Why on earth do narwhals have tusks?

Courtesy of alert reader QOS., I learned about an old bit of research from 2005, reported in the Harvard Gazette, purporting to answer the question above—but probably not succeeding. But let’s leave that aside for the nonce and learn a bit about narwhals. For reasons that elude me, they seem to have become the iconic species of atheists, perhaps because they’re a living unicorn.

The narwhal (Monodon monoceros) is a small toothed whale (odontocete) that lives in the Arctic portions of the Atlantic ocean. It’s an odontocete but has lost all its teeth but one—the tusk. The tusk is a very long canine tooth, and is found only in males.  It’s a spiral tooth and is placed not only asymmetrically (off to one side), but directionally asymmetrically—that is, it’s almost invariably on the left side. There’s a ton of good narwhal information at the “Narwhal FAQ” site of Kristin Laidre, a research scientist at the University of Washington. Here’s a few things you might not have known about the creature:

  • The tusk is actually a tooth that protrudes through the gum, and one of the few spiral teeth in nature. It’s also directionally asymmetrical; that is, it is off-center and nearly always on the left side of the body. Such directional asymmetries are not that common in animals (we, of course have them: in the placement of our internal organs, for instance), but to me they pose an evolutionary conundrum. How did a directional asymmetry evolve in the first place? Given an anterior-posterior and a top-bottom developmental gradient, left and right should then be embryologically symmetrical, so how does a gene “know” if it’s on the left or right from developmental cues? Once there’s an initial directional asymmetry, of course, then further asymmetries can cue on that one, but it’s a mystery to me how the first directional asymmetry (i.e., left vs. right) got off the ground in evolution.
  • Nota bene: by and large, only males have tusks, and (as the photo below shows) are often seen “jousting” with them while females linger nearby.  Only rarely does one see a tusked female, and two-tusked males are known as well.
  • Narwhals can live to be up to 90 years old.  This has been determined using “amino acid racemization” of aspartic acid.  That is, all of the amino acids we incorporate into our body are the L-form, but in tooth enamel of mammals, L-forms convert to D-forms (“racemization”) at a constant rate with age. Thus the relative proportion of D- versis L-aspartic acid gives you an idea of how old a mammal is.
  • “The narwhal is one of the deepest diving whales, with a record dive depth of approximately 1800 m (5905 ft., over one mile). . . Narwhals typically dive to at least 800 metres between 18 and 25 times per day every day for 6 months; over half reach at least 1,500 meters (4,500 feet). In addition to making their 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 pressure can exceed 2200 PSI (1500 atmospheres) and life exists in complete darkness.”
  • Finally, narwhals eat well, when they do eat (they largely fast during summer). Their diet, which explains why they’re always diving so deep, consists of fine Greenland halibut and squid, with a soupçon of other seafood like skate eggs.

A while back I did a long post on narwhal biology, concentrating on their tusks and teeth. (The tusk is the only tooth that erupts, but they have vestigial teeth, remnants of their ancestry, embedded in the jawbone. Fetuses also begin to develop teeth but abort them.  I included a video of narwhal behavior.)

Here’s a picture of male narwhals “tusking,” from the Narwhal News Network, which has a bazillion other narwhal photos:

narwhals-tusking

An article by Leah Gourley in the Harvard Gazette: “Marine biology mystery solved“, suggested that the question “Why tusks” had been answered, with the solution given at a meeting by Martin Nweeia at the Harvard School of Dental Medicine. The supposed answer involved chemosensory and proprioceptive properties of the tusk:

Nweeia has discovered that the narwhal’s tooth has hydrodynamic sensor capabilities. Ten million tiny nerve connections tunnel their way from the central nerve of the narwhal tusk to its outer surface. Though seemingly rigid and hard, the tusk is like a membrane with an extremely sensitive surface, capable of detecting changes in water temperature, pressure, and particle gradients. Because these whales can detect particle gradients in water, they are capable of discerning the salinity of the water, which could help them survive in their Arctic ice environment. It also allows the whales to detect water particles characteristic of the fish that constitute their diet. There is no comparison in nature in tooth form, expression, and functional adaptation.

“Why would a tusk break the rules of normal development by expressing millions of sensory pathways that connect its nervous system to the frigid arctic environment?” asks Nweeia. “Such a finding is startling and indeed surprised all of us who discovered it.” Nweeia collaborated on this project with Frederick Eichmiller, director of the Paffenbarger Research Center at the National Institute of Standards and Technology, and James Mead, curator of Marine Mammals at the National Museum of Natural History of the Smithsonian Institution.

Well, that sounds well and good, but there’s one problem.  If the tusk evolved for this reason, why is it missing in females? Presumably females would have the same need to detect particle gradients and salinity.

That suggests two things. First, that the initial evolution of the structure was driven not by general adaptation to the environment, but by sexual selection. Males could either fight with the things, with the bigger or more deftly wielded tuskers winning, or simply “feel each other out” by mutual tusking; in both cases the winner gets reproductive success by winning nearby females.

Then why the chemosensory and proprioceptive capacities?  Well, there are two possibilities. First, these could have evolved for the reasons suggested after the tusk had already arisen by sexual selection. Then would then be a general adaptation made possible by the prior appearance of an “outside sensor platform”—in other words the capacities discovered by Nweeia et al. could be what Steve Gould called a exaptations.  Or, their chemosensory functions might have evolved simply to detect other males’ scents when jousting; after all, the males don’t really battle it out when jousting, but seem to gently rub tusks while making noises.

In either case, it appears that the adaptive value of chemosensory teeth hasn’t been sufficient to allow their appearance in females; which, after all, carry the same genes that are present in males—including the genes for tusks. Those genes simply aren’t activated in females, ergo no tusks.  There must be a disadvantage for females to have tusks that outweighs their chemosensory advantage. Whatever the “advantage” for females (who don’t have the extra benefit of jousting for mates), it could be outweighed by having to carry a cumbersome apparatus that could get injured.

I’m surprised, actually, that neither the author of the paper nor of the article mention this obvious problem. Gourley does allude to sexual selection as previous ideas for the evolution of tusks, but gets even that wrong:

In the past, many theories have been presented to explain the tooth’s purpose and function, none of which have been accepted as definitive. One of the most common is that the tooth is used to display aggression between males, who joust with each other for social hierarchy. Another is that the tooth is a secondary sexual characteristic, like a peacock’s feathers or a lion’s mane.

But these are not distinct “theories”!  If a character evolved by sexual selection, it is a secondary sexual characteristic. They are not competing explanations. A lion’s mane, after all, probably serves as a kind of plumage that attracts females, showing something about the male’s age or condition.

Finally, in arguing that the mystery of the tusk has been “solved,” Gourley conflates two different explanations—explanations that impelled Gould to distinguish between “adaptations” and “exaptations”. Gourley says that the “solved” mystery is this:

“. . . why does the narwhal, or “unicorn,” whale have an 8-foot-long tooth emerging from its head, and what is its function?

There are two separate questions here.  First, what were the selective pressures that caused the structure to appear in the first place (note to Larry Moran: I seriously doubt that genetic drift is responsible for this trait!)? In my view, it’s clearly sexual selection, though exactly how it worked is a mystery (are males with bigger tusks simply older, in better condition, or do they have better genes?).  Second, how do the tusks function now?  For a feature can acquire new functions after it arose for other reasons, just as the penguin’s “wings” now enable it to swim.  The flippers originally arose by natural selection for flight, and after penguin ancestors lost that function (or during its loss), the vestigial wings became useful for swimming. In the case of the narwhal, the tusk could have both the original evolutionary function and also a new one.  The point is that current function may bear little relationship to the selective pressures that led a trait to evolve in the first place.

I notice that in the intervening eight years since this report was made, Nweeia and his colleagues have published a nice article on the vestigial teeth of narwhals, but have apparently published nothing about this remarkable discovery of the chemosensory and tactile functions of the tusk. And even in an interview about that article, Nweeia goes wrong about evolution:

“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.”

Well, the asymmetry can make sense. If only one tusk was useful in jousting, and if it’s simply an enlarged canine tooth, well, evolution has to choose either a left or right canine. As for the sexual dimorphism, explanations for that are well known. We may not know exactly how the tusk is used, or how it evolved, but the path for understanding is wide open.

29 Comments

  1. gbjames
    Posted May 10, 2013 at 8:54 am | Permalink

    Interesting post. And they clearly are atheists, these narwals. Very strident.

  2. John Harshman
    Posted May 10, 2013 at 8:58 am | Permalink

    One conceivable explanation for male-only tusks would be niche partitioning. Perhaps males feed in a different way from females, on different resources or in different places, and the sensory abilities of the tusks are useful only in male-type feeding. There are certainly examples from other species of sexual dimorphism being explained in this way.

    And it should be easily testable, to the extent that any hypothesis about things that happen deep underwater is testable. Do males dive to different depths than females? Do their stomach contents differ? I have no idea, but it’s something to consider.

    • gillt
      Posted May 10, 2013 at 9:45 am | Permalink

      And it should be easily testable,
      Kristin Laidre from the OP must have thought the same thing almost a decade ago

      WINTER FEEDING INTENSITY OF
      NARWHALS (MONODON MONOCEROS)
      http://staff.washington.edu/klaidre/docs/LaidreandHJ_2005b.pdf

      • John Harshman
        Posted May 10, 2013 at 3:23 pm | Permalink

        The only relevant bit:
        “Sex and body size measurements were not taken by hunters for some of the narwhals in this study and consequently, sex-based dietary comparisons were not possible. Finley and Gibb (1982) reported no sex or age group differences between the diet, numbers of prey items, or prey sizes taken by narwhals in Pond Inlet, Canada. No differences were found between stomach contents of males and females when sex was available and no differences in deep-diving capabilities have been detected (Laidre et al. 2003). Records from the winter harvest in Disko Bay between 1990 and 1994 report 41% of the harvest was females (Heide-Jørgensen, unpublished data), indicating no large bias in the hunters selection of whales.”

        This would seem to be evidence against the hypothesis, though it doesn’t seem strong enough to exclude it entirely.

    • Sagra
      Posted May 11, 2013 at 10:44 am | Permalink

      If they live in pods, the females wouldn’t necessarily need the adaptation. They could just follow a male.

      • Joel Pelissier
        Posted May 12, 2013 at 11:59 am | Permalink

        “During the upper pleistocene era ,a flying man (Homo Alatum)came on the scene.Unfortunately,homo alatum had an evolutionary deffect:during coupling he had to stay on the ground in some kind of daze,and the aggressive cruelosaurus took advantage of that flaw.Consequently, homo alatum disappeared with few traces.
        Some scientists however, believe that there were a few specimen that survived all the way to the bronze age.And that is probably where, those stories of angels came from.”
        Prof Jopelski

  3. Posted May 10, 2013 at 9:01 am | Permalink

    Jousting? The photo you have appears to show many narwhals coming up for air in a narrow break in the ice. Yes, I see that those teeth do look rapier-like, but I don’t think that jousting/fighting is the idea here.

    BTW, the name means “corpse whale” because of their coloring.

  4. Posted May 10, 2013 at 9:08 am | Permalink

    Why on earth do narwhals have tusks?

    To properly understand this question, of course, one must put it in its proper context by contrasting M. monoceros with its extraterrestrial cousins.

    The Martian — or, more properly, Barsoomian narwhal, as is well known, is a flying frugivore. It uses its horn to spear qualia fruits from the jubjub tree, which it must do at high velocities to avoid predation by bandersnitchen.

    And the purpose of the four-footed land-dwelling albino Narnian narwhal’s horn is, by convention, never actually stated on account of its obvious salacious nature in conjunction with its fondness for young…erm…”inexperienced” human females.

    I’m still not exactly sure how this helps explain the function of terrestrial narwhal tusk purpose, but I’m sure the world’s greatest minds are working long and hard at this pointy problem and will soon set us straight with an upright answer.

    Cheers,

    b&

    • RFW
      Posted May 11, 2013 at 10:29 am | Permalink

      It was a Boojum, you see.

  5. Posted May 10, 2013 at 9:16 am | Permalink

    I had no idea teeth amino acids (or some of them) racemize in teeth. What’s the mechanism?

    • Gregory Kusnick
      Posted May 10, 2013 at 11:50 am | Permalink

      I’m no biochemist, but I imagine it’s just thermal agitation flipping them randomly back and forth between the two enantiomorphs. Since the tooth enamel is dead tissue, there’s no countervailing biological process to de-randomize them.

      • Torbjörn Larsson, OM
        Posted May 10, 2013 at 2:30 pm | Permalink

        I think that must be the dominant mechanism (another non-biochemist here), but if they are locked within a protein structure or by a cold environment I imagine it is also possible that quantum tunneling between configurations would contribute over long times such as in fossils respectively asteroids.

        • Torbjörn Larsson, OM
          Posted May 10, 2013 at 2:35 pm | Permalink

          Re the asteroid example, the reason why a racemization rate is interesting is because there are excesses of enantiomers found, believed to be promoted by polarized light that can occur around stars and other astronomical phenomena.

      • Torbjörn Larsson, OM
        Posted May 10, 2013 at 2:40 pm | Permalink

        And the elephant in the room would be water that most proteins are immersed in, jostling the molecules further.

  6. lkr
    Posted May 10, 2013 at 9:37 am | Permalink

    Concerning penguins:

    “The flippers originally arose by natural selection for flight, and after penguin ancestors lost that function, the vestigial wings became useful for swimming”

    I haven’t seen anything recent on evolution of penguins, but I would be surprised if they were flightless first, and THEN took up underwater flying. Much more likely they took the same path as alcids — wing-swimming, wings shortened as compromise with swimming/flying, and finally aptery with possibility for large size [as Greak Auk]

    • lkr
      Posted May 10, 2013 at 12:34 pm | Permalink

      NOT “aptery” — I should have said “loss of flight” — my entomologist nature was typing!

  7. Posted May 10, 2013 at 10:08 am | Permalink

    Don’t narwhals live in social groups? I agree, the fact it is only seen in males is a strong indication of sexual selection. But if the males can then seek out good feeding grounds, the whole pod (presumably including many of their own kin and offspring) benefits; plus there is no pressure for females to have them.

    • Marcoli
      Posted May 10, 2013 at 5:34 pm | Permalink

      Yes, I was thinking the same thing. Trying to remember instances where group foraging helps those less fortunate…. hmmmm..
      anyway, one can see that the female narwhals would benefit hanging out with big tusker males, and the males will have females around so it is not all one way. Meanwhile, this could continue to support the sexual selection aspect at the same time.
      Oh yes! New world primates have a situation where many individuals are rather color blind– sort of like red-green color blind. I think the thinking was that those who have a harder time picking out ripe fruit might follow the cues of those who see those colors better.

  8. Rick Wayne
    Posted May 10, 2013 at 10:33 am | Permalink

    Dr. Coyne, I must admit I am surprised that you fail to understand. It could not be more obvious why the narwhal is a beloved, iconic figure to anyone (well, almost anyone — see below) in what we satirically refer to as the “atheist community”.

    1) PZ Myers is a famously irascible, formidable antagonist in any kind of debate; attendees are frequently heard to comment that Myers “ate his opponent alive”

    2) Myers, aka “the tentacled one”, is known to personify certain legendary evils out of H.P. Lovecraft and scarcely bothers to hide the fact

    3) Narwhals, as the eponymous song clearly and explicitly explains, are an effective defense against Myers

    QED.

    Notes:

    • Marcoli
      Posted May 10, 2013 at 5:35 pm | Permalink

      Yeesh, my kids have that on the ipod. *shudder*.

  9. Gregory Kusnick
    Posted May 10, 2013 at 11:34 am | Permalink

    The tusk is actually a tooth that protrudes through the gum

    Don’t all teeth protrude through the gum? What am I missing here?

    • Greg Fitzgerald
      Posted May 10, 2013 at 1:53 pm | Permalink

      Technically, you’re right that all teeth protrude through the gum. The important difference for the narwhal’s tusk is that it protrudes anteriorly instead of ventrally. Imagine if our incisors jutted forward rather than downward!

      • Gregory Kusnick
        Posted May 10, 2013 at 2:32 pm | Permalink

        OK, but surely that’s not unique to narwhals. Elephants have forward-facing tusks too.

        • Marcoli
          Posted May 10, 2013 at 5:36 pm | Permalink

          I think its through their LIP.

  10. Greg Fitzgerald
    Posted May 10, 2013 at 2:00 pm | Permalink

    An interesting (though highly impractical) way to test the sexual selection theory would be to artificially extend a male narwhal’s tusk with a prostheses, and see if it is preferred by females. Conversely, one could shorten a narwhal’s tusk and see if it is ignored by females.

    This idea has famously been tested with widowbirds, whose tail length was manipulated by the experimenters. The results showed that male widowbirds with artificially lengthened tail feathers were significantly preferred by the females.

  11. Torbjörn Larsson, OM
    Posted May 10, 2013 at 2:21 pm | Permalink

    Nary a whale has tusks, maybe that is why they become icons.

    That is, all of the amino acids we incorporate into our body are the L-form

    Noo… It is most often one or the other, but D-forms appears. As interested in astrobiology I have had to keep track of this as indicators for life, and the small but persistent use of D-forms now even appears in Wikipedia:

    “Of the standard α-amino acids, all but glycine can exist in either of two enantiomers, called L or D amino acids, which are mirror images of each other (see also Chirality). While L-amino acids represent all of the amino acids found in proteins during translation in the ribosome, D-amino acids are found in some proteins produced by enzyme posttranslational modifications after translation and translocation to the endoplasmic reticulum, as in exotic sea-dwelling organisms such as cone snails.[30] They are also abundant components of the peptidoglycan cell walls of bacteria,[31] and D-serine may act as a neurotransmitter in the brain.[32]”

    [ http://en.wikipedia.org/wiki/Amino_acid ]

    During a discussion on Panda’s Thumb someone who had done a lot of literature work described it (as I remember it) like something of an explosion in medical literature happened in the early -00s, as the human enzymes that converts to and from L-form to D-form were elucidated. Since one form can have a different biochemical response than the other (which is what the bacterial cell wall change is based on), I assume it is vital to keep track of them in medicine.

    They are rare though which I just found out:

    Out of an organism proteome database with ~ 200 million AAs, ~ 1 000 were found to be D-forms (as of 2011), so occurs at a frequency of ~ 5*10^-6.

    [ http://en.wikipedia.org/wiki/Chirality_(chemistry) ]

  12. Hempenstein
    Posted May 10, 2013 at 4:05 pm | Permalink

    It also allows the whales to detect water particles characteristic of the fish that constitute their diet.

    Say whaa?? This sounds very homeopathic.

  13. marksolock
    Posted May 10, 2013 at 7:36 pm | Permalink

    Reblogged this on Mark Solock Blog.


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