Mysteries of evolution: the narwhal’s “tusk,” or rather, tooth

For some reason, atheists are obsessed with narwhals, a fascination that I can’t quite understand.  I mean, I do like them, but why do they come up so often, invariably accompanied by incredibly annoying narwhal songs?  At any rate, let us leave this persiflage behind and deal with some real narwhal science, reported in a new journal paper. It’s an investigation of the nature of their tusks, and gives some fascinating results.

But first, a bit about the beasts.  The species is Monodon monoceros (Greek for “one tooth, one horn—a clue to what its “horn” is), and its closest relative is the beluga whale (Delphinapterus leucas), with these two species constituting the entire family Monodontidae.  Narwhals are small, with males growing up to only sixteen feet long, but they’re heavy: although three times as long as a human, they can weigh up to 4,000 pounds!

Their signal attribute is, of course, their “tusk,” which is really an upper tooth that protrudes not from the mouth, but from a hole in the upper “lip”.  We’ll talk about this tooth in a minute:

 

The narwhal lives in the Arctic; here’s its range:

Narwhal’s range; dark blue shows where they’re commonly found, while stripes denote rare occurrences. From Wikipedia

They subsist on squid, fish, and shrimp, which they swallow whole. Why?  Because, with the exception of its tusk, the narwhal has no teeth.  Here’s a shot of an adult narwhal mouth, as toothless as that of a newborn baby, taken from a Smithsonian Science report on this week’s paper (reference below):

There has never been a single observation of narwhals feeding in the sea; all dietary information comes from analyzing stomach contents.  They’re really a largely unstudied species.

It’s also one of the deepest-diving whales: the Narwhal FAQ from the University of Washington says this:

Narwhals typically dive to at least 800 meters between 18 and 25 times per day every day for 6 months. Many of these dives go even deeper than 800 meters: over half reach at least 1,500 meters (4,500 feet).  Dives to these depths last around 25 minutes, including the time spent at the bottom and the transit down and back from the surface.  In addition to making remarkably deep dives, narwhals also spend a large amount of their time below 800 meters (>3 hours 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 (making them vulnerable to entrapment; see below), and there are about 80,000 of them worldwide.

On to the “tusk”. Wikipedia says this:

The most conspicuous characteristic of the male narwhal is its single 2–3 meter (7–10 ft) long tusk, an incisor tooth that projects from the left side of the upper jaw and forms a left-handed helix. The tusk can be up to 3 meters (9.8 ft) long—compared with a body length of 4–5 meters (13–16 ft)—and weigh up to 10 kilograms (22 lb). About one in 500 males has two tusks, which occurs when the right incisor [JAC: as we'll see below, it's really a canine tooth, so some Wikipedia editor should correct this], normally small, also grows out. A female narwhal has a shorter, and straighter tusk. She may also produce a second tusk, but this occurs rarely, and there is a single recorded case of a female with dual tusks.

The most broadly accepted theory for the role of the tusk is as a secondary sexual characteristic, similar to the mane of a lion or the tail feathers of a peacock. This hypothesis was notably discussed and defended at length by Charles Darwin, in The Descent of Man, and Selection in Relation to Sex (1871). It may help determine social rank, maintain dominance hierarchies, or help young males develop skills necessary for performance in adult sexual roles. Narwhals have rarely been observed using their tusk for fighting, other aggressive behavior or for breaking sea ice in their Arctic habitat.

This specimen clearly shows the spiral nature of the “tusk”:

From the Smithsonian piece: “he dissection team at the Osteo-Prep Lab of the Smithsonian’s National Museum of Natural History begins dissection on a male narwhal specimen. From left, James Mead, curator emeritus, Museum of Natural History; Ted Cranford, San Diego State University; and Martin Nweeia. (Photo by Chip Clark, Smithsonian Institution)”

According to one source, the narwhal’s “tusk” is the only straight tusk in the world, and one of the only spiral teeth.  It’s also flexible, can bend a foot either way.  The placement of the tusk on the left side of the body is a case of directional asymmetry: an animal asymmetry that goes in only one direction, as opposed to “fluctuating asymmetry” in which the asymmetry is random with respect to side. The lobster claw is an example of the latter: the claws are asymmetrical (a “grinder” and a “crusher”), but random with respect to side.  Many species show directional asymmetries, including humans. Our heart, for example, tilts over to our left side, which is why we put our hands there to feel our heartbeat, and our right clavicle (“shoulder bone”) tends to be shorter than our left.

Here’s a double-tusked narwhal skull from the Smithsonian piece:

A rare double-tusked narwhal in the collection of the National Museum of Natural History is examined by Martin Nweeia, left, and Charles Potter, collections manager, Smithsonian Marine Mammal Program. (Photo by Joseph Meehan)

A brief digression: directional asymmetry has fascinated me because it implies that a gene somehow “knows’ whether it’s on the right or left side of the body. Once evolution has established a dorsoventral and an anterior-posterior body axis, then left and right are completely specified, and presumably in the ancestral animal the developmental cues were the same on both sides of the body. For a directional asymmetry to evolve, then, the gene for such a trait has to be activated consistently on the same side of the body, and hence be activated by some developmental “cue” that differs consistently between left and right. But the ancestor wouldn’t seem to have such cues!  How this happens is a mystery, though work on mice has shown that it may all stem from the direction in which the cilia of the embryo beat.  And, of course, once you’ve evolved one directional asymmetry, that sets up a left-right difference that can serve as a developmental cue for the evolution of further directional asymmetries.

But let’s proceed to the new data, given in a new paper in The Anatomical Record (reference below; free download) by M. T. Nweeia et al. Martin Nweeia is a New England dentist who also has appointments at the Smithsonian and Harvard’s Museum of Comparative Zoology, and his dentistry training plays a big role in the paper.   The authors examined 131 narwhal skulls (110 from museums, the rest from kills), and did fine-structure dissection of the skull and “tusk” as well as CT imaging.  And they found some cool things:

  • Fetal narwhals have six pair of tooth buds (this has been known for a while), but four pair disappear before maturity.  This itself shows that narwhals, though toothless, show vestigial traits indicating that their ancestors had teeth. And of course the ancestral whale did have teeth, for they all descend from a land-dwelling artiodactyl (even-toed mammal), probably something like a deer.  Toothless baleen whales also show embryonic tooth buds that disappear. I mention this in WEIT as evidence for evolution and common ancestry.
  • Of the two pair of tooth buds that remain, two go toward forming the tusks (only one of which erupts through the upper mandible), while the other pair remain vestigial (more on that below).
  • The authors have established convincingly that the tusk is not only an enlarged tooth, which has been known for a while, but a canine tooth.  That determination was made from both the anatomy and the way they develop from the maxillary bone plate.  The tusk has always been described as an enlarged incisor, but that’s wrong. Note as well that this tooth doesn’t grow out of the mouth, but erupts right through the animal’s upper jaw.  It’s also horizontal rather than vertical.  When we imagine how it evolved, we have to imagine intermediate conditions (presumably favored by selection) in which a tooth in the jaw changes its orientation and, while the other teeth disappear, starts poking through the front of the head.  That’s a challenge!
  • Narwhals have 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 (0.04-1.2 inches). They lie behind and to the left of the sockets for the two tusks (only one of which usually erupts).  Here are two pictures of the vestigial teeth in situ (note how they’re embedded in the bone and lie horizontally):

The next photo shows two vestigial teeth embedded in the bone horizontally, along with enlargements:

And here are some of these vestigial teeth, with a scale to show size. Note that although the teeth are deformed, they have roots and crowns.  Their status as “teeth” is also confirmed by histological studies showing a pulp chamber inside that is surrounded by dentin and cementum.

These are true vestigial features, testifying to the descent of narwhals from toothed ancestors.  And they’re clearly nonfunctional. Let some creationist come out of the woodwork and explain how they’re really useful after all (though a vestigial trait doesn’t have to be nonfunctional, viz., the ostrich’s wings).  I’m sure one will. Nweeia et al. say this:

The small size of some vestigial teeth, 3 mm by 1 mm, makes them difficult to locate during dissection, even with good imaging aids. Their location in subadult and adult maxillae is posteroventral and lateral to the tusks and their varied expressions in root (Fig. 13) and crown (Fig. 14) morphology suggest a lack of any defined functional significance.

  • The authors make some evolutionary speculation about the tusk, which I think is a bit misguided.  Here’s what they say about it:

“As the erupted tusk of Monodon monoceros is distinguished by unique morphology and expression (Arvy, 1978; Hay and Mansfield, 1989) including an extreme example of dental asymmetry (Arvy and Pilleri, 1977; MacLeod et al., 2007) and sexual dimorphism in mammalian teeth, horizontal impaction in the maxilla, eruption through the upper lip, and a cementum covering over its entire exposed length, it can be considered a novel innovation (Nitecki, 1990; Muller and Wagner, 1991). As there is no ancestral condition that accounts for this expression, conventional mechanisms of evolution do not help explain this organ system. Behavioral observations are also difficult to collect and piece together in a conclusive discussion of functional significance (Silverman, 1979; Best, 1981). Such a phenotypic novelty is more likely explained as an epigenetic byproduct of selection.”

I’m not sure what the authors mean by saying “conventional mechanisms of evolution do not help explain this organ system.”  Do they mean that the tusk was not subject to selection for its presence as a large protruding tooth, but for some other reason? Or that there are other mechanisms of evolution (genetic drift?) that are involved?

Nweeia said something similar in his interview in the Smithsonian piece:

“The whole thing that is great about the teeth of the narwhal is that nothing makes sense,” Nweeia adds. “The tusks are an extreme example of dental asymmetry. They exhibit uncharacteristic dimorphic or sexual expressions since females do not exhibit erupted tusks as commonly as males. Also, the tusk has a straight axis and a spiraled morphology.  Conventional mechanisms of evolution do not help explain these expressions of teeth.”

It seems likely to me that a form of selection was involved here: sexual selection.  And that’s because the “tusk” is found only in males.  When one sees an ornament or excrescence like this that’s limited to males, the immediate hypothesis is sexual selection, often based on female choice. Now it’s not clear whether in this case the males actually joust with their tusks to win females, or that females use the size or presence of tusks as a way to judge mates, or whether some other form of sexual selection is going on, but I think that sexual selection is ultimately responsible.  That could be tested in principle: by seeing whether females prefer tusked over untusked males, two-tusked over one-tusked males, or males with longer rather than shorter tusks. We can’t do this of course, but perhaps there are indirect ways to test for sexual selectin.

To me the real mystery is how the toothed ancestor developed into the narwhal: how all the teeth but one canine tooth disappeared, and how that tooth changed its development from erupting from the gums to erupting through the upper lip.  It’s hard to imagine the intermediate stages of that transformation, yet if it was impelled by sexual selection, each step in this process must have incurred a selective advantage over its predecessor. I have no clue here, as I haven’t thought about it deeply and at any rate don’t have the dental expertise. Perhaps one clue is that the vestigial teeth are, like the tusk, oriented horizontally: perhaps this is a sign of how it might have happened developmentally.

Intelligent design advocates may see this as some kind of “irreducible complexity,” in which the intermediate stages weren’t adaptive and hence required the hand of God.  Readers may want to weigh in below with their scenarios of how evolution produced the tusk. But when the authors say, “Such a phenotypic novelty is more likely explained as an epigenetic byproduct of selection,” they should provide some clues about what they mean. What kind of selection: sexual or natural? What is the real trait that was subject to selection?  And how can such a bizarre malformation of a tooth be a mere “epigenetic byproduct”?  This is all fertile ground for evolutionary biologists, but my guess, again, is sexual selection, which has caused the evolution of many bizarre traits in male animals (have a look at the New Guinea birds of paradise, for instance).

*****

Finally, here’s a National Geographic video I found of a narwhal pod (they travel is groups of variable size), including an “ice entrapment,” which the narwhal FAQ describes this way:

Q. What is an ice entrapment?

Although narwhals spend much of their time in heavy ice, they are vulnerable to unique events called ice entrapments or ‘sassats’.  During an ice entrapment, hundreds of whales might become trapped in a small opening in the sea ice and they often die.  This occurs when sudden changes in weather conditions (such as shifts in wind or quick drops in temperature) freeze the open water and the leads and cracks are sealed shut. Narwhals occupy dense pack ice for half of the year and are incapable of breaking holes in dense ice.  There have been no direct observations of narwhal ice entrapments in central Baffin Bay because the area they routinely occupy is hundreds of kilometers from shore and is rarely visited by humans.  There are, however, reports of large coastal ice entrapments in areas near where humans live.

Note the highly misleading statement in the video: “They use it for jousting, but it doesn’t seem to serve any evolutionary purpose.”  Well, I don’t know what the term “evolutionary purpose” means, but I think they mean it doesn’t have any obvious function.  My own guess is that it’s a sexually selected ornament, and thus does have a function: to attract female narwhals.

h/t: Michael

________

M. T. Nweeia. F. C. Eichmiller, P. V. Hauschka, E. Tyler, J. G. Mead, C. W. Potter, D. P. Angnatsiak, P. R. Richard, J. R. Orr, and S. R. Black. 2012  Vestigial tooth anatomy and tusk nomeclature for Monodon monoceros.  Anat. Rec. online: DOI: 10.1002/ar.22449

70 Comments

  1. Posted April 22, 2012 at 7:39 am | Permalink

    fascinating. Thanks.

    • newenglandbob
      Posted April 22, 2012 at 9:58 am | Permalink

      Ditto.

  2. Posted April 22, 2012 at 7:40 am | Permalink

    I have no novel hypotheses to offer, but one thing I didn’t see mentioned in the above is that the spiral morphology most likely is adaptive; a spiral forms a much stronger structure than a completely straight protrusion.

    And I whole-heartedly agree about the most likely explanation is sexual selection. I remember as a child that I pitied the whales for being stuck with such an ill-fitting tooth (at least from my human perspective!); and I have thought of it as a handicap ever since. That, and it’s pretty. It’s interesting how humans seem as mesmerised by secondary male sexual characteristics as the intended females; we just can’t help helping ourselves to spiral teeth and flashy feathers!

    • ivo
      Posted April 22, 2012 at 1:29 pm | Permalink

      And don’t forget female baboons’ red buttocks. Mesmerizing indeed.

  3. Tyro
    Posted April 22, 2012 at 7:49 am | Permalink

    Very cool, thanks. Once again I learned how much I don’t know.

  4. Boko999
    Posted April 22, 2012 at 7:53 am | Permalink

    Form now on I’m going to refer to whales such as the sperm and beluga as teethed whales on accounta the narwhal being the only true toothed whale.

  5. BilBy
    Posted April 22, 2012 at 7:54 am | Permalink

    Why do you say the “tusk” is really a tooth? Aren’t tusks teeth? Elephants tusks are. Is there a specific meaning of tusk that precludes them being teeth?

    • whyevolutionistrue
      Posted April 22, 2012 at 8:06 am | Permalink

      Yes, you’re right; I’m just trying to clarify the origin of tusks here. All tusks are indeed teeth, though some things that might be informally called “tusks”, like the rhino’s horn, aren’t teeth.

      • s.k.graham
        Posted April 22, 2012 at 8:59 am | Permalink

        Can’t say as I’ve ever heard the word “tusk” ever used for a rhino horn, or anything that is not a tooth, no matter how informal the context. (though I am sure there are cases of extremely uninformed individuals conflating “tusk” and “horn”)

        • BilBy
          Posted April 22, 2012 at 11:09 am | Permalink

          Me neither. Though ‘horn’ is often used for antlers

      • Posted April 22, 2012 at 9:34 am | Permalink

        “Tusk” and “tooth” are etymologically related.

  6. Ty
    Posted April 22, 2012 at 7:57 am | Permalink

    Given the orientation of the vestigial teeth it seems possible that all of the anterior teeth may have enlarged and been oriented forwards, perhaps to scrape or break pack ice as weddell seals do (perhaps relating to the strength of spiral structures as mentioned by Josephine). Large teeth may then have become the target of sexual selection. I think however that we will remain in the realm of speculation until we have a better idea of what these animals do, particularly at depth. perhaps genetic analyses can get at the issue of paternity.

  7. mb
    Posted April 22, 2012 at 7:59 am | Permalink

    Seems reasonable to me that a creature spending it’s life among the ice as the narwhale does would benefit from some kind of hard or boney prominence that would aid in breaking ice. It also seems like an evolutionary advantage to be able to make, for example, air holes in ice if you’re an air breathing animal. The ‘tusk’ seems like it’d serve that purpose pretty well, among others.

    • gravelinspector
      Posted April 22, 2012 at 8:10 am | Permalink

      . It also seems like an evolutionary advantage to be able to make, for example, air holes in ice if you’re an air breathing animal. The ‘tusk’ seems like it’d serve that purpose pretty well, among others.

      Other whales (belugas?) seem to manage perfectly well without such a “ram”. Same for various seals, tipped with a “doggy” nose (some scrape at the ice with their teeth).

  8. gravelinspector
    Posted April 22, 2012 at 8:05 am | Permalink

    They are very peculiar beasts, that’s for sure, and there’s lots in the post I don’t have time to address before it’s time to start packing my stuff.
    But one small thing … why is “tusk” in quotes. Is there another example of a (mammalian) tusk which isn’t a tooth? Aren’t the two terms synonymous? Or does the general population (sofa, six-pack and football on the TV 6 hours/day and 2 hours for the missus to watch soap operas) so ignorant that they only know of elephant tusks, but not walrus, warthog or muntjac-deer tusks?
    Are there tusks which are not teeth? Some lizards have skull projections called tusks ; some beetles too. But within the mammals? I can’t think of any. (Giraffe “horns” might be a test case ; I don’t know their anatomy.)

  9. The whole truth
    Posted April 22, 2012 at 8:08 am | Permalink

    “And of course the ancestral whale did have teeth, for they all descend from a land-dwelling artiodactyl (odd-toed mammal), probably something like a deer.”

    Aren’t artiodactyls even toed?

    • gravelinspector
      Posted April 22, 2012 at 8:15 am | Permalink

      Whales closest living relatives are hippopotamus, I think. This was proposed a long time ago on the basis of blood chemistry, but has since been supported by genetics and more recently by fossil anatomy of the “walking whales” (Ambulocetus, IIRC).

    • Posted April 22, 2012 at 9:27 am | Permalink

      I think so too. Perissodactyls are the odd toed ungulates.

  10. Posted April 22, 2012 at 8:16 am | Permalink

    wonderful article. nice change from teh kittehs, and yet another reminder that Nat Geo desperately needs to either die quietly or get some serious scientists among their ranks of bad editors and pretty picture takers.

  11. Grania Spingies
    Posted April 22, 2012 at 8:19 am | Permalink

    That’s a fascinating read. Thank you.

  12. ekinodum
    Posted April 22, 2012 at 8:21 am | Permalink

    I read somewhere that the tusks are extremely enervated, and reading here that narwhals eat squid (and dive to teh depths to find them) makes me wonder if the tusks are used to find their prey in the dark? Sperm whales do this by echolocation, but I dunno if narwhals do; there does not appear to be much of a melon compared to other whales. But I’m stumped here- do only the males have the “tusk” as stated at one point in this post, or do the females have a shorter (straighter) tusk as stated in another point?

    If only the males have the tusk, then my suggestion is weak (unless only the males bring home the squid, as it were). If both have a tusk, then maybe narwhals only need a shorter tusk to find squid, and the longer one is to impress the girls?

    And more discussion is needed of the claim that “atheists are obsessed with narwhals”- I’ve been interested in them since I saw a tusk for sale in a shop in Canada (I was about 7 years old; call it the early 60′s) long before I even knew what an atheist was.
    But I have to admit that this is one of the longer comments i have posted here… does that make me obsessive? Should I be insulted?

    • Achrachno
      Posted April 22, 2012 at 8:39 am | Permalink

      I’d never noticed the obsession with narwhals either — maybe we both missed the same memo. It seems to me there’s much more obsession with cats.

      But, if atheists are obsessed with narwhals, might it be because they contain so much evidence of evolution in their teeth? Why would a “god” make all those vestigial teeth? Not a chewing or biting one in the set.

      • Posted April 22, 2012 at 12:35 pm | Permalink

        Since narwhals are the “unicorns of the sea”, perhaps the obsession (such as it is) focuses on the invisible pink narwhal… ?

        Fascinating article.

        /@

    • Torbjörn Larsson, OM
      Posted April 22, 2012 at 10:17 am | Permalink

      But I’m stumped here- do only the males have the “tusk” as stated at one point in this post, or do the females have a shorter (straighter) tusk as stated in another point?

      “A female narwhal has a shorter, and straighter tusk.” [Wikipedia]

      “It seems likely to me that a form of selection was involved here: sexual selection. And that’s because the “tusk” is found only in males.” [Coyne]

      Coyne is correct of course. I was stumped ["tusked"? "tusked with this"?] too, so I found this:

      “While both male and female narwhals can grow tusks, generally only male narwhals do so.”

      FWIW (not much), google also turned up some unreliable Q&A page that claims “Almost all male narwhals have a tusk, but only about 15 percent of female narwhals do.”

      • Chris B
        Posted April 23, 2012 at 8:21 am | Permalink

        “While both male and female narwhals can grow tusks, generally only male narwhals do so.”

        And this of course ties in with the free will discussions – if the females _can_ grow tusks, but choose not to, then in what sense _could_ they have done so? ;)

        • whyevolutionistrue
          Posted April 23, 2012 at 8:27 am | Permalink

          Everybody’s a damn comedian here! :-)

    • Tim
      Posted April 22, 2012 at 10:41 am | Permalink

      As an atheist chemist who learned more about narwhals from reading this post than I’ve learned in the rest of my 57 years, I can say that ‘atheist narwhal obsession’ is new to me as well. Let me offer an alternative hypothesis: Jerry is a biologist, biologists are disproportionately atheistic, so Jerry’s perceptions of atheists are skewed by his hanging around a lot of biologists.

      • Tim
        Posted April 22, 2012 at 10:42 am | Permalink

        Very interesting post, BTW!

      • ekinodum
        Posted April 22, 2012 at 10:48 am | Permalink

        I think I saw Atheist Narwhal Obsession open for Bonny Raitt

  13. Avis
    Posted April 22, 2012 at 8:30 am | Permalink

    It was wonderful to wake up to reading this interesting essay. Thanks, Jerry.

  14. DiscoveredJoys
    Posted April 22, 2012 at 9:00 am | Permalink

    Looking at the Wikipedia (I know) Water Deer and Musk Deer have tusks which are canine teeth.

    In Water Deer:

    “These canines are held loosely in their sockets, with their movement controlled by facial muscles. The buck can draw them backwards out of the way when eating. In aggressive encounters, he thrusts his canines out and draws in his lower lip to pull his teeth closer together. He then presents an impressive two-pronged weapon to rival males.”

    I can imagine sexually dimorphic land-going tusked proto-narwhale with such tusks; these tusks gradually becoming permanently ‘out’, passing through the upper lip, due to pressures of selection both sexual and natural (due to shape of food and capturing it at considerable depths).

    Why only one major tusk? Nothing obvious springs to mind, but if the tooth is costly to grow, and the species passed through a population bottleneck, a early mutation for suppressing one tooth could spread through the population.

    How you prove this is, of course, another matter.

    • DiscoveredJoys
      Posted April 22, 2012 at 9:22 am | Permalink

      Thinking about it a bit more, if the length of the tooth is the sexually selected signal, it may grow longer a la Costly Signalling Hypothesis (Peacocks Tail). However two extra long tusks would be difficult to swim with (otherwise the signal wouldn’t be costly), but an extra long single tooth may be a sufficient signal. After all the signal, face on, would not be so obvious. From the side “Would you look at the size of that, baby!”

  15. chascpeterson
    Posted April 22, 2012 at 9:11 am | Permalink

    Very cool indeed.
    I know of one other example of teeth growing in the ‘wrong’ direction and poking out of the head elsewhere than in the mouth: the babirusa.

    small correction: the clavicles are the ‘collar bones’; ‘shoulder bones’ are scapulae.

  16. s.k.graham
    Posted April 22, 2012 at 9:33 am | Permalink

    About “eruption” and intermediate evolutionary stages:

    I envision the tooth projecting gradually more horizontally, and higher, out of the mouth, with the lips gradually surrounding and meeting and closing beneath the tooth. Later stages would have a sort of bump/loop/kink in the shaped of the lip where the tooth erupts and over time this distortion of the lip would no longer be obvious, until it looks like the tooth erupts through ordinary skin above the lip. But I suspect that a close look at the fetal development of the tissue around the gums/lips/cheek where the tooth erupts will still show evidence of such a kink or loop in the cells destined to form the lip, but that the cells which are part of the kink or loop then go on to form something more like normal skin — but adapted to withstand/facilitate the eruption of the tooth.

    Alternatively the sharp tooth began scraping/cutting the lip, which became tougher (thicker, less likely to bleed, fast-healing) at that location, and as the tooth moved higher, the adapted skin moved with it, until the tooth was piercing the lip, and later piercing the skin above the lip.

  17. Posted April 22, 2012 at 9:40 am | Permalink

    Couldn’t the orientation of the teeth have been a epigenetic change (that changed them all)? Then selection could go from there. Would an epigenetic change be so likely to persist in a population for so long?

    But, I suppose it could just as easily have been a regular nucleotide mutation.

    I don’t see how they could have made such a statement about an epigenetic explanation for the teeth. Wouldn’t they need some very firm genetic data to support this?

    • Thanny
      Posted April 22, 2012 at 12:34 pm | Permalink

      As I learned in the not-so-distant past, the word epigenetics used to simply refer to development (i.e. embryology). It may be in this sense that the authors are using the term, not the new sense of something outside the genome (which itself is misleading, since sensitivity to “epigenetics” in that sense is genetic).

  18. ToffeeMoon
    Posted April 22, 2012 at 9:42 am | Permalink

    This was a really great article thanks!

  19. s.k.graham
    Posted April 22, 2012 at 9:50 am | Permalink

    About function of the tusk:

    The straightness of the tusk strongly suggests design for head-on impact with the full weight and momentum of the narwhal behind it.

    This is all but useless offensively against prey or defensively against predators, as sharp turns and maneuvers put the line-of-momentum out of line of the tusk. But for punching through ice it would be ideal.

    While they obviously cannot punch through ice which is “too thick”, they can equally obviously punch through ice which is much thicker than any other marine mammal can punch through. This would extend their range under arctic ice far beyond other marine mammals.

    Since observation of the narwhals in the wild is pretty limited, and as the use of this skill would take place in areas where it would be the most difficult to observe them, I suspect that this function of the tusk simply has not been observed.

    I would certainly agree that female mate preference has probably had a major selective impact. But the restriction of the tusk to males does not rule out function — perhaps females have lost their tusk because they can rely on males (who are already bigger) to break the ice for them.

    • questionauthorities
      Posted April 22, 2012 at 12:20 pm | Permalink

      There is no way that the tusk evolved for breaking through ice. Ice is hard. Attempting to break it with a long, thin tusk would destroy the tusk very quickly. And all the poor Narwhal would get for his troubles is a 5 cm hole in the ice. An icebreaking adaptation would be short and very wide at the base, more like an axe head than a spear. You would also expect to see adaptations to the skeleton to support high impact and protect the brain.

      • s.k.graham
        Posted April 22, 2012 at 9:58 pm | Permalink

        (1) Ice hardness is extremely variable, depending on conditions, but typically has much lower hardness than tooth enamel.

        (2) Sonar is an ideal sensor for determining the thickness/hardness of ice, thus allowing the narwhal to find weak spots and avoid any ice that is too hard thick.

        (3) This is not like the head-crashing of battling rams. The impact is on a tiny point and the ice gives way. Far less compression would be transferred back to the base of the tooth than you might imagine. Basically if the ice can’t break the tip, then the force transferred back is not going to break any thicker part of the tooth, or the skull.

        (4) Compare hammer vs. ice-pick — you might first think how much better the hammer would be o smash through some ice. But now think about how much *less* speed and force you need with an ice-pick — it’s wooden handle has much less mass than the hammer head, and you also have less speed without the mechanical advantage of the hammer-swing. So for a fair comparison, remove the hammer’s head, and cap the end of its wooden handle with a small steel disk. Now with equal mass and leverage, which will get through ice more effectively?

        (5) The flexibility of the tooth was mentioned (“can bend a foot”), which suggests to me that the tusk has undergone some selection pressure to bend rather than break under impact.

        (6) Numerous cracks (in the ice) will accompany the “small holes” punched by the tusk, and several small holes will add up to a weak patch of ice that can easily be pushed through.

        Given the apparent dearth of observational data on narwhal behavior, it does not seem to me that ice-breaking can be seriously ruled out. Again note that the behavior is most likely to occur in locations they are least likely to be observed. Note that the article says “rarely observed” in regard to ice-breaking, so apparently they do at least occasionally use it for that purpose. Perhaps if we attached a camera to narwhal, or sent little robot-submarines to follow them, we would observe such behavior occurring much more in the places where it was needed most — under the ice flows, far far from open water.

        A detailed analysis of the bio-mechanics involved might refute my off-the-cuff analysis here, but has such been done?

  20. Achrachno
    Posted April 22, 2012 at 10:30 am | Permalink

    Alright. So I am becoming obsessed with narwhals.

    There’s a good bit of information out on the web, some of it apparently reliable:

    “Canadian scientists Michael C.S. Kinglsey and Malcom A. Ramsay, writing in the journal Arctic, suggest that the twisted tusk is what lets narwhals swim straight. They point out that, like all teeth, the narwhal’s tusk grows from new material deposited at its root. Unless the rate at which new material is added is perfectly even all around the root, one side of the tooth would grow faster. A tusk unevenly grown like that would necessarily be curved. That is what happens with other tusk-bearing mammals, like walrus and elephants.

    The likelihood of one section of the root growing just a bit faster than another is great, and a long curved tusk would hamper a narwhal’s swimming ability. But if the tusk twists as it grows, each point on it passes in turn over regions of faster and slower growth in the socket. Any imbalance in the rate of growth evens out, and the result is a tightly twisted tusk with a straight axis.

    Source: http://www2.gi.alaska.edu/ScienceForum/ASF8/895.html

    • Hempenstein
      Posted April 23, 2012 at 9:40 am | Permalink

      Cool explanation, thx for posting it!

  21. Gregory Kusnick
    Posted April 22, 2012 at 10:45 am | Permalink

    Narwhals are small, with males growing up to only sixteen feet long, but they’re heavy: although three times as long as a human, they can weigh up to 4,000 pounds!

    4000 pounds is just what you’d expect from something three times the linear size of a human, since mass increases with the cube of linear size. So 27 * 150 = 4050. No surprise there.

    Pragmatically, narwhals couldn’t be much denser than water without expending a lot of excess energy to stay afloat. Neutral buoyancy (or pretty close to it) is what you’d expect.

  22. Thanny
    Posted April 22, 2012 at 12:42 pm | Permalink

    I can see no reason why the tusk couldn’t have started as a pair of tusks facing downward (like with a walrus), which gradually migrated forward, to allow easier hunting while still having the sexually attractive feature. The great lengthening would then probably occur only after the horizontal orientation made it less a liability.

    I’m sure what actually happened was more complicated than that simplified scenario, but my point is that I don’t see any great strain to the imagination.

    • John Scanlon, FCD
      Posted April 23, 2012 at 9:59 am | Permalink

      What this discussion has lacked is any mention of the fossil record. There is indeed a Pliocene relative, Odobenocetops, where the tusks point backwards.

      (Note that the wiki article on Odo gets mixed up, confusing obvious convergent nature of resemblance to walrus, vs falsely inferred convergence with narwhal – another one for you ‘editors’ to fix!)

      There must be a fossil whale expert around who knows of other observed colours in the tusk-orientation spectrum…?

  23. Reginald Selkirk
    Posted April 22, 2012 at 1:08 pm | Permalink

    and one of the only spiral teeth.
    .
    Of course it is. That would be true if 99% of teeth were spiral. t would be true if < 1% of teeth were spiral. This is part of my personal crusade to show just how pathetic and uninformative the phrase "one of the only" is.

  24. emmageraln
    Posted April 22, 2012 at 1:55 pm | Permalink

    Reblogged this on emmageraln.

  25. bsk
    Posted April 22, 2012 at 2:12 pm | Permalink

    “I mean, I do like them, but why do they come up so often…?”

    Like an underwater unicorn, they’ve got a kickass facial horn! They’re the Jedi of the sea! They stop Cthulhu eating ye! WHAT MORE DO YOU WANT??!1

    Sorry, couldn’t resist :/

  26. MadScientist
    Posted April 22, 2012 at 3:00 pm | Permalink

    Directional asymmetry has its exceptions of course. I had known these monozygotic twins who looked like identical copies but one had his internal organs on the other side (heart on the right, appendix to the left) – by coincidence their mom gave them palindromic names.

  27. JohnnieCanuck
    Posted April 22, 2012 at 4:42 pm | Permalink

    All occurrences of incisor have been changed to canine on the Wikipedia entry. It was a team effort.

    • Tim
      Posted April 22, 2012 at 7:01 pm | Permalink

      I was thinking, “the Wikipedia editors?”, that could be one of us, right? Well done.

      Anyone can edit Wikipedia (though knowlegeable people might undo mischief). A couple of years ago, when teaching a graduate chemistry course, I assigned term papers and I made them do Wikipedia pages – a couple of them were quite good. It seems like a much more constructive thing to do than having them just write something that only I would read.

      • JohnnieCanuck
        Posted April 23, 2012 at 2:09 pm | Permalink

        That’s brilliant. I hope that idea spreads. Even asking them to rewrite some of the initial entry with the results of their research would be really great.

        Professors, teachers; are you listening?

        I find Wikipedia on balance to be very good. The one aspect that grates is when more than one person has worked on the sections and they pull in duplications from the other parts of the article.

    • Achrachno
      Posted April 22, 2012 at 7:39 pm | Permalink

      Thank you and the team.

  28. Hempenstein
    Posted April 22, 2012 at 7:09 pm | Permalink

    And also fascinating:

    The lobster claw is an example of the latter: the claws are asymmetrical (a “grinder” and a “crusher”), but random with respect to side.

    I had never noticed that, but by strange coincidence had a chance to confirm it this evening.

  29. Mark
    Posted April 22, 2012 at 7:17 pm | Permalink

    *speculation ahead*
    Forward-protruding teeth are present in some artiodactyls, so it seems reasonable that oddly-oriented teeth could be selected for in the same way and that the genetic raw material could already be there.
    A genetic abnormality could have produced the first pod or so of tusks after that, which I realize is pretty highly suspect because we’d be looking for a single ancestor or family rather than a population.
    Such a bony protrusion might make such a creature harder to predate upon.
    Tack sexual selection on top of that…

  30. Posted April 23, 2012 at 6:52 am | Permalink

    It may be just me, but the map of “where commonly found” and “rare occurrences” seems similar to a map “where there are commonly people who might see them” and “rare individuals” as much as it is to actual population statistics.

  31. Achrachno
    Posted April 23, 2012 at 8:16 am | Permalink

    Odd creature, and the tusk may be the least of it: large toothless mouth, dives very deeply where it feeds on flounder, cod, squid and shrimp, feeds in darkness at that depth, often under or near floating pack ice.

    Why preferentially near ice? Prey richness? Cold = slow prey?

    Why have the teeth been lost? Is it some sort of “vacuum” feeder? Is the mouth peculiar in any other way? Animals feeding on slippery prey often have numerous needle-like teeth for grip. Why not here?

    Presumably using sonar to detect prey? Tusk presumably not an organ for detection of prey or both sexes would have it.

  32. Posted April 23, 2012 at 8:20 am | Permalink

    I read it through twice & learned some good stuff. Thanks JAC!! [just so you know]

  33. Chinahand
    Posted April 23, 2012 at 8:30 am | Permalink

    I’ve just read Prof Coyne’s request in the Albino Orca post to go and read about Narwhals – as it “took hours”.

    I am sure it must have done – thanks for the effort – very interesting and informative.

    I did read this post in its entirity last night, but didn’t click into it as it was all available on the “front page”, so to speak.

    I suppose that is why people segment their postings – “more below the fold” – to get usage stats concerning who is reading what.

    With having the entire post available to read upfront there was no need to click “in” to it.

    I’m leaving this comment now – to say thanks and also to encourage more hard science postings. I realize they take a long time, and appreciate the effort to inform us.

    Cats, freewill, and the politics of religion and evolution are worthwhile subjects – but it is real science education I get the most from on here – please do keep spending those hours doing it.

    It is worth it.

    Thanks again!

  34. Hempenstein
    Posted April 23, 2012 at 9:38 am | Permalink

    Is it known how fast these tusks grow? And, being mammals, I assume they nurse. Do nursing males have tusks at all?

    Also, since they can bend, is anything known of the molecular structure of the dentin/enamel that enables them to bend?

    And unrelated to all of this only as weird things, are there any marine marsupials? If not, is there any evidence that any ever existed? I guess that would have to be pretty difficult if any did exist since they’d have to depend on the mother surfacing to take a breath.

    • John Scanlon, FCD
      Posted April 23, 2012 at 6:21 pm | Permalink

      Marine marsupials?

      No. None, ever, as far as we know.

      But I think there’s a freshwater aquatic ‘opossum’ in South America, where the pouch has a sphincter. Freshwater (amphibious) niches are open to mammals of all sizes, but there don’t seem to be any small marine mammals, so the volume (air capacity) of the air-filled pouch may be a relevant limit.

    • Marella
      Posted April 23, 2012 at 6:54 pm | Permalink

      Well there is the Platypus, but it’s a monotreme rather than a marsupial, it’s an egg-laying mammal but it doesn’t have a pouch, the young are left in a burrow. No marsupials that I’m aware spend any time in water to speak of. I guess pouches are not suitable for underwater, they would be very hard to get water-tight and there is so little water in Australia, (dryest continent on Earth except for Antarctica) that it would not provide much in the way of a benefit really.

      • Marella
        Posted April 23, 2012 at 6:54 pm | Permalink

        And not marine, riverine, sorry.

  35. Marella
    Posted April 23, 2012 at 7:38 pm | Permalink

    Interesting the the tooth/tusk is off-centre, this makes more sense evolutionarily. I had always wondered how a single horn had developed in a bilaterally symmetrical animal, though rhinos have mid-line horns it’s pretty unusual. I wonder how recent a development this horn is, it seems rather labile, 15% of females have it which is odd in a sexually determined character. And all those vestigial teeth seem to indicate recency too. Do we know how long it is since the separated from the belugas? And why don’t females like two horned males, why make them go to the trouble of only growing one? Or maybe they do but two is too much trouble to swim around with.

  36. Asura
    Posted April 24, 2012 at 11:11 pm | Permalink

    What of this hypothesis I’ve heard going around (dunno the source) that the tusk has sensory elements to it? The narwhal can detect salinity changes in the environment for example via its tusk.

    If anybody (please Jerry?) do a follow-up on what the current state of this hypothesis is, that would be sweet.

    • ekinodum
      Posted April 25, 2012 at 8:53 am | Permalink

      Yes, more, please!

  37. Posted April 30, 2012 at 9:03 pm | Permalink

    NY Times article from 2005 that discusses the receptors in the tusk: http://www.nytimes.com/2005/12/13/science/13narw.html?pagewanted=all

    “The find came when the team turned an electron microscope on the tusk’s material and found new subtleties of dental anatomy. The close-ups showed that 10 million nerve endings tunnel from the tusk’s core toward its outer surface, communicating with the outside world. The scientists say the nerves can detect subtle changes of temperature, pressure, particle gradients and probably much else, giving the animal unique insights.”

    and

    “In the Canadian wilds, the team recently conducted a field study on a captured narwhal, fitting electrodes on its head. Changes in salinity around the animal’s tusk, Dr. Nweeia found, produced signs of altered brain waves, giving preliminary support to the sensor hypothesis. The unharmed whale was then released.”

    Surely there’s been more development in the last seven years, but maybe it’s since been debunked and that’s why there’s no mention of it?

  38. samantha vicksenburg
    Posted May 9, 2012 at 7:59 am | Permalink

    I think people should cut out killing these gorgous creatures that never did anything to you!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!


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