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The
movie Legend starred Tom Cruise and Mia Sara,
as
well as a bunch of little people – you know, actors
that
were small, not small actors. The unicorn pair
represented
light and goodness, and kept the devil
at
bay. Until Mia got cocky and touched one. Then
Cruise
had to save the day.
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Not to be outdone, Kim Jung Un made an amazing announcement in 2012. He and his archeologists discovered a unicorn lair. Yep, North Korea’s
twenty-something leader proved the existence of unicorns. The lair was
supposedly the resting place of the unicorn ridden by the great King
Dongmyeong, around the year 0 CE.
The earliest writings that describe unicorns
were those of the Greek, Ctesias, in the late 5th century BCE. He
described the Indian Ass, an animal with a white, strong body and perhaps a red
head from which sprung a long single horn of red, white, and black. It was said
that a cup made from the horn could neutralize any poison.
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There
are real animals with one horn, like the
unicorn
leatherjacket fish in the top left, and the
Indian
rhinoceros at the bottom left. The rhinoceros
beetle
has one big horn and fairly large part of his
jaw
below, so I don’t know if he counts. On the top
right
is the Meller’s chameleon. They say he a has a
horn
on his nose, but you have to look close and
want
to see it.
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Pliny wrote, “The unicorn (uni = one, and ceros =
horn) is the fiercest animal, and it is said that it is impossible to capture
one alive. It has the body of a horse, the head of a stag, the feet of an
elephant, the tail of a boar, and a single black horn three feet long in the
middle of its forehead. Its cry is a deep bellow.” Uh-huh. That doesn’t sound
much like an antelope or a rhino, so I guess he meant the Indian Ass.
Soon, Romans were trading long spiral tusks, but no one was telling where exactly they had come from. These “unicorn” horns were snow white with
a tight spiral. As a result of these horns, the unicorn in the West settled
down to be a pure white horse with a very long, pure white, spiraled horn. This
is the image we generally see in tapestries and illustrations.
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Kirin
Beer from Japan uses a unicorn (kirin) as its
logo.
Look closely and you can see the single
horn
on its head.
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But what about real life? Most likely, those horns in the
Roman markets were really narwhal
tusks, as discussed in a 2011 paper. It is very likely that the narwhal played
into the unicorn legend, as their tusks could be offered as concrete proof of
unicorn existence.
The narwhal (Monodon
monoceros) is an amazing animal, and fits into our recent theme of animals
that abandon bilateral symmetry. Monodon
means one tooth, and monoceros means
one horn; a pretty accurate name, all in all.
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Our
post today uncovers many of the problems
with
these cartoon narwhals. Yes, they love where
there
is ice. But they don’t have all those teeth, the
tusk
isn’t centered and doesn’t come out of their
forehead,
and they don’t have a dorsal fin
to
speak of.
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This is a steady number because it’s so hard to get to where
they live. Consequently, narwhals haven’t been hunted into extinction. They
spend a lot of their time on deep dives under the ice floes, so they aren’t
seen often. No narwhal has ever been seen feeding; we only know what they
eat from examining stomach contents.
Their most distinctive feature is the long (up to 10 ft/3 m)
tusk on the males. Just one tusk, mind you, like a unicorn horn. The narwhal tusk -
like elephant, walrus or warthog tusks - is a tooth.
Very young narwhals have six maxillary (upper jaw) tooth
buds and two pairs of tooth buds in the lower jaw (mandible). However, only one
pair develops any further. A tooth bud is what you find on an X-ray of a child
(see picture).
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You
can see the teeth developing from crown to
root
in the darker tooth buds. The pulp is usually
dark,
but the middle tooth has had a root canal
and
a filling has been placed in the whole pulp
chamber.
The large tooth to eh left is the first
molar.
It doesn’t have a baby tooth to push out
of
its way.
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Just one tooth, almost always the left cuspid (most people
call it a canine), does develop. Hold on though, it isn’t that simple. Instead
of developing in a vertically directed tooth bud and erupting down through the
jaw, the left canine stays horizontal and erupt right through the front of the
jaw and through the narwhals lip!
Since the tusk is derived from the left cuspid, it erupts
left of center, making the narwhal bilaterally asymmetric! A 2012 study showed
that the bony attachment and length proves that the narwhal tusk is a canine,
not an incisor as so many people think. But, it’s not just the location that
makes the narwhal tusk amazing, it’s how it’s made and what it can do.
A 1988 study suggests that the tight spiral as it grows keep
the tusk from curving. A curved tusk would make it hard of the narwhal to swim in a straight line.
Whatever the reason, the spiral is an iconic image for both narwhals and
unicorns.
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The
top image shows how the narwhal tusk is off
center.
The bottom image is my analogy. The tusk
is
offset like a knight with his jousting lance. This
is
Heath Ledger in A Knight’s Tale. Um….why
isn’t
he
wearing armor?
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Teeth are normally built with the hard enamel on the
outside. Enamel is harder than bone and protects the teeth from breakage when
chewing. The mouth is a rough environment and teeth have to put up with a lot
of abuse.
Deep to the enamel is a material called dentin. This stuff
has a lot of similarity to bone, although it isn’t quite as hard and doesn’t
have living cells within it (like osteocytes – see this post). The dentin does
contain millions of tubules that go from the enamel junction all the way to the
pulp in the center. The pulp has a nerve and blood vessels.
The dentinal tubules have fluid and small processes
of the neuron in them. When you eat something cold or have a cavity, the fluid in these
tubules moves and changes the pressure in the pulp chamber. The single neuron in the
tooth is a pain neuron, so any pressure change is interpreted by your brain as
pain. It teaches you to take care of your teeth, but it ain’t the most pleasant
of all evolutionary adaptations.
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The
cartoon on the left shows the enamel crown
covering
the dentin and the dentinal tubules.
Inside
the tubules are the odontoblasts that lay
down
dentin all during the life of the tooth and the
nerves
that go into the tubules. The right image is
an
electron photomicrograph of the tubules.
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A 1990 study compared calcium content and hardness between human
teeth and narwhals. The narwhal cementum was more mineralized than human, but
the dentin of narwhals was less mineralized than human dentin and was softer.
This may be why the narwhal tusk is so flexible.
The tubules of the narwhal tusk dentin connect to channels in the
cementum, so there is a communication to the outside. A group in 2014 showed
this and used the information to hypothesize that the tusk is a mechanosensor. Experiments
showed that their heart rate changed when the water touching the tusk was
switched from freshwater to salt water. They hypothesize that the tusk senses
temperature, salinity, pressure, and perhaps touch to help in navigation and
hunting.
But if that’s the case, why do only males have them? Females
have to hunt too. The group from the 2014 paper offers that males and females
have sexually dimorphic foraging techniques – they eat different things and
hunt differently, so females don’t need horns. This is not well-supported. Many
scientists believe the long tusk is a sign of health and genes and is therefore
an ornament for mate selection.
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The
dorsal fin of the narwhal is greatly reduced. It
has
notches that scientists hop to use to identify
individuals.
The lack of a dorsal fin is believed to
be
so they don’t injure it on the under side of the
ice
floes when they surface, but it could also be so
they
don’t run it into the ocean floor as they feed
upside
down.
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One last thing. The offset tusk lead to another weird
narwhal behavior. A group in 2007 put cameras and positional monitors on some
narwhals and found that they tend to swim upside down a lot. Almost 70% of
their time on the ocean floor was spent in the supine position. Since the tusk
points down just slightly, scientists believe they hunt upside down so that the tusk won’t get stuck in the ocean floor
and break! The tusk must be pretty important - or they just like lounging on
their backs.
Next week – another whale has become asymmetric, but in a
completely different way. This time, it’s the nose that goes.
For
more information or classroom activities, see:
Narwhals
–
Tooth
structure –
