Introduction

Diving to depth is a typical behaviour observed in many aquatic mammals such as; whales, otters, seals, walrus and many others. Such mammals are well adapted at diving. The Weddell seal for example, is able to dive for periods of 80 minutes and more. Compare this to the average 1-3 minutes in untrained humans and you can see that diving mammals have developed great abilities for diving to depths.

The two major problems associated with diving are; limited oxygen stores (limiting the length of the dive) and hydrostatic pressure (limiting the depth of the dive). Oxygen is an important factor during diving, an absence of oxygen (anoxia) means anaerobic respiration becomes the only source of energy. Anaerobic respiration consists only of glycolysis, which produces around 5% of the energy (ATP) produced during aerobic respiration. Such a great deficit in ATP production can damage brain cells and have fatal consequences. In anaerobic respiration, lactate is produced as a waste product. Normally (in aerobic conditions) the lactate is metabolised by the presence of oxygen, in its absence however, the lactate builds up and dissociates into lactic acid. This causes fatigue, which decreases swimming ability.

The dangers of hydrostatic pressure come include; physical damage as a result of the pressure (barotrauma). Especially gas filled spaces of the body such as the lungs and ears. As well as decompression sickness (referred to as ‘the bends’ in humans), caused by rapid changes in pressure.

Oxygen Stores

The main stores or oxygen in mammals are; the lungs, blood and muscle. Oxygen is also present dissolved in body tissue. In adept diving mammals such as the Weddell seal the primary store of oxygen is the blood (in haemoglobin), followed by the muscle (in myoglobin) and then the lungs. The Weddell seal actually exhales before performing a dive.

Having a large store of oxygen in the lungs causes some problems. A large lung volume can make the dive more energetically costly due to the extra energy spent negating the effect of increased buoyancy. Exhaling pre-dive decreases energy required to dive. Another factor to consider is that, being a gas filled space, the lungs are susceptible to hydrostatic pressure. At large enough depths, the pressure can cause the lungs to collapse. So, a large lung volume is not necessary for accomplished diving mammals and some will exhale before diving to reduce the lung volume further.

The blood has an excellent oxygen carrying capacity, oxygen binds with haemoglobin in the blood and this essentially dissolves the gas, thus reducing the volume and associated problems. Adept divers will have a greater volume of blood (and haemoglobin) and therefore are able to store more oxygen. Humans are typically able to store 15ml of oxygen per Kg of body weight, whereas accomplished divers can store anything between 40-70+ ml Kg-1.

Myoglobin is similar to haemoglobin, it is found in the muscles and transports oxygen, but it has a much higher affinity for oxygen. Large concentrations of myoglobin mean diving mammals are able to dive for much longer, therefore high levels of myoglobin are associated with accomplished divers.

No matter how great the ability to store oxygen, there will always be a time when oxygen stores are exhausted. This is known as the aerobic diving limit. It is the length of time an organism is able to dive and respire aerobically from stored oxygen. Dives longer than the aerobic diving limit are possible, but are often rare even in capable diving mammals. Taking the Weddell seal as an example, 80% of dives occur within the aerobic diving limit (about 18-20 minutes) but some still dive for periods of 80 minutes or more. This shows they have developed systems to help combat the buildup of lactate and the lack of oxygen which occurs during anaerobic dives of 20 minutes or more.

Extending Dives beyond the Aerobic Diving Limit

The body is still able to produce energy (ATP) without oxygen via glycolysis during anaerobic respiration but this is at an effective efficiency of 5%. Therefore glucose and glycogen stores (the body’s fuel) would be depleted at 20x the normal rate if glycolysis increased to match aerobic ATP production. Such an increase is known as the ‘Pasteur Effect’, but if the diving mammal is able to decrease metabolism, this might not be necessary.

Diving mammals are able to undergo metabolic depression, which reduces the energy demand of certain body systems by reducing their output. The only problem is that the mammalian brain is very energy hungry and requires a lot of ATP to remain functional.

The ‘Diving Reflex’ is observed when diving past the aerobic diving limit. This reflex includes the slowing of the heart rate (bradycardia) and regional vasoconstriction to produce metabolic depression.

The Diving Reflex

The bradycardia seen in the diving reflex, reduces the heart rate from around 60-70 to 5-10 beats per minute. This is combined with regional vasoconstriction to increase the length of dives. The regional vasoconstriction is seen across the body except the brain as it would be unable to survive any reduction in blood flow. The amount by which blood flow is reduced across the rest of the body varies from 80-95%. This essential sets up a near closed blood flow from the lungs, to the heart, to the brain. Blood flow from the heart to the rest of the body is minimal.

Bradycardia prevents a change in stroke volume, blood pressure and blood flow, which if altered may damage the body. So even though the dimensions of the cardiovascular system have been reduced, the blood flows around the ‘closed circuit’ of the heart, lungs and brain as it would flow round the complete cardiovascular system under aerobic conditions.

The ‘closed circuit’ is able to remain aerobic from oxygen stores in the blood, muscles and lungs. The rest of the body remains aerobic only for as long as the oxygen bound to the myoglobin of the muscles lasts, barely any oxygen is received from the blood or lungs due to the extreme vasoconstriction. This means the rest of the body becomes anaerobic much faster than the ‘closed circuit’, the anaerobic respiration results in production of lactate.

Because of this, high levels of lactate are observed in the muscles, whereas levels are low in heart, lung and brain. Lactate produced by anaerobic respiration of the rest of the body is stored in the muscles until resurfacing. Lactate remains in the muscles due to the regional vasoconstriction, which essentially has disconnected them from the blood flow.

Upon resurfacing, regional vasoconstriction and bradycardia cease. This results in a ‘Wash out’ of lactate. All the lactate stored in the muscles during the dive is released into the bloodstream, thus increasing blood lactate levels. The lactate is metabolised however, in the presence of oxygen. The lactate produced during the dive must be metabolised before another anaerobic dive may occur. Therefore the longer the dive, the longer it takes before another dive may occur. The period of time between dives spent metabolising lactate, is known as the recovery period. Because of this, long dives are not desirable (unless necessary, such as escaping a predator) as it means less time can be spent beneath the water.

Energy Saving Behaviour

To further preserve energy, diving mammals will perform diving locomotion which reduces muscle energy requirements as much as possible. Diving mammals will use a combination of actively stroking for propulsion and gliding to preserve energy. Typically the descent of a dive requires stroking (combined with brief glides) to reach large depths due to buoyancy of the animal. The ascent however consists almost completely of long gliding behaviour in an attempt to expend as little energy as possible. Stroking it is not needed due to buoyancy, which causes the animal to rise to the surface.

Another energy saving behaviour is the induction of voluntary hypothermia. Whilst diving, mammals actively seek cooler waters, as a decrease in temperature causes a decrease in metabolic rate. If the animal is able to decrease their metabolic rate, the length of the dive can be extended.

Hydrostatic Pressure

Pressure can be measured in terms of atmospheres. At sea level (0m), pressure is the equivalent of 1 atmosphere or 1atm.Underwater, the pressure increases by 1atm per 10 meters, therefore at 10 meters below sea level, the pressure is 2atms. At 100 meters below sea level, the pressure rises to 11atms.

Boyle’s law states that pressure multiplied by volume equals a constant value (at a constant temperature) or PV= Constant. This means, if pressure increases, volume decreases (and vice versa). This is important because the body contains many gas filled spaces such as the lungs. Gas is easy to compress, and so as pressure increases (i.e. As we travel deeper into the ocean) the volume of these gas filled spaces within the body decrease in volume.

In order to understand the changes of the body with pressure, there are two other important pressure laws, these are:

Dalton’s Law – The total pressure of a gas constitutes all of the partial pressures within the gas. For example, the total pressure of air at sea level (or Ptot) = 1atm. This means all partial pressures of gases within air must total to 1atm. PN2 (or the partial pressure of nitrogen) = 0.78atm and PO2 = 0.21atm therefore the partial pressures of all the other gases in air must equal 0.01atm.

Henry’s Law - The solubility of a gas in a liquid is proportional to the partial pressure of the gas (above the liquid). This means that if the PN2 in the lungs increases, the amount of nitrogen which dissolves into the blood also increases.

Problems Associated with Hydrostatic Pressure

The main concerns of hydrostatic pressure are; the toxicity of the gases, decompression sickness and barotrauma. The potential of damage being caused by these factors increases as hydrostatic pressure increases.

At certain depths, nitrogen and oxygen can become toxic. At around 20m depth nitrogen can begin to have affect consciousness – altering perception. At around 50m oxygen can cause damage to the central nervous system, lungs and eyes.

Barotrauma is physical damage as a result of pressure. Gas filled spaces are prone to such damage. Major targets of barotrauma are the thorax and the trachea, if the trachea is not strengthened at great depths, the high pressure can cause the trachea to collapse. This is similar with the thorax, as pressure increases, the size of the lungs decreases. At a depth of 40m the lungs are 1/5 of their size at sea level. Such a large decrease in size could cause damage to thoracic muscles. By having a strengthened trachea, elastic diaphragm and strong sternum, it is possible to prevent such damage.

Decompression Sickness

Decompression sickness occurs due to resurfacing too quickly, this is a result of how pressure of nitrogen changes within the body at depth:

Pre-dive

PN2 (0.8atm) is equilibrated between the lungs blood and tissues.

Dive to 30 meters (Compression)

Ptot has increased to 4atms. Due to Dalton’s law, PN2=3.2atms. Due to Boyle’s law, the volume of the lungs has decreased.

PN2 in the lungs quickly reaches 3.2atms, due to their compression. PN2 in the blood slowly equilibrates followed by the tissues. Eventually lungs, blood and tissues have all equilibrated to PN2=3.2atms.

Resurfacing (Decompression)

Resurfacing too quickly means PN2 in the blood and tissues remains at a value near 3.2atms as there was not a chance to equilibrate with the lungs. At the surface, PN2=0.8atms in the lungs, whilst blood and tissues PN2 still equals 3.2atms. This results in a rapid release in pressure resulting in the formation of bubbles in the blood. The bubbles consist of nitrogen at a partial pressure of 0.8atms and so it is energetically favourable for any remaining dissolved nitrogen to diffuse into these bubbles. Because of this, the bubbles can quickly grow in size.

Decompression sickness arises due to any rapid change in pressure (not just changes in pressure when diving). It is also known as either ‘Caisson’s Disease’ or the ‘Bends’. The only treatment available is to contain the sufferer in a hyperbaric chamber – a chamber where pressure can be controlled and allowed to rise steadily, preventing further formation of nitrogenous bubbles in the blood.

Decompression sickness can arise due to the rapid resurfacing from large depths underwater, this results in a large gradient of partial pressures between the blood, lungs and tissues. The offloading of nitrogen from the blood into the lungs is a relatively slow process and therefore takes time. By resurfacing too fast, the pressure is quickly released causing gases (nitrogen) to go out of solution. This results in the formation of nitrogen bubbles in the blood. The bubbles can join together to become larger and more dangerous, whilst nitrogen is able to diffuse in, further increasing the size. It is energetically favourable for nitrogen to diffuse into the bubbles as they are at normal atmospheric pressure. If decompression sickness reaches the spinal cord, it can result in paralysis.

Avoiding Decompression Sickness

There are basically two ways to combat decompression sickness; limit the load of nitrogen into the body and prevent its distribution. To limit the load of nitrogen, diving mammals have evolved specialised alveoli.

An alveolus consists of a sac-like, bulbous area (where gas exchange takes place) and a terminal bronchiole (a tube connecting to the alveolus where no gas exchange takes place). In terrestrial mammals, pressure (such as that experienced whilst diving) causes the bronchiole to collapse, trapping nitrogen within the bulbous end of the alveoli. With nowhere to go, the nitrogen moves out into the tissues and blood (increasing nitrogen load and the possibility of decompression sickness).

The bronchioles of diving mammals on the other hand consist of strengthened cartilage, preventing them from collapse. Under pressure it is the alveoli which compresses first. Nitrogen in the alveolus moves out into the bronchiole. As no gas exchange occurs in the bronchiole, nitrogen is not able to move into the surrounding blood and tissues. Nitrogen load is therefore decreased as is the possibility of suffering from decompression sickness upon resurfacing. Such alveolar collapse occurs at a depth of 30m in Weddell seals, meaning the partial pressure of nitrogen in the tissues does not increase further than 3.2atms.

The other method of avoiding decompression sickness is to limit the distribution of nitrogen. This is done by peripheral vasoconstriction. As with oxygen limited dives that extend past the aerobic diving limit, regional or peripheral vasoconstriction result in decreased blood flow to the body. Blood flow is only preserved to the lungs, heart and brain. The benefit of this is that fatty tissue (i.e. the areas to where blood flow is restricted) has high nitrogen solubility, so reducing the blood flow to these areas also reduces the amount of nitrogen able to dissolve into the tissue. The brain on the other hand consists mainly of watery tissue which has much lower nitrogen solubility; it also offloads nitrogen much faster and therefore has a reduced risk of bubble growth.

References: Voelkel, S (2010), “Diving Mammals”, BIOL 445, University of Liverpool, Unpublished.

About The Author

VetSci

One Response to Strategies of Diving Mammals

  1. Believe it or not, the answer to the mystery of whales beach themselves can stated in only one

    word:  BAROSINUSITIS

    Barosinusitis in diving sea mammals is a pressure-related injury in the sinuses and air sacs

    located inside their heads. 

    It is well-known that rapid and excessive changes in the surrounding water pressure can cause

    physical trauma in all diving mammals, including man. 

    Severe oscillations in pressure are common above the epicenter of certain shallow-focused

    undersea earthquakes, especially those located in the rift valley of mid-ocean ridges. Scientists

    called these seismoacoustic waves seaquakes until the 1950s when the name was changed to

    T-Phase Waves. 

    But not all earthquakes generate whale-dangerous T-Phase Waves (aka: seaquakes). Only

    events that occur in specific places and in a specific manner generate T-Phases that are

    dangerous to diving whales and dolphins.

    Navy sonar, oil industry airguns, and underwater explosives also induce rapid and excessive

    changes in pressures surrounding the diving whales and dolphins.  These man-made devices

    cause the exact same barosinusitis injury as caused by seismoacoustic waves generated by

    undersea earthquakes.

    In toothed whales, the sinuses and air sacs serve as acoustic mirrors reflecting sound inside their

    heads in such a fashion to enable their echo-navigation system to function properly.  An injury in

    this critical part of their biosonar system naturally disrupts echo-navigation, causing the animals

    to lose their normally excellent sense of direction.  It also prevents them from diving and feeding

    themselves.

    Lost at sea, the flow of the surface currents direct the injured whales/dolphins downstream from

    the point of injury.  This control over the swim path of the injured sea mammals happens

    because water is 800 times denser than air.  The increased density induces a powerful drag

    (resistance) to swimming in any direction except downstream with the flow.  Thus, surface

    currents quickly point lost whales and dolphins headfirst into the path of least resistance or least

    drag.

    The whales/dolphins will recover from a slight barotraumatic injury within a week or so.  On the

    other hand, surface currents are likely to deposit those that do not recover on a sandy beach

    because current just happens to be the same energy that carries each grain of sand to build the

    beach in the first place. In general, whales/dolphins are directed to beaches that are building

    sand; not to beaches that are eroding.

    Hungry sharks sense the whales/dolphins are in trouble. They move in close and wait for an

    opportunity to snatch any weakened pod member that falls behind.

    Unable to navigate or dive and terrified by the pack of starving predators trailing them, the

    wounded whales/dolphins huddle together in a tight group for protection against sharks and

    killer whales. They swim downstream with the flow of the surface currents. The idea that

    individuals will follow a pod leader to the beach, or be drawn in by the distress calls of a

    beached member out of some sort of strong social bond is an over-glamorized false concept.

    Rather, individuals are highly-stressed and have no idea which way to swim to safety.  They will

    follow any whale that ventures out in hopes that this individual knows the way to open water. 

    They abide by a herd instinct, remaining close to their pod mates because they are in dreadful

    fear of becoming the next shark attack victim if they swim away from the herd.  It appears as if

    the Pod has close social ties but in fact the action of each individual is focused on self

    preservation.

    Said differently, it is the whale with the least fear that appears to human observers as a pod

    leader when in fact this individual is just as lost and confused as the rest of the pod.

    Landmasses that extend out to sea opposing the flow of the surface currents, serve to trap sand,

    flotsam, seaweed, and lost sea mammals swimming with the flow.  Cape Cod is the best

    example of such a natural trap in the United States.  Cape Sorrel in Tasmania and Golden Bay

    in New Zealand are also natural traps for non-navigating whales/dolphins.

    The reason for the increased strandings in Cape Cod during the 2011/2012 stranding season

    was the drastic increase in oil survey activity off the coast of Canada and West Greenland.

    Normally the dolphins swim away from the oil survey boats long before they are injured;

    however, the rocky coast of northeast Canada and western Greenland consist mainly of one

    small cove after the next.  The dolphins dart into these coves to avoid the loud booms.  The

    survey vessel blocks their escape route when it tows the airguns along the openings of the

    coves.

    The survey crews could prevent these deaths if they would simply reduce the volume of air

    supplied to the airguns as they cross the mouth of the coves.

    I do not advocate the halt of oil exploration; however, I only ask that the oil industry and the US

    Navy stop denying the existence of barosinusitis and start listening to ways to prevent it.  We

    can solve the problem by owning up to the truth and using simple precautions in the operations

    of oil survey vessels and sonar ships.

    The reason the Navy and the oil industry will not admit barotrauma involves the numerous “best

    available scientific information” clauses in the Marine Mammal Protection Act of 1972.  This

    law mandates that our governments protect marine mammals to the limits of the “best available

    scientific information.”

    However, the control over developing “best available scientific information” is now solely in the

    hands of the US Navy and the oil industry since these two organizations fund 98% of all marina

    mammal research worldwide.

    It’s like putting the tobacco industry in charge of lung cancer research.

    The Navy and oil industry are not going to fund a study into barosinusitis since they are afraid

    they will shoot themselves in the foot.  Instead, they fund research that covers up barotrauma in

    whales/dolphins.

    As long as they can muddy the waters on the “best available scientific information,” they can

    skirt around the laws and do as they please.  On the other hand, if they would fund research on

    preventing barotrauma and barosinusitis in marine mammals, simple procedures could be put in

    place that would allow the oil industry to extract offshore oil and the Navy to practice using

    sonar and still save the lives of the thousands of whales and dolphins that are killed every month.

    Capt. David Williams, Chairman
    Deafwhale Society, Inc.
    http://www.deafwhale.com
    (a 501-c non-profit whale research corporation devoted solely
    to understanding why marine mammals beach themselves.)