Diving mammals do not have larger lung volume to body size ratios than terrestrial mammals, yet they can dive for extended periods without breathing because they have evolved alternative mechanisms for increasing the amount of oxygen they carry. This article examines some of those mechanisms.
Unlike their terrestrial freediving cousins, seals, sea-lions and whales perform breath-hold dives for practical reasons such as feeding and escaping from predators. Like their land-based counterparts, these dives are accompanied by physiological changes that require certain adaptations.
The magnitude of adaptation is more pronounced than those observed in even the most elite human freedivers, this enhanced adaptive response providing a partial explanation for the depth and descent durations performed by these mammals. For example, the current ‘No Limits’ record of 163m is relatively shallow in comparison with depths achieved by bottlenose whales (Hyperoodon ampullatus). Using time-depth recorders and acoustic transponder tags these whales have been tracked down to 1450m. By way of comparison, the northern elephant seal (Mirounga angustirostris) has been tracked to depths of 1500m, although it should be noted these depths are not representative of normal diving behavior.
Perhaps the most effective physiological ‘equipment’ belongs to the New Zealand sea lion (Phocarctos hookeri), a mammal that performs more prolonged and continuous diving than any other species, typically diving to depths of 120m (maximum recorded depth 474m) and routinely staying submerged for five minutes. Although these depths and durations are exceeded by other aquatic mammals, it is the diving pattern that sets this sea lion apart since they perform almost continual dives. What is of interest to human freedivers is that nearly half the dives performed by this sea lion exceed the theoretical aerobic dive limit (ADL) [see inset below].
Calculating your aerobic dive limit
In theory, if a freediver starts a dive at total lung capacity (TLC), the maximal theoretical depth could be predicted by the ratio of TLC to residual lung volume (RLV). Based on these calculations it is possible to predict the maximum ‘theoretical depth’ or ‘breakpoint’ that may be achieved by Pipin Ferreras, a diver with a TLC of 9.6L and a RLV of 2.2L. Applying Boyles Law, the safe limit of compression for Pipin would be about 4.4 atm. (absolute pressure), corresponding to a depth of 34m. Fortunately, for the sport of freediving freedivers have little regard for the laws of physics, since Pipin has descended 128m below his maximum predicted depth! Clearly there are accessory mechanisms that permit freedivers — and freediving seals! – to exceed these laws.
For freedivers wishing to calculate their theoretical depth limit the following formula is presented. [For practical use only]
Estimating Residual Lung Volume from Age, Stature, and Body Mass
In freediving, RLV affects the depth a diver may achieve without experiencing problems associated with a ‘thoracic squeeze’. Normally, the free diver’s TLC: RLV ratio at the surface will determine the maximum diving depth before experiencing a thoracic squeeze. One method a freediver is able to calculate RLV value is by performing the following calculations.
RLV Prediction Equations
Variables: age (y); St, stature (cm); BM, body mass (kg).
RLV (L) = (0.022 x Age) + (0.0198 x St) – (0.015 x BM) – 1.54
Normal-weight females (only age and stature used)
RLV (L) = (0.007) x Age) + (0.0268 x St) – 3.42
The mechanisms by which these ‘divers’ resolve the conflict between the energetic demands of diving and conserving limited oxygen stores are similar to the problems faced by terrestrial freedivers and are not fully understood. However, our marine counterparts do have some physiological advantages at their disposal.
For example, the maximum dive time is not determined solely by the seal’s oxygen storage capacity because seals can function anaerobically. However, aerobic metabolism is favored over anaerobic because it is much more efficient. Lowering their metabolic rate allows seals to increase the amount of time they maintain aerobic respiration while diving because it allows the oxygen store to last longer. Also, by selectively perfusing tissues the seal is able to increase the duration of oxygen stores. The point at which a freediving seal — or ‘regular’ freediver – must either take in oxygen or switch to anaerobic respiration is the ADL. Levels of lactate in the blood will increase above resting levels once the ADL has been reached and will result in a burning sensation in the muscles.
So how do seals function anaerobically? Unlike human tissues, seal tissues have a high tolerance for the ‘asphyxial triad’: low oxygen, high carbon dioxide, and low pH. The low oxygen concentration is caused by oxygen consumption through aerobic respiration, the carbon dioxide is waste created by working muscles and the low pH results from lactic acid produced by anaerobic respiration. A high tolerance for this triad allows a seal to function anaerobically after oxygen supplies have been depleted.
Longer dives usually cause seals to exceed the ADL and use anaerobic respiration. Experimentally this is determined by blood sampling – an increase in blood lactate indicating the seal has used anaerobic respiration. Seals use different diving patterns in order to recover from any lactate that accumulates during an anaerobic dive. Weddell seals for example, will vary the length of the recovery period depending on the length of the dive. A series of long (about twenty minutes each) dives by these seals is followed by a sequence of short, aerobic dives that allow the accrued lactate in the blood to be gradually flushed out.
Energetic effectiveness is another strategy employed by seals, sea lions and whales in conserving oxygen. As might be expected, dive depth, and therefore distance traveled, affects the percentage of time available for gliding, the primary oxygen-conserving behavior employed by marine mammals. The percentage of time spent gliding during a descent increases significantly and nonlinearly with increasing dive depth and equates to a considerable energetic saving in terms of oxygen use.
Another mechanism available to seals is how it stores oxygen. Seals do not use their lungs to store oxygen. As seen in the graph below, when diving, there is significantly less oxygen in the lungs of a seal than the lungs of a human. The lungs cannot store air as a seal dives because of the increased risk of decompression sickness it would impose on the seal.
So how does a seal store the oxygen? The answer can be found in the blood and tissues.
A seal’s blood has a higher oxygen-carrying capacity than a human’s partly because a seal has a greater blood volume and partly because of a higher hematocrit (concentration of hemoglobin). Because there is more blood in a seal there are also more red blood cells (RBCs). The increased number of RBC’s increases the amount of hemoglobin, a blood pigment found in RBC’s that carries oxygen. However, a seal’s RBC is composed of less water than a terrestrial mammal’s, so even at the cellular lever, this mammal is designed to carry more oxygen — this accounts for the higher hematocrit. Of course the amount of RBC’s the blood can carry is limited since we know that if there are too many RBC’s the blood becomes too viscous for the heart to pump effectively. However, marine mammals overcome this be resorting to accessory methods for storing oxygen at their disposal.
One of these methods is myoglobin, a compound found in muscle tissue that binds oxygen. In fact, the myoglobin is so highly concentrated in seal muscle that when viewed under a microscope it appears almost black! Humans also have myoglobin but sadly — for freedivers – its storage capability is far inferior to a seals.
Myoglobin concentrations of marine mammals
Myoglobin (g 100 g-1)
Northern fur seal
Finally, marine mammals are able to store more oxygen in other tissues of the body than humans do, thus giving them the ability to store more oxygen, most notably, the spleen. The mechanism by which the spleen stores oxygen is similar to those found in humans and was explained in the previous artcicle ‘Spotlight on the Spleen’. However, the splenic capacity of marine mammals far exceeds that of a human.