A single paragraph to the French journal Comptes rendus de l'Académie des Sciences in 1884 by an anonymous author told how, in a letter to renowned French mammologist Alphonse Milne-Edwards, French Jesuit missionary and zoologist Pierre Marie Heude announced to him that he'd received various specimens of a water deer from Korea. These specimens differed in skull morphology and fur colour to such an extent from that described by Swinhoe 14 years earlier, that Heude advocated this Korean water deer be considered a new species, proposing the binomial Hydropotes argyropus. There's no information in this short note from where this binomen originates, although it seems plausible it's derived from the Greek argyreios meaning “silvery” and pous meaning “foot”, implying the specimens exhibited lighter coloured feet/legs.
In the 1920s, the US National Museum acquired 13 specimens of water deer from China and 11 from Korea. Zoologist Alfred Howell compared these as part of his project to reassess mammal specimens in the museum's collection. Howell was driven to assign the Chinese and Korean samples subspecific standing. In other words, Chinese water deer would be Hydropotes inermis inermis and Korean water deer Hydropotes inermis argyropus, a division commonly used today. In his paper to the Proceedings of the United States National Museum during 1929, he noted:
“This fine series shows that there is much individual variation in color, and in size of skull, within this genus.”
Nonetheless, it seems that the chief separator here was the colour of the coat, and Howell went on to describe how:
“The form is recognized on the basis of average darker and richer coloration, with more reddish about the head. The skull exhibits no consistent differences.”
Much more recently, Yung Kun Kim and colleagues carried out univariate and multivariate analyses of the cranial morphology of 50 Chinese and 45 Korean water deer. The results, published in the Journal of Veterinary Medical Science in 2015, show that the two groups were very similar, with at least 75% overlap, leading them to conclude:
“… these 2 subspecies are not well-differentiated, suggesting that individuals of the 2 populations share common morphological traits”.
In 2009, Hung Sun Koh and colleagues at Chungbuk University in South Korea published their genetic analysis of 75 water deer specimens, 30 from three sites in China and 45 samples from five sites in South Korea. The mtDNA control region and cyt b data suggest two distinct forms distributed in two populations: a major group that includes individuals in both China and Korea, and a minor clade that appears unique to Korea. In other words, this sample set implies that Chinese water deer are found in both China and Korea, while “Korean water deer” are found only in Korea. It's important to recognise that these data suggest only that there has been reproductive separation between some populations of water deer in Korea; they don't support the classification of these clades as separate subspecies. Indeed, in their paper to Biochemical Genetics, Koh and his team note that a cyt b derived genetic distance of less than 2% is considered typical of the natural variation expected within a population (i.e., intraspecific variation) and that their data show the major and minor clades had a genetic distance of only 1.3%.
The findings of Koh and his colleagues were substantiated by a team led by Baek-Jun Kim at South Korea's National Institute of Ecology using different genetic markers (D-loop haplotype and median-joining network data). These data, published in Genes and Genetic Systems during 2014, also indicate two water deer lineages: A, which is present in both China and Korea; and B, which is present only in Korea, where it presumably originated. Molecular clock analysis carried out as part of the investigation suggests these lineages separated at some point in the early Pleistocene (around 1.8 mya), at which time South Korea was connected with China and undergoing a period of significant climatological fluctuation. South Korea was cut off from China by the Yellow Sea about 10,000 years ago, preventing further gene flow between the lineages. Again, the genetic distance of these lineages calculated in this study was less than 2%.
While it is possible to identify differences in the Chinese and Korean lineages using some genetic markers, the data from the 2009 and 2014 studies, as well as the microsatellite analysis published by Seoul National University researchers in 2011, suggest that there's still very little regional or genetic structure among water deer populations within Korea.
Overall, water deer from China and Korea cannot be confidently distinguished by their morphology and, while there may be populations undergoing speciation (there are some data suggesting water deer in South Korea have lower genetic diversity than those in China), the available genetic data suggest they are best considered the same species at the present time.