Water Deer Mortality - Non-predatory

During their study of free-ranging water deer at France's Branféré Zoological Park, Gérard Dubost and colleagues noted that about 20% of mature individuals died, on average, each year, despite there being no recorded predation. This figure is roughly half the estimated mortality for wild populations, suggesting non-predatory losses can be significant. Indeed, in 1997, Richard Champion summarised the mortality reports for 134 water deer (56 bucks and 78 does) that died at Whipsnade between 1991 and 1994: 33 were killed by predators, while the remaining 75% met their end through a variety of other causes, including falling foul of parasites, cars and other residents of the park, including other water deer.

We've touched on the observation that in most populations (see: Longevity) mortality is highest among the youngest, and the chances of survival are significantly improved upon reaching maturity. Dubost and his team, for example, observed how, across three years in the Branféré population, 48% of fawns died during their first month and most of those in the first week. After fawns reached a month old, mortality generally occurred at between two and four months old. The researchers calculated that 50-70% of fawns died before reaching six months old, but if they made it through that then the main periods of natural mortality were at between two and three, and again at four to six years old. At Whipsnade in the early 1990s, where predation was a significant source of mortality for fawns, Endi Zhang found that about 86% died before they were two months old, while Arnold Cooke and Lynne Farrell reported a mortality rate of 75% at 6-9 months in the wild in Britain.

Some adults appear to fall victim to stress, sheer exhaustion and fawning. Several researchers working at Whipsnade have noticed how water deer, much like roe deer, appear particularly susceptible to cardiomyopathy and this is especially so while being handled (so-called “capture myopathy”). According to Kenneth Whitehead, capture myopathy seems to be caused by “systematic acidosis” as lactic acid builds up in the muscle. In his 1993 Encyclopaedia of Deer, Whitehead noted that this frequently occurs between one and two weeks after release, but can take a month to manifest, presumably depending on the level of trauma. More recent research on the condition in roe deer by Jordi Montané and colleagues in Spain indicates that the acidosis leads to the breakdown of muscle tissue and myoglobinaemia, which subsequently damages the kidneys. Arnold Cooke has found otherwise healthy deer dead at Woodwalton Fen and, in Muntjac and Water Deer, implies myopathy may have been the cause in some cases, given that it “can be caused by factors apart from capture”.

Writing in 1950, Whitehead commented how:

“Even during a mild winter, for some unaccountable reason a few bucks seem to die as soon as the rut is over, and it is not for want of food.”

Such sudden deaths may be a result of the rigours of the rut and most (83%) of the deer Cooke and Farrell found dead at Woodwalton Fen over a six-year period died between October and March because of inclement weather and/or the rut. Equally, Dubost and his team observed that reproductive females at Branféré suffered significant mortality during September “probably from exhaustion due to suckling”. The researchers also recorded six females (4%) that died from complications during, or very shortly after, fawning. At West Midlands Safari Park, Robert Lawrence found that 40% of fawns died in their first month, mainly from various complications during parturition (particularly malpresentation or death of the mother) and from exposure, while birthing complications were largely responsible for the death of 25% of fawns at Raymond Chaplin's Woburn study site, although hyperthermia was also a significant problem (see below).

Water deer appear susceptible to the elements, particularly very hot and dry weather as fawns and inclement winters as adults, the latter in spite of their dense fur. Indeed, Cooke and Farrell found that mortality at Woodwalton Fen in Cambridgeshire was highest during cold and wet winters, with bucks particularly susceptible, presumably because post-rut energy reserves are low.

Sharon Scott told me that, in her experience, fawns tend not to do well during cold and wet summers, and the science we have suggests temperature is a more significant concern than rainfall. At Branféré, Gérard Dubost his co-workers failed to find any apparent influence of rain on the percentage of young dying each month, but fawn mortality was weakly correlated with the mean air temperature, most occurring during August, the hottest month of the year.

At Woodwalton Fen, Arnold Cooke and Lynne Farrell found that cold summers generally meant few young deer the following winter, but that summer rainfall didn't impact that observation - they also point to summers in China being hotter and wetter than ours. In August 2018, Arnold told me:

“At Woodwalton, I knew that amount of summer rain had no overall effect, but I checked results that were readily to hand for 1976-2008. Five summers stood out as being drier than the rest, including 1976, which had the highest level of recruitment for the whole of that period (36%). On the other hand, recruitment was zero following the dry summer of 2001. The average for the 5 years was 20% which was also roughly the average for the whole period. So I've no clear evidence for dry summers leading to breeding failure at Woodwalton Fen.”

From the Woodwalton Fen dataset, it appears that there is a significant correlation between the average temperature between May and August and the proportion of young deer seen during the following winter; the warmer the better. There does, however, appear to be a caveat to this: the presence of shelter.

Alongside the observations of Dubost and his team at Branféré, in his 1977 book Deer, Raymond Chaplin recounted how fawns born on open ground with little or no cover at Woburn were found panting during very hot weather. The small size, relatively dark coat and infrequent suckling by the mother could, he suggests, compound overheating and dehydration. Consequently, in more open habitats, fawns can be very vulnerable to overheating and dehydration, while hot weather appears to favour fawn survival in reedbed habitats.

During periods of cold and wet or snowy weather, water deer are susceptible to both hypothermia and starvation. Indeed, Cooke and Farrell, in their 1983 booklet for the British Deer Society, describe prolonged hard weather (i.e. ice and snow) as being potentially devastating. Similarly, Kenneth Whitehead noted how, during the bitter winter of 1946/47, it was feared that the whole stock at Woburn would succumb. A significant factor in this was that the deer refused to eat hay, even when they were starving, and they generally failed to recover even when they were caught and moved to a paddock with plenty of food (chopped roots) and shelter. In Muntjac and Water Deer, Cooke recounts a report from Norfolk by Ted Ellis, the late Broadland Haven naturalist, of water deer coming out of cover during hard weather in starving condition. Despite a thick winter coat, their small size means a larger surface area to volume ratio and faster metabolism, which in turn means they are very vulnerable to food shortages, particularly during cold weather.

In some circumstances individuals, and bucks more than does, may succumb to a combination of exhaustion and hypothermia during the winter, and many of the carcasses examined from Woodwalton Fen have been in reasonably good condition and with digestible material in the stomach. There is also at least one report of a water deer that died at Whipsnade as a result of over-eating grain, and two more that succumbed to suspected enterotoxaemia, thought to have arisen after the deer over-fed on concentrates.

Interestingly, the data from the fens suggest that young animals are no more susceptible to bad weather than adults. Early in Cooke and Farrell's study, the main periods of mortality were when bad weather and flooding forced the deer out of the reserve and into peripheral farmland. In the case of 16 found dead during the severe winter of 1978/79, which saw 60 days of air frost and protracted snow cover, 11 were adults, and there were hardly any first-year deer in the population the following winter, implying there had probably been an appreciable mortality of adults. Adult susceptibility to hard weather may be one explanation for the slow spread of water deer from their release sites. One of six deer sent from Farleigh House near Basingstoke by the Earl of Portsmouth to the Cwmllecoediog estate in Montgomeryshire during December 1953, for example, escaped and lived feral in the surrounding woodland until succumbing to a hard winter with heavy snowfall the following February.

An additional consequence of cold weather is that it may drive deer to roads where snow cover may be reduced and a ready supply of salt may be accessed. Indeed, Bernard Nau, in his 1992 report for the Bedfordshire Natural History Society, included a report of one animal recorded on the Woburn-Steppingley road licking salt during January 1988. He also noted that animals were frequently seen licking salt from roads in winter 1995, with records from Dunstable, Toddington and Totternhoe.

Personally, I've seen very few water deer carcasses on the side of the road, certainly compared with other species, but equally water deer are present at low density and carcasses may be removed in the boots of cars. I have observed them grazing by the roadside in Woburn and the annual reports of the Bedfordshire Natural History Society suggest they get run over reasonably frequently. Similarly, Cooke and Farrell note that road deaths have provided them with many records, sometimes in places where the species had not previously been recorded.

Formal data on water deer road casualties is scant. The Deer on Roads survey published in 2011 suggests that a couple of hundred water deer are killed in traffic each year, and they thus represent fewer than 1% of collisions involving deer in Britain annually. More recently, Jochen Langbein, a West Country-based deer biologist who holds the deer-vehicle collision (DVC) data for England since 2000, told me that while the dataset extends to some 95,000 mapped records, only just over 29,400 (31%) have a reliable species allocation. Of these reports, however, 168 were of Chinese water deer up to the end of 2019, suggesting water deer account for about 0.5% of the DVCs in England for which a species is known. Overall, Deer Aware estimate there are around 74,000 deer-related traffic accidents in the UK each year, resulting in several hundred human injuries and several fatalities.

Interestingly, the work of Dubost and his team suggested that the mortality rate at Branféré Zoological Park wasn't correlated with the population density, as one might anticipate, particularly given the population is almost certainly descended from only a couple of introductions and probably has low genetic variability. At Whipsnade, a significant mortality event was observed during the winter of 1933/34 when the population was high. Reports from keepers described animals moving erratically and with seemingly paralysed hind quarters. In his 1938 paper to the Proceedings of the Zoological Society of London, Adrian Middleton wrote of how an estimated 140 water deer died between December and January and the epidemic necessitated sending a lorry around the fields to collect the carcasses lying about. Two animals were autopsied at London Zoo and found to have died from enteritis, while inspection by the superintendent at Whipsnade found large numbers of nematode worms. Similarly, in his review of the cause of death for 134 deer at Whipsnade between 1991 and 1994, Richard Champion lists 42 as having heavy parasite burdens. Significant mortality events appear to have followed booming populations in the late 1990s, the mid-2000s and again around 2010.

Other sources of misfortune that have reportedly resulted in the death of water deer include two in Champion's group that died following interaction with other species at Whipsnade, both kicked by an onager (Equus hemionus), and an account from Michael Clark in his 1981 Mammal Watching of one of his tame muntjac (Muntiacus reevesi) bucks killing a water deer it shared an enclosure with while excited about a doe in oestrous—the water deer apparently didn't fight back. In 1994, Endi Zhang reported that more than one hundred deer drowned when a typhoon struck the coast of the Yancheng Reserve in September 1990, and an assessment by Hongfa Xu and Houji Lu at Shanghai's East China Normal University, published in 1996, suggested the population would become extinct within a couple of decades at the current rate of poaching and tidal inundation. They may be somewhat accident prone, too, and in March 2022 Nature Ecology 365 on YouTube posted a video of a buck that almost drowned after falling through ice while licking water from the surface of a frozen lake. Marine litter may also be an issue and in a 2013 paper to the Marine Pollution Bulletin, Sunwook Hong and colleagues described:

“A water deer H. inermis was a victim of a derelict gill net in the Han River estuary in the Gyeonggi province. Its legs were entangled in the net. This led to its death because it drowned during the flood tide.”

Being a “hider species” that leave their young lying up in tall vegetation, I suspect that harvesting of crops may result in the death of many fawns by mowers in agricultural settings, as they do for roe deer, although I know of no data quantifying this. Home Counties farmer Sharon Scott walks her fields ahead of the tractor and tells me she has saved many fawns that would otherwise have died. The loss of cover associated with grass harvesting may also increase fawn susceptibility to predators, and for this reason Sharon leaves a small section of the field uncut for a couple of weeks fawns are more mobile. Finally, during the rut, bucks will fight among themselves for access to females and their use of long, sharp canine teeth can inflict significant injury, even death (see: Fight for the one you love).