The narwhal, an Arctic whale often called the “unicorn of the sea” due to the long tusk projecting from the male’s head, is a unique marine creature. These toothed whales live entirely in the frigid, ice-covered waters of the Arctic Ocean. Narwhals are mammals and must surface to breathe atmospheric air. This requirement dictates their existence, influencing their deep-diving physiology and their constant navigation of the challenging sea ice environment.
The Mammalian Necessity Why Narwhals Need Air
Narwhals belong to the order Cetacea, the group of marine mammals that includes whales, dolphins, and porpoises. Since they do not possess gills, they cannot extract oxygen from water. Like all mammals, they have lungs and must regularly exchange air at the water’s surface. Their breathing apparatus is a single blowhole, which is a modified nostril located on the top of their head.
Breathing for the narwhal, and all cetaceans, is a conscious and voluntary action, unlike the involuntary reflex seen in terrestrial mammals. They must actively contract muscles to open the blowhole and take a breath. When the narwhal prepares to dive, a muscular flap automatically seals the blowhole, preventing water from entering the respiratory tract.
The act of breathing is rapid, beginning with an explosive exhalation followed immediately by a quick, deep inhalation. This efficient exchange maximizes oxygen uptake before submerging again. Since there is no connection between their mouth and respiratory system, the blowhole is the sole entry point for air.
Specialized Adaptations for Deep Diving
Despite relying on atmospheric air, narwhals are among the deepest-diving marine mammals. They routinely descend to depths exceeding 1,500 meters and can remain submerged for up to 25 minutes. This capability is supported by physiological mechanisms designed to manage oxygen stores and pressure changes.
A major adaptation is the high concentration of myoglobin found within their muscles, which acts as a localized oxygen reserve. This protein stores oxygen directly in the muscle tissue, supporting muscle function during extended breath-holds. Narwhal muscle tissue possesses myoglobin concentrations that are among the highest measured in marine mammals, reaching up to 7.87 grams per 100 grams of wet muscle.
During a dive, the narwhal initiates the mammalian dive reflex, which includes a slowing of the heart rate, known as bradycardia. The resting heart rate of about 60 beats per minute at the surface can drop to as low as 10 to 20 beats per minute during a dive. This slowed heart rate is coupled with peripheral vasoconstriction, which restricts blood flow to peripheral tissues. Blood is redirected primarily to the brain, heart, and other oxygen-sensitive organs.
To cope with the hydrostatic pressure at depth, which can be over 150 times greater than at the surface, the narwhal’s lungs are designed to collapse. This collapse pushes residual air out of the gas-exchange surfaces and into the cartilaginous airways. By preventing the absorption of nitrogen gas into the bloodstream at high pressure, this mechanism protects the narwhal from decompression sickness, or “the bends.”
Breathing in the Ice Navigating the Arctic
The narwhal’s need for air creates a vulnerability in their Arctic habitat, where the ocean surface is frequently covered by thick sea ice. They rely on naturally occurring openings in the ice, such as polynyas, which are large areas of open water maintained by winds and currents. Narwhals also utilize small cracks and holes, called breathing holes, to quickly surface and replenish oxygen.
The requirement to access the surface means narwhals must remain near open water features, particularly during the winter when sea ice is extensive. They maintain small breathing holes in thin ice by repeatedly surfacing and breaking the new ice layer with their heads. This behavior is necessary for survival.
A sudden change in weather, such as an abrupt drop in temperature or a shift in wind, can cause rapid ice formation, quickly sealing off open water and trapping the whales. This phenomenon is known as “Sikku,” or ice entrapment, and it poses a threat to narwhal populations. Hundreds of whales may crowd into a single, shrinking opening, leading to high mortality if the ice does not break up.