129
These include ion gradients, such as water salinity, pressure gradients caused by
dive depth or atmospheric pressure changes, air temperature and movement, or possibly other chemical stimuli specifi c to food sources or environment. Due to these
results, the narwhal dentin is proposed to be a unique example of the naturally
designed tissue with sensory abilities to detect, pressure, osmotic gradients and temperature. The role of sexual selection cannot be excluded because erupted tusks are
usually associated with males (Nweeia et al. 2005 , 2009 ).
Of the total radius of the narwhal tusk, the dentine occupies about 45 %, the
cementum about 15 % and the pulp cavity about 40 % (Brear et al. 1990 ). Of course,
the chemical content, size and morphology of narwhal tusk prompted numerous
comparative studies on mechanical and material properties of this interesting
biological material (Brear et al. 1990 , 1993 ; Currey et al. 1994 ; Zioupos and
Currey 1996 ; Nweeia et al. 2009 ). For example, Currey ( 2006 ) suggested three
quite different mechanisms for toughening of the narwhal dentine:
• “ One makes use of the difference in properties of fully mineralized dentine and
the hypomineralized, interglobular dentine.
• The second mechanism makes it easier for a crack to form along the fi brillar
direction.
• A third, more classical mechanism makes use of the fact that dentine is arranged
in layers, coaxial to the long axis of the tusk, ” (Currey 2006 ).
Microhardness and Young’s Modulus measured by nanoindentation both
appear to be correlated with the mineral-to-collagen ratios (MCR) in narwhal
tooth tissues away from the tusk base. It was showed that these properties and the
degree of correlation are location specifi c (Eidelman et al. 2005 ). They show tendency to decline in moving from the tip to the base. The modulus demonstrated a
strong decline moving from pulp to the outer surface in the two cross sections
away from the tip. The fl exural strength and work of fracture both increased for
dentin when comparing the tusk base to the midsection. The fl exural strengths of
95 MPa at mid tusk and 165 MPa at the base compare to approximately 100 MPa
for human dentin. These are adaptations for a tooth that must withstand high
fl exural stresses and deformation rather than the compressive loads of chewing
(Nweeia et al. 2009 ).
As reviewed by Freeman et al. ( 1998 ), narwhals have been hunted in Greenland
and eastern Canada for centuries, and may have brought the Greenlandic Inuit in
close contact with the Norse in Greenland beginning in the tenth century. Narwhal
ivory was bartered among Inuit long before European contact. Narwhal tusks were
highly valued by European traders in the Middle Ages, who sold the tusks in Europe
mislabeled as unicorn horn, sometimes for their weight in gold. The royal throne
of Denmark, made in the fi fteenth century, is made almost entirely of narwhal
ivory. Numerous handmade articles made from narwhal tusks are still present in
museums and collections worldwide (Fig. 3.20 ). However, Inuit in Greenland and
Canada used the tusks to create durable and functional tools, especially harpoon
fore shafts.
3.2 Teeth
These include ion gradients, such as water salinity, pressure gradients caused by
dive depth or atmospheric pressure changes, air temperature and movement, or possibly other chemical stimuli specifi c to food sources or environment. Due to these
results, the narwhal dentin is proposed to be a unique example of the naturally
designed tissue with sensory abilities to detect, pressure, osmotic gradients and temperature. The role of sexual selection cannot be excluded because erupted tusks are
usually associated with males (Nweeia et al. 2005 , 2009 ).
Of the total radius of the narwhal tusk, the dentine occupies about 45 %, the
cementum about 15 % and the pulp cavity about 40 % (Brear et al. 1990 ). Of course,
the chemical content, size and morphology of narwhal tusk prompted numerous
comparative studies on mechanical and material properties of this interesting
biological material (Brear et al. 1990 , 1993 ; Currey et al. 1994 ; Zioupos and
Currey 1996 ; Nweeia et al. 2009 ). For example, Currey ( 2006 ) suggested three
quite different mechanisms for toughening of the narwhal dentine:
• “ One makes use of the difference in properties of fully mineralized dentine and
the hypomineralized, interglobular dentine.
• The second mechanism makes it easier for a crack to form along the fi brillar
direction.
• A third, more classical mechanism makes use of the fact that dentine is arranged
in layers, coaxial to the long axis of the tusk, ” (Currey 2006 ).
Microhardness and Young’s Modulus measured by nanoindentation both
appear to be correlated with the mineral-to-collagen ratios (MCR) in narwhal
tooth tissues away from the tusk base. It was showed that these properties and the
degree of correlation are location specifi c (Eidelman et al. 2005 ). They show tendency to decline in moving from the tip to the base. The modulus demonstrated a
strong decline moving from pulp to the outer surface in the two cross sections
away from the tip. The fl exural strength and work of fracture both increased for
dentin when comparing the tusk base to the midsection. The fl exural strengths of
95 MPa at mid tusk and 165 MPa at the base compare to approximately 100 MPa
for human dentin. These are adaptations for a tooth that must withstand high
fl exural stresses and deformation rather than the compressive loads of chewing
(Nweeia et al. 2009 ).
As reviewed by Freeman et al. ( 1998 ), narwhals have been hunted in Greenland
and eastern Canada for centuries, and may have brought the Greenlandic Inuit in
close contact with the Norse in Greenland beginning in the tenth century. Narwhal
ivory was bartered among Inuit long before European contact. Narwhal tusks were
highly valued by European traders in the Middle Ages, who sold the tusks in Europe
mislabeled as unicorn horn, sometimes for their weight in gold. The royal throne
of Denmark, made in the fi fteenth century, is made almost entirely of narwhal
ivory. Numerous handmade articles made from narwhal tusks are still present in
museums and collections worldwide (Fig. 3.20 ). However, Inuit in Greenland and
Canada used the tusks to create durable and functional tools, especially harpoon
fore shafts.
3.2 Teeth