As the late Archie McDiarmid pointed out in his review on the subject of deer mortality for Mammal Review in 1974, all deer must die sooner or later and death is an important facet of their ecology. Overall, it is estimated that about 3% of the adult Red deer population currently die each year from ‘natural causes’, although this is dependent upon habitat and age. Indeed, Red deer exhibit two ‘critical periods’ for mortality – early and late in life. Studies on the Red deer of Rum by the Red Deer Research Group (RDRG – a collaboration between Cambridge and Edinburgh Universities) have provided a comprehensive picture of survival rates for this wild population.
In a paper to the Journal of Animal Ecology published during 1969, Victor Lowe constructed life tables (showing the mortality rates of each age group of a population at a given time) of the deer on Rum in a bid to get a general picture of the population structure. Lowe found a gentle reduction in survivorship up to around eight years old (around 1% per year for 2 – 7 year olds), after which there were two years of very heavy mortality (i.e. most of the adult deer dying in the population were in their ninth or tenth year) and then a return back to lower, but increasing, level. From his reconstruction of the 1957 population, Lowe estimated that only 8.4% of the population were older than eight years.
The perils of being young
In a subsequent paper to the Journal of Animal Ecology (during 1978), Fiona Guinness, Tim Clutton-Brock and Steve Albon presented data on calf survival between 1971 and 1976 on the North Block of Rum. The biologists found that, on average, 18% of calves died before they reached five months old (i.e. by September of their first year), most (78%) within their first week, and a further 11% failed to survive the following winter, most dying during March. Overall, the researchers found that anywhere between 19% and 35% (the average being 28%) of calves died before their first birthday, which compares favourably to Lowe’s data, which record 37% of calves dead before they reached a year old. It should be mentioned, incidentally, that mortality can be much higher, and more recent data suggest that as many as 65% of calves may fail to survive their first winter if conditions are particularly bad.
The level of mortality seen in any given year is the product of a complicated interrelationship between, amongst other influences, weather, population density, birth weight and time of birth. Guinness and her team found that light-born calves were less likely to survive their first winter than heavier ones, although light-born hinds were more likely to die than light born stags, while heavy-born stags were less likely to survive than heavy-born hinds. Additionally, calves born late in the year were more likely to die than those born earlier in the year or during the main birthing period because they have less time to increase their body weight in time for winter.
The 1971-76 dataset also revealed that calves born to young or old hinds were more likely to die during their first six months than those born to hinds between seven and 10 years old – it appears that old and young hinds produced smaller than average calves. Indeed, the age and thus experience level of the mother can be a crucial factor in calf survival. Dr Guinness and her colleagues found that 10 (32%) of the 31 calves, for which cause of death could be firmly established, died because they were deserted or killed by their mothers.
The influence of weather
Stags born on Rum during cold springs are less likely to survive their first few years of life than those born under mild conditions – it seems that cold temperatures lead to reduced birth weights, which in turn lead to lower survival rates. In general, the data from Rum show that the faster growth and development of stags (see Breeding Biology) makes them more sensitive to environmental conditions and thus more likely to die than hinds; there is some indication that this may be equally true of stag foetuses.
Work by the RDRG (the 1971-76 data set and subsequent studies) has also shown that deer survival is closely related to both population density and weather conditions. On parts of the island where population densities were high during the 1979 study, so too was calf mortality; increases in winter (although not summer) mortality were associated with increases in population density. This is to be expected because as a population grows, so too does competition for food. Indeed, a study published in the journal Ecology during 1997 presented population data for Red deer on Rum between 1984 and 1993 and concluded that local population density was more important in determining calf survival than total population density. The authors, led by Zoological Society of London biologist Tim Coulson, wrote:
“We propose that high local density of deer occurs on herb-rich Agrostis-Festuca grassland. Calves born here are more likely to die due to high levels of competition for food than in other areas of poorer grazing and low local density.”
In other words, the deer flock to the areas where bentgrass and fescue grasses grow because they’re a good quality food. Consequently, each blade of this grass has more hungry mouths going for it here than in areas of lower quality grazing, which means that the youngsters have more competition for food. According to the study, the main factor influencing whether or not an individual survived the winter was its condition at the end of summer – the calves must gain weight and grow rapidly during the summer months if they’re to survive winter. The authors go on to mention that males are more likely to die than females, because they grow faster and therefore require more food, making them potentially more susceptible to competition. Indeed, another of the RDRG’s findings has been that stags seem more sensitive to environmental conditions than hinds.
Deformities and still birth
In addition to low birth weights and inclement weather, calves can suffer accidents and are sometimes born with congenital defects that can be fatal, if they are born alive at all. In their 1979 paper the RDRG biologists note that five (16%) of the 31 calves for which a cause of death could be reliably established were stillborn. The authors also provide examples of calves born with various congenital disorders including shortened hoof tendons (which cause the feet to curl up and prevent the animal from walking), shortened lower jaw, perforated skull and muscle wastage – all conditions were invariably fatal (typically from malnutrition), although some calves with skeletal defects lived for several months. The study records other calves that died from various causes, including failing to suckle properly, their mother’s milk drying up, knocking themselves unconscious and drowning while crossing a burn or succumbing to heavy parasite infection (see below).
In his 1977 book Deer in the New Forest, John Jackson notes that:
“various congenital abnormalities have been recorded in New Forest deer, the most bizarre of which was a red deer born without eyes or eyesockets.”
Jackson doesn’t mention whether the calf survived, but the prognosis doesn’t seem good. Despite the foregoing, accidental deaths and predation (see below) account for relatively few calf deaths and Lowe found that malnutrition killed the majority (almost 90%) of the unfortunate yearlings.
Mortality in later life
Much work has gone into assessing the survival rate of calves, but it is also important to look at adult death rates. As we have seen, if a calf makes it to its first birthday its continued survival is considerably more likely and there is a ‘levelling out’ of mortality (i.e. mortality of two-year-olds, three-year-olds, etc. are roughly the same and ‘absolute levels’ of mortality are fairly low) until the animal reaches old age. Generally speaking, however, stags are less likely to reach old age than hinds; there are several reasons for this, but perhaps the most apparent is that they undergo the strenuous physiological changes and increased activity associated with rutting – moreover, they are often involved in fights that may prove fatal. Indeed, Tim Clutton-Brock and colleagues, during their studies of the Red deer population on Rum, estimated that 23% of red deer stags over the age of five years were injured during the rut each year; 6% were permanently injured.
For hinds, survival can be closely linked with the time they first produce a calf. We have already seen, in the ageing section, how recent work by the RDRG has established that “the rate of senescence in maternal performance increases with early-life fecundity” (the title of a 2006 paper to Ecology Letters). In other words, the sooner (i.e. younger) a hind produces her first calf, the more rapidly she shows signs of senescence (ageing) after the age of about nine years.
In their various papers on the subject, the RDRG biologists don’t speculate about whether this is likely to translate to an earlier death, but it seems likely. Data on other animals (including a study, published in the journal Nature during 1998, that looked at a large collection of human genealogical records) has linked, albeit rather tenuously in some cases, low fertility with increased longevity. The idea is referred to as the ‘disposable soma hypothesis’—sometimes also referenced under the ‘umbrella theory’ of antagonistic pleiotrophy—and while the details don’t concern us here, the basic premise is that reproduction is an energetically costly undertaking and producing a baby diverts resources away from the cellular ‘construction crews’ that maintain and repair the body. Ergo, the sooner you start reproducing, the sooner this energy budget is exhausted.
Energetic expenses aside, the act of calving can pose a threat to a hind and the RDRG have found that each year a few hinds will die giving birth. Death during parturition (i.e. giving birth; from the Latin parturire, meaning ‘to be in labour’) is generally uncommon and usually requires abnormal presentation of the foetus, but if the mother is in poor condition and the weather is inclement as many as 44% can die during childbirth.
Accidents and traffic
Fighting, childbirth, population levels and just being very young or old aren’t the only mortality sources that affect Red deer. Weather is highly significant and the RDRG have found that wet springs and autumns lead to an overall reduction in both adult and calf survival (presumably because high rainfall has a detrimental impact on plant growth and a lot of rain in the autumn thus reduces the growth of grass, and hence food, for the winter). Road traffic may also represent an important threat and although the first report of the National Deer-Vehicle Collisions Project found that Red deer are involved in fewer collisions than other species (presumably because of their limited distribution), it is estimated that around 1% of the total UK population are killed on roads each year and locally it can be a serious problem. The subject of deer deaths on roads is covered in greater detail in the main deer article.
Deer can also die as a result of various accidents, including falls and drowning and are susceptible to items such as discarded netting/twine, poorly maintained fences, snares, barbed wire and, according to Norma Chapman in her book Deer, one (she doesn’t mention the species) was found with a discarded lens hood caught around its leg. Discarded fishing nets can also be problematic for deer in coastal regions and, in his A Life for Deer, John Fletcher recounts several harrowing stories of deer that had become entangled in fishing wire and drowned or been latched to rocks and starved to death. Chapman also points out that some deer are killed by trains while crossing the tracks.
In his review of deer mortality, McDiarmid wrote:
“Deer on the whole, are extremely healthy animals and nowadays we do not have the dramatic ‘die-offs’ which occurred in and before the nineteenth century. It is worth remembering that serious diseases such as rinderpest [a lymphatic virus primarily affecting cattle], contagious bovine pleuropneumonia [a bacterial infection causing inflammation of the lungs] and foot and mouth disease used to be rife. … Nowadays our main problems are associated with parasitism…”
Indeed, although free-ranging deer are largely disease-free (most of the data we have come from captive populations), they are susceptible to various parasites and infections. It is important to recognise that, although the majority of parasitic infections aren’t fatal in themselves (after all, it’s seldom in the parasite’s best interest to kill its host), during the winter and early spring, when a deer may be malnourished and battered by the elements, parasites can be an additional drain on the animal’s resources – this can be a ‘tipping point’, overwhelming the animal and leading to its death. Owing to the potential zoonosis (i.e. diseases that can be transferred to humans) of certain deer parasites and diseases, the subject is discussed further in the Interactions with Humans section.
The impact of predators
Finally, and to a lesser extent in the UK, predators can influence deer survival. To the exclusion of humans, there is only one predator in the UK that is probably capable of bringing down an adult Red deer: the golden eagle (Aquila chrysaetos). Golden eagles will take calves and just such an attempt was photographed by amateur wildlife photographer Dawid Banasiak while on a nature tour on the Isle of Mull during October 2017. One might think an adult Red deer would be too large a target for an eagle, but I have seen footage of an eagle attempting to drive a herd of adult Red deer over a cliff in the Scottish Highlands. Similarly, the BBC filmed an eagle hunting reindeer (Rangifer tarandus) in northern Finland as part of their Life documentary series, although the footage was unfortunately left out of the final cut owing to the distance the camera crew were from the action.
There are few data available on how significant eagles are as predators of deer, but in his 1969 paper Lowe estimated that, based on bruises in the meat of carcasses, just over 13% of calf deaths were caused by eagles. In a 1969 paper to the Journal of Zoology, A.B. Cooper described an attack on a “healthy and vigorous” calf, weighing 20.5kg (45 lbs), by an immature eagle on Plat Reidth in Sutherland during July 1968, after it became separated from the main herd:
“The calf tried to dodge but collapsed when the eagle seized it by the neck. It gave a long and very loud howl, and tried to rise several times, but the eagle had a firm hold and retained its balance by beating its wings. The eagle was not seen to lower its head or use its bill. The bird now sat on the calf and waited, and after three minutes the calf threshed its legs briefly, its last sign of life. Then the eagle twice tried to lift or drag the calf, still holding it by the neck, but it only succeeded in raising the head off the ground.”
In Europe, grey wolves (Canis lupus), lynx (Lynx lynx) and brown bears (Ursus arctos) may take deer as, occasionally, do wolverines (Gulo gulo). A study on the diet of lynx in Poland’s Bialowieza Primeval Forest between 1985 and 1996 revealed that these cats killed between 42 and 70 deer per hundred sq-km, which represented between 6% and 13% of the total spring population. The study, published in the journal Acta Theriologica during 1997, also found that although wolves and hunters took more deer each year, lynx most often targeted calves and similar studies elsewhere have shown that Red deer calves can be an important component of a lynx’s diet.
Calves are vulnerable to domestic dogs and potentially other smaller carnivores – red deer meat has been identified in the diet of buzzards, foxes, badgers and pine martens, although it is unclear how much represents direct predation and how much is scavenged. In his fascinating 1999 book Kia: A study of Red deer, Ian Alcock, talks of the ‘intrinsic aggression’ that Red deer hinds show towards foxes and writes:
“Foxes may kill a few Red deer calves during their first day or two, but probably after that the calves are too big for them to tackle.”
I have come across a couple of casual references to wild boar (Sus scrofa) being predators of calves, although I have been unable to track down a source for them. There are several photos online showing wild boar either feeding on a deer carcass or carrying a deer calf/fawn, but it is unclear whether the boar killed the deer or found it already dead. Nonetheless, wild boar are certainly capable of killing a deer calf and have been known to take smaller mammals such rabbits if the opportunity presents itself. However, wild boar biologist Martin Goulding recently told me:
“I am not aware of a reputable, or even anecdotal, reference reporting that wild boar will predate deer calves. There is plenty of evidence that wild boar will eat road kill deer, and wild boar are reported in eastern Europe to have driven lynx away from their deer kills.”
Calves may also be killed by their mother and at least one death on the North Block of Rum during the mid-1970s was attributed to attack by feral ponies. Outside of the UK and excluding man, wolves are probably the most significant predator of deer and there has been some work recently looking at the physical condition of the deer taken by wolves.
Selective predation by wolves
Between 1984 and 1988, Henryk Okarma of Jagiellonian University in Poland studied the carcasses of 90 deer killed by wolves during winter in the Carpathian Mountains of southeastern Poland. Okarma found that wolves predominantly killed calves and hinds, representing 44% and 40% of the total, respectively – stags were only represented in 16% of carcasses. The majority of calves were taken by wolves in the late part of the winter (i.e. February and March), with calves consisting 32% of kills in early winter and 51% in late winter – adult hinds were killed in roughly proportionate numbers during both periods. The data also showed that wolves killed ‘prime’ adults—the average ages of hinds and stags were 7.2 years and 5.3 years, respectively—with old animals (those of 10 years or above) accounting to only 13% of kills.
From the carcasses, Okarma was able to assess marrow fat content of the animals, and found that adults had highest marrow fat content during the early winter (76% in early winter, 52% by late winter) – calves showed a similar utilisation of fat during the winter months. Overall, Okarma concluded that wolves affected the young classes of deer more significantly, with calves being particularly vulnerable in late winter, and that hinds were more vulnerable to wolf predation than stags. Calves are probably more susceptible during late winter because they’ve used much of their fat reserves and begin to lose condition.
Given the pronounced sexual segregation in Red deer outside of the breeding season, it may also be that wolves actively seek female groups with calves – the author suggests that this might explain why hinds seem more susceptible to wolf predation than stags.
More recently, a team of Italian biologists studied the physical condition of Red deer killed by wolves in an area of the western Alps between November 2003 and April 2004. The data, published in the journal Folia Zoologica during 2007, showed that most of the 14 Red deer killed by wolves were in poor physiological condition, with low levels of fat in the femur (leg) marrow; almost 40% had less than 25% fat content. The biologists tentatively suggested that, unlike for the sympatric Roe deer which were the primary focus of the study, body condition may have played a role in the wolves’ choice of Red deer prey.
Thus, taking these and other data together, it seems likely that in areas where predation is a significant source of mortality, body condition (and specifically the conditions in which the deer spend the summer and autumn) may play an important role in influencing the likelihood of being killed.
Overall, in Britain, deer that die of natural causes and aren’t killed by another deer, predator or in an accident, die of starvation, exposure, disease/infection or physiological failure (i.e. ‘old age’) – the majority of deaths occur during March and April. Starvation is often associated with the wearing/loss of teeth in old animals. In addition, it is worth mentioning that we should be careful not to lump cars and predators together as a source of mortality – cars are entirely unselective in their actions. Deer populations are also heavily managed by humans and where shooting is employed as a method of control, allowing sickly animals to be killed and thereby reducing competition, natural mortality can be significantly reduced. The management of deer populations by man is discussed in greater detail in the Interaction with Humans section.
Parasites & Diseases
Lice and ticks are probably the most common ectoparasites (ecto- from the Greek ektos, meaning ‘outside’ or ‘external’) and the deer tick Ixodes ricinus, which can carry the Lyme Disease bacterium Borrelia, is probably the most common of these. Lyme Disease is transferrable to humans, although it seems that the ticks are generally picked up by walkers from vegetation rather than via any direct contact with the deer themselves. Tiny crab-like insects called deer keds (Lipotena cervi) feed on skin and blood, while sucking lice (Solenopotes burmeisteri) and various biting lice species are also fairly common. These parasites are rarely more than an irritant to the deer. There are, however, some that can potentially do more damage.
A significant parasite of Red deer is the warble fly (Hypoderma diana), which is most problematic during the spring when a single deer may be infested with more than a hundred larvae. The adult fly lands on the deer and pierces its hide to lay its eggs under the skin. The larvae of the fly don’t burrow into the deer’s flesh, but remain under the skin – as the larvae develop they form lumps (called ‘warbles’) under the skin which remain through the winter until the maggots drop from the deer’s back in early spring the following year. The condition isn’t generally fatal as the lumps will heal once the larvae have hatched, although the open sores can sometimes become infected.
Another significant parasite of Red deer is the nasal-bot fly (Cephenemyia species), which lays eggs in the nose of the deer; the eggs develop into maggots roughly three centimetres (one inch) long with well-developed bristles that allow them to cling to the base of the nostril. The maggots can then migrate up the nostril to over-winter, feeding on blood and mucus, before crawling back down and dropping out in the spring – the maggot typically cause the deer irritation (causing considerable coughing and sneezing), but in sufficient numbers they can block the nasal passages and suffocate the deer.
Deer can also suffer from a range of endoparasites (endo- from the Greek endon meaning ‘within’ or ‘internal’) including the tissue worm Elaphostrongylus cervi, lungworms of the genus Dictylocaulus (these are of considerable concern to the deer farming industry and can heavily infest malnourished wild calves causing appreciable mortality), liver flukes (Fasciola hepatica) and various tapeworms. As with the ectoparasites, these endoparasites generally don’t lead to the death of their host, but under conditions of hardship when the deer is undernourished, they may represent an additional drain that can prove fatal.
There are several viruses and bacterial infections that have been documented in deer—including rhinotracheitis, bovine herposvirus, Mycobacterium (including M. bovis and M. avium) and Salmonella—but they are rare and cases of clinical infection are exceptional. The Mycobacterium bacteria have been in the headlines for some time now over their ability to cause tuberculosis; most of the attention has focused on M. bovis, which causes bovine tuberculosis (bTB) - see Badgers & Bovine TB.
The nervous system disorder Chronic Wasting Disease (CWD) is well known from deer and further details can be found in the associated QA. Tumours occur very rarely, as do cases of infection with the protozoan parasites Babesia and Toxoplasma, some species of which can be passed to humans. There are no records of bovine spongiform encephalopathy (better known as BSE) in wild deer, although laboratory studies have shown that they are susceptible to it – thus far, symptoms (which include anorexia, blindness, ‘panic attacks’ and failure to moult) have only been induced by direct injection of infected material into the deer’s brain.
Farmed deer have died as a result of vitamin deficiency. During the late autumn of 1999 three adult hinds in a Norwegian deer park died shortly after presenting as ‘generally thin and unthrifty animals’ with very dull light-coloured hair and diarrhoea. A necropsy was carried out on the animals and it revealed that all three were suffering from copper and selenium deficiencies. The biologists who conducted the post mortems tried an experiment, the results of which were published in the journal Acta Veterinaria Scandinavia during 2008. The scientists found that just giving the deer a copper-enriched salt lick in their enclosure wasn’t sufficient to maintain their copper requirement and they had to give the animals copper oxide capsules every couple of months to maintain their condition. When copper levels returned to normal, the deer’s coat condition dramatically improved, as did their overall body condition; the biologists also observed increased fertility and reduced parasite load.
Much of our understanding of deer diseases, indeed of deer biology in general, has come from studies conducted on deer in parks, farms and in nature reserves and there are some diseases that have caused appreciable mortality to captive herds. Malignant catarrhal fever is one such condition. A fatal viral infection caused by the ovine herpesvirus-2, it presents with symptoms of blood-shot eyes and blood and foaming around the mouth following an incubation period that can last from six weeks to five months, and has led to high mortality of Reds on New Zealand deer farms.