Aerobic Capacities of Freediving Mammals
Posted By Erik Seedhouse on 31 March 2003
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]
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