The narwhal (Monodon monoceros) is a cetacean best known for the "tusk" from a protruding canine tooth. It is the only member of the genus "Monodon".
Taxonomy and etymology[]
The narwhal was one of many species originally described by Carl Linnaeus in his landmark 1758 10th edition of Systema Naturae. Its name is derived from the Old Norse word nár, meaning "corpse", in reference to the animal's greyish, mottled pigmentation, like that of a drowned sailor and its summertime habit of lying still at or near the surface of the sea (called "logging"). The scientific name, Monodon monoceros, is derived from Greek: "one-tooth one-horn".
The narwhal is most closely related to the beluga whale. Together, these two species comprise the only extant members of the family Monodontidae, sometimes referred to as the "white whales". The Monodontidae are distinguished by their medium size (at around 4 m (13.1 ft) in length), pronounced melons (round sensory organs), short snouts and the absence of a true dorsal fin.
Although the narwhal and the beluga are classified as separate genera, with one species each, there is some evidence that they may, very rarely, interbreed. The complete skull of an anomalous whale was discovered in West Greenland circa 1990. It was described by marine zoologists as unlike any known species, but with features midway between a narwhal and a beluga, consistent with the hypothesis that the anomalous whale was a narwhal-beluga hybrid; in 2019, this was confirmed by DNA and isotopic analysis.
The white whales, dolphins (Delphinidae) and porpoises (Phocoenidae) together comprise the superfamily Delphinoidea, which are of likely monophyletic origin. Genetic evidence suggests the porpoises are more closely related to the white whales and that these two families constitute a separate clade which diverged from the rest of Delphinoidea within the past 11 million years. Fossil evidence shows that ancient white whales lived in tropical waters. They may have migrated to Arctic and sub-Arctic waters in response to changes in the marine food chain during the Pliocene.
Description[]
Narwhals are medium-sized whales and are around the same size as beluga whales. Total length in both sexes, excluding the tusk of the male, can range from 3.95 to 5.5 m (13 to 18 ft). Males, at an average length of 4.1 m (13.5 ft), are slightly larger than females, with an average length of 3.5 m (11.5 ft). Typical adult body weight ranges from 800 to 1,600 kg (1,760 to 3,530 lb). Male narwhals attain sexual maturity at 11 to 13 years of age, when they are about 3.9 m (12.8 ft) long. Females become sexually mature at a younger age, between 5 and 8 years old, when they are around 3.4 m (11.2 ft) long.
The pigmentation of narwhals is a mottled pattern, with blackish-brown markings over a white background. They are darkest when born and become whiter with age; white patches develop on the navel and genital slit at sexual maturity. Old males may be almost pure white. Narwhals do not have a dorsal fin, possibly an evolutionary adaptation to swimming easily under ice, to facilitate rolling, or to reduce surface area and heat loss. Instead, narwhals possess a shallower dorsal ridge. Their neck vertebrae are jointed, like those of land mammals, instead of being fused together as in most whales, allowing a great range of neck flexibility. Both these characteristics are shared by the fellow beluga whale. The tail flukes of female narwhals have front edges that are swept back and those of males have front edges that are more concave and lack a sweep-back. This is thought to be an adaptation for reducing drag caused by the tusk.
Tusk[]
The most conspicuous characteristic of the male narwhal is a single long tusk, which is in fact a canine tooth that projects from the left side of the upper jaw, through the lip and forms a left-handed helical spiral. The tusk grows throughout life, reaching a length of about 1.5 to 3.1 m (4.9 to 10.2 ft). It is hollow and weighs around 10 kg (22 lb). About one in 500 males has two tusks, occurring when the right canine also grows out through the lip. Only about 15 percent of females grow a tusk, which typically is smaller than a male tusk, with a less noticeable spiral. Collected in 1684, there is only one known case of a female growing a second tusk (image).
Scientists have long speculated on the biological function of the tusk. Proposed functions include use of the tusk as a weapon, for opening breathing holes in sea ice, in feeding, as an acoustic organ and as a secondary sex character. The leading theory has long been that the narwhal tusk serves as a secondary sex character of males, for nonviolent assessment of hierarchical status on the basis of relative tusk size. However, detailed analysis reveals that the tusk is a highly innervated sensory organ with millions of nerve endings connecting seawater stimuli in the external ocean environment with the brain. The rubbing of tusks together by male narwhals is thought to be a method of communicating information about characteristics of the water each has travelled through, rather than the previously assumed posturing display of aggressive male-to-male rivalry. In August 2016, drone videos of narwhals surface-feeding in Tremblay Sound, Nunavut showed that the tusk was used to tap and stun small Arctic cod, making them easier to catch for feeding. The tusk cannot serve a critical function for the animal's survival, as females — which generally do not have tusks — typically live longer than males. Therefore, the general scientific consensus is that the narwhal tusk is a sexual trait, much like the antlers of a stag, the mane of a lion, or the feathers of a peacock.
Vestigial teeth[]
The tusks are surrounded posteriorly, ventrally and laterally by several small vestigial teeth which vary in morphology and histology. These teeth can sometimes be extruded from the bone, but mainly reside inside open tooth sockets in the narwhal's snout alongside the tusks. The varied morphology and anatomy of small teeth indicate a path of evolutionary obsolescence, leaving the narwhal's mouth toothless.
Genome[]
A 2.3 GB genome sequence has been assembled from multiple Illumina libraries. The genome consists of 37.9% repetitive elements and encodes 21,785 protein-coding genes (similar to many other mammals). The genome will help to place the narwhal both into the evolutionary context of other whales but also will help to understand the evolution and embryonic development of features such as the striking tusk and its sexual dimorphism.
Distribution[]
The narwhal is found predominantly 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, Hudson Strait, Baffin Bay; off the east coast of Greenland; and in a strip running east from the northern end of Greenland round to eastern Russia (170° East). Land in this strip includes Svalbard, Franz Joseph Land and Severnaya Zemlya. The northernmost sightings of narwhal have occurred north of Franz Joseph Land, at about 85° North latitude. Most of the world's narwhals are concentrated in the fjords and inlets of Northern Canada and western Greenland.
Behavior[]
Social[]
Narwhals normally congregate in groups of about five to ten and sometimes up to 20 individuals outside the summer. Groups may be "nurseries" with only females and young, or can contain only post-dispersal juveniles or adult males ("bulls"), but mixed groups can occur at any time of year. In the summer, several groups come together, forming larger aggregations which can contain from 500 to over 1000 individuals.
At times, a bull narwhal may rub its tusk with another bull, a display known as "tusking" and thought to maintain social dominance hierarchies. However, this behaviour may exhibit tusk use as a sensory and communication organ for sharing information about water chemistry sensed in tusk microchannels.
Migration[]
Narwhals exhibit seasonal migrations, with a high fidelity of return to preferred, ice-free summering grounds, usually in shallow waters. In summer months, they move closer to coasts, often in pods of 10–100. In the winter, they move to offshore, deeper waters under thick pack ice, surfacing in narrow fissures in the sea ice, or leads. As spring comes, these leads open up into channels and the narwhals return to the coastal bays. Narwhals from Canada and West Greenland winter regularly in the pack ice of Davis Strait and Baffin Bay along the continental slope with less than 5% open water and high densities of Greenland halibut. Feeding in the winter accounts for a much larger portion of narwhal energy intake than in the summer.
Diet[]
Narwhals have a relatively restricted and specialized diet. Their prey is predominantly composed of Greenland halibut, polar and Arctic cod, cuttlefish, shrimp and armhook squid. Additional items found in stomachs have included wolffish, capelin, skate eggs and sometimes rocks, accidentally ingested when whales feed near the bottom. 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. It is thought that the beaked whales, which have similarly reduced dentition, also suck up their prey. The distinctive tusk is used to tap and stun small prey, facilitating a catch.
Narwhals have a very intense summer feeding society. One study published in the Canadian Journal of Zoology tested 73 narwhals of different age and gender to see what they ate. The individuals were from the Pond Inlet and had their stomach contents tested from June 1978 until September 1979. The study found in 1978 that the Arctic cod (Boreogadus saida) made up about 51% of the diet of the narwhals, with the next most common animal being the Greenland halibut (Reinhardtius hippoglossoides), consisting of 37% of the weight of their diet. A year later, the percentages of both animals in the diet of narwhals had changed. Arctic cod represented 57% and Greenland halibut 29% in 1979. The deep-water fish – halibut, redfish (Sebastes marinus) and polar cod (Arctogadus glacialis) – were found primarily in the diet of the males, which means that the narwhals can dive deeper than 500 m (1,640 ft) below sea level. The study found that the dietary needs of the narwhal did not differ among genders or ages.
Diving[]
When in their wintering waters, narwhals make some of the deepest dives recorded for a marine mammal, diving to at least 800 metres (2,620 feet) over 15 times per day, with many dives reaching 1,500 metres (4,920 feet). Dives to these depths last around 25 minutes, including the time spent at the bottom and the transit down and back from the surface. Dive times can also vary in time and depth, based on local variation between environments, as well as seasonality. For example, in the Baffin Bay wintering grounds, narwhals farther south appear to be spending most of their time diving to deeper depths along the steep slopes of Baffin Bay, suggesting differences in habitat structure, prey availability, or innate adaptations between subpopulations. Curiously, whales in the deeper northern wintering ground have access to deeper depths, yet make shallower dives. Because vertical distribution of narwhal prey in the water column influences feeding behaviour and dive tactics, regional differences in the spatial and temporal patterns of prey density, as well as differences in prey assemblage, may be shaping winter foraging behaviour of narwhals.
Communication[]
As most toothed whales, narwhals use sound to navigate and hunt for food. Narwhals primarily vocalise through "clicks", "whistles" and "knocks", created by air movement between chambers near the blow-hole. These sounds are reflected off the sloping front of the skull and focused by the animal's melon, which can be controlled by musculature. Echolocation clicks are primarily produced for prey detection and for locating obstacles at short distances. It is possible that individual "bangs" are capable of disorienting or incapacitating prey, making them easier to hunt, but this has not been verified. They also emit tonal signals, such as whistles and pulsed calls, that are believed to have a communication function. The calls recorded from the same herd are more similar than calls from different herds, suggesting the possibility of group or individual-specific calls in narwhals. Narwhals may also adjust the duration and the pitch of their pulsed calls to maximise sound propagation in varying acoustic environments Other sounds produced by narwhals include trumpeting and squeaking door sounds. The narwhal vocal repertoire is similar to that of the closely related beluga, with comparable whistle frequency ranges, whistle duration and repetition rates of pulse calls, however beluga whistles may have a higher frequency range and more diversified whistle contours.
Breeding and early life[]
Females start bearing calves when six to eight years old. Adult narwhals mate in April or May when they are in the offshore pack ice. Gestation lasts for 14 months and calves are born between June and August the following year. As with most marine mammals, only a single young is born, averaging 1.6 metres (5.2 feet) in length and white or light grey in colour. During summer population counts along different coastal inlets of Baffin Island, calf numbers varied from 0.05% to 5% of the total numbering from 10,000 to 35,000 narwhals, indicating that higher calf counts may reflect calving and nursery habitats in favourable inlets. Hybrids have been documented between the narwhal and beluga (specifically a beluga male and a narwhal female), as one, perhaps even as many as three, were killed and harvested during a sustenance hunt. Whether or not these hybrids could breed remains unknown. The unusual dentition seen in the single remaining skull indicates the hybrid hunted on the seabed, much as walruses do, indicating feeding habits different from those of either parent species.
Newborn calves begin their lives with a thin layer of blubber which thickens as they nurse their mother's milk which is rich in fat. Calves are dependent on milk for around 20 months. This long lactation period gives calves time to learn skills needed for survival during maturation when they stay within two body lengths of the mother.
Lifespan[]
Narwhals can live an average of 50 years, however research using aspartic acid racemization from the lens of the eyes suggests that narwhals can live to be as old as 115 ± 10 years and 84 ± 9 years for females and males, respectively Mortality often occurs when the narwhals suffocate after they fail to leave before the surface of the Arctic waters freeze over in the late autumn. As narwhals need to breathe, they drown if open water is no longer accessible and the ice is too thick for them to break through. Maximum aerobic swimming distance between breathing holes in ice is less than 1,450 m (4,760 ft) which limits the use of foraging grounds and these holes must be at least 0.5 m (1.6 ft) wide to allow an adult whale to breathe. The last major entrapment events occurred when there was little to no wind. Entrapment can affect as many as 600 individuals, most occurring in narwhal wintering areas such as Disko Bay. In the largest entrapment in 1915 in West Greenland, over 1,000 narwhals were trapped under the ice.
Despite the decreases in sea ice cover, there were several large cases of sea ice entrapment in 2008–2010 in the winter close to known summering grounds, two of which were locations where there had been no previous cases documented. This suggests later departure dates from summering grounds. Sites surrounding Greenland experience advection (moving) of sea ice from surrounding regions by wind and currents, increasing the variability of sea ice concentration. Due to strong site fidelity, changes in weather and ice conditions are not always associated with narwhal movement toward open water. More information is needed to determine the vulnerability of narwhals to sea ice changes. Narwhals can also die of starvation.
Predation and hunting[]
Major predators are polar bears, which attack at breathing holes mainly for young narwhals and Greenland sharks. Killer whales (orcas) group together to overwhelm narwhal pods in the shallow water of enclosed bays, in one case killing dozens of narwhals in a single attack. To escape predators such as orcas, narwhals may use prolonged submergence to hide under ice floes rather than relying on speed.
Humans hunt narwhals, often selling commercially the skin, carved vertebrae, teeth and tusk, while eating the meat, or feeding it to dogs. About 1,000 narwhals per year are killed, 600 in Canada and 400 in Greenland. Canadian harvests were steady at this level in the 1970s, dropped to 300–400 per year in the late 1980s and 1990s and rose again since 1999. Greenland harvested more, 700–900 per year, in the 1980s and 1990s.
Tusks are sold with or without carving in Canada and Greenland. An average of one or two vertebrae and one or two teeth per narwhal are carved and sold. In Greenland the skin (muktuk) is sold commercially to fish factories, and in Canada to other communities. One estimate of the annual gross value received from narwhal hunts in Hudson Bay in 2013 was CA$530,000 for 81 narwhals, or CA$6,500 per narwhal. However, the net income, after subtracting costs in time and equipment, was a loss of CA$7 per person. Hunts receive subsidies, but they continue as a tradition, rather than for the money and the economic analysis noted that whale watching may be an alternate revenue source. Of the gross income, CA$370,000 was for skin and meat, to replace beef, pork and chickens which would otherwise be bought, CA$150,000 was received for tusks and carved vertebrae and teeth of males and CA$10,000 was received for carved vertebrae and teeth of females.