One of the most remarkable organs in nature might have one of the most remarkable functions, if the results of a recent study are to be accepted…
I lectured this weekend on the evolutionary history of whales, so am feeling pretty inspired about cetaceans and their history. Actually, it’s always a good time to get inspired about the evolution of whales, given that this is such a happening area in vertebrate palaeontology nowadays. Recent years have seen the description of multiple new fossil ziphiids and mysticetes, and a huge amount of new work on stem-group cetaceans like pakicetids and basilosaurids. Bizarre and exciting new taxa like the walrus whales, the ‘killer sperm whale’ Zygophyseter and the predatory stem-group mysticete Janjucetus have only turned up recently [image at top from here].
Inspired by recent comments about hypothetical intermediate fossil taxa, I’ve been thinking a lot lately about the ancestry of one of the most remarkable of whales, the Narwhal Monodon monoceros, an Arctic odontocete sometimes called the Unicorn whale and widely thought to be part of the explanation for the unicorn myth. Narwhals are one of only two modern species of the delphinoid clade properly called Monodontidae; the other living member is the White whale or Beluga Delphinapterus leucas. A few other extant monodontids have been recognized at times, but all are either not really of monodontid affinity, or are synonymous with D. leucas (I’ll blog about these ‘other’ monodontids at some stage). My plan today was to write about ‘proto-narwhals': as you’ll see, however, I got distracted…
Monodontids have unusual flattened skulls that are neither short-beaked like those of porpoises nor long-beaked like those of most dolphins. Their unfamilair appearance led to the 2006 misidentification of one rotting beluga from Sakhalin Island (off Russia), widely hailed in the media as an unidentified sea monster [image at left]. In the beluga the upper surface of the snout is flat or concave while it is convex in narwhals. Belugas possess up to 40 conical homodont teeth, all of which are functional and visible in a normal individual. Narwhals are stranger. While embryos possess six pairs of teeth in the upper jaws and two pairs in the lower jaw, only one pair is present in the adult, though whether they are incisors or canines remains undetermined so far as I know (and the exact homology of these teeth would be difficult to establish anyway). In females the two teeth normally remain embedded within the skull, making the animal functionally toothless, but in males the left tooth erupts and extends to form the remarkable tusk. As it grows it spirals to the left, and quite why it does this is unknown (Kingsley & Ramsay (1988) argued that, while the direction of the spiral was unimportant, the tooth has to spiral in order to grow straight). This immense straight tooth lacks enamel and has a pulp cavity that extends for its entire length. It can exceed 2.6 m and 10 kg, with an additional 30 cm or so embedded within the alveolus.
The exact function of the tusk remains uncertain. Suggestions include that it might be used to break through ice, to pierce the bodies of prey, to disturb sediment on the sea floor when foraging, to disperse heat, or as a specialized sort of sound-gun. The last idea was proposed by Peter Beamish of the Marine Ecology Laboratory in Nova Scotia after he noticed how the tusks of males ‘throbbed in a disturbing way’ (to quote Lyall Watson) while the whales made high-frequency noises. Watson (1981) was obviously quite keen on the idea, and wrote ‘this opens up the possibility of rival male Unicorn Whales fighting acoustic duels, jousting and lashing at each other with their sonic lances’ (p. 164) [in the adjacent photo I'm posing with a narwhal tusk I discovered in Jon McGowan's living room].
Maybe the tusks do ‘throb’ when the males make noises, but subsequent studies have not demonstrated an acoustic role for the tusk, nor do the noises produced by narwhals appear to be particularly remarkable compared to those of other odontocetes: narwhals produce narrow-band pulses that range from 1.5 to 24 kHz (Ford & Fisher 1978). It seems more likely that the ‘throbbing’ is an incidental effect and that there is a better explanation. Indeed there is: the high incidence (61.5% of 39 sampled males) of broken tusks, and the occasional presence of tusk fragments embedded within the tusks and heads of other males (some broken tusk tips have been discovered jammed within the tips of other tusks), and of narwhals stabbed to death by others, strongly suggests that the tusk is used aggressively as a weapon in combat (Silverman & Dunbar 1980, Best 1981, Gerson & Hickie 1985). Some studies on the mechanical strength of the tusk indicate that it is well able to withstand a significant amount of stress and strain without breaking and is about similar in Young’s modulus, yield stress, ultimate stress and ultimate strain to deer antlers (Brear et al. 1990, Currey et al. 1994, Zioupos & Currey 1996), suggesting to the workers concerned that male narwhals engage in vigorous sparring that far exceeds anything yet observed by scientists.
However, a novel hypothesis was proposed in 2005 by Martin Nweeia of the Harvard School of Dental Medicine and his colleagues. You can read many reports of this hypothesis on the internet (and the abstract is viewable here) but, so far as I can tell, their study still hasn’t been published as a full paper. Noting the presence of over 10 million tiny nerve tubules connecting the tooth’s pulp cavity to its external surface, Nweeia and colleagues have argued that the tusk must be hyper-sensitive and thus act as a hydrodynamic sensor that can detect changes in temperature, pressure and salinity gradients. Even more remarkably, Nweeia et al. contend that the tusk isn’t stiff as assumed but with flexible outer layers that provide it with resilience and flexibility. In fact its structure indicates that it can bend about 30 cm in either direction, apparently. Nweeia et al.’s hypothesis is, obviously, fascinating and incredible. I look forward to reading more about it, but I can’t help but be sceptical in view of the fact that the tusk is unique to males; some narwhal specialists have expressed similar concern (Milius 2006). Or mostly unique to males: read on [adjacent image from here].
Not all narwhal males have just the one emergent tooth, and not all females lack emergent teeth, as individuals of both sexes may sometimes be equipped with two parallel tusks. The right tusk is usually shorter and slimmer than the left when present. Intriguingly, in two-tusked individuals the right tusk spirals to the left as does the left tusk. Tuskless males have been reported, but only one individual has ever been mentioned in which a right tusk was present but a left one was absent. There is also just a single report of an individual that sported a single small tusk in the lower jaw. While every published discussion on narwhals mentions anomalous tusked females and bidental males and females, Hay & Mansfield (1989) emphasized that such individuals are remarkably rare and not normally encountered in narwhal populations. The remarkable anatomy of the narwhal’s tusk has resulted in a voluminous literature that I won’t begin to review here: for a bibliography go here.
Both belugas and narwhals have a reasonable fossil record: there are Pleistocene narwhals from England and Germany, and Pleistocene belugas from the eastern USA and Canada (Reeves & Tracey 1980, Stewart & Stewart 1989). There are also fossil monodontids, including Denebola brachycephala from Miocene California. However, to date there are no fossil intermediates hinting at the evolution of the most morphologically bizarre member of the group, the narwhal, leaving us to speculate as to the course of evolution that resulted in its remarkable tusk. However, a poorly known modern case might shed some light on this, and it concerns some aberrant and mysterious monodontids observed from Greenland in the late 1980s. I’m aiming to cover this topic in the next post: watch this space…
Refs – –
Best, R. C. 1981. The tusk of the narwhal (Monodon monoceros L.): interpretation of its function (Mammaia: Cetacea). Canadian Journal of Zoology 59, 2386-2393.
Brear, K., Currey, J. D., Pond, C. M. & Ramsay, M. A. 1990. The mechanical properties of the dentine and cement of the tusk of the narwhal Monodon monoceros compared with those of other mineralized tissues. Archives of Oral Biology 35, 615-621.
– ., Currey, J. D., Kingsley, M. C. S. & Ramsay, M. 1993. The mechanical design of the tusk of the narwhal (Monodon monoceros: Cetacea). Journal of Zoology 230, 411-423.
Currey, J. D., Brear, K. & Zioupos, P. 1994 Dependence of mechanical properties on fibre angle in narwhal tusk, a highly oriented biological composite. Journal of Biomechanics 27, 885-897.
Ford, J. K. B. & Fisher, H. D. 1978. Underwater acoustic signals of the narwhal (Monodon monoceros). Canadian Journal of Zoology 56, 552-560.
Gerson, H. B. & Hickie, J. P. 1985. Head scarring on male narwhals (Monodon monoceros): evidence for aggressive tusk use. Canadian Journal of Zoology 63, 2083-2087.
Hay, K. A. & Mansfied, A. W. 1989. Narwhal Monodon monoceros Linnaeus, 1758. In Ridgway, S. H. & Harrison, R. (eds) Handbook of Marine Mammals, Volume 4. Academic Press, pp. 145-175.
Kingsley, M. C. S. & Ramsay, M. A. 1988. The spiral in the tusk of the narwhal. Arctic 41, 236-238.
Milius, S. 2006. That’s one weird tooth. Science News 169, 186.
Reeves, R. R. & Tracey, S. 1980. Monodon monoceros. Mammalian Species 127, 1-7.
Silverman, H. B. & Dunbar, M. J. 1980. Aggressive tusk use by the narwhal (Monodon monoceros L.). Nature 284, 57-58.
Stewart, B. E. & Stewart, R. E. A. 1989. Delphinapterus leucas. Mammalian Species 336, 1-8.
Watson, L. 1981. Whales of the World. Hutchinson, London.
Zioupos, P. & Currey J. D. 1996. Pre-failure toughening mechanisms in the dentine of the narwhal tusk: microscopic examination of stress/strain induced microcracking. Journal of Materials Science Letters 15, 991-994.