In studying the breathing patterns of cetaceans we must consider both the anatomy of their respiratory system and their behavioral characteristics. Common amongst all cetaceans is the dorsal positioning of the nares. This blowhole, which leads through the trachea to the lungs, is ideally located so the animal can breathe in rhythm with its ordinary motion. The blowhole is surrounded by large muscles around the skull that are attached to a thick piece of flesh, the nasal plug. These muscles must be contracted to remove the plug and open the airway. When closed, the nasal plug shuts out water from the respiratory tract even under extreme pressure. The cetacean respiratory tract does not contain turbinate bones like other mammals, which enables them to forcefully exhale without risk of injury. Cetaceans can only breathe through their blowhole. Their oral cavity leads only to their digestive tract and their nares lead only to their respiratory tract. This specificity ensures that neither food nor water obstruct the respiratory tract (Marshall 2002).
Dorsal position of the nares can be seen on this whale skull. Photo provided by USFWS.
The cetacean breathing cycle consists of a powerful and rapid exhalation, an equally rapid inhalation, and a period of breath-holding, apnea. The duration of the inhalation and exhalation are fixed, lasting only fractions of a second for dolphins and porpoises and up to 1 or 2 seconds for large whales (Sumich 2002). The duration of apnea is dependent on the animal’s activity level and whether or not he is diving. Apnea may last several seconds in resting dolphins and porpoises or up to several hours in diving whales. The time a cetacean’s body is exposed during the breathing cycle is dependent upon its speed. If the animal is moving slowly it will break the surface very slightly. If it is moving very rapidly it will breach (jump from the water) so that it does not inhale any water during the breathing cycle (Hui 1989).
Research has shown that cetaceans are active breathers who must consciously exhale to initiate respiration (Gierak 1986). Dolphins, whales and porpoises have control of their intercostal muscles, larynx and blowhole and must actively use them when above the ocean's surface. McCormick (1969) revealed how an anesthetized porpoise simply stops breathing once it loses conciousness. This cetacean characteristc diverges from humans and other mammals whose pneumotaxic and apneustic brain centers unconciously control respiration (McCormick 1969). The ability to control their own breathing functions underscores the importance of respiration to cetaceans.