The Order Cetacea is one of the most distinctive and highly specialized groups of mammals. Over the course of evolutionary history, cetaceans have become adapted to surviving and prospering in a vast aquatic environment. One of the most important obstacles they have overcome is the ability to be obligate ocean dwellers while still breathing oxygen. Cetaceans are able to remain underwater for extended periods of time, returning only to the surface to exhale. Diving beneath the surface is an essential component of life for these species since almost all of their food sources are found under the surface of the ocean.
One of the most-well known diving cetaceans is the sperm whale (Physeter macrocephalus). The world’s largest carnivore, this massive cetacean makes extremely deep dives for food, mainly squid and octopus, but also fish and sometimes giant squid (Burnie and Wilson 2005). Sperm whales can dive to depths of 400 to 1200 meters, and for durations of up to 138 minutes (Watwood et al. 2006). The majority of sperm whale dives have been reported to last from 33 to 53 minutes (2006). Sperm whales have to overcome several fundamental problems while diving at such great depths: the effects of pressure and the need to actively forage while holding their breath. Adaptations to pressure have to deal with the mechanical effects of pressure and the increased solubility of gas at depth (Costa 2007). Adaptations to breath-holding diving center around modifications in metabolism, blood flow, and an increased oxygen storage capacity (2007).
Effects of Pressure
As sperm whales dive, they must deal with the direct effect of pressure that is associated with the volume change of air-filled spaces in their bodies. Changes in air volume are most dramatic in the first 10 meters of the water column, but the direct effect of pressure on cellular processes is only an issue at relatively deep depths of 500 to 1,000 meters (Costa 2007). Pressure and volume are inversely related, so as the pressure increases the volume of the whale’s lungs will decrease. Lung capacities of marine mammals are larger than terrestrial mammals, and they must overcome the problems associated with lung collapse (Kooyman 1973). They have compliant chest walls that allow for complete lung collapse and possess specialized structures in their lungs that allow the alveoli to collapse first, followed by terminal airways (Costa 2007). The water pressure compresses the lungs, which forces air into the trachea and nasal passages where some of the air is absorbed by foamy secretions along the respiratory tract wall (Burnie and Wilson 2005). When the whale resurfaces, hyperventilation occurs so the frequency of individual breaths increase in order to refill the lungs with oxygen (Butler and Jones 1997).
At high partial pressures, both nitrogen and oxygen become toxic to marine mammals. At higher pressures, the body tissues absorb greater amounts of these gasses, which increases gas...