Superficial similarities with musk deer aside, most naturalists were quick to recognise that the Chinese water deer shares a great many anatomical and behavioural traits in common with the “true deer”. Today, the 50 or so species that lack a gall bladder and share various consistencies in the arrangement and anatomy of the bones in their legs, are lumped together in a family called the Cervidae. In 1991, Florida-based zoologists Fred Kraus and Michael Miyamoto's presented their molecular analysis showing that the Cervidae was only monophyletic (i.e., complete) if Hydropotes was included, confirming the thoughts of most naturalists for the past century. While the placement of Hydropotes in this family is now widely accepted, however, its specific relationship to other deer species has been more contentious.
The Cervidae Way
For decades now we have recognised that the Cervidae can be split into two broad groups: about 32 species of “Old World” and 22 species of “New World” deer - the Cervinae and Capreolinae (Odocoileinae in some older texts), respectively. In his original description of Hydropotes, Swinhoe didn't suggest a taxonomic placement, but it was clear early on that, despite its origin, it had a lot in common with the New World (telemetacarpalian) species. Indeed, writing in 1882, William Forbes noted how the morphology of the penis, lack of Cowper's (or bulbourethral) glands and the organisation of the brain aligned water deer with Capreolus - the roe deer. The construction of the ear and the retention of the lower second and fifth metapodia (in the cannon bones in the legs) is also typical of telemetacarpalian deer such as roe.
In 1898, French naturalist Édouard Troussart proposed that the water deer should be placed within its own subfamily, separate from the roe deer, and created Hydropotinae. This placement of Hydropotes was subsequently presented by Australian zoologists Colin Groves and Peter Grubb during the 1980s, largely based on its lack of antlers (see below). George Simpson, in his 1945 opus on the classification of mammals, also considered some separation between roe and water deer was required, proposing the tribe (a taxonomic grouping that splits families) Hydropotini. Both Hydropotinae and Hydropotini appear in the literature today, although they have largely fallen out of use. Indeed, genetic, behavioural and hair structure studies all suggest a very close relationship between roe and water deer and indicate such a split is unwarranted.
Emmanuel Douzery and Ettore Randi, Université Montpellier used a mitochondrial gene called cytochrome b (cyt b) as a molecular marker to infer the relationships within the Cervidae and found that water deer were nested within the Capreolinae, with “pronounced affinities” to roe deer. In other words, Hydropotes and Capreolus are what we call sister taxa (i.e., more closely related to each other than they are anyone else) and together they form the sister clade to the North American mule and white-tailed deer (Odocoileus). The following year, Ettore Randi and colleagues identified a rare configuration of one mitochondrial protein that was shared by roe and water deer, providing additional support for a Capreolus-Hydropotes clade. In their paper to the Proceedings of the Royal Society of London, the researchers advocated placing Hydropotes in the subfamily Capreolinae alongside the roe deer. A team of German anatomists, led by Wilfried Meyer at the School of Veterinary Medicine Hannover, reached the same conclusion based on their analysis of hair cuticle structure published in 2001.
Two analyses published in 2006, one by Memorial University biologist Steven Carr and colleagues and the other from a team led by Clément Gilbert and co-workers at the Muséum National d'Histoire Naturelle in Paris, arrived at the same rough grouping of telemetacarpalian deer. Both suggested that the Capreolinae consists of three clusters, or tribes: Capreolini (Capreolus and Hydropotes); Alceini (Alces); and Odocoileini (Rangifer + American genera). This broad arrangement was also arrived at by Museum für Naturkunde Berlin researcher Nicola Heckeberg. Heckeberg's exhaustive analysis of the systematics of the Cervidae, published in PeerJ during 2020, applied morphological, nuclear, mitochondrial, molecular combined, and total evidence analysis; all supported Hydropotes as the sister taxa of Capreolus and that together these genera formed the Capreolini.
Genetically and anatomically water deer clearly align with roe, and there is some evidence that this can be seen from their behaviour, too. In 2002, a French research team led by Université Paul Sabatier behaviouralist Henri Cap proposed that water deer were, in fact, the sister species to an assemblage (the Capreolini) containing roe, moose, mule, white-tailed and reindeer/caribou (for the taxonomists among you: Hydropotes [Capreolus [Alces [Odocoileus, Rangifer]]]). This arrangement was based on the distribution of four particular behaviours: never walking backwards; shaking head and body at the same time; showing circular head movement; and licking the anogenital region. In a subsequent analysis involving their vocalisation behaviour, however, Cap and colleagues constructed a cladogram supporting much of the molecular data in indicating Capreolus and Hydropotes were sister genera and together (i.e. as the Capreolini) they're sister to the European moose (Alces alces).
Taking the morphological, genetic, and behavioural data together, the current state of play is broadly that three tribes make up the Capreolinae subfamily: the Rangiferini (reindeer, brockets, white-tailed, mule, marsh, pampas, pudu and taruca/huemul); the Alceini (moose or Eurasian elk); and the Capreolini (roe and water deer). As such, the taxonomic structure of the Chinese water deer is considered to be:
Kingdom: Animalia (Animals)
Phylum: Chordata (Possess a basic 'backbone')
Class: Mammalia (Mammals)
Order: Artiodactyla (Even-toed ungulates)
Family: Cervidae (True deer)
Subfamily: Capreolinae (New World deer)^
Tribe: Capreolini (Roe deer)
Genus: Hydropotes (from the Greek hydor or hudr- for “water” and potes, “drinker”)
Species: inermis (the Latin masculine adjective inermis means “unarmed”, “weaponless” or “defenceless” in reference to the lack of antlers)*
^ This follows the primary molecular data and the scheme laid out by Colin Groves and Peter Grubb in their Ungulate Taxonomy, published in 2011. Some authors maintain the water deer belong to their own subfamily, the Hydropotinae.
* Note that some texts incorrectly translate the binomial name as “the unspotted water-deer”.
Finally, it is worth mentioning that some authors have suggested a connection between water deer and the rusine deer of southern Asia (i.e. members of the Rusa genus, which includes the Philippine spotted deer and several sambar species).
Lydekker, in his 1898 opus, mentioned that Sir Victor Brooke originally believed water deer related to the rusine before revising his opinion and placing it within the New World cervids. At the time of writing this isn't a well-supported arrangement, with most schemes grouping both Rusa and Rucervus (the barasingha or swamp deer of India) together with the Axis and Cervus species. Some genetic markers do, nonetheless, imply an intriguing connection and, in their brief note to Mitochondrial DNA in 2020, Zongzhi Li and colleagues presented the complete mitochondria genome of the water deer, pointing out its remarkable similarity to that of Rucervus duvaucelii branderi, a subspecies of barasingha found in Kanha National Park. If borne out, this means Rucervus actually belongs within the Capreolinae and implies we should revisit our current thinking of New and Old World cervids.
For more information on how species are classified see: Taxonomy