Narwhal

This narwhal skull has rare double tusks. Usually, the canine tooth only on the left side of the upper jaw becomes a tusk. Rarely, males develop two tusks. This specimen, however, was of a female (Zoologisches Museum, Hamburg; collected in 1684)

The most conspicuous characteristic of the male narwhal is a single long tusk, which is in fact a canine tooth[19][20] 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,[21] which typically is smaller than a male tusk, with a less noticeable spiral.[22][23][24] Collected in 1684, there is only one known case of a female growing a second tusk (image).[25]

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.[26] 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.[27][28][29][30] 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.[29] 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.[31][32] 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.[33]

The tusks are surrounded posteriorly, ventrally and laterally by several small vestigial teeth which vary in morphology and histology.[19] These teeth can sometimes be extruded from the bone, but mainly reside inside open tooth sockets in the narwhal's snout alongside the tusks.[19][34] The varied morphology and anatomy of small teeth indicate a path of evolutionary obsolescence,[19] leaving the narwhal's mouth toothless.[34]

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.[15] In the summer, several groups come together, forming larger aggregations which can contain from 500 to over 1000 individuals.[15]

At times, a bull narwhal may rub its tusk with another bull, a display known as "tusking"[28][36] and thought to maintain social dominance hierarchies.[36] However, this behaviour may exhibit tusk use as a sensory and communication organ for sharing information about water chemistry sensed in tusk microchannels.[27][28]

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.[37] As spring comes, these leads open up into channels and the narwhals return to the coastal bays.[38] 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.[39] Feeding in the winter accounts for a much larger portion of narwhal energy intake than in the summer.[39][37]

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.[15][39][37][36] 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.[40] The distinctive tusk is used to tap and stun small prey, facilitating a catch.[31][32]

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.[41]

Upside-down swimming behaviour of narwhals

Narwhal tail fluke

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.[42] 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.[42]

As in 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.[43] 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 [44] Other sounds produced by narwhals include trumpeting and squeaking door sounds.[9] 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.[43]

Females start bearing calves when six to eight years old.[9] 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.[9][45] 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.[45] 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.[46][47]

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.[9] 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.[9][45]

A polar bear scavenging a narwhal carcass

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 [48] 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.[15][49] 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.[50] 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.[51]

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.[49] 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.[15]

Major predators are polar bears, which attack at breathing holes mainly for young narwhals and Greenland sharks.[15][52] Killer whales (orcas) group together to overwhelm narwhal pods in the shallow water of enclosed bays,[53] in one case killing dozens of narwhals in a single attack.[54] To escape predators such as orcas, narwhals may use prolonged submergence to hide under ice floes rather than relying on speed.[50]

Beluga and narwhal catches

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.[55]

Tusks are sold with or without carving in Canada[56][57] and Greenland.[58] An average of one or two vertebrae and one or two teeth per narwhal are carved and sold.[56] In Greenland the skin (muktuk) is sold commercially to fish factories,[58] and in Canada to other communities.[56] 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.[56]

The head of an Inuit lance made from a narwhal tusk with a meteorite iron blade (British Museum)

In Inuit legend, the narwhal's tusk was created when a woman with a harpoon rope tied around her waist was dragged into the ocean after the harpoon had struck a large narwhal. She was transformed into a narwhal and her hair, which she was wearing in a twisted knot, became the characteristic spiral narwhal tusk.[66]

Some medieval Europeans believed narwhal tusks to be the horns from the legendary unicorn.[67][68] As these horns were considered to have magic powers, such as neutralising poison and curing melancholia, Vikings and other northern traders were able to sell them for many times their weight in gold.[69] The tusks were used to make cups that were thought to negate any poison that may have been slipped into the drink. A narwhal tusk exhibited at Warwick Castle is according to legend the rib of the mythical Dun Cow.[70] In 1555, Olaus Magnus published a drawing of a fish-like creature with a horn on its forehead, correctly identifying it as a "Narwal".[67] During the 16th century, Queen Elizabeth I received a narwhal tusk worth 10,000 pounds sterling—the 16th-century equivalent cost of a castle (approximately £1.5–2.5 million in 2007, using the retail price index[69])–from Sir Humphrey Gilbert, who proposed that the tusk was from a "sea-unicorne". The tusks were staples of the cabinet of curiosities.[67]

Image of narwhal from Brehms Tierleben (1864–1869)

The narwhal was one of two possible explanations of the giant sea phenomenon written by Jules Verne in his 1870 novel Twenty Thousand Leagues Under the Sea. Verne thought that it would be unlikely that there was such a gigantic narwhal in existence. The size of the narwhal, or "unicorn of the sea", as found by Verne, would have been 18.3 m (60 ft). For the narwhal to have caused the phenomenon, Verne stated that its size and strength would have to increase by five or ten times.[71]

Herman Melville wrote a section on the narwhal (written as "narwhale") in his 1851 novel Moby-Dick, in which he claims a narwhal tusk hung for "a long period" in Windsor Castle after Sir Martin Frobisher had given it to Queen Elizabeth. Another claim he made was that the Danish kings made their thrones from narwhal tusks.[72]