Deep Diving

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What allows a sperm whale to dive to a depth of 3000m and stay submerged for more than two hours while a killer whale can only reach depths of 250m and stays submerged for 15 minutes? (ACS 2005) A combination of anatomical advantages and behavioral modification allow some cetaceans to dive deeper and longer than others. We already learned how increased volumes of hemoglobin and red blood cells enable some porpoises to be more active divers. Shaffer’s et. al work looks at the behavioral modifications and dive patterns surrounding deep diving in the white whale (Delphinapterus leucas). These cetaceans are relatively small but can still reach depths of nearly 650m and dive-durations of almost 20 minutes. In their study, Shaffer found the aerobic dive limit (ADL) of a white whale to be eight to 10 minutes. This value is defined as, “the diving duration beyond which blood lactate levels increase above resting levels” (Shaffer et. al. 1997). Those white whales whose dives exceeded the ADL were forced to increase their respiratory rate significantly once they resurfaced in order to pay the oxygen-debt incurred by the lactate. The respiratory rate increased by 6 times to 9.6 breaths per minute in one whale after a particularly long dive. These results allowed Shaffer to conclude that marine mammal dive patterns do not include a series of dives to maximum depth and duration but rather, “strategies that maximize underwater time by minimizing the number of dives that exceed the ADL” (Shaffer et. al. 1997).

White whales swimming at greater speeds breath fewer times per minute. As a result, they will have to raise their respiratory rate even more to account for their physical exertion and breath-holding. Figure adapted from Shaffer et. al. 1997.

Shaffer’s preceding research suggests how very taxing a dive to maximum depths may be on an animal’s body. During the final two minutes of a bottlenose porpoises’ maximum dive, the cardiovascular system essentially ceases function as an oxygen-transport mechanism. After a 300m dive its blood contained only 2% oxygen, meaning a porpoise can survive without oxygen for at least the one or two final minutes of a dive (Ridgway and Johnston 1969).

Bottlenose porpoises use less oxygen during deep dives than when holding their breath at the surface because at deep depths some of the oxygen becomes inaccessible when the lungs collapse. Oxygen content is lowest at 20 m depths because exertion is greater than at the surface and all oxygen is accessible. Figure adapted from Ridgway 1969.

While bottlenose porpoises, white whales and other cetaceans are clearly able to push the limits of their respiratory systems to survive without oxygen for extended periods, they must pay the price by greatly increasing their respiratory rate following the dive. The maximum dolphin dive of 10 minutes is far removed from their average respiratory rate of 3.9 breaths per minute (Williams et. al. 1999). Such a radical shift from equilibrium is only remedied by adopting a respiratory rate far above the average after the dive. By staying at the surface while they recover from a deep dive, these marine mammals must temporarily forfeit the resources and security of the ocean’s depths.