127
to making remarkably deep dives, narwhals also spend a large amount of their
time below 800 m (>3 h per day). This is an incredible amount of time at a depth
where the pressure can exceed 2200 PSI (150 atmospheres) and life exists in complete darkness. Finally, they can live up to 90 years in the wild, are the only whales
that overwinter in the Arctic pack ice. There are about 80,000 of them worldwide, ”
(Laidre, “Narwhal FAQ” published online; for more information Laidre and
Heide- Jørgensen 2005 , 2011 ; Laidre et al. 2003 ).
The very characteristic morphological feature of the male narwhal is its single
2–3 m long tusk, a canine tooth (Nweeia et al. 2012 ). This unique structure projects
from the left side of the upper jaw and forms a left-handed helix (Kingsley and Ramsay
1988 ; Brear et al. 1990 ). The left-hand helical nature of tooth development has been
hypothesized to be a functional adaptation to maintain the overall concentric center of
mass during growth (Nweeia et al. 2009 ). This hypothesis certainly makes a great deal
of sense when one considers the hydrodynamic loads that would develop if the tooth
were curved or skewed to one side. The animal with a body length of 4–5 m, and
weigh up to 10 kg, can possess tusk up to 3 m long (see for review Best 1981 ; Milius
2006 ). A female narwhal are characterized by a straighter and shorter tusk. Also a
female with dual tusks have been observed, however it may be an exception. In males
and females, the left tooth erupts at the end of the fi rst year (Silverman and Dunbar
1980 ). The development of two tusks has been also reported (Clark 1871 ).
As recently described by Coyne ( 2012 ):
“Narwhals have also vestigial teeth that lodge (horizontally) in the maxillary
bone. Although they rarely erupt in some males, they don’t erupt into the mouth;
they lodge between the palatal tissue and underlying maxillary bone. These teeth
are very small, from 1 to 30 mm long. They lie behind and to the left of the sockets
for the two tusks (only one of which usually erupts), ” (Coyne 2012 ).
Numerous hypothesis have been proposed (Gervais 1873 ; Silverman and Dunbar
1980 ; Best 1981 ; Gerson and Hickie 1985 ; Nweeia et al. 2009 , 2012 ) for better
understanding of the purpose and function of the tusk. These suggestions include
use as a secondary sexual characteristic in males an instrument for breaking ice, a
spear for hunting, a ritualistic appendage in establishing male hierarchy, a weapon
of aggression between males, a breathing organ, a swimming rudder, a thermal
regulator, a tool for digging, and an acoustic organ or sound probe. Examination of
tusk anatomy, histology, and biomechanics combined with traditional knowledge of
Inuit elders and hunters has revealed features that support a new sensory hypothesis
for tusk function (Nweeia et al. 2005 ).
The microanatomy of the tusk provided insight into potential function. SEM
micrographs of the pulpal surface revealed tubule features that are similar in size
and shape to the dentin tubules found in masticating teeth (see for details Nweeia
et al. 2009 ). The tubule diameters are similar to those observed in human teeth, but
the spacing of these tubules across the pulpal wall is three to fi ve times wider than
that seen in human dentin (see for comparison Zaslansky 2008 ). The polished cross
sections show that these tubules run continuously throughout the entire thickness of
dentin, just as they do in human dentin. A surprising fi nding, however, was the presence of tubule orifi ces on the outer surface of the cementum. This indicates that the
3.2 Teeth
to making remarkably deep dives, narwhals also spend a large amount of their
time below 800 m (>3 h per day). This is an incredible amount of time at a depth
where the pressure can exceed 2200 PSI (150 atmospheres) and life exists in complete darkness. Finally, they can live up to 90 years in the wild, are the only whales
that overwinter in the Arctic pack ice. There are about 80,000 of them worldwide, ”
(Laidre, “Narwhal FAQ” published online; for more information Laidre and
Heide- Jørgensen 2005 , 2011 ; Laidre et al. 2003 ).
The very characteristic morphological feature of the male narwhal is its single
2–3 m long tusk, a canine tooth (Nweeia et al. 2012 ). This unique structure projects
from the left side of the upper jaw and forms a left-handed helix (Kingsley and Ramsay
1988 ; Brear et al. 1990 ). The left-hand helical nature of tooth development has been
hypothesized to be a functional adaptation to maintain the overall concentric center of
mass during growth (Nweeia et al. 2009 ). This hypothesis certainly makes a great deal
of sense when one considers the hydrodynamic loads that would develop if the tooth
were curved or skewed to one side. The animal with a body length of 4–5 m, and
weigh up to 10 kg, can possess tusk up to 3 m long (see for review Best 1981 ; Milius
2006 ). A female narwhal are characterized by a straighter and shorter tusk. Also a
female with dual tusks have been observed, however it may be an exception. In males
and females, the left tooth erupts at the end of the fi rst year (Silverman and Dunbar
1980 ). The development of two tusks has been also reported (Clark 1871 ).
As recently described by Coyne ( 2012 ):
“Narwhals have also vestigial teeth that lodge (horizontally) in the maxillary
bone. Although they rarely erupt in some males, they don’t erupt into the mouth;
they lodge between the palatal tissue and underlying maxillary bone. These teeth
are very small, from 1 to 30 mm long. They lie behind and to the left of the sockets
for the two tusks (only one of which usually erupts), ” (Coyne 2012 ).
Numerous hypothesis have been proposed (Gervais 1873 ; Silverman and Dunbar
1980 ; Best 1981 ; Gerson and Hickie 1985 ; Nweeia et al. 2009 , 2012 ) for better
understanding of the purpose and function of the tusk. These suggestions include
use as a secondary sexual characteristic in males an instrument for breaking ice, a
spear for hunting, a ritualistic appendage in establishing male hierarchy, a weapon
of aggression between males, a breathing organ, a swimming rudder, a thermal
regulator, a tool for digging, and an acoustic organ or sound probe. Examination of
tusk anatomy, histology, and biomechanics combined with traditional knowledge of
Inuit elders and hunters has revealed features that support a new sensory hypothesis
for tusk function (Nweeia et al. 2005 ).
The microanatomy of the tusk provided insight into potential function. SEM
micrographs of the pulpal surface revealed tubule features that are similar in size
and shape to the dentin tubules found in masticating teeth (see for details Nweeia
et al. 2009 ). The tubule diameters are similar to those observed in human teeth, but
the spacing of these tubules across the pulpal wall is three to fi ve times wider than
that seen in human dentin (see for comparison Zaslansky 2008 ). The polished cross
sections show that these tubules run continuously throughout the entire thickness of
dentin, just as they do in human dentin. A surprising fi nding, however, was the presence of tubule orifi ces on the outer surface of the cementum. This indicates that the
3.2 Teeth