The narwhal is extraordinary among marine mammals and exhibits four distinguishing features of teeth. First, the adult male tusk erupts through the left side of the upper jaw plate and lip, extending 8-10 feet (Tomilin, 1957) and forming a unique left handed helix (Kingsley and Ramsay, 1988; Brear et al., 1990). Spiral tusk morphology is rarely observed with isolated examples found primarily in elephants undergoing trauma during tooth development (Goethe, 1823 and 1949; Busch, 1890; Colyer, 1915). Second, the tusk is prevalent in males and rarely exists among females, and is thus an unusual example of sexual dimorphism in mammalian teeth. Comparative findings are limited for other beaked whales that exhibit sexually dimorphic teeth (Heyning, 1984; Mead, 1989). The presence of a tusk among females is reported to be 15% (Roberge and Dunn, 1990) and is smaller in size and more narrow in form than male tusks (Clark, 1871; Pederson, 1931). Third, in cases of a single tusk expression, the tooth protrudes from the left side, distinguishing the narwhal with an extreme example of dental asymmetry in the teeth of mammals (Hay and Mansfield, 1989; Harrison and King, 1965). Only in rare instances, such as the fossil record of Odobenscetops peruvianus, does such asymmetry in tusk expression exist (de Muizon, 1993). This walrus-like species is hypothesized to be in the Delphinoidae family and may be related to Monodontidae
. Fourth, the asymmetry is evident in size and morphology. Approximately 1.5% of males exhibit double tusks (Hay, 1984). In such instances the right tusk is often shorter (Fraser, 1938) and the helix also left handed (Clark, 1871; Gervais, 1873; Thompson 1952). Normally, the tusk antemere would measure equal in size and be identified with a mirror imaged morphology that would include a right handed helix (Reeves and Tracy, 1980).
Myriad theories have been proposed to explain the purpose and function of the tusk (Helffich, 1988; Rosing, 1999). They include use as a weapon of aggression between males (Brown, 1868; Beddard, 1900; Lowe, 1906; Geist et al., 1960), a secondary sexual characteristic in males (Scoresby, 1820; Hartwig, 1874; Mansfield et al., 1975) an instrument for breaking ice (Scoresby, 1820; Tomilin, 1957), a spear for hunting (Vibe, 1950; Harrison and King, 1965; Bruemmer, 1966; Ellis, 1980), a ritualistic appendage in establishing male hierarchy (Geist, 1966, 71; Best, 1981; Pilleri, 1983), a breathing organ, a thermal regulator, a swimming rudder (Kingsley and Ramsay, 1988), a tool for digging (Freuchen, 1935; Peterson, 1960; Newman, 1971), and an acoustic organ or sound probe (Best, 1981; Reeves and Mitchell, 1981). Several theories have been offered to explain the unique tusk helix formation but none are postulated with conclusive evidence (Thompson, 1939 and 42). Most marine mammal researchers acknowledge that no explanation of the male tusk has received wide acceptance.
Limited publications are available documenting the anatomy, histology, and cell and molecular biology of narwhal teeth. Studies are inconsistent and can contradict one another. For example, there are reports of double tusked narwhal being primarily female (Eales, 1950; Thomson, 1952; Nowak and Paradiso, 1997), which contrasts with Inuit observations (Nweeia, video interviews with elders, 2003) and some published accounts as identifying only males with this trait (Scoresby, 1820; Bruemmer, 1966). Inuit elders interviewed have never seen nor heard of a narwhal with a tusk on the right side. Estimates of broken tusks range from 30% (Silverman and Dunbar, 1980) to 66% (Gerson and Hickie, 1985), though photographs from the 1915 ice entrapment (Born et al., 1994) of over 1000 narwhal in Greenland show far less than 30 broken tips of 200 tusks (Nweeia, photographic observation). Though previous studies of narwhal dentin describe a less mineralized tissue than human or cattle dentin (Brear et al., 1990), further analysis will be undertaken as preliminary SEMís reveal a highly mineralized tissue. The presence of enamel will also be investigated as conflicting results are reported in the literature.
Behavioral observations are also inconsistent. Though studies report the aggressive use of the male tusk between males or in defense against predators (Buckland, 1882; Gray, 1889; Freuchen, 1935; Breummer, 1966), other observations indicate that aggressive behavior and tusk combat in narwhal is rare (Silverman and Dunbar, 1980; Gerson and Hickie, 1985) and sometimes described as delicate and sensual (Porslid, 1922).
Evolutionary biology provides little insight into the appearance of and explanation for the narwhal tusk (Winge, 1921; Kule, 1972; Evans, 1989; Heide-Jorgensen and Reeves, 1993). From hypothesized artiodactyl origins (Gatesy, 1999) to the appearance of Monodon monoceros, there is limited precedence for the expression of the male tusk, and only speculation to explain how or why it exists. The fossil record for Monodontidae dates to the Miocene era approximately 11-15 million years ago (Waddell et al., 2000). Isolated hybrid whales have been discovered with narwhal-like traits (Mitchell and Kemper, 1980; de Muizon, 1993; Heide-Jorgensen and Reeves, 1993) but none has provided an evolutionary link or insight into the existence of the narwhal tusk other than describing it as an example of Darwinian sexual selection.