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:
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.