Water Deer Reproduction - Gestation & Litter Size


Understandably, most of what we know about water deer fecundity comes from the observation of captive individuals, the picture in the wild being less clear. At Branféré Zoological Park, Christiane and Robert Mauget reported that 70% of mature does gave birth. More specifically, at the same site, Gérard Dubost and his team observed that, overall, about 64% of females gave birth over the three years of their study, with older females more likely to have fawns than younger ones. The researchers found that 31% of juvenile does (i.e., those mated at six months old), 47% of yearlings (mated at 18 months) and 75% of adults gave birth. In 1937, Adrian Middleton reported higher pregnancy rates at Whipsnade in old (80%) versus young (67%) animals, while Stefan Stadler found that about half of yearling does at the zoo gave birth. In their 1983 British Deer Society booklet Chinese Water Deer, however, Arnold Cooke and Lynne Farrell note that fecundity of first year does appears similar to that of mature females, although pre- and perinatal losses are apparently high. In his 1977 book, Raymond Chaplin mentioned how there is no obvious decline in fecundity with age in water deer, although this was based on a sample of only 24 animals.

A heavily pregnant Chinese water deer doe. Multiple fawns are common in litters, with up to five live fawns on record, and in such a small cervid this means signs of pregnancy are difficult to hide. - Credit: Marc Baldwin

The only study I'm familiar with from the wild was conducted by Helin Sheng and Houji Lu on the Zhoushan Islands between December 1982 and April 1983, and was published in Acta Theriologica Sinica during August of the following year. Sheng and Lu found the fecundity in mature does to be slightly higher than that in one-year-olds, 2.73 vs. 2.17 fawns per pregnancy, respectively. They also observed pregnancy rates increasing from winter into spring. None of the 21 adult females shot during December and January were pregnant, while five (55%) of the nine shot in February were, as were all six of those taken in March. Interestingly, however, none of the eight one-year-old does collected in February showed signs of pregnancy, while half of yearlings taken in the first half of March and all yearlings taken from 16th March onwards were pregnant. This implies the same late, post-rut conceptions in younger females that we sometimes see in other deer species, such as fallow (Dama dama). Researchers at Branféré Zoological Park reported that older females gave birth earlier than younger (i.e., first year) does, probably reflecting later mating in the latter. In their 2008 paper to Mammalia, Dubost and his colleagues suggest coming into season first, being mated first and giving birth earlier may give fawns of mature does a head start over others in the population.

Part of the reason for the variation in fecundity between young (i.e., less than two years old) and mature females may lie in body condition. Several studies have demonstrated a link between body condition and fertility (e.g., Czech researchers working with domestic cattle). Data are lacking for water deer, but while studying protein digestibility in animals kept at Nanjing Hongshan Forest Zoo during 2013, Zhao Ling-Ling and colleagues found that increasing the protein and fat content of the food offered through inclusion of soybean also increased the fertility rate of does by almost three percent. This aligns with observations at Branféré by Dubost and his team, who reported that larger litters were more likely to be produced by older females (which are more likely to be in good condition) and litter size declined with increasing density. As the population increases, so the resources become spread more thinly and the condition of each animal tends to decline. The researchers also observed that as the number of adult females in the population during the rut increased, so the number of fawns produced per litter declined, perhaps because the bucks struggled to cover them all.

Litter size & gestation

A litter of triplets born to a water deer doe in the Home Counties. Twins and triplets are not uncommon in this species. - Credit: Sharon Scott

Hydropotes is what we call a polytocous species, from the Greek poly “many” and tokos “offspring”, meaning females routinely produce more than a single fawn in a litter. Indeed, much has been expounded about the fecundity of this species, with Duff Hart-Davis, in his 2002 Fauna Britannica, describing this species as “the most prolific of all deer”. Writing in his 1998 Deer of the World, Valerius Geist notes that multiple births indicate a long evolution in unstable, productive environments, such as riparian floodplains.

Robert Swinhoe made mention of their fecundity in his original description of the species, noting how 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, in which he wrote:

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 by a friend of his in winter each had seven embryos, while Hamilton recounted a report of a doe being shot in late February of the same year that was found to be pregnant with seven foetuses sufficiently advanced to distinguish their feet and eyes. Similarly, in Shooting in China, published in 1908, US consul general in Shanghai Thomas Jernigan described how a deer shot by William Cooper of the China Inland Mission near Shanghai, gralloched by Dr Henderson and Jernigan, was found to contain seven embryos.

Not everyone was initially convinced of the breeding potential of this species and, writing in the Journal of Anatomy and Physiology in 1878, Scottish zoologist James Cossar Ewart noted how initial reports of the very prolific nature of the water deer “were received with much doubt”. Having received 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”.

More recently, in his 1977 book Deer, Raymond Chaplin mentions three viable records of six healthy foetuses in Woburn animals, although no details were recorded, 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, note that up to seven foetuses have been found in a single female in China, but Cooke goes on to mention in Muntjac and Water Deer that while litters of six have been reported from China, they remain very rare. Helin Sheng and Houji Lu, in their 1984 paper to Acta Theriologica Sinica, reference some “elderly hunters” who told them that, when they were young, they had shot does with six and seven embryos. The largest number of foetuses I have come across in the literature was eight, given by Czech zoologist Luděk Dobroruka in a short note to the journal Mammalia in 1970, although he doesn't mention specifically where this report originated or how many foetuses made it to term.

The uterus of a Chinese water deer during the second trimester showing triplicate foetuses. In the literature there are reports of up to seven foetuses in a single uterus and Raymond Chaplin described conditions as "crowded". The scale bar is approx. 10mm. - Credit: Raymond Chaplin

It's interesting that the bulk of records of large litters seem to derive from late 19th century sources, while more recent accounts typically present much lower numbers. At Whipsnade, for example, Adrian Middleton found that of the 12 pregnant does he examined, three-quarters (nine) carried twins, while only two (17%) had triplets and one (8%) had quadruplets. These does were shot as part of a culling programme in March, so it remains unknown how many of the fawns would've made it to birth, although in early June 1963, quadruplets were born to captive doe “Suki” at the Battersea Park Children's Zoo in London, all of which apparently did well and were a draw for visitors. A similar analysis of 95 pregnancies by Raymond Chaplin revealed 17 to be singletons, 42 twins, 28 triplets and only eight quadruplets. Chaplin's unpublished research notes mention that age of the mother did not appear to influence the number of fawns in a litter.

In China, Sheng and Lu found that among the 17 pregnant does (six juveniles, 11 adults) taken by hunters on the Zhoushan Islands during their study, most (79%) had between one and three embryos; in fact, nine (53%) does carried twins and five (29%) had triplets. Only one (adult) doe had four embryos, and one (also adult) had five. A study at the Shengzhou Chinese Water Deer Breeding Center in China's Zhejiang Province between May and July 2004 recorded that twins and triplets were most common, together accounting for 76% of births, with quadruplets (14%) and single fawns (9%) less common. My experience in Bedfordshire and Norfolk has been largely of twins, and in the Home Counties Sharon Scott also sees predominantly twins and triplets.

To my mind, there seems two likely explanations for the variation seen in the literature. First, we know that populations tend to be more productive in terms of litter or clutch sizes when in a productive habitat and subject to substantial mortality. During the 1800s, the Chinese population was under considerable hunting pressure, but much of their ancestral habitat remained. In more recent times, not only has hunting pressure increased and become more efficient, much of the habitat has been lost or suffered significant degradation, which is likely to stress the population and reduce fecundity. Indeed, in their study at Branféré Zoological Park, published Current Zoology during 2009, Christiane and Robert Mauget reported that the average litter size represented about 12% (range of 5-19%) of maternal body weight and was significantly correlated with the doe's body mass during the previous autumn. Similarly, studies of free-range deer at Branféré Zoological Park and Whipsnade suggest the average litter size declines with increasing density, as is anticipated.

A water deer doe exhibiting a swollen udder, indicative of lactation, feeding on a grazing marsh in East Anglia. Habitat quality, particularly the quality and quantity of food, is one of several factors influencing litter size in this species. - Credit: Marc Baldwin

Alternatively, water deer may experience high in utero mortality and the idea of high pre- and peripartum (i.e., before, and just prior to birth) loss aligns with Chaplin's observations that conditions intra utero can become very crowded. He reports having found both a dead embryo along with three larger healthy ones, and one pregnancy in which a restricted placental attachment resulted in an embryo that was about half the size of the others. It is perhaps also interesting to note that, in their haematological study of 72 does from Huaxia Park of Shanghai, published in Veterinaria México during 2023, Dayi Nie and his colleagues reported pregnant animals with a significantly depressed white blood cell count versus non-pregnant females, and no sign of haemodilution. Pregnant mammals often show an elevated white blood cell count, while haemodilution during pregnancy is thought to substantially enhance blood flow in the capillaries of the placenta, especially in late-stage pregnancy, thereby increasing nutrients and oxygen to the foetus. One might expect, given the litter size of water deer, that both elevated white blood cell count and haemodilution might be observed.

Does have four inguinal mammae (teats), each protruding about 6 mm, and while this is twice as many as most other species, it suggests raising a litter of five or more fawns would be difficult at best. Nonetheless, in his 1966 booklet Reproduction in British Deer, Chaplin published a photo of five fawns born to a doe at Whipsnade and Paul Childerley has reported litters of five from his estate in Bedfordshire, suggesting that, while perhaps somewhat exaggerated by early foetal counts, water deer certainly seem to have the highest reproductive potential of any cervid.

Gestation in water deer appears to be around 175 days (i.e., about 26 weeks or six months), although there is variation. In the captive setting of Whipsnade, Stadler recorded a doe giving birth on 26th May 1988, 167 days after she was observed mating, while Endi Zhang estimated gestation to take 165 days. At Branféré Zoological Park, Christiane and Robert Mauget used faecal progesterone metabolite profiles to calculate a mean gestation length of 177 days (25 weeks), while Gérard Dubost and his team found the gestation to range from 161 to 176 days (23-25 weeks), the mean being 169 days. In their chapter in Mammals of the British Isles, published in 2008, Arnold Cooke and Lynne Farrell give the range between 165 and 210 days, with an average of 180.