The narwhal (Monodon monoceros) is a species of toothed whale native to the Arctic. It is the only member of the genus Monodon and one of two living representatives of the family Monodontidae. The narwhal is a stocky cetacean with a relatively blunt snout, a large melon, and a shallow ridge in place of a dorsal fin. Males of this species have a spiralled tusk, which is a protruding left canine, that is long and thought to function as a secondary sexual characteristic, a weapon, a tool for feeding, or a sensory organ. Specially adapted slow-twitch muscles, along with the jointed neck vertebrae and shallow dorsal ridge allow for easy movement through the Arctic environment, where the narwhal spends extended periods at great depths. The narwhal's geographic range overlaps with that of the similarly built and closely related beluga whale, and the animals are known to interbreed.
Narwhals inhabit the Arctic waters of Canada, Greenland and Russia. Every year, they migrate to ice-free summering grounds, usually in shallow waters, and often return to the same sites in subsequent years. Their diet mainly consists of polar and Arctic cod, Greenland halibut, cuttlefish, shrimp, and armhook squid. Diving to depths of up to , the narwhal is among the deepest-diving cetaceans. The animals typically travel in groups of three to eight, with aggregations of up to 1,000 occurring in the summer months. Narwhals mate among the offshore pack ice from March to May, and the young are born between July and August of the following year. When communicating amongst themselves, narwhals use a variety of clicks, whistles and knocks.
There are an estimated 170,000 living narwhals, and the species is listed as being of least concern by the International Union for Conservation of Nature (IUCN). The population is threatened by the effects of climate change, such as reduction in ice cover and human activities such as pollution and hunting. Narwhals have been hunted for thousands of years by Inuit in northern Canada and Greenland for meat and ivory, and regulated subsistence hunting continues to this day.
Taxonomy
The narwhal was scientifically described by Carl Linnaeus in his 1758 publication Systema Naturae. The word "narwhal" comes from the Old Norse , meaning 'corpse-whale', which possibly refers to the animal's grey, mottled skin and its habit of remaining motionless when at the water's surface, a behaviour known as "logging" that usually happens in the summer. The scientific name, Monodon monoceros, is derived from Ancient Greek, meaning 'single-tooth single-horn'.
The narwhal is most closely related to the beluga whale (Delphinapterus leucas). Together, these two species comprise the only extant members of the family Monodontidae. Monodontids are distinguished by their pronounced melons (acoustic sensory organs), short snouts and the absence of a true dorsal fin.
Although the narwhal and beluga are classified as separate genera, there is some evidence of interbreeding between the two. Most prominent are the remains of a whale, described by marine zoologists as unlike any known species, which were found in West Greenland around 1990. It had features midway between a narwhal and a beluga, indicating that the remains belonged to a hybrid between the two species (a 'narluga'); this was confirmed by a 2019 DNA analysis. Whether the hybrid itself could breed remains unknown.
Evolution
Results of a genetic study reveal that porpoises and monodontids are closely related, forming a separate clade which diverged from other dolphins about 11 million years ago (mya). A 2018 molecular analysis of monodontid fossils indicates that they separated from Phocoenidae (porpoises) around 10.82 to 20.12 mya, and they are considered to be sister taxa. A later phylogenetic study conducted in 2020 suggested that the narwhal split from the beluga whale around 4.98 mya, based on data from mitochondrial DNA.
The fossil species Casatia thermophila of early Pliocene central Italy was described as a possible narwhal ancestor when it was discovered in 2019. Bohaskaia, Denebola and Haborodelphis are other extinct genera known from the Pliocene of the United States. Fossil evidence shows that prehistoric monodontids lived in tropical waters. They may have migrated to Arctic and subarctic waters in response to changes in the marine food chain.
The following phylogenetic tree is based on a 2019 study of the family Monodontidae.
Description
The narwhal has a robust body with a short, blunt snout, small upcurved flippers, and convex to concave tail flukes. Adults measure in length and weigh . Male narwhals attain sexual maturity at 12 to 20 years of age, reaching a length of . Females reach sexual maturity at a younger age, between 8 and 9 years old, when they are about long. On average, males are about longer and more than 75% heavier than females.
The colouration of the narwhal consists of a mottled pattern, with blackish-brown markings over a white background. At birth, the skin is light grey, and when sexually mature, white patches grow on the navel and genital slit; such whitening occurs throughout life, resulting in aged narwhals being almost purely white. Unlike most whales, the narwhal has a shallow dorsal ridge, rather than a dorsal fin, possibly an evolutionary adaptation to make swimming under ice easier or to facilitate rolling. The neck vertebrae are also jointed, instead of being fused as in most whales, which allows for a greater range of neck flexibility. These characteristics are shared by the beluga whale. Furthermore, male and female narwhals have differently shaped tail flukes; the former are bent inward, while the latter are swept back on the front margins. This is thought to be an adaptation for reducing drag caused by the tusk.
The skeletal muscles of narwhals are highly adapted for prolonged periods of deep-sea foraging. During such activities, oxygen is reserved in the muscles, which are typically slow-twitch, enabling greater endurance and manouverability. Narwhals also have a comparatively high amount of myoglobin in their body, which helps to facilitate deeper dives. It has a dense layer of blubber, around thick. This fat accounts for a third of the body mass and helps insulate from cold ocean temperatures.
Tusk and dentition
The most conspicuous trait of male narwhals is a long, spiralled tusk, which is a canine tooth that projects from the left side of the upper jaw. Both sexes have a pair of tusks embedded in the upper jaw, which in males erupt from the lip somewhere between two and three years of age. The tusk grows throughout the animal's life, reaching lengths of . It is hollow and weighs up to . Some males may grow two tusks, occurring when the right canine also protrudes through the lip. Females rarely grow tusks: when they do, the tusks are typically smaller than those of males, with less noticeable spirals.
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. According to Martin Nweeia, male narwhals may rid themselves of encrustations on their tusks by rubbing them together, as opposed to posturing displays of aggressive male-to-male rivalry. 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. Females, who usually do not have tusks, live longer than males, hence the tusk cannot be essential to the animal's survival. It is generally accepted that the primary function of the narwhal tusk is associated with sexual selection.
Alongside its tusk, the narwhal has a single pair of small vestigial teeth that reside in open tooth sockets in the upper jaw. These teeth, which differ in form and composition, encircle the exposed tooth sockets laterally, posteriorly, and ventrally. Vestigial teeth in male narwhals are commonly shed in the palate. The varied morphology and anatomy of small teeth indicate a path of evolutionary obsolescence.
Microscopic structure The tusk consists of two opposing helical structures formed from mineralised collagen fibrils. The outer layer forms a left-handed helix, while the underlying layer forms a right-handed helix. This opposing helical arrangement helps the tusk to grow straight while maintaining its characteristic outer large-scale left-handed spiral.
Distribution
The narwhal is found in the Atlantic and Russian areas of the Arctic Ocean. Individuals are commonly recorded in the Canadian Arctic Archipelago, such as in the northern part of Hudson Bay, in Hudson Strait, in Baffin Bay, off the east coast of Greenland and in a strip running east from the northern end of Greenland to eastern Russia (170° east). Land in this strip includes Svalbard, Franz Joseph Land and Severnaya Zemlya. The northernmost sightings of narwhals occurred north of Franz Joseph Land, at about 85° north. There are an estimated 12,500 narwhals in the northern Hudson Bay, whereas around 140,000 reside in Baffin Bay.
Migration Narwhals exhibit seasonal migration, with a high fidelity of return to preferred ice-free summering grounds, usually in shallow waters. In summer months, they move closer to the coast, often in pods of 10–100 individuals. In the winter, they move to deeper waters offshore, under thick pack ice, surfacing in narrow fissures or in wider fractures known as leads. As spring comes, these leads open up into channels and the narwhals return to the coastal bays. Narwhals in Baffin Bay typically travel to northern Canada and Greenland between June and September. After this period, they travel about south to the Davis Strait, and stay there until April. During winter, narwhals from Canada and West Greenland regularly visit the pack ice of the Davis Strait and Baffin Bay along the continental slope which contains less than 5% open water and hosts a high density of Greenland halibut.
Behaviour and ecology
Narwhals normally congregate in groups of three to eight individuals. Groups may be "nurseries" with only females and young, or can contain only juveniles or adult males ("bulls"); mixed groups can occur at any time of year. In the summer, several groups come together, forming larger aggregations which can contain 500 to over 1,000 individuals. Male narwhals have been observed rubbing each other's tusks, a behaviour known as "tusking".
When in their wintering waters, narwhals make some of the deepest dives recorded for cetaceans, diving to at least over 15 times per day, with many dives reaching . The greatest dive depth recorded is . 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. This is mainly attributed to prey being concentrated nearer to the surface, which causes narwhals to alter their foraging strategies.
Diet