Chinese water deer don't hold the vernacular name of “vampire deer” for nothing, and it's perhaps the enlarged canines, commonly referred to as tusks or (incorrectly, given that they neither inject nor suck liquid) “fangs”, for which they are best known. Leaving aside the musk and mouse deer, both of which grow large canines but, despite their deer-like appearance, aren't true deer, the presence of such hypertrophied dentition is uncommon among the Cervidae, Hydropotes being one of only three members of the family sporting them; the others being the muntjacs (Muntiacus spp.) and the Chinese tufted deer (Elaphodus cephalophus), native to northeast Myanmar and southern and central China.
While it is conceivable that the canines evolved for use in general defence, water deer make no obvious attempt to fight back when caught by humans or dogs. Instead, tusks seem primarily a weapon of territorial combat in this species, having evolved in lieu of antlers, and are consequently sported primarily by the males. That said, however, Bedfordshire-based deer manager Paul Childerley has come across a few does with reasonably well-developed canines on his ground, although it should be mentioned that this is uncommon (see: Sexing).
Writing in the Journal of Mammalian Evolution during 2020, Doreen Cabrera and Theodore Stankowich point out that tusks are a better choice of weapon for the smaller, slinking artiodactyls that live in well-vegetated environments with low visibility, while larger species inhabiting more open ground can bear the cost of elaborate headgear. Furthermore, for a largely antisocial group, small weapons that could inflict maximum damage on an opponent without risking being tangled among tall vegetation were perfect. As deer grew larger and more sociable, they could ill afford to do each other serious damage as wounds could draw predators to the herd, so antlers evolved as a way of managing largely bloodless combat.
In her 2020 paper to PeerJ, Museum für Naturkunde Berlin biologist Nicola Heckeberg suggests that the presence, and possibly also the size, of upper canines is genetically linked with antlers, such that the underlying mechanism prompting their development is the same for both structures. This is an interesting idea, although it poses the question of why female water deer grow canines, albeit it typically only very small ones, given that upper and lower canines are lost in almost all female ungulate species in which the males use them in combat (e.g. horses).
During the winter rutting season, there is much chasing and bucks may fight one another (see: Rutting and Courtship). During battle, the bucks setup a circular “dance” and each tries to land a canine blow to the head, neck or rump of the other in a way that James Aitchison described as “raising their heads and striking downwards at their opponents with both tusks, viper-fashion”. During the late spring, mature bucks can also be territorial, chasing smaller males and landing deep gashes on their rumps in the process. Indeed, not only do the tusks taper to a point, they're also very sharp. The tusks point inwards slightly, bringing them into contact with the lower lip, and it's the rubbing of the inside surface of the tusk against the lip that polishes the surface and keeps them sharp. Arnold Cooke and Lynne Farrell, in their British Deer Society booklet published in 1983, note that this rubbing against the side of the lower lip can progress to such an extent that it wears through the enamel, and in older bucks the central cavities are exposed as grooves in the tooth. The outside surface of the tooth, by contrast, remains rough.
Keeping up appearances
Concurrent with the tusk itself, in both their summer and winter coats water deer have a dark patch of fur directly behind the canine as it protrudes from beneath the lower lip. This small area is known as the labial spot and, in a paper to the Journal of Mammalogy during February 1971, University of Alaska zoologist Russell Guthrie proposed that the darkly coloured hairs contrast with the white canine, making it more visible to potential competitors. Guthrie wrote:
“The dark labial spot probably began as an emphasis of the shadow effect of the canine that protruded from below the lower lip. It increased the threat value of the canine from a distance by making it look larger, or at least by calling attention to it.”
In addition to the darkly pigmented fur, the arrangement of the hair around the labial spot fans out away from the canine in such a way as to produce a noticeable tuft, which further dramatizing its presence. Arnold Cooke has mentioned to me how old bucks can be particularly whiskery, and one wonders whether such a tuft may also help hide a broken tusk from a potential competitor or predator. Alternatively, perhaps it just provides some additional padding to reduce wear of the inside edge of the tusk against the lower jaw.