The narwhal is a medium-sized toothed whale, with males reaching a maximum length of about 460 cm and a maximum weight of 1645 kg, and females 400 cm and 900 kg (NAMMCO 2013, Garde et al. 2007). Narwhals are very long-lived animals: recent research looking at chemical changes in the eye lens has demonstrated that narwhal can live for over 100 years (Garde et al. 2007), making the narwhal the longest-lived of the toothed whales and one of the longest lived of all mammals.
The narwhal reaches sexual maturity between the ages of 7 and 9. They mate from late May to early June, and calves are born about a year later, during the time the animals are migrating towards their summering areas. On average narwhal have a single calf every 3 years throughout their lifetime (NAMMCO 2013, Garde et al. 2007).
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The Unicorn of the sea
The most distinguishing feature of the narwhal is the male’s long tusk, which has led them to be called the “unicorn of the sea”. The tusk is actually a modified canine tooth. Usually only one tusk erupts from the left side of the upper jaw, but a small percentage of male narwhal have a tusk on both sides of the jaw. Other than the single tusk, narwhals are toothless. The tusk is spiral in form, almost always in an anti-clockwise direction. In some narwhal the tusk may exceed 2 m in length (NAMMCO 2013, Garde et al. 2007).
Photo: Zoologisches Museum Hamburg
Narwhal feed primarily on fish and squid throughout their range. Both the diet and the intensity of feeding vary seasonally.
During the summer, narwhal apparently feed very little; hence the stomachs of narwhal harvested at this time of year are usually empty. They begin to feed more heavily in the fall during migration and especially during the winter at their deep-water wintering grounds. In Baffin Bay, the winter diet is dominated by the Greenland halibut (Reinhardtius hippoglossoides) (Laidre and Heide-Jørgensen 2005b). To access this bottom-dwelling species, narwhals must make repeated dives to depths of between 800 and 1600 m, each dive taking up to 25 minutes, with about half of that spent near the bottom (Laidre et al. 2003, 2004). Arctic cod (Arctogadus glacialis), polar cod (Boreogadus saida), and squid of the genus Gonatus also form an important part of the winter diet, and may be available in mid-water in areas too deep for narwhals to reach the bottom (Laidre et al. 2003, 2004, Laidre and Heide-Jørgensen 2005b).
During the spring, as narwhals follow the retreating ice edge, polar and Arctic cod become more important in the diet (Laidre et al. 2004).
Like all toothed whales, narwhals use echolocation to find prey. This is particularly important for a species that does most of its feeding in very deep waters during the long nights of the Arctic winter. A recent study in which a camera was attached to free ranging narwhals demonstrated that narwhal spend a high proportion of their time upside down while diving and swimming at the sea bottom (Dietz et al. 2007). This may allow them to better project their echolocation clicks to locate prey, and perhaps also to protect their lower jaw and tusk from contact with the bottom.
There have been some attempts to determine the importance of narwhals as predators, by modelling how much fish they consume. Laidre et al. (2004) used a bioenergetic model to estimate the biomass of Greenland halibut needed to sustain narwhals during the five months that they spend on their wintering grounds. Any energy budget model requires certain assumptions to be made, as not all the needed information is available. The authors developed what they termed a “minimum realistic” model with results that compared well with a similar model developed for beluga whale (Welch et al. 1993 in Laidre et al. 2004). Estimates were made for varying levels of Greenland halibut in the narwhal diet: 25%, 50% and 75%. Assuming a diet comprised of 50% Greenland halibut, the 32,000 narwhals in the southern overwintering area would eat approximately 576 t per day, or about 86,000 t of Greenland Halibut for the five-month winter period (DFO 2014). The area classified as the northern over-wintering ground supports a larger number of whales (approx. 45,000) and was estimated to require 700 t per day with a mean consumption over five months of 110,700 t (95% CI of 53,000 t - 310,300 t)(DFO 2014, DFO 2007). This amount is, however, larger than the estimated biomass of Greenland halibut in that area, suggesting that narwhal in this area make greater use of other prey species.
Narwhals show sexual dimorphism, with several differences between males and females. Adult male narwhal are on average longer and heavier than females, reaching a maximum length of about 460 cm and a maximum weight of 1645 kg. Adult females have a maximum length of 400 cm and weight of 900 kg (NAMMCO 2013, Garde et al. 2007).
The other obvious difference between the sexes is the presence of the tusk in males. While a small percentage of females may also have a tusk (roughly 6% of tusked narwhals in the Canadian Inuit harvest were females (Petersen et al. 2012)), it is a feature found in all adult males. The tusk is actually a modified canine tooth, and is in fact the only tooth these whales possess. Usually only one tusk erupts from the left side of the upper jaw, but in a small percentage of male narwhals a second tusk erupts from the right side of the jaw. The tusk is spiral, almost always in an anti-clockwise direction (NAMMCO 2013, Garde et al. 2007).
Narwhal breach. Photo: Gl. Williams, NIST, NOAA.
Recently Nweeia et al. (2014) suggested that the tusk may have another function altogether: as a sensory organ. The tusk is innervated and small channels in the tusk put the inner, pulpy core of the tusk in contact with the marine environment. A tusked (usually male) narwhal may therefore be able to directly sense changes in the marine environment, such as variations in salinity and temperature, through its tusk. This may aid in navigation and in avoiding being caught in rapidly freezing seas. Nevertheless it is difficult to explain why only male narwhals would have evolved or retained this capability.
Apart from man, the main predator of narwhal is the killer whale. Narwhal react to the presence of killer whales by moving close to land or sea ice, and forming tight groups. They may move into very shallow water and even occasionally become beached (Laidre et al. 2006).
|From Laidre et al. (2006) Killer whales attacking narwhals. Left: When killer whales arrived, narwhals formed tight groups and remained close to the shore lying still. Photo: M. P. Heide-Jørgensen. Right: during the killer whale attack, narwhals beached themselves in sandy areas and made tail slaps. Photo: K. L. Laidre|
Watch a video of killer whales attacking a pod of narwhals!
Because of their association with heavy ice cover, narwhals are vulnerable to becoming trapped in ice if high winds move ice around or the temperature drops suddenly. Narwhals require leads or open areas in the ice to breathe, since they are unable to open or maintain a breathing hole through thick ice. A cetacean entrapment in ice is called a savsaat in Inuktitut, and is a relatively common if unpredictable occurrence. Such entrapments may be a major source of natural mortality for narwhal (Heide-Jørgensen et al. 2002). Many of these events probably go undetected, especially if they are small and in a remote location.
A savsaat in 2008 in Eclipse Sound saw at least 629 narwhals entrapped. Six hundred and twenty-nine narwhal were harvested from the savsaat, but more animals may have drowned (DFO 2008). Such a large entrapment of animals is rare in Nunavut, having been recorded only once before in the previous century (Heide-Jørgensen et al. 2002).