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:
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”:
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.
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.
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