Water Deer Reproduction - Gestation & Litter Size

Fertility

Most of what we know about water deer fecundity comes from captive animals, with data from wild populations remaining sparse. At Branféré Zoological Park, Christiane and Robert Mauget reported that 70% of mature does gave birth. A subsequent study at the same site by Gérard Dubost and colleagues found that, across three years, around 64% of females gave birth overall, reproductive success increasing with age: 31% of juvenile does (mated at six months), 47% of yearlings (mated at 18 months), and 75% of adults produced fawns. Similar age-related variation was recorded at Whipsnade, where Adrian Middleton reported in 1937 that 80% of older does were pregnant compared with 67% of younger animals, and Stefan Stadler later found that roughly half of yearling does at the same zoo gave birth. Arnold Cooke and Lynne Farrell, writing in their 1983 British Deer Society booklet Chinese Water Deer, noted that fecundity in first-year does appears broadly similar to that of mature females, though pre- and perinatal losses are apparently high. Raymond Chaplin, in his 1977 book, observed no obvious decline in fecundity with age, though his sample comprised only 24 animals. An outlier of sorts comes from Frankfurt Zoo, where Lutz Heck recorded in 1972 that the female kept there produced smaller litters as she aged – though he acknowledged this may have been a consequence of captive conditions rather than representative of the species more broadly.

The only published study from wild populations known to me was conducted by Helin Sheng and Houji Lu on the Zhoushan Islands between December 1982 and April 1983, and published in Acta Theriologica Sinica the following year. Sheng and Lu found fecundity in mature does to be slightly higher than in one-year-olds: 2.73 versus 2.17 fawns per pregnancy respectively. They also observed pregnancy rates increasing from winter into spring. None of the 21 adult females collected in December and January were pregnant. By February, five of nine (55%) were, and all six taken in March were pregnant. Among yearlings, however, the picture was different: none of the eight one-year-old does collected in February showed signs of pregnancy, while half of those taken in the first half of March were pregnant, and all yearlings collected from 16th March onwards were. This mirrors the pattern of delayed conception in younger females seen in other deer, including fallow (Dama dama). Researchers at Branféré similarly reported that older females gave birth earlier than first-year does, most likely reflecting later mating in younger animals. In their 2008 paper in Mammalia, Dubost and colleagues suggest that fawns of mature does may get a head start over others by their dams coming into season earlier, being mated sooner, and giving birth first.

Some of the variation in fecundity between females under two years old and mature does may reflect differences in body condition. A link between body condition and fertility is well established across ungulates (e.g., domestic cattle), and while data specific to water deer are limited, a study by Zhao Ling-Ling and colleagues offers some insight. Examining protein digestibility in deer kept at Nanjing Hongshan Forest Zoo in 2013, they found that enriching the diet with soybean -- thereby increasing protein and fat content -- raised the fertility rate of does by almost three percent, although it's worth noting that "fertility rate" here likely refers to conception or birth rate rather than litter size. This is consistent with the findings of Dubost and his team at Branféré, who reported that larger litters were more likely in older, better-conditioned females, and that litter size declined as population density increased – presumably because resources became more thinly distributed and individual body condition declined as a result. They also found that as the number of adult females in the population during the rut increased, litter size fell, perhaps because the bucks were unable to mate with all available females.

Litter size & gestation

Hydropotes is what we call a polytocous species -- from the Greek polys ("many") and tokos ("offspring") -- meaning females routinely produce more than a single fawn. Much has been made of this capacity, with Duff Hart-Davis, in his 2002 Fauna Britannica, describing water deer as "the most prolific of all deer". In his 1998 Deer of the World, Valerius Geist noted that multiple births suggest a long evolutionary history in unstable but resource-rich environments – the kind of productive floodplain habitat where high adult mortality selects for high reproductive output.

Robert Swinhoe mentioned their fecundity in his original description of the species, noting that one doe was found to contain six embryos. The following year, in a note to The Proceedings of the Zoological Society of London, Edward Hamilton recounted a letter from Mr J.A. Arnott of Shanghai:

"Do you know that the doe of this species has constantly five or six young at a birth? We often find it so when the animal is opened, as is customary immediately after it is shot."

In a short paper to the same journal in 1873, Swinhoe reported that two females shot in winter each carried seven embryos, while Hamilton described a doe shot in late February of the same year that was found to be pregnant with seven foetuses, already developed enough that their feet and eyes were visible. In a similar vein, US consul general Thomas Jernigan, writing in Shooting in China (1908), described a deer shot near Shanghai and gralloched by Dr Henderson that was found to contain seven embryos.

Not everyone was initially convinced. Writing in the Journal of Anatomy and Physiology in 1878, Scottish zoologist James Cossar Ewart noted that early reports of the water deer's prolific nature "were received with much doubt". Having examined a uterus containing four embryos sent from Shanghai, however, he conceded "the question practically settled, that the Shanghai river deer is much more prolific than other deer we are acquainted with".

Raymond Chaplin, in his 1977 book Deer, mentioned three verified records of six healthy foetuses in Woburn animals, as well as a single report of a doe carrying seven. Arnold Cooke and Lynne Farrell, in their 2008 contribution to Mammals of the British Isles, noted that up to seven foetuses have been recorded in a single female in China, though Cooke adds in Muntjac and Water Deer that litters of six, while reported, remain very rare. Helin Sheng and Houji Lu, in their 1984 paper in Acta Theriologica Sinica, cite elderly hunters who recalled, from their younger days, shooting does with six and seven embryos. The largest litter count I have encountered in the literature is eight, reported by Czech zoologist Luděk Dobroruka in a short note to Mammalia in 1970, though he gives no details of where the record originated or how many foetuses survived to term.

It's striking that the bulk of large litter records come from late nineteenth-century sources, while more recent accounts tend to report considerably lower numbers. At Whipsnade, Adrian Middleton found that of 12 pregnant does examined, nine (75%) carried twins, two (17%) had triplets, and one (8%) had quadruplets. These does were shot during a culling programme in March, so we don't know how many fawns would have survived to birth. Nevertheless, in early June 1963, quadruplets were born to a captive doe named "Suki" at Battersea Park Children's Zoo in London, all of which apparently did well and drew considerable interest from visitors. A separate analysis of 95 pregnancies by Chaplin revealed 17 singletons, 42 twins, 28 triplets, and only eight quadruplets. His unpublished research notes suggest that the age of the mother did not appear to influence litter size.

In China, Sheng and Lu found that among 17 pregnant does taken by hunters on the Zhoushan Islands -- six juveniles and 11 adults -- most (79%) carried between one and three embryos: nine (53%) had twins and five (29%) had triplets. Only one doe, an adult, had four embryos, and one other had five. A study at the Shengzhou Chinese Water Deer Breeding Centre in Zhejiang Province between May and July 2004 recorded twins and triplets as the most common litter sizes, together accounting for 76% of births, with quadruplets (14%) and single fawns (9%) less common. My own experience in Bedfordshire and Norfolk has been largely of twins, and in Buckinghamshire, Sharon Scott reports predominantly twins and triplets.

There seem to be two likely explanations for the variation seen across the literature. The first concerns habitat and population pressure. Populations tend to be more productive, in terms of litter size, when living in good habitat and subject to significant mortality pressure. During the nineteenth century, Chinese water deer faced heavy hunting, but much of their ancestral habitat remained intact. In more recent times, hunting has intensified and become more efficient, while large areas of suitable habitat have been lost or degraded. That stress on body condition directly affects litter size is supported by research at Branféré Zoological Park: Christiane and Robert Mauget, writing in Current Zoology in 2009, reported that average litter size represented around 12% of maternal body weight (range 5-19%) and was significantly correlated with the doe's body mass the previous autumn. Studies of free-ranging deer at both Branféré and Whipsnade similarly show that average litter size declines as population density increases – as resources become more thinly spread and individual condition falls.

The second explanation involves in utero mortality. The idea of significant pre- and perinatal loss (that is, losses before and around the time of birth) aligns with Chaplin's observation that conditions inside the uterus can become very crowded. He recorded one pregnancy containing a dead embryo alongside three healthy, larger ones, and another in which a restricted placental attachment had left one embryo roughly half the size of its siblings. It may also be relevant that, in a haematological study of 72 does from Huaxia Park in Shanghai -- published in Veterinaria México in 2023 -- Dayi Nie and colleagues found that pregnant animals had a significantly depressed white blood cell count compared with non-pregnant females, and showed no sign of haemodilution. In most pregnant mammals, white blood cell counts rise and haemodilution occurs, the latter thought to improve blood flow in the placental capillaries and so increase the delivery of nutrients and oxygen to the foetus. Given the litter sizes water deer are capable of producing, we might expect both responses to be present – their apparent absence is a curious finding that warrants further investigation.

Does have four inguinal mammae (teats or nipples), each protruding about 6 mm. This limits the number of fawns that can be raised simultaneously, and suggests that a litter of five or more would be very difficult to sustain. Nonetheless, Chaplin published a photograph of five fawns born to a doe at Whipsnade in his 1966 booklet Reproduction in British Deer, and Paul Childerley has reported litters of five from his estate in Bedfordshire. An even more striking data point comes from Jonny Stephenson who, in Field Food & Country's film Hunting Chinese Water Deer (January 2026), described gralloching a large, mature doe and finding seven foetuses. More remarkably, he recounted a conversation with a gamekeeper who claimed that a doe on his beat had successfully reared seven – extraordinary if true, given that does have only four teats. Whatever inflation may have crept into the earliest foetal counts, the evidence is sufficient to suggest that water deer have the highest reproductive potential of any cervid.

Gestation in water deer is around 175 days -- approximately 25 weeks -- though there is some variation. At Whipsnade, Stadler recorded a doe giving birth 167 days after an observed mating, while Endi Zhang estimated gestation at 165 days. At Branféré, Christiane and Robert Mauget, using faecal progesterone metabolite profiles, calculated a mean of 177 days, while Dubost and his team found a range of 161 to 176 days with a mean of 169 days. Cooke and Farrell, in Mammals of the British Isles (2008), give a broader range of 165 to 210 days with an average of 180, the upper end of which represents a notably long gestation for a deer of this size.