Water Deer Taxonomy - True Deer
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, the Cervidae have been divided into two broad groups: around 32 species of Old World deer and 22 species of New World deer — the Cervinae and Capreolinae (Odocoileinae in some older texts), respectively. In his original description of Hydropotes, Swinhoe did not suggest a taxonomic placement, but it was apparent early on that, despite its geographic origin, the species had much in common with the New World (telemetacarpalian) deer. Writing in 1882, William Forbes noted the absence of a gall bladder -- consistent with the true deer -- and observed that the morphology of the penis, the absence of Cowper's (bulbourethral) glands, and the organisation of the brain all aligned water deer with Capreolus, the roe deer. The structure of the ear and the retention of the lower second and fifth metapodia in the cannon bones are also typical of telemetacarpalian deer such as roe. More recently, Elena Artemeva's 2018 study demonstrated the presence of haemolymph nodes in water deer for the first time. These miniature spleens, distributed throughout the body, further reinforce the taxonomic placement of Hydropotes: despite the species' ancient outward appearance, its internal immune architecture is as sophisticated as that of modern livestock and other deer.
In 1898, French naturalist Édouard Trouessart proposed that water deer should be placed in their own subfamily, separate from the roe deer, and established Hydropotinae. This arrangement was subsequently supported by Australian zoologists Colin Groves and Peter Grubb in the 1980s, largely on the basis of the species' lack of antlers. George Simpson, in his 1945 classification of mammals, also considered some separation between roe and water deer warranted, proposing the tribe Hydropotini – a taxonomic rank that subdivides families. Both Hydropotinae and Hydropotini appear in the literature, though neither is now widely used. Genetic, behavioural, and hair-structure studies all point to a very close relationship between roe and water deer, suggesting that such a separation is unwarranted.
The molecular evidence is particularly compelling. Emmanuel Douzery and Ettore Randi at the Université de Montpellier used the mitochondrial gene cytochrome b (cyt b) as a molecular marker to reconstruct relationships within the Cervidae, finding that water deer nested firmly within the Capreolinae, with pronounced affinities to roe deer. In other words, Hydropotes and Capreolus are sister taxa -- more closely related to each other than to any other genus -- and together form the sister clade to the North American mule deer and white-tailed deer (Odocoileus). The following year, Randi and colleagues identified a rare mitochondrial protein configuration shared by roe and water deer, providing further support for a Capreolus-Hydropotes clade; in their paper in the Proceedings of the Royal Society of London, they advocated placing Hydropotes within the 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 from an analysis of hair cuticle structure, published in 2001.
Two analyses published in 2006 -- one by Memorial University biologist Steven Carr and colleagues, the other by a team led by Clément Gilbert at the Muséum National d'Histoire Naturelle in Paris -- arrived at the same broad grouping of telemetacarpalian deer. Both proposed that the Capreolinae consists of three tribes: Capreolini (Capreolus and Hydropotes); Alceini (Alces); and Odocoileini (Rangifer and the American genera). This arrangement was corroborated by Nicola Heckeberg of the Museum für Naturkunde Berlin, whose exhaustive analysis of cervid systematics, published in PeerJ in 2020, drew on morphological, nuclear, mitochondrial, and combined total-evidence approaches; all supported Hydropotes as the sister taxon of Capreolus, and both genera together as the Capreolini.
The genetic and anatomical affinity between water deer and roe is also reflected, to some extent, in their behaviour. In 2002, a French research team led by Henri Cap of the Université Paul Sabatier proposed, on the basis of four shared behavioural traits -- never walking backwards, shaking the head and body simultaneously, performing circular head movements, and licking the anogenital region -- that water deer were in fact sister to an assemblage comprising roe, moose, mule deer, white-tailed deer, and reindeer (Hydropotes [Capreolus [Alces [Odocoileus, Rangifer]]] for the taxonomists amongst you). In a subsequent analysis of vocalisation behaviour, however, Cap and colleagues constructed a cladogram more consistent with the molecular evidence, placing Capreolus and Hydropotes as sister genera and the Capreolini as a whole as sister to the European moose (Alces alces).
Taking the morphological, genetic, and behavioural data together, the current situation is broadly one of three tribes making 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 can be considered as follows:
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, but this is not widely supported.
* Note that some texts incorrectly translate the binomial name as "the unspotted water-deer".
Finally, it is worth noting that some authors have suggested a connection between water deer and the rusine deer of southern Asia; members of the genus Rusa, which includes the Philippine spotted deer and several sambar species.
Lydekker, in his 1898 opus, mentioned that, before revising his opinion and placing the species among the New World cervids, Sir Victor Brooke had originally considered water deer to be related to the rusines. This alignment has little support in the current literature, where most schemes group both Rusa and Rucervus -- the barasingha or swamp deer of India -- with the Axis and Cervus species. Some genetic markers do, nonetheless, point to an intriguing connection. In a brief note in Mitochondrial DNA in 2020, Zongzhi Li and colleagues presented the complete mitochondrial genome of the water deer, highlighting its remarkable similarity to that of Rucervus duvaucelii branderi, a subspecies of barasingha from Kanha National Park. If confirmed, this would mean that Rucervus actually belongs within the Capreolinae – with significant implications for our current understanding of the New World and Old World cervid divide.
For more information on how species are classified see: Taxonomy
Bibliography
Artemeva, E.A. 2018.
Histomorphology of Haemolymph Nodes of Water Deer (Hydropotes inermis argyropus): Novel Study.
Basrah Journal of Veterinary Research. 17(1): 315-325.