Dolphins are air-breathing mammals, sharing the same fundamental respiratory system as terrestrial life, which means they can drown. Drowning occurs when respiratory impairment results from being submerged in a liquid, which can lead to death by suffocation. An inability to reach the surface to exhale waste air and inhale fresh oxygen is the primary mechanism by which a dolphin succumbs to the water, though specialized biology makes drowning rare under natural, healthy conditions.
Why Dolphins Must Breathe Air
Dolphins are classified as marine mammals, meaning they possess lungs for gas exchange and must breathe air. To survive, a dolphin must regularly surface to expel carbon dioxide and take in a fresh supply of oxygen through its blowhole. Their warm-blooded nature and active lifestyle dictate a high demand for oxygen and a high metabolic rate. They are highly efficient breathers, exchanging up to 80% of the air in their lungs with each breath, and can hold their breath for five to 20 minutes before resurfacing.
The Mechanics of Voluntary Breathing
The primary biological defense against drowning is that dolphin respiration is a conscious, voluntary process, unlike the automatic breathing of humans. The dolphin must actively decide to open its blowhole, exhale, and then inhale, ensuring it never attempts to breathe while submerged. The blowhole, which is the dolphin’s single nostril, is a muscular valve positioned on top of the head. This valve remains sealed watertight by default, and internal flaps create multiple airtight seals against water entering the trachea and lungs.
This voluntary control extends into periods of rest through unihemispheric slow-wave sleep (USWS). During USWS, the dolphin rests only one half of its brain at a time, allowing the other half to remain awake and alert. The active hemisphere manages the voluntary act of surfacing and breathing, preventing the dolphin from losing consciousness and failing to initiate a breath while asleep.
Circumstances That Lead to Drowning
External threats can override the voluntary respiratory system. A primary cause of drowning is entanglement in fishing gear, known as bycatch, where nets prevent the dolphin from swimming to the surface. The dolphin then dies from suffocation as its oxygen reserves are depleted.
Internal biological failures, such as illness or physical trauma, can also compromise the conscious control required for breathing and swimming. Neurological damage, severe infections, or injuries from boat strikes can impair the dolphin’s ability to coordinate surfacing and opening the blowhole. If the animal is too weak or its conscious control is compromised, the voluntary breathing mechanism fails.
Newborn dolphin calves are particularly vulnerable immediately after birth. The calf relies on its mother to quickly guide it to the surface for its first breath of air. If the mother is separated or unable to assist, the calf may lack the necessary strength and coordination to initiate breathing alone.