In 1758, Linnaeus used the scientific name Monodon mo-noceros for the whale with one tooth and one horn. Together with the close relative the white whale or beluga, Delphinapterus leucas, the narwhal now forms the two-species family of Monodontidae (Fig. 1).
I. External Appearance and Dentition
Newborn narwhals are evenly gray or dark-brownish gray. While nursing for 1-2 years, the coloration changes gradually to a dark background color with white patches that give a mottled appearance. When adult, the animals are completely mottled on the dorsum but with increasing white fields on the ventral side. Old adult males only maintain a narrow dark-spotted pattern on the top of the back, whereas the rest of the body is white. Unlike in other cetaceans, the tail flukes are concave in fully grown narwhals and a low ridge replaces the dorsal fin.
The most conspicuous feature of the narwhal is the up to 3-m-long spiraled tusk. Six pairs of maxillary and two pairs of mandibulary teeth are present in early narwhal embryos, but only two maxillary pairs persist and develop. Of these the two anterior teeth develop into an elongated tooth that is the start of the tusk. The other two teeth remain vestigial. In males, the left of the two elongated teeth grows and protrudes through the maxillary bones and skin of the rostrum of the whale. During growth the tusk spirals and grooves to the left. In males, the right of the elongated maxillary teeth and in females both maxillary teeth remain inside the skull, sometimes just protruding through an opening in the maxillary bone. Irregularities in the development of tusks are frequently seen: females sometimes attain a tusk, males occasionally have no tusk, and narwhals with two tusks (so-called “double tuskers”) are not rare.
There is great variability in the shape and dimensions of the protruding tusk. Some are fairly straight and others corkscrew like: some are thin and fragile, whereas others are short and thick. The largest tusk measured was 267 cm, but a full-grown male usually carries a tusk of about 200 cm. Tusks are sometimes broken, and there are records of tusk from another narwhal sitting inside the broken tip. The purpose of the tusk has been much disputed, but because both females and males without tusks thrive, tusks do not seem critical for survival. The tusk is more likely a secondary sexual character that is related to the hierarchy of male narwhals. Displays and crossing of tusks are frequently seen on narwhal summering grounds and it is likely that this activity determines social rank.
Figure 1 The narwhal, Monodon monoceros, occurs in the remote North Atlantic and Arctic Oceans and is conspicuous with a long tusk in males, usually formed from one tooth in the left upper jaw.
II. Fossil Records
There are several records of narwhal fragments from Pleistocene deposits in England and Germany. Bones found along the Russian Arctic coasts—both on the mainland and on the Russian Arctic Islands—also suggest a different occurrence of narwhals before or during the most recent glaciation. In Canada, bone remains from early postglacial times have also been found both north (Ellesmere Island) and south (Gulf of Saint Lawrence) of present narwhal distribution.
III. Distribution and Abundance
The main reason the narwhal remained a legendary animal for so long may be because of its preference for remote and inaccessible habitats, usually in areas over deep water that is covered with heavy pack ice during dark winter months. Europeans did not visit most of these areas until the 19th century, and even though Inuit hunters traded the tusks with whalers, precise descriptions were lacking.
The narwhal essentially inhabits the Atlantic sector of the Arctic Ocean with few records of stragglers from the Pacific sector (Fig. 2). During the last glaciation, narwhals were restricted to the North Atlantic but with the retreating ice they inhabited the archipelago of the Canadian High Arctic, north-em Hudson Bay, Davis Strait, Baffin Bay, the Greenland Sea, and the Arctic Ocean between Svalbard and Franz Josef Land. Today, low numbers of narwhals are found offshore in deep-water areas of the Eurasian sector of the Arctic Ocean, where they are seen most frequently around Franz Josef Land and Svalbard. The northernmost recordings of narwhals are from the area between 84°N and 85°N northeast of Franz Josef Land at 70-80°E. In the Greenland Sea, narwhals are widely distributed in the pack ice but probably in low numbers. Along the coast of East Greenland, narwhals are found during the open water season in fjords from 65°N to 81°N, with particularly large concentrations in Scoresby Sound and Kangerlus-suaq. No complete abundance estimates are available from any of the Northeast Atlantic areas, but in 1983 an estimate of 300 narwhals was derived from a survey in Scoresby Sound.
In West Greenland, narwhals visit coastal areas in northwest Greenland (Inglefield Bredning and Melville Bay) during summer and central West Greenland during autumn (Uumman-naq) and winter (Disko Bay). Up to 4000 narwhals have been counted in Inglefield Bredning in August and 3000 in Disko Bay in March. Offshore, narwhals are abundant in the heavy consolidated pack ice in northern Davis Strait and Baffin Bay from late November through May, and the number of narwhals wintering in this area has been estimated at 35,000 whales.
During ice break-up, narwhals move into the Canadian High Arctic through Lancaster Sound and Pond Inlet. They visit the fjord systems of Eclipse Sound, Admiralty Inlet, Prince Regent Inlet, and Peel Sound during the open water season from June to September. The abundance in these areas was estimated at 18,000 narwhals in 1984. With the formation of fast ice in October, narwhals move east toward Baffin Bay and Davis Strait.
In northern Hudson Bay and Foxe Basin, an apparently isolated group of 1300 narwhals persists. It is believed that they move south to Hudson Strait in winter.
Narwhals follow the distribution of the ice and move toward coastal areas in summer when these are ice free. During freeze-up the coastal areas are abandoned and the narwhals move offshore. In winter they stay in very heavy consolidated pack ice, usually in leads or holes in 10/10 of ice. When ice breaks up in the spring, narwhals penetrate north through narrow leads and open water channels.
Movements from summer through winter have been monitored by tracking of narwhals instrumented with satellite-linked radio transmitters attached to the tusk of males. At summering grounds in West Greenland and Canada, narwhals moved back and forth between glacier fronts, offshore areas, and neighboring fjords. When fast ice formed the whales moved out to deeper water, usually up to a 1000-m water depth. In October the whales moved southward toward the edge of the continental shelf where the water depth increases over a short distance from 1000 to 2000 m. This slope was also used as a wintering ground, and even though the whales seemed stationary in this area, they still conducted shorter movements along this steep slope. Narwhals tracked from Canada and West Greenland were within a few kilometers from each other at these wintering grounds at the deep slope at the edge of the continental shelf in central Baffin Bay. The importance of this area as a wintering ground has also been confirmed by aerial surveys. No satellite trackings of whales have been conducted so far in spring and early summer. The mean swimming speed of traveling narwhals is 5 km/hr.
V. Growth in Length and Weight
Length at birth is approximately 160 cm. The tusk erupts at a body length of 260 cm and attains a length of 150 cm at sexual maturity. Body length at sexual maturity is around 360 and 420 cm for females and males, respectively. Mean length and weight at physical maturity are around 400 cm and 1000 kg for females and 475 cm and 1600 kg for males.
No reliable methods are available for estimating the age of narwhals; both the protruding tusk and the embedded teeth contain distinctive growth layers in both dentine and cementum, but with increasing age the growth layers apparently collapse and become unreadable. Also, there is no empirical way to determine how many growth layers are deposited annually. So far, no narwhals have been kept successfully in captivity.
The gestation period of the narwhal is subject to some uncertainty, as mating probably occurs in inaccessible areas in March-May. Calving seems to occur in July-August in both Greenland and Canada, and with a mating season early in spring, this implies a gestation period of 13-16 months. Lactation lasts 1-2 years, and females are generally believed to calve every 3 years, but data supporting this seem inadequate.
Figure 2 Distribution of the narwhal.
The diet of narwhals has been studied in Greenland and Canada and they typically prey on high Arctic fish species such as polar cod, Boreogadus saida, and Arctic cod, Arctogadus glacialis; both are pelagic species that are often associated with the underside of ice. Narwhals also take demersal species that are found at great depths, such as Greenland halibut, Rein-hardtius hippogbssoides; redfish, Sebastes marinus; and bottom-dwelling cephalopods. In some areas the narwhals seem to be feeding exclusively on schools of squids, Gonatus fabricii, which can be found at variable depths. Apparently little feeding takes place during the open water season in August.
VIII. Population Structure
Narwhals are usually found in small groups of 5-10 whales migrating together. Sometimes larger herds are formed that consist of several smaller groups often all on a directional movement along a coastline or toward the head of a fjord. The narwhal groups are usually segregated with adult males in separate groups and females with calves sometimes together with immature males. Mixed groups occur especially in large herds, but single animals, particularly males, can also be found.
Studies of mitochondrial DNA have revealed a low level of nucleotide and haplotype diversity in narwhals. This is probably the result of a rapid expansion of the population after the last glaciation from a small founding population. Despite the low variation in narwhal mtDNA, there are still genetic differences between narwhals from different areas. Not so surprisingly, narwhals from East Greenland are different from those inhabiting Baffin Bay, but more surprising was the distinctness of narwhals at two summering grounds (Inglefield Bredning and Melville Bay) and one autumn ground (Uummannaq) in West Greenland. Apparently, narwhals have annual fidelity to certain summer and autumn feeding localities, but the extent of mixing on the wintering grounds is unknown.
IX. Diving Behavior
Data on narwhal diving have been collected from whales instrumented with satellite transmitters in both Canada and Greenland. Narwhals are able to dive to depths exceeding 1000 m, and the deepest dive recorded was 1164 m. However, the dives are usually completed within 20 min and never exceed 25 min, so the whales only have a short time at the bottom as ascent-descent rates for deep dives are 2 and 1 m/sec for shallow dives. Narwhals apparently reduce their diving activity during autumn and early winter and make more deep dives.
Narwhals are known to make a variety of noises. Clicks that are believed to be used for echolocation have been measured to have their maximum amplitudes at 48 kHz with rates of 3-10 clicks/sec. Faster click rates of 110-150 clicks/sec had maximum amplitudes at 19 kHz. Whistles or pure tones in frequencies from 300 Hz to 18 kHz have also been recorded and they are suspected to serve as social signals among the whales.
XI. Unicorn Myth and Systematics
The narwhal is the animal behind the legend about the mysterious unicorn: a horse-like creature with a spiraled horn protruding from the forehead. The horn was supposed to have healing abilities, and the wild and shy animal could only be captured with a virgin as bait. Based on narwhal tusks that were brought south from Arctic coasts, this was essentially how narwhals were perceived in western civilization until the 17th century when the first descriptions of a fish-like sea monster appeared.
XII. Human Effects and Interactions
Narwhals have never been a target for commercial whaling probably because of their skittishness and the difficulties involved in catching them. Inuit hunters in Greenland and Canada hunt narwhals for their valuable tusks and the highly prized skin that is considered a delicacy throughout the Inuit communities. The harvest level was on average 550 and 280 during 1993-1995 in Greenland and Canada, respectively, and it is considered small relative to the population size in most areas; however, depending on the population structure, some subpopulations may be overexploited.
Narwhals have high levels of some organochlorines and heavy metals where at least the first are of anthropogenic origin. Possible effects of these contaminants have not been studied in narwhals.
Because of their prevalence for high-density pack ice, narwhals are susceptible to climatic changes that influence the water currents and thereby ice formation in the Arctic. Whether it is naturally occurring or human-induced climate changes, narwhals may become entrapped or lose access to important feeding areas if ice conditions change.
XIII. Ice Entrapments
A peculiar feature of the natural history of narwhals is their susceptibility to being entrapped in ice. Because of their preference for heavy pack ice, large schools of narwhals are occasionally caught in ice that freezes rapidly during intense cold, thereby preventing the whales from getting enough air to breathe. This happens particularly often in areas where unpredictable ice conditions persist due to the mixing of warm and cold water masses of variable strength, e.g., Disko Bay in West Greenland. Large numbers of narwhals may succumb during such an ice entrapment, and in January 1915, more than 1000 narwhals died in a well-known ice entrapment in Disko Bay. If the whales are discovered, Inuit hunters may also prey upon them, using the word “sassat” for the event.