Water Deer - Tusk Movement

While any large teeth are beneficial in the right circumstances, they can also be an impediment. One way these teeth can reach their full potential is if they're mobile. If the tusks aren't fixed to the jaw they're flexible to move back and forth and side to side, allowing them to move out of the way while feeding and, of particular importance for a ruminant that spends a lot of its time chewing cud, they don't get in the way of sideways jaw movements. It is just such mobility that we see in the tusks of mature Chinese water deer, although the mechanics of it remains a matter of debate. It is worth noting, at this point, that only fully developed tusks (i.e., completely erupted canines with closed roots) are mobile, immature tusks are not.

The sockets in the skull in which the tusks sit are referred to as maxillary alveola, basically 'air sacks' within the maxilla (upper jaw), and those of water deer are exceptionally long and wide. The upper reaches of this are often visible as a mild bulge in the skull of the male and, in the chapter on artiodactyl teeth in their The Teeth of Mammalian Vertebrates, Barry Berkovitz and Peter Shellis include a radiograph of the skull of a mature buck highlighting just how large the alveola are; their rear wall extend back to the juncture between the second and third premolar. There's no scale on the radiograph but, based on the tusks and other cranial proportions, I would estimate that each alveola was approximately 50 mm (2 in.) long and 20mm (0.8 in.) at the widest, which corresponds with the handful of skulls I have seen.

Despite having a short root and being remarkably mobile, they are surprisingly difficult to extract. Indeed, former Zoological Society of London veterinary officer James Aitchison, writing in 1946, recounted how, on one occasion, a water deer at Whipsnade tried to climb a strong wire-mesh fence and ended up hanging by its tusks on one of the wires; testament, if any were needed, to how strongly embedded the tusks are. In his 1973 book, Beasts in My Belfry, Gerard Durrell recounts what appears to be the fence-hanging story, which began with him and three colleagues trying to net a buck who had escaped the park's perimeter fence and managed to get into a chicken run in a nearby field during the winter of 1945/46:

“The Chinese water deer became more and more panic-stricken and started hurling himself at the tall wire fence in an effort to break through. He made one prodigious leap, hit the fence and hooked himself neatly on to the mesh by his two tusks and hung there kicking and struggling. We made a concerted rush but at the last minute he managed to unhook himself with a magnificent display of muscular contraction and, landing on the grass, turned with tremendous speed and broke through out ranks.”

The reason behind the security of the tusk is a very thick pericementum, the collagen-rich fibrous connective tissue otherwise known as the periodontal membrane or periodontal ligament surrounding the tooth. This ligament holds the tooth extremely securely and is itself firmly attached to the walls and outside edge of the socket. The only measurements for the periodontal ligaments surrounding the root inside the sockets that I have come across in the literature to date were given for a mature buck by Todd Wheeler, a palaeontologist at the Powell Museum in Los Angeles, in his chapter on the killing bites of sabre-toothed cats that was published in 2011 as part of The Other Saber-tooths: Scimitar-tooth Cats of the Western Hemisphere. Wheeler reported the ligament to be just over 3 mm (0.1 in.) thick and nearly 17 mm (0.7 in.) wide, covering virtually the entire root.

While the alveolar capacity and the periodontal ligament permit some mobility, it is nonetheless rather limited and there is a 'hard stop' as the root connects with the walls of the socket. One of the earliest and most detailed studies on the tusks of deer was carried out by Zoological Society of London vet James Aitchison based on two fresh heads, one skull and “several live” water deer at the Society's Whipsnade Zoo. Aitchison published his findings in a paper to the Proceeding of the Zoological Society of London in 1946, in which he notes that the slightest pressure moves the tusks by 15 mm (0.6 in.) front-to-back and 25 mm (1 in.) side-to-side, which corresponds with observations made by Barry Berkovitz and Peter Shellis in 2018 and some video footage I've seen. Aitchison observed that when the pressure was removed, the tusk didn't return immediately to any fixed position, but moved very slowly towards the centre of the ellipse of movement.

An area of some contention has been how much of the tusks' movement is under the control of the buck. In the late 1800s, Swinhoe noted how, during his observations of a captive animal, he never once saw it move its tusks, which always appeared to be held back. In dead animals, by contrast, Swinhoe had observed the tusks were always vertical, suggesting the “pulling back” was an active engagement by the buck. As such, he sent a fresh head of a buck to anatomist Dr. R. Jamieson in Shanghai who dissected the tusk and examined the surrounding tissue under a microscope. Jamieson's findings failed, however, to support this hypothesis, concluding that:

“... the movement of the tooth moves the surrounding gum, which firmly clasps it; but neither the gums nor lips have any power to move the teeth.”

Some seven decades later, Aitchison described how the gum terminates along its lower border, next to the gum pad, in a thickened band that formed a loop, also freely movable, around the distal surface of the tusk. In other words, this tough band forms a “lasso” around the neck of the tusk and in his paper, Aitchison proposed:

“When the animal snarls, the snarling muscles raise the upper lip and pull the moveable gum forwards and upwards. The thickened band and loop of the gum, in turn, hinge the tusk forwards into its most 'erect' position before the animal 'strikes'.”

The handful of examples of bucks fighting of which I'm familiar support Aitchison's description and writing in their Mammals of the British Isles: Handbook, 4th Edition chapter, Arnold Cooke and Lynne Farrell describe the same scenario:

“This snarling action pulls forward a section of movable gum which brings the tusk into its most forward position.”

In discussing this with Dr Cooke, he pointed out how the snarling action results in a change in the profile of the nose and, at the same time, a parting of the lips and rotation of the ears to point forwards. Indeed, in the superb series of images captured by Mike McKenzie at Claxton in south-east Norfolk during February 2013, the shortening of the nose and pulling back of the lower jaw, slightly parting the lips, can be clearly seen. This has the effect of bringing the tusks into a near-vertical position and actually gives the illusion of more movement in the tusks than the anatomy would seem to allow.

Based on Aitchison's findings, once the buck relaxes, the thickened gum pad (i.e., the periodontal ligament) then slowly returns the tusk to its resting position which, again going by Aitchison's observations, is around the middle of its full range of movement. This corresponds with video footage of freshly shot bucks Dr Cooke and I have seen. Some observers suggest that the tusks are actively “pulled back” out of the way while grazing, but to the best of my knowledge this has never been documented. Indeed, my observations and those of Dr Cooke indicate that the relaxed/grazing position may be at or just behind the middle of the orbit, and any movement further backwards might simply provide resistance against impact or be a reflexive response to mouth movements associated with grazing, rather than being voluntary. During our discussion on the topic, Dr Cooke suggested to me:

“Maybe the animal cannot control further backward movement, but such involuntary movement exists to reduce the risk of the tusk breaking when fighting or simply going about its usual business. We know tusk breakage happens fairly frequently, so a mechanism that reduces the risk must be a good thing. In other words, the grazing position is the central usual position; the animal can bring the tusks forward by snarling; the buck cannot make them go further back from the central position, but if a tusk is knocked or if someone pushes it, such movement is possible.”

Subsequently, Dr Cooke went on to muse about whether the stretching down of the neck while the buck is feeding might affect tusk position. Conceivably, he postulated, stretching the neck might “shift things around in the jaw of the deer so the tusk goes back”.

Additionally, based on video stills and photos, the trailing edge of the tooth exits the top lip at an angle of some 40-degrees in grazing bucks, about 50-degrees in relaxed (e.g., resting, walking or recently shot) males and around 70-degrees when pulled forward by the snarl. As we've mentioned, different bucks have different tusk lengths and curvatures, but this crude analysis would seem to hint at the tusks being further back while grazing than when relaxed, and that in their relaxed position they're around just behind the centre of the range of movement. Whether this rear grazing position is an active exercise by the buck or simply lowering the head to feed acts on the muscles/ligaments to pull the tusks back a bit further is unknown.

Certainly, the observations of Swinhoe and, more recently, Todd Wheeler, a palaeontologist at Page Museum in Los Angeles, suggest that the tusks are not voluntarily moveable by their owners. That is to say, to bring the tusks forward the buck must snarl and to return them to rest he must relax; he cannot move them back and forth without either one of the aforementioned actions. Dr Wheeler told me:

“I couldn't find, and can't imagine, any provision for voluntary movement within the periodontal ligament.”

In addition to the ability to the tusk to be pulled a short way forward prior to combat, we have some anatomical data to suggest weak chewing (masseter) muscles may facilitate tusk use by allowing the owner to open their mouth wide. In their 2013 study of the water deer masseter muscle, Motoki Sasaki at Obihiro University of Veterinary Medicine and colleagues found a weakly developed maxillo-mandibularis tendon, the tendon that attaches the masseter muscles to the lower jaw, which they suggest may allow for wider-than-normal jaw opening. If so, this could maximise the use of tusks during combat in a manner similar to that observed among Bactrian camels. Unfortunately, Sasaki and his team only studied two female deer found dead in Gyeonggi-do, so we don't know if the same applies to bucks.