The narwhal, scientifically known as Monodon monoceros, is a medium-sized toothed whale inhabiting the Arctic waters. Often called the "unicorn of the ocean" due to the male's prominent tusk, a spiraled, elongated tooth, the narwhal is an elusive and mysterious creature. Its diet plays a critical role in its survival in the harsh Arctic environment. This article explores the intricacies of the narwhal's diet, its feeding habits, and their relationship with the surrounding ecosystem.
Narwhal: Physical Characteristics and Habitat
Narwhals are characterized by a robust body, a small, bulbous head with little to no beak, and an upturned mouthline. Adult narwhals typically have black and white spots on their dorsal parts, and older narwhals can be almost entirely white. They possess short, blunt flippers, lack a dorsal fin, and have flukes with a straight to concave leading edge and a convex trailing edge with a deep notch. Adult females can reach lengths up to 13.8 feet and 2,204 pounds, while adult males can reach lengths up to 15.7 feet and 3,527.4 pounds.
These whales are scattered throughout Arctic waters and the North Atlantic Ocean, predominantly in Greenland and the eastern part of the Canadian Arctic Ocean, typically between 70° and 80°N latitude. They exhibit seasonal migration, returning with high fidelity to preferred, ice-free summering grounds, usually in shallow waters. In summer, they move closer to coasts, often in pods of 10-100. In winter, they move to offshore, deeper waters under thick pack ice, surfacing in narrow fissures in the sea ice, or leads. Narwhals remain near pack ice for the whole year, creating breathing holes through sheets of ice with thrusts of their thick head, sometimes by several of them at the same time.
Dietary Preferences and Feeding Habits
Narwhals are carnivores (piscivores) with a restricted and specialized diet. Their diet includes Greenland halibut, polar and Arctic cod, cuttlefish, shrimp, and armhook squid. A study of the stomach contents of 73 narwhals revealed that Arctic cod (Boreogadus saida) was the most commonly consumed prey, followed by Greenland halibut (Reinhardtius hippoglossoides). Large quantities of Boreo-Atlantic armhook squid (Gonatus fabricii) were also discovered. Male specimens had a higher likelihood of showing two additional prey species within their stomach contents: polar cod (Arctogadus glacialis) and redfish (Sebastes marinus), both of which are found at depths of more than 500 m (1,600 ft).
Due to the lack of well-developed dentition in the mouth, narwhals are believed to feed by swimming towards prey until it is within close range and then sucking it with considerable force into the mouth. They create suction using their mouth to capture prey. The distinctive tusk is used to tap and stun small prey, facilitating a catch, as revealed by drone footage from August 2016 in Tremblay Sound, Nunavut.
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Narwhals feed at or near the ocean bottom in deep water, diving down to depths of 3,937 feet for up to 25 minutes. Dives last up to 25 minutes, and vary in depth depending on the season and local variation between environments. For example, in the Baffin Bay wintering grounds, narwhals tend to dive deep within the steep coasts, typically south of Baffin Bay. This suggests differences in habitat structure, prey availability, or genetic adaptations between subpopulations. In the northern wintering grounds, narwhals do not dive as deep as the southern population, in spite of greater water depths in these areas.
Seasonal Variations in Diet
Narwhal diet varies between seasons. In winter, narwhals feed on demersal prey, mostly flatfish, under dense pack ice. During the warmer summer season, the narwhal eats extraordinarily little. This migratory seasonal feeding pattern is the opposite of the migratory patterns of other marine mammals in the summer season.
The lack of feeding during the summer has been confirmed by examinations of stomach contents, which are often found to be empty during this period. This unique seasonal feeding pattern distinguishes narwhals from other marine mammals.
Social Hunting and Communication
Narwhals are gregarious and commonly occur in pods of between 6 and 20 animals, though most groups number between 3 and 8. Narwhals travel and hunt in groups of up to twenty members called pods. They use a complex combination of whistles, clicks, and knocking sounds using chambers located near the blowhole.
Like most toothed whales, narwhals use sound to navigate and hunt for food. They primarily vocalise through clicks, whistles and knocks, created by air movement between chambers near the blowhole. The frequency of these sounds ranges from 0.3 to 125 hertz, while those used for echolocation typically fall between 19 and 48 hertz. Echolocation clicks are used for detecting prey and locating barriers at short distances. Whistles and throbs are most commonly used to communicate with other pod members. Calls recorded from the same pod are more similar than calls from different pods, suggesting the possibility of group- or individual-specific calls.
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The Role of the Tusk in Feeding
The tusk of the narwhal is its most unique and distinctive feature. Current scientific consensus indicates that narwhal tusks are secondary sexual characteristics which indicate social status. Further functions of the narwhal tusk are debated: while some biologists suggest that narwhals use their tusks in fights, others argue that they may be of use in feeding. The tusk is also a highly innervated sensory organ with millions of nerve endings, allowing the narwhal to sense temperature variability in its surroundings. These nerves may also be able to detect changes in particle concentration and water pressure.
Drone footage from August 2016 in Tremblay Sound, Nunavut, revealed that narwhals used their tusks to tap and stun small Arctic cod, making them easier to catch for feeding. However, females, who usually do not have tusks, live longer than males, hence the tusk cannot be essential to the animal's survival.
Nutritional Needs of Narwhal Calves
Narwhals give birth to a single calf and they are born tail first. After a gestation of about 13 to 16 months, narwhals give birth to a single calf in summer (July through August). Calves nurse from their mother for at least a year. Newborn calves begin their lives with a thin layer of blubber. The blubber thickens as they nurse their mother's milk, which is rich in fat; calves are dependent on milk for about 20 months. This long lactation period gives calves time to learn the skills they will need to survive as they mature.
Threats to Narwhal Diet and Survival
Narwhals are susceptible to climate fluctuations as well as long-term climate change. Their range declines as pack ice recedes. They have sometimes been trapped under the ice which forms too quickly for them to make a breathing hole. Climate change poses a significant threat to narwhals due to sea ice decline, especially in their northern wintering grounds such as the Baffin Bay and Davis Strait regions. Satellite data collected from these areas shows the amount of sea ice has been markedly reduced from what it was previously. It is thought that narwhals' foraging ranges…
Industrial extraction, marine construction, shipping, and military activities cause noise pollution under the water. As narwhals grow, bioaccumulation of heavy metals takes place within their bodies. It is thought that pollution in the ocean is the primary cause of bioaccumulation in marine mammals; this may lead to health problems for the narwhal population.
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Conservation Status and Human Interactions
According to the IUCN Red List, the global narwhal population size is around 80,000 individuals, including estimates for this species in specific regions: Canadian High Arctic - 70,000 animals; northern Hudson Bay - 3,500 animals; West Greenland - over 2,000 animals; East Greenland - fewer than 1,000 animals. The narwhal is listed as a species of least concern by the IUCN Red List. But the World Wildlife Federation considers it to be near threatened.
Narwhals are hunted for meat, their skin, called "maktaq", and their ivory tusks, sold as curios or for carving. About 1,000 narwhals are killed per year: 600 in Canada and 400 in Greenland. In Canada and Greenland, Narwhal tusks are sold both carved and uncarved. In Greenland, the skin (muktuk) is sold commercially to fish factories, and in Canada to other communities.