In common with most mammals, the lowest survivorship is among the very young, with survival prognosis improving past the first birthday. At Branféré Zoological Park in Brittany, France, for example, Gérard Dubost and his team found that nearly 60% of water deer died before they reached four months old, while only about 20% of mature animals died each year. At Whipsnade, Endi Zhang observed that some 86% of deer failed to make it past two months old, while Arnold Cooke and Lynne Farrell calculated 75% died before nine months old in the wild in Britain.
Writing in 1911, Sir Peter Chalmers Mitchell, the eminent Scottish zoologist, gave the maximum longevity of water deer as 129 months (almost 11 years), with an average of 70 months (just over 5.5 years), but this was based on only three records. Indeed, there are several reports of deer surviving for 11 or 12 years and even beyond in captivity.
The oldest record I have come across so far is given by Richard Weigl, in his 2005 Longevity of Mammals in Captivity, and involves an individual of unknown sex born at Whipsnade Zoo on 12th October 1969 that died at the park on 1st October 1983, a few days shy of its 14th birthday. I confess, however, to being rather suspicious of this record.
Water deer are almost invariably born during June or early July, and while I'm aware of one report of a very young fawn seen accompanying a female in early August, I know of no records from October. Nick Lindsay, Conservation Breeding Programme Manager at Whipsnade, told me that he wasn't aware that water deer were ever tagged at the park, and it's certainly not something they have done in recent memory, bar the fawns hand-reared in 2020, 2021 and 2022. Endi Zhang conducted his Ph.D thesis at the zoo in the mid-1990s, however, and writing about one particular area of the grounds he noted that “About 150 water deer live in this part and around 80 of them were ear-tagged”. Emma Milnes, ZSL's Deputy Librarian, was able to clarify the situation when she kindly reviewed the original registrar cards held by the zoo. The dates recorded on these cards aren't birth dates but the date the animal was first recorded at the park - the cards have “recorded dd/mm/yy” in their Date of Arrival section. Unfortunately, we know nothing more about this animal. If ear-tagging was implemented as early as the 1960s, it seems straightforward enough to understand how the keepers were able to chart its lifespan. Zhang only mentions, however, the 13 animals being tagged during Stadler's study in the mid-1980s, and 104 in 1993 for genetic analysis, while Nick's comments suggest any tagging was at best intermittent or inconsistent. While I remain somewhat sceptical of the record, if we nonetheless consider that this individual was likely born in June 1969, it appears to have lived to be about 14 years and three months old.
In his article on ungulate longevity to the International Zoo Yearbook in 1993, Marvin Jones gave the maximum longevity as 13 years and 11 months. I presume this to be the same individual to which Weigl refers. Jones doesn't cite a source, but the age fits and he does note the animal died in 1983. James Carey and Debra Judge, in their Longevity Records: Life Spans of Mammals, Birds, Amphibians, Reptiles, and Fish, published in 2000, listed the oldest captive record for this species (unknown sex) as 12 years old, with another at 11.5 when their survey was conducted, although they don't give any details.
As we've touched upon, life in the wild is considerably more dangerous and few water deer are likely to make it to double figures. In a 2011 paper to Mammalian Biology, Gérard Dubost and colleagues suggested that 11 years was the theoretical maximum age, although eight years was the oldest individuals were observed to reach at Branféré Zoological Park. Indeed, during their studies, the researchers estimated most of the dead adults they assessed at between three and seven years old and, in the aforementioned paper, they noted how “... the stage of wear of their teeth indicated that they could not be much older than 9 years without having difficulty in grazing.” Yeong-Seok Jo, John Baccus and John Koprowski, in their 2018 Mammals of Korea, considered the lifespan in the wild as 10 to 12 years old, with 14 reached in captivity (again, presumably Weigl's record).
Overall, it seems reasonable that the upper age for this species, under ideal conditions, is probably 10 to 12 years, but few will surpass eight years in the wild and most bucks probably make only four or five. As with almost all other mammals, does are likely to outlive bucks but, as Arnold Cooke summed up in Muntjac and Water Deer: “... water deer do not generally enjoy long, healthy lives even if they have the security of a zoo or nature reserve.”