The main sources of mortality for both adults and fawns is discussed in more detail elsewhere (see: Predators and Non-Predatory mortality and Medicine sections), so here we'll just briefly review the statistics on infant mortality among water deer. Suffice to say, stillbirth (including malpresentation, which can also be fatal for the mother), the prevailing weather (i.e., hyper- and hypothermia, the latter leading to pneumonia), and predators appear to be the most significant threats. In China, unweaned fawns suffer heavy mortality from poaching for “naikuai”, the colostrum in their stomachs that is used in traditional folk medicine as a cure for indigestion in children. We have no current data on the scale of this, but in the 1990s it was estimated that the number of fawns taken from the Zhoushan Islands had reached 1,500 per year.
Fawn survival appears to be low, and in their 1983 British Deer Society booklet on Chinese water deer Arnold Cooke and Lynne Farrell note that, in farmland, losses during the first three days of life may be as high as 25%. They further point to it being potentially much higher in some years:
“At Woodwalton Fen, during two of the five years of our study, losses from the foetus stage until maturity at six months of age are believed to have exceeded 90%. ... At Woodwalton, reproductive failures were most noticeable in 1977 and 1979, and it is possible that weather-mediated spring mortality of adults may be followed by a high incidence of abortion amongst the surviving does.”
Subsequently, in his 2019 opus Muntjac and Water Deer, Cooke gave the average recruitment per female (i.e., the number of fawns making it to their first winter) across 42 years at Woodwalton Fen as 0.56. In other words, 22% of first year deer were in the winter population on average, suggesting that more than three-quarters of fawns either die of disperse before their first winter. There was variation in the recruitment index at the fen, as we might expect, and the range covered zero (i.e., no fawns in the winter population) in 1984/85 and 1995/96 to a maximum of 1.7 fawns per female in winter of 2016/17, a survival rate of 46%.
Even in captivity, where occupants are relatively more protected than in the wild, mortality is still high. At Whipsnade in the early 1990s, Endi Zhang calculated that the mortality of 28 ear-tagged fawns was as high as 86%, with predation accounting for nearly 60% of losses. Christiane and Robert Mauget alongside Gérard Dubost and colleagues, all working at Branféré Zoological Park in France, have reported fawn survival to be lowest during the first 10 days after birth, with the mortality rate decreasing thereafter. By the end of the third month about 40% of the 155 fawns being monitored were still alive, and this had dropped to just over 31% at a year old. Hence, even in the confines of this zoo some 70% of the fawns had died within a year. In their 2008 paper to Mammalia, Dubost and his team noted particular periods of high mortality within these first 12 months. Forty-eight percent of fawns died during their first month, most within their first week, but after this, mortality occurred particularly between two and four months old, after the weaning period. In the two-acre broadleaf woodland enclosure at West Midlands Safari Park, Robert Lawrence found the mortality rate among fawns to be 40% during the first four weeks.
The Branféré dataset show an interesting difference in mortality between males and females. During the first month's mortality, more males than females died (55% versus 41%), while does were apparently more likely than bucks to die at between two and three, and four to six, years old. Another mortality period for calves occurred in August (i.e., the post-weaning phase), and for reproductive females in September, probably from exhaustion due to suckling. Dubost and his co-workers noted that if a fawn lost its mother or was abandoned within the first three weeks of life it was almost certain to die, while those losing their mother after at least 23 days survived. Allosuckling (i.e., a female suckling an unrelated fawn), as previously noted, was relatively common in the park, but this never replaced true maternal care and all fawns losing their mother within eight days of birth died regardless. In their 2008 paper to Mammalia, the researchers suggested that the high and perhaps unusual density at Branféré might have engendered less care, more confusion and desertion of young by mothers, particularly during the first week of life. Coincidentally, a note in the Daily Telegraph on 6th June 1957 alludes to something similar in the UK:
“The birth-rate among the Chinese water deer at Whipsnade is so high this season that the keepers cannot keep count of them. Several mother deer have deserted their young, six of whom are being bottle-fed in the Children's Zoo.”
Alongside dereliction, it is interesting to note that water deer seem less devoted mothers than many other cervids. While studying water deer at Poyang National Nature Reserve in Jiangxi during 1988 and 1989, Li-xing Sun observed that females with fawns were no more vigilant than those without and, at Whipsnade, Stadler found that while dams were slightly more vigilant the difference was not significant. Likewise, in an article to Deer in 1988, Stadler noted that during his study they caught and measured fawns, many of which would utter “a loud scream of varying intensity”, but unlike mothers of roe (Capreolus capreolus) or muntjac (Muntiacus reevesi), water deer dams never approached a screaming fawn.
The age and nutritional condition of the mother may influence the prospects for her fawns. During their studies on protein digestibility among water deer kept at Nanjing Hongshan Forest Zoo in 2013, Zhao Ling-Ling and colleagues found that adding soybean to the food to boost the protein and fat content increased the survival rate of fawns by 19%. It's not entirely clear whether this improvement was a result of a higher quality diet for the fawn, or the improved condition of its mother, although the researchers do mention that there were no post-partum deaths during the trial and the condition of the mothers was significantly enhanced. Also in China, in an experimental enclosure at Huxia Park in Pudong New Area between 2007 and 2009, Fei Yan Ma observed that fawn survival increased with the age of the mother - older females tend to be in better condition than younger ones.