Wednesday, March 18, 2015

The Search For The Unicorn - Slightly Off Center

Biology concepts – teeth, narwhals, unicorns, bilateral symmetry, evolution, mechanosensing, asymmetry

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.
It’s no secret that some pretty odd and awful stories have come out of North Korea in the past few years. Kim Jung Un and his recent ancestors have done some amazing things….. supposedly. Un’s father, Kim Jung Il apparently invented the hamburger, and he shot 11 hole-in-ones in his first round of golf.

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.

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.
Four hundred and fifty years later, Pliny the Elder, historian of Rome, also wrote about a very strong animal with a single horn protruding from its forehead. He described an oryx (an antelope with a single horn), an Indian Ox (probably a rhinoceros – rhino = nose and ceros = horn), and the same Indian Ass with a horse-like build and a single horn.

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.

Kirin Beer from Japan uses a unicorn (kirin) as its
logo. Look closely and you can see the single
horn on its head.
In the Far East there were unicorns as well. Known as the qilin (pronounced chee-lin) in China, there was a version in Japan too, the kirin. This was a benevolent animal, with shiny scales like a dragon and one or perhaps two horns. It avoided fighting and walked so softly that it would not disturb or harm a blade of grass. An animal like this (perhaps the saola) is most likely the one referred to in the North Korea stories.

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.

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.
Narwhals are a species of whale, meaning that they are mammals. They live way up north. From Baffin Bay, around Greenland, to the north of Russian, they swim in pods of 10-100, but you’ll rarely see them even if you live near there. There are perhaps 45,000-50,000 narwhals today.

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).

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.
Teeth form in the jawbones as tooth buds. Most narwhal teeth never go past the tooth bud stage, but occasionally a tooth will erupt where one shouldn’t. These are often misshapen or caught between the bone and the palate, or in the wrong place. This is all good evidence that the teeth are vestigial; they serve no functional purpose for the normal narwhal.

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.

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?
Despite being a tooth, the tusk is quite flexible. It can bend up to a foot (0.3 m) in any direction without breaking. It’s awfully long, we said 10 ft. above, but most are in the 8-9 foot range. This is huge when you think that most male narwhals are only about 15 foot long in the body.

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.

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.
The narwhal tusk is different. It is the only tooth known that has the dentin on the outside, although a 1987 study showed that it has no enamel, so it isn’t really an inside out tooth. The dentin is covered by a thin layer of cementum. This is what normally covers the roots of the teeth and helps attach them to bone. The dentin of the narwhal tusk has about 10 million of those tubules, but it is different from human dentin.

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.

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.
Occasionally, one will see females with a tusk, but like with many tusked females (elephants, etc), they are usually shorter. You can also find narwhal males with two tusks. But two tusks doesn’t mean that they are returned to bilateral symmetry. Both tusks spiral to the left! There must be some strong left-hand genes at work.

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.

Christen AG, & Christen JA (2011). The unicorn and the narwhal: a tale of the tooth. Journal of the history of dentistry, 59 (3), 135-42 PMID: 22372187

Kingsley, M., & Ramsay, M. (1988). The Spiral in the Tusk of the Narwhal ARCTIC, 41 (3) DOI: 10.14430/arctic1723

Nweeia, M., Eichmiller, F., Hauschka, P., Donahue, G., Orr, J., Ferguson, S., Watt, C., Mead, J., Potter, C., Dietz, R., Giuseppetti, A., Black, S., Trachtenberg, A., & Kuo, W. (2014). Sensory ability in the narwhal tooth organ system The Anatomical Record, 297 (4), 599-617 DOI: 10.1002/ar.22886

Dietz, R., Shapiro, A., Bakhtiari, M., Orr, J., Tyack, P., Richard, P., Eskesen, I., & Marshall, G. (2007). Upside-down swimming behaviour of free-ranging narwhals BMC Ecology, 7 (1) DOI: 10.1186/1472-6785-7-14

For more information or classroom activities, see:

Narwhals –

Tooth structure –