Dolphins are obligate air-breathing mammals that must regularly surface to inhale oxygen. The body part a dolphin uses to breathe air is the blowhole, a muscular respiratory opening located strategically on the dorsal surface of its head. This placement allows the animal to take a full breath by exposing only a small portion of its body above the water’s surface.
The Anatomy of the Blowhole
The blowhole is a highly modified external nostril that migrated from the snout to the top of the head. In toothed whales, or odontocetes, which include dolphins, the blowhole presents as a single, crescent-shaped opening supported by a complex arrangement of heavy muscles.
These muscles are responsible for maintaining a watertight seal when the dolphin is submerged, preventing water from entering the respiratory tract. Beneath the blowhole, a system of nasal plugs or valves acts as a further safeguard, sealing the internal airways. This robust anatomical design ensures the dolphin can dive to significant depths without compromising its lungs.
The Mechanism of Voluntary Respiration
Unlike human respiration, which is involuntary, dolphin breathing is entirely voluntary and conscious. The dolphin must actively decide to open its blowhole, exhale, and inhale, a process controlled by higher brain functions.
The physical act of ventilation is remarkably fast and efficient, minimizing the time spent exposed at the surface. A dolphin can complete an entire breath exchange in approximately 0.3 to 0.5 seconds after breaking the water’s surface. Dolphins exchange between 80 to 95 percent of the air in their lungs, far exceeding the 10 to 17 percent typical for humans during a resting breath. This high efficiency allows the dolphin to maximize oxygen intake and conserve energy before diving again.
Managing Breath During Sleep
The necessity of conscious breathing presents a unique challenge for dolphins when they need to rest, as they cannot afford to lose consciousness completely. To solve this, dolphins employ a neurological state known as unihemispheric slow-wave sleep (USWS). During USWS, only one half of the brain enters a deep sleep state while the other hemisphere remains awake and alert.
The active half of the brain is responsible for monitoring the environment, maintaining awareness of the pod, and initiating the voluntary act of surfacing to breathe. In a 24-hour period, each cerebral hemisphere typically receives about four hours of slow-wave sleep. The hemispheres alternate, ensuring the dolphin is continuously rested and vigilant.