The prominent, spiraling tusk that emerges from the head of the unusual whales called narwhals may serve a variety of uses for these animals, including acting as a water-salinity detector, according to recent studies.
The narwhal (Monodon monoceros) ranks among the rarest and most elusive whales in the world. It inhabits the North Atlantic Ocean and the Arctic waters of Canada, Greenland, Norway, and Russia. It tends to remain above the Arctic Circle throughout the year, preferring areas with deep waters and loose pack ice (the animal's migration is typically triggered by the seasonal advance and retreat of the sea ice). As an adult, the narwhal can attain a body length of 4–6 m (13–20 ft) and a weight of 1600 kg (3520 lb), with females being slightly smaller than males. The life span of the narwhal has been estimated to be more than 50 years. See also: Arctic Circle; Arctic Ocean; Atlantic Ocean; Cetacea; Diving animals
The most notable anatomical feature of the narwhal is its ivory tusk, which is a long, twisted, pointed canine tooth, growing directly through the whale's upper lip. The tusk often reaches a length of 2.7 m (8.8 ft). Because of the tusk's appearance, the narwhal is often referred to as the "unicorn of the sea." The tusk is typical of male narwhals; it appears at a less prominent size in about 15% of female narwhals. The tusk is located on the left-hand side, and it always spirals to the left (counterclockwise). In rarer cases in which a right tusk emerges in addition to the left one, it too spirals in the same counterclockwise direction. Otherwise, the narwhal is toothless, having only vestigial tooth stubs. Another distinguishing anatomical characteristic of narwhals is their lack of a dorsal fin. See also: Animal evolution; Dentition; Tooth
Researchers have investigated the function of the narwhal's tusk and drawn various conclusions. The tusk may serve as a sexual trait among males for determining social rank, exerting dominance, and competing for females. Its use has been observed in mating rituals, and researchers surmise that it also may be employed as a weapon in fights between rival animals. See also: Reproductive behavior; Sexual dimorphism; Social hierarchy
In a recent issue of The Anatomical Record (April 2014), a sensory function for the tusk was also proposed by a team led by Martin Nweeia. Investigations have determined that the narwhal's tooth has a unique sensory capability, enabling it to detect water-salinity changes. The outer layer of the tooth has no enamel and thus is porous, allowing the salinity levels of the surrounding seawater environment to be communicated to the brain (via an extensive sensory network in the tusk, which contains approximately 10 million nerve endings). This sensory feature has been demonstrated by the observation of significant changes in the narwhal's heart rate when the external surface of the tusk was exposed to alternating solutions of high-salt water and freshwater. This ability to sense the salinity of the surrounding seawater might help these marine mammals to navigate properly or to locate food. See also: Nerve; Nervous system (vertebrate); Neurobiology; Seawater; Sensation
Narwhals are assigned a near-threatened status by the Red List of the International Union for Conservation of Nature. Their worldwide population is estimated to be 45,000 to 80,000. Although narwhals are preyed upon by polar bears and killer whales, and indigenous Arctic communities are permitted to hunt them, the effects of climate change and water pollution are the greatest threats to the narwhal's survival. In particular, the narwhal depends on sea ice for its existence; thus, any changes in the amount of sea ice as the result of global warming can have a tremendous impact on the continued existence of this unique creature. See also: Climate modification; Effects of global warming on polar bears; Endangered species; Global climate change; Global warming; Ocean warming; Sea ice; Species and global climate change; Water pollution