Korea are pretty startling, with the ability to stay under water and remain active for long periods of time, so the phenotypic adaptations involved may be quite dramatic.

Sometimes intermittent adaptive pressure, especially severe and gradually increasing pressure, can have greater effect than constant but less severe environmental effects. Remember, I’m talking about phenotypic adaptation, not genetic selection over multiple generations.

The BBC website, for example, mentions the possibility of the spleen contracting to squeeze out more haemoglobin, an effect seen in some research on the Ama:

During breath-holding, oxygen stores reduce and the body starts diverting blood from hands and feet to the vital organs.

Our bodies have a way to compensate. Underwater pressure constricts the spleen, squeezing out extra haemoglobin, the protein in red corpuscles that carry oxygen around the body.

“Not enough research has been done to know if it wears off when you’re not diving,” says Farrell. “But I know people who do a lot of deep training – as Sulbin does – whose blood is like that of people living at high altitude.”   In high altitudes, there is less oxygen and so the amount of haemoglobin in blood increases.

This increasing efficiency and trainability (Schagatay et al. 2000) is what makes me a little uncomfortable with referring to the changes as part of a ‘dive reflex,’ in part because I’m not entirely sure that the term ‘reflex’ is universally parsed in the same way.  Clearly, bradycardia and peripheral vasoconstriction are common responses; heck, you can even argue that vasoconstriction is not so much an adaptation as a direct consequence of the mechanical and hydraulic ways that pressure affects the body (in other words, it might be hard to call vasoconstriction an ‘adaptation’ given some definitions of the word – you could call it a ‘consequence’).

A fully blown human ‘dive reflex,’ with profound bradycardia, vasoconstriction, cardiac dysrhythmia, (possibly) splenic contraction, even eardrum rupture, because the complex really requires priming from multiple dives, is rather a phenotypic adaptation, with all the messy complexity that I have suggested is implied in the term.  That is, the dive adaptation builds upon innate reflexes, but it also requires cultural niche construction and social support and results in physiological change.

In other words, my problem with ‘reflex’ as a description of what happens to the body during an oxygen-depriving is not merely semantic.  I think it misrepresents the role of consciousness and experience in the physiological response to a very basic sensation: if you’re not ready and accustomed to hypoxia and pressure, immersion is not going to produce the fully blown ‘reflex,’ in part because your interpretation of the event (‘crap, I need to breathe’) may not allow you to grapple with your body’s involuntary reflexes (such as spasms in the diaphragm).  I daresay it’s quite possible that some conscious interpretations of what is happening (‘crap, I am DROWNING!’) may even completely unravel even the basic ‘dive reflex,’ for example causing the heart rate to spike in spite of the tendency toward bradycardia.

The commentator on the video clip says that Sulbin’s heart rate can drop to 30 beats per minute, and competitive free-divers can achieve even lower rates.  But they do so, in part, by seizing willful control of the autonomic system through proxy variables, specifically emotional states. They don’t just benefit from the ‘dive reflex’; they drive a consciousness-to-emotion-down-to-autonomic chain to get more out of the dive reflex than just the 10-25% reduction in heart rate that most of us would get (if we don’t panic, in which case we might not even get that).

Will Trubridge, for example, like many competitive free divers, uses meditative and mind-body techniques borrowed from yoga.  Sulbin uses his pre-dive routine, including a smoke to ‘relax his chest’ and air gulping.  That is, the ‘reflex’ can be elaborated into a well-schooled top-down technique for self-management that ends up exerting control over autonomic systems like the cardio-pulmonary system.

Loose notes

As I write this, I realize that I’ve got a couple more things to add about this video that I can’t really fit into my central argument, so I’m just going to put them here. Although I’m fascinated by diving and could go on, I just want to quickly highlight a couple of the ways that life and water can lead to human adaptation to a more amphibious existence:

1.  Buoyancy

One of the strangest things about the footage of Sulbin is his apparent negative buoyancy: he walks along the bottom and doesn’t just bob up to the surface.  When most people hold their breath, they are positively buoyant; the air inside the torso offsets the slightly greater-than-water density of bodily tissue so that most people will bob to the surface when they hole their breath.

As you hold your breath, the total air in the lungs does gradually decrease, so you become slightly less buoyant near the end of a breath-hold.  But for most people, the only way to really sink is to exhale a bit to get to negative buoyancy.

Being extremely lean can make the body sufficiently dense that a person is negatively buoyant, even when holding a full breath.  Being lean also helps you to stay under water longer because, if your lung volume remains constant, decreasing your overall weight means more oxygen for every kilo of oxygen-burning bodyweight.  So if you want to hold your breath a long time, it helps to diet.

2. Body temperature

Spending a lot of time in water can play havoc with the human body’s ability to maintain a constant temperature.  The thermal conductivity of water is twenty-five times greater than air (Reilly and Waterhouse 2005: 74), so being dipped in water below body temperature can quickly lead to hypothermia. Without insulated gear, however, Korean divers can spend hours in the water at 10 degrees Celsius in January, conditions under which hypothermia should have been likely.  Hong and Rahn (1967) found, however, that divers did not have thicker layers of subcutaneous fat.  In fact, during the coldest seasons, an elevation in their basal metabolic level—an incredibly rare seasonal variation in human metabolism—left them unable to eat enough to keep from slowly losing weight.  They turned up the internal furnace.

In addition, the divers’ vascular systems appeared to adapt, restricting heat loss from blood vessels near the skin by constricting them (ironically, to below the level of obese individuals).  Their skin became cool to the touch; if the furnace was up, they were also closing off less important rooms in the house.  In addition, the shiver response was suppressed because it speeds up the body’s radiation of heat.  Ferretti and Costa (2003: 208) note that adaptation to cold has also been found in Australian aborigines who sleep nearly naked in the cold; in trained Arctic scuba divers, even when they wore wetsuits (it’s still bloody cold); and in Canadian fishermen who repeatedly immerse their hands in water at 9-10 degrees.  The adoption of wetsuits by the Ama, however, has led to a decrease in their bodily adaptation to cold-water resistance.

3. Underwater vision

In research that’s far too interesting for me to just discuss in passing, Anna Gislén and collaborators (2003) found that children among the Moken, one of the indigenous groups called ‘sea gypsies,’ developed the ability to see nearly twice as well as European children under water.   Because water has a higher density than air, light passing from water into the eye does not refract as much, so our pupils do not redirect it sufficiently to focus the image on the retina (instead, the light converges beyond the back of the eye, where the accurate image cannot be perceived).  Moreover, since our pupils respond to the decreasing light that strikes the eye as we dive deeper by dilating, the lens flattens and exacerbates the distortion still further.

Gislén found that Moken children’s pupils responded in the opposite way when diving, constricting (which the eye would normally do in bright light) so that the lens caused the image to converge on the retina.  Gislén and a team was able to train European kids to do the same thing upon her return (Gislén et al. 2006).  Again, an ‘automatic’ system or reflex could be re-directed through training, whether or not the individuals involved were explicitly aware of what their nervous system was learning to do.

Like I said, way too interesting for a little note, but this will have to suffice for now…

Links:

• The BBC’s Human Planet page on ‘Oceans’
• A beautiful pilot of a documentary project that was posted on YouTube in 2010: Early Pilot for The Moken Kabang Documentary Film in pre-production, spring-2010.
• The Sea Gypsies of Surin Island: The Wake of the Tsunami Brings Both Help and Harm to Thailand’s Mogen People.  By Antonio Graceffo (February 2007)
• Rebecca Leung, Sea Gypsies Saw Signs In The Waves: How Moken People In Asia Saved Themselves From Deadly Tsunami, CBS News, 10 June 2007.
• Johnny Langenheim, The last of the sea nomads, The Guardian 18 September 2010.
• The Sea Gypsies of the Andaman Sea, Sonia Y. Dong, Environmental Graffiti
• Where the sea whistle echoes, video on the Ama, ‘the legendary women divers of Japan,’ from the United Nations University
• The AmaSan Japanese Diving Women, by Jolie Bookspan, M.Ed, PhD, FAWM on HealthLine.com  (there’s a whole series of articles on the Ama and Haenyeo of Korea linked on this site)
• Megan Lane, What freediving does to the body
• James Morgan Photography, provides beautiful photo essay on the Bajau Laut.  If I had more time and patience, I would write to him and beg to reproduce some of his images here.
• William Trubridge freedives THE ARCH at Blue Hole, Dahab, video on YouTube.
  ‘The first no fins, no suit freedive through the Arch, a 30m long tunnel connecting Dahab’s Blue Hole to the Red Sea at a depth of 55m.  William Trubridge is the world record holder in unassisted freediving. Website: www.verticalblue.net’
• AIDA — Association Internationale pour le Développement de l’Apnée – the international association for breath-hold diving.
• United States Apnea Association

Image:

Photo, ‘Freediving: Ewens Ponds’ by Saspotato (no real name given) on Flickr.com, Creative Commons license.

Saspotato also tells us, ‘This photo was taken on March 20, 2010 in Mount Gambier, South Australia, AU, using a Sony DSC-P93A.’

Photo of multiple divers, Robert Berman/HA’a, from Remember the Water

Appendix:

Ferretti (2001: 267-268) provides an English translation of the medical report done on the Greek fisherman who successfully dove 70 m for the lost anchor, and I feel I must reproduce it in its entirety:

Haggi Statti Giorgios, born in Simi, sponge diver, 35-year-old, married, four children, all alive and healthy. He is 1.70 m tall and weighs 65 kg. His resting thoracic perimeter is 0.92 m, being 0.98 m after a maximal inspiration, and 0.90 m after a maximal expiration. Dark-skinned, slim, he has an ordinary muscle mass. Although an examination of the thorax reveals a remarkable lung emphysema, the upper part of the thorax has not yet reached a large size, even if it is somewhat convex and rigid. The heart tones are far, but regular. The pulse rate is 80-90, and the respiratory rate is 20-22. Nothing abnormal in the nervous system, nor in the eyes. He has impaired auditory function because of the complete lack of the eardrum in one ear, and only the remnants of one in the other. He sfufered from no illness, except for a trachoma, healed after surgery. He reports only pain in his back, which he tolerates resignedly. When asked to hold his breath in the ordinary ambient, he first refused, claiming that the test had no value because he could resist much more under water. Then he accepted, and it resulted that his capacity under these conditions is only 40 s. Yet in the rescue operations he dived to depths varying from 40 to 60 m, and even to 80 m, staying under water for 1.30-3.35 min. He claims that he has reached 110 m, and that he can stay at 30 m for up to 7 min. Statti emerged from all dives in good shape and vigour, as demonstrated by the way he jumped into the boat and released the water that had entered his nose and ears. When questioned on the phenomena he feels during the dives, he says he perceives none. Probably accustomed since childhood, he does not perceive them. He only says he feels all pressure on his shoulders. Nothing on his eyes. He also claims that at 80 m, despite the weakening of light, one can see enough to work, if the water is clear.

On a similar note, one of the accounts of Sulbin on the BBC website reports that his abilities do not come from his clean living.  According to their website: “Anyone who thinks this is an example of what a non-smoker’s lungs can do will be disappointed,” says Hugh-Jones. “Sulbin smokes like a chimney. He says it relaxes his chest.”

Like I said: ‘adaptation’ is a pretty neutral word for what can be a far more complicated reality.

References:

Bavis, R., Powell, F., Bradford, A., Hsia, C., Peltonen, J., Soliz, J., Zeis, B., Fergusson, E., Fu, Z., Gassmann, M., Kim, C., Maurer, J., McGuire, M., Miller, B., O’Halloran, K., Paul, R., Reid, S., Rusko, H., Tikkanen, H., & Wilkinson, K. (2007). Respiratory plasticity in response to changes in oxygen supply and demand Integrative and Comparative Biology, 47 (4), 532-551 DOI: 10.1093/icb/icm070

Ferretti, G. (2001). Extreme human breath-hold diving European Journal of Applied Physiology, 84 (4), 254-271 DOI: 10.1007/s004210000377

Fitz-Clarke JR (2006). Adverse events in competitive breath-hold diving. Undersea & hyperbaric medicine : journal of the Undersea and Hyperbaric Medical Society, Inc, 33 (1), 55-62 PMID: 16602257

Ferretti G, & Costa M (2003). Diversity in and adaptation to breath-hold diving in humans. Comparative biochemistry and physiology. Part A, Molecular & integrative physiology, 136 (1), 205-13 PMID: 14527641

Gislén A, Dacke M, Kröger RH, Abrahamsson M, Nilsson DE, & Warrant EJ (2003). Superior underwater vision in a human population of sea gypsies. Current biology : CB, 13 (10), 833-6 PMID: 12747831

Gislén A, Warrant EJ, Dacke M, & Kröger RH (2006). Visual training improves underwater vision in children. Vision research, 46 (20), 3443-50 PMID: 16806388

Hong, Suk Ki, and Hermann Rahn.  1967.  “The Diving Women of Korea and Japan.”  Scientific American 216 (5): 34-43.

Park YS, Shiraki K, Hong SK (1990) Energetics of breath-hold diving in Korean and Japanese professional divers. In: Lin YC, Shida KK eds.  Man in the sea.  Best, San Pedro, Calif., pp 75-87.

Parkes, M. (2005). Breath-holding and its breakpoint Experimental Physiology, 91 (1), 1-15 DOI: 10.1113/expphysiol.2005.031625

Reilly, Thomas, and Jim Waterhouse.  2005.  Sport, Exercise and Environmental Physiology. Edinburgh: Elsevier.

Schagatay E, van Kampen M, Emanuelsson S, & Holm B (2000). Effects of physical and apnea training on apneic time and the diving response in humans. European journal of applied physiology, 82 (3), 161-9 PMID: 10929209

SCHOLANDER PF, HAMMEL HT, LEMESSURIER H, HEMMINGSEN E, & GAREY W (1962). Circulatory adjustment in pearl divers. Journal of applied physiology, 17, 184-90 PMID: 13909130

Speck, D. F. and D. S. Bruce.  (1978) Effects of varying thermal and apneic conditions on the human dive reflex.  Undersea Biomedical Research 5(1): 9-14.