In their overall breeding biology, Chinese water deer do not appear to deviate significantly from the general ruminant condition, although relatively little has been published on their reproductive anatomy and hormone profiles.
While King's College zoologist Alfred Garrod provided some preliminary data on the uterus of Hydropotes in 1877, and Edinburgh University zoologist James Cossar Ewart gave a detailed assessment of the placenta the following year, the only formal description of the female reproductive tract of which I'm aware was published by Primorsk State Agricultural Academy biologist Elena Lyubchenko and colleagues in 2021. This survey was, unfortunately, based on only a single mature doe, found recently dead in a forest near Khasan in Russia during February 2020, so we cannot know how representative it is. The reproductive tract consisted of a vagina measuring approximately nine centimetres (3.5 in.) in length, the uterus body about 6.5cm (2.6 in.) long, and the right and left horns some seven centimetres (2.8 in.) each. The ovaries and follicles were small, measuring 1x1 cm (0.4 in.) and 0.5x0.5 cm, respectively.
In common with all higher ruminants, water deer exhibit a cotyledonary placenta type; specifically an oligocotyledonary type. Unlike most mammals, where the endometrium (uterus lining) appears uniform throughout the uterus, in ruminants it's differentiated into two distinct types. Some of the tissue is devoid of uterine glands and appears as pale, raised areas broadly elliptical in shape - these are called caruncles, from the Latin caruncula, which translates roughly to “little piece of flesh”. The tissue in between these caruncles is classed as “intercaruncula area”. During pregnancy, the caruncles (maternal side) go on to merge with the finger-like villi of the cotyledon (foetal side) to form structures called placentomes. Essentially, therefore, a placentome is the merging of maternal and foetal tissues that forms a connection between doe and fawn. In their presentation on the placenta of the Korean water deer, given at the Vietnam National University of Agriculture in 2014, Shota Yamane and co-workers noted that the number of placentomes was between five and nine, typically six, indicating that each foetus occupied two or three - far fewer than in other ruminants (e.g., sheep, goats and cattle have between 75 and 125), but in line with other deer.
In their study of the placenta of ten water deer killed in road accidents in South Korea, Joon Hyuk Sohn and colleagues described the placentomes as slightly convex in shape exhibiting wide ranging volumes: between 3.6 and 82 cubic centimetres (0.12-2.8 fl. oz.), with the overall mean being about 24 cubic cm (0.8 fl. oz.). Typically, each female had five or six well-developed placentomes in their uterus, although up to nine were found in one doe. This is in line with Ewart's dissection 143 years earlier, during which he found six large spongy placentomes measuring between 25 and 44 mm (1-1.75 in.) in length, 13 mm (0.5 inch) in width and projecting 13 mm above the membrane of the uterus. Interestingly, while caution must be applied given the small sample size of the Korean study, the distribution of the placentomes between the uterine horns appeared remarkably even given the occurrence of odd number totals. Where five were present (four females), for example, three were in one uterine horn and two in the other, and where six were counted (two females), three were in each horn. There was no tendency for one horn (right or left) to always have more than the other.
The data presented by Sohn and his team showed no correlation between the number of placentomes in a uterine horn (i.e., left or right) and the number of foetuses found there. They also illustrate how, despite both being well known for multiple births, the branching of the villi in the placentome of the water deer is better developed than in the roe. In their paper to the Journal of Veterinary Medical Science during 2021, the authors speculate that water deer may “possess special mechanisms or structures in the attachment site of the foetus to maintain the unusually high number of foetuses”.
In part two of his The Mammals of China and Mongolia, published in 1938, Glover Allen wrote that the penis of the water deer is simple and slightly attenuate, without the filiform (threadlike) outgrowths we see in some cervids, and with the opening terminal. When not in use, the penis is sheathed within a fur-covered foreskin called a prepuce and, in some species, fallow in particular, this tissue is studded with scent glands, preputial glands, that are most active during the rut. In a 1996 paper to the Journal of Mammalogy, however, Stewart Odend'hal and colleagues described sebaceous glands around the roots of the hairs at the tip of the water deer prepuce without sweat glands, suggesting they only serve to provide lubrication between the penis and wall of the preputial canal in this species.
The male reproductive system was described in detail by Joon Sohn and Junpei Kimura, also at the Seoul National University, in a 2012 paper to the Asian Journal of Animal and Veterinary Advances. Sohn and Kimura studied the testes of 25 adult male deer in Korea, 16 collected during the winter rut (Oct-Dec) and nine during the summer (April-July). The four accessory reproductive glands typical of ruminants were identified during the study - ampulla, vesicular gland, prostate gland and bulbourethral (Cowper's) gland - and the testes were descended and positioned vertically between the hind legs, each testicle being oval. In my experience, the genitals of the buck are not easily identified at distance, being largely concealed by the long fur of the underbelly. Similarly, there is no obvious staining of the testicles or prepuce, as seen in other species (e.g., fallow), nor does the size increase appreciably during the rut. Indeed, overall there's relatively little change in the appearance of bucks during the rut compared with other times of the year. Some perhaps have a stockier appearance, although it's difficult to be certain given the much thicker winter coat.
Sohn and Kimura concluded that spermatogenesis (sperm production) in water deer occurs between October and December, although no specimens were collected during January or February, and while there was no obvious change in the size or volume of the accessory glands, they were more active during the winter, corresponding with the rut. Sperm development was taking place in the seminiferous tubules only between October and November and the testes were much smaller and lighter in the summer. Correspondingly, testes were significantly larger and heavier during the late autumn and early winter - based on 16 samples, the mean (and median) testes weight and volume was just over 11 grams and 18 millilitres, respectively, with a range of 9-19 g and 9-32 ml. In summer, the mean (and median) was approximately six grams and 9 millilitres, with a range of 4-8 g and 6-13 ml based on nine bucks. Spermatids and convoluted seminiferous tubules were only observed between October and November.
The data provided by Sohn and Kimura strongly suggest bucks are non-reproductive during spring and summer, and this corresponds with Raymond Chaplin's observations during the late 1960s. In his unpublished research notes, Chaplin presents the results of the examination of the testes and epididymides of 25 bucks killed in Woburn during March, all of which showed little or no spermatic activity. The testes weighed 7-8 grams and the epididymides about two grams. Chaplin noted that, based on this, reproduction appears to shut down relatively soon after the rut and during this time none of the animals studied could've effected fertile matings come March.