The New Naturalist Library: Hedgehog

LIKE A LOT OF OTHER species, the hedgehog has survived for millions of years in the face of many natural hazards. The problem now is that it is being confronted with serious threats that are the result of comparatively recent human activity, to which it is not adapted and which require rapid responses if the species is to thrive. However, although these may be the most pressing issues of today, we should not overlook the natural processes that also impact on the hedgehog population. Foremost among these is the challenge of hibernation, a major physiological readjustment that is needed every year to cope with food shortages in winter, but it also renders the animal helpless in the face of flooding or an inquisitive dog. Whilst relatively few hedgehogs probably die during hibernation itself, there is a critical time when they arouse in the spring. Food may be short and cold nights often resume, restricting the availability of many of the hedgehog’s natural prey. Those that do successfully survive the winter will have depleted fat reserves and little to help tide them over until better weather resumes. They will be very vulnerable when metabolising what fat remains and any harmful chemicals that may have accumulated within it. Another period of extreme vulnerability occurs when young hedgehogs depart from their maternal nest. They leave behind the protection of the nest itself and the warmth of their mother and siblings. Poorly insulated and with little or no fat, survival can be very precarious, especially if the weather turns cold or dry. Parasites represent another natural challenge, with some of them posing a real risk to survival (see Chapter 11). However, these are all normal challenges and the hedgehog has had plenty of time for natural selection to adjust its population dynamics to cope with the inevitable mortality. It is the newly arrived dangers that threaten its long-term survival.

Hedgehogs do seem rather prone to falling into things, and they may die simply because of their inability to climb out. However, many of those hazards are created by humans, such as trenches on building sites, and I once found the remains of several hedgehogs that had fallen into the underground explosives store of a wartime blockhouse. Cattle grids certainly claim numerous hedgehog victims, with as many as 52 being reported when one was cleaned out. I have several times seen hedgehogs drowned in cattle grids that had filled with water and a campaign to get escape ramps fitted was begun by Willie Euman after he had found ten of them in the pit below a cattle grid (Euman, 1979) (Fig. 183). This idea was later carried forward to great effect by Adrian Coles (see Chapter 13), who succeeded in getting the British Standards Institute to include an escape ramp in cattle grids built in conformity with a new official British Standard 4008. Despite this, a countywide survey in 1990 by the Lancashire Wildlife Trust found that 80 per cent of installed grids lacked an escape ramp. I have noticed a similar lack of escape routes, even among some of the cattle grids on National Trust properties. The trouble is that new cattle grids are built infrequently and many were installed long ago without any escape route for animals that fall in. A retro-fitted ramp is not expensive and a small pile of bricks will do instead, but it needs an army of volunteers and a campaign among landowners to make all existing cattle grids safe, not just for hedgehogs but also for toads, shrews, newts and many other creatures that must be trapped in them, year in, year out.

FIG 183. There are thousands of cattle grids installed in the British countryside, permanently open pitfall traps for a wide range of animals that fall in and drown or starve if they cannot get out. Hedgehogs appear to be a frequent victim.

Garden ponds (and swimming pools) are another danger. One correspondent told me that six were drowned in a neighbour’s pond within a short time and four others rescued from it alive. Hedgehogs can swim, but they cannot get out if the pond has a smooth fibreglass or plastic lining unless there is a shallow escape route or some other structure that allows them to climb free. There are many other hazards around the garden too. Strawberries and peas are often protected by covering the plants with netting, cricket and tennis nets are frequently set out for a game in the summer, but not put away afterwards. Baggy piles or lines of netting are a threat because hedgehogs will push underneath, perhaps to hide there during the day, and their legs and spines become entangled (Fig. 184). There is no escape and they will die a slow death. These dangers can be easily mitigated (see Chapter 13).

FIG 184. Hedgehogs easily become entangled in garden netting and will die if not soon discovered and released.

Hedgehogs also seem to be rather prone to accidents. A few years ago, it was reported that some of them had been feeding on the dregs in plastic coffee cups discarded by staff outside the Rutherford and Appleton Laboratories. Backing out, the animals jammed their spines into the soft plastic and ended up unable to feed or move about owing to the large container covering their head. A similar problem affected the polystyrene packaging used for ‘McFlurry’, and the BHPS launched a successful campaign to have McDonald’s modify them or take them off the market.

There is a problem with mowing machines too. Unlike other animals the hedgehog’s natural response to the approaching noise and disturbance is to roll up and sit tight, not run away, so they do not flee to safety as a mowing machine approaches. Domestic lawnmowers pose no threat but gardeners, park managers and farmers often use strimmers to cut back weedy growth that intrudes into places meant for flowers or mown turf (Fig. 185). Following radio-tagged hedgehogs reveals that they often chose exactly this type of rank vegetation in which to lie up on warm summer days. There may be no visible sign of the animals, making it hard to avoid them when using these machines. Strimmers have a whirling cord that cuts back plant stalks, removing the cover and nesting material that hedgehogs need. They also inflict terrible injuries on the animals, severing limbs and noses or cutting deeply into the skin. Such damage may well be fatal, but traumatised survivors are now one of the most frequent arrivals at the door of wildlife hospitals and rescue centres.

FIG 185. Hedgehogs often nest in rank vegetation, exactly the type of growth that is cleared away by gardeners and maintenance workers using a strimmer. The whirling cord in one of these machines inflicts appalling damage on any hedgehogs that are resting there.

Larger mowing machines are a common sight in the summer, cutting back the grass and wildflowers along road verges. These are usually flails towed by a tractor, the cutting blade being invisible as it is covered by a protective metal hood about 1.5 m wide (Fig. 186). It simply smashes anything in its path, leaving a mulch of mangled vegetation. This will bury the remains of any hedgehogs that are encountered, so we have no way of knowing how many animals might be killed by these machines and what effect that might have on their population as a whole. Roadside vegetation has to be managed for safety and amenity reasons, so there must be some hedgehogs lost in this way, perhaps quite a lot. Hedgehogs will be among the many creatures that benefit from campaigns to protect insects, wild flowers and nesting birds that also inhabit roadside vegetation. These campaigns have mainly focused on delaying the onset of mowing in early summer and reducing its frequency, but increasing fuel and labour costs also mean that road verges seem to be mown less often now than in the past. This is a welcome development, but we still have no idea of how big an impact mowing machines make on the national population of hedgehogs.

FIG 186. Flail mowers are used to keep areas of long grass under control, especially road verges. Hedgehogs often lie up in such habitats during summer days. It is impossible to know how many are killed by these mowers, as they will leave little in the way of conspicuous evidence.

FIG 187. Foraging hedgehogs often fall foul of litter like this plastic six-pack holder or the thousands of elastic bands discarded by postmen.

Fires, natural and otherwise, also result in hedgehog casualties and this is a particular problem around Guy Fawkes’ night. Shortly before 5 November each year, the BHPS issues a warning about the threat posed by bonfires and urges care over lighting them or the piles of garden refuse in which hedgehogs may have chosen to hibernate. The danger was even mentioned in Parliament in 2015. Flooding probably accounts for a few more deaths each year, but this is likely to be a relatively minor issue and anyway hard to quantify, just like the victims of fires and mowing machines. However, there are several other clear and distinctive threats to hedgehogs that pose a serious challenge to survival, killing hedgehogs both directly and also indirectly in ways that may not always be immediately obvious.

Flattened hedgehogs are one of the more distinctive forms of road casualty and the animal’s remains are easily recognised, so people are very conscious of having seen them. Most other species would go unrecognised and without comment. In the past, whenever anyone made lists of species killed on the roads, hedgehogs were usually one of the two or three commonest casualties recorded, but this could simply be because their tough skin and robust spines mean the carcass persists for longer than those of say rabbits or rats. It is possible that the ease of detection and recognition exaggerates the apparent importance of road traffic accidents for this species. Nevertheless, roadkill is a matter of high public awareness and also prompts obvious questions about the numbers killed and what threat it poses to the population as a whole (Fig. 188).

It is popularly supposed that the apparent high numbers of dead hedgehogs seen on roads is a consequence of their natural habit of rolling up when confronted by danger, in this case an approaching vehicle. Actually, unless they are physically touched, hedgehogs do not roll up completely when threatened, but lower the head and hunch the body whilst still remaining on their feet. Either way, rolled up or not, they are no match for an approaching vehicle. A beguiling theory has been aired suggesting that some hedgehogs might be learning to run away instead of staying still. Natural selection would then favour them and we may see the evolution of a race of speedy hedgehogs that take avoiding action when confronted by approaching danger, the way that other mammals do, rather than standing their ground. This idea has an appealing Darwinian flavour, but is basically flawed on two counts. Firstly, we have no data on what hedgehogs did before the invention of fast cars and lorries, so that proving an evolutionary trend by making ‘before and after’ comparisons is not possible. Secondly, if an animal runs away obliquely in the path of the vehicle, it actually spends longer in the track of the oncoming wheels. If it runs off to one side, at right angles to the approaching danger, it could easily cross the path of a tyre which would otherwise have missed it had the animal stayed still.

FIG 188. In the past, the hedgehog has been one of the top three mammal casualties killed by road traffic (rabbit and brown rat being the other two). Many are still killed on the roads, but badgers and foxes are often more numerous victims these days.

Nevertheless, this story gained a lot of press attention and remains a favourite source of conjecture when the issue of roadkill is discussed. My colleague Steve Carter carried out some experiments (carefully!) to observe how hedgehogs behave when confronted by an approaching vehicle at night. His hedgehogs often did not attempt to avoid roads, but on the road itself they tended to move faster than when on the grassy verge. When a slow-moving car approached, about half of the experimental animals reacted by running off, while the rest adopted the typical ‘freeze’ reaction, waiting for the threat to recede. Those that ran away reacted when the car was so close that under normal circumstances they would probably have been run over anyway. These observations suggest that when a vehicle approaches a hedgehog, it makes little difference whether the animal freezes or runs – they are likely to become traffic victims either way. There is no evidence that we are observing natural selection promoting an evolutionary trend towards fleet-footed hedgehogs.

Why should they get killed on roads at all? Why would a hedgehog set out to cross a road, especially a wide one? The road surface is inherently hostile, being smelly with oil and rubber residues and also very rough on the hedgehog’s soft feet. It is unlikely it can see anything attractive on the far side, in the dark and with eyes barely 2 cm off the ground. In fact there is evidence that they will frequently turn away from a road and refrain from crossing it (Rondinini & Doncaster, 2002). It is rare to see a dead hedgehog on a British motorway, with its wide carriageways and heavy traffic. Hedgehogs are far more commonly killed on our smaller roads, but even these may act as a disincentive to dispersal, with animals being able to cross but reluctant to do so. This is certainly the case with other small mammals (MacPherson et al., 2011) and there are significant implications for fragmenting existing populations. Hedgehogs may cross roads less often than we think, in spite of the numbers we see dead. Nevertheless many get killed doing so. Maybe traffic density reaches a level at which it becomes a deterrent and will cause a hedgehog to turn back rather than venture forth. In New Zealand, it is asserted that roads carrying more than 3,000 vehicles per day form barriers to larger mammals, and it is the less busy roads that claim the most victims (Brockie et al., 2009). However, an analysis of roadkill in Britain showed no correlation between traffic flow and counts of dead hedgehogs or rabbits (Bright et al., 2015) (Fig. 189).

FIG 189. Roads are smelly, noisy and often have approaching vehicles, so hedgehogs often turn away rather than cross. Many are killed there nevertheless, with much debate about whether they would be better off running away rather than curling up in a defensive posture.

Driving a regular route does create the impression that hedgehogs are run over more often in certain places than along the route as a whole and this has been confirmed by statistical analysis in Ireland (Haigh et al., 2014a). That might suggest hedgehogs are using their own regular pathways, but it might also be because fences and other obstacles prevent access elsewhere and funnel the animals into crossing at particular places. The numbers we see dead on the road do vary seasonally, often with peaks in April and May, which is when the males are especially active seeking mates. There is another peak in late summer as the population reaches its maximum before winter begins (a pattern also observed in Ireland and other countries).

Regular counts following a standard protocol can provide data from which conclusions may be drawn regarding comparative abundance of hedgehogs at different times and in different parts of the country (see Chapter 2), but trying to estimate the total number killed in Britain each year is not easy. It is important because we need to assess whether roads represent a serious threat to the population or merely kill a small proportion of the total numbers, with no long-term impact on the population as a whole.

The obvious way to estimate the total number killed in this country is to count how many are seen dead on a sample of roads and scale up that number to take account of the total mileage of roads in the country as a whole. The problem here is that if the original sample count happened to be in an area where hedgehogs were unusually abundant, scaling it up will result in an excessively large estimate for the country overall. This is why the figure of 1 million killed on the roads each year, published in a Sunday newspaper in the 1960s and widely publicised since, is likely to be a significant overestimate. In an attempt to approach this and other hedgehog questions more rigorously, the People’s Trust for Endangered Species (PTES) instigated regular counts of road casualties by volunteers, the Mammals on Roads (MoR) surveys. In the four years 2001 to 2004, there were 6,411 dead hedgehogs counted on a total of 270,000 miles of roads surveyed. This works out at an average of about one hedgehog per 42 miles of road. Since there are about 245,000 miles of roads in Britain, one might estimate an annual mortality of about 5,900 during the three months that the survey was undertaken annually (July–September inclusive). Scaling this up to take account of hedgehogs being active for at least six months of the year, the annual mortality seems likely to be a minimum of 12,000 animals, probably nearer 15,000 and more still because the survey counts excluded any killed on motorways and also the large numbers that are killed in urban areas (because these were excluded from MoR surveys). By this line of reasoning, the annual toll on the hedgehog population would be between 15,000 and 20,000, perhaps more. A survey on the roads of Suffolk, conducted between January and October, found 1,488 hedgehogs. Suffolk is about 3 per cent of England, so scaled up to take account of the rest of Britain, there would be over 50,000 deaths on roads. But Suffolk is not typical of the country as a whole. It has also been suggested that the density of roads is a key factor; more roads means more chance that a hedgehog will get run over, so comparisons between different parts of the country with different densities of roads is unreliable. If so, then scaling up from any local survey to a national level will give invalid estimates for total roadkill.

However, it is important to take account of the practical and methodological details of the MoR surveys. For example, when driving on dual carriageways, observers were told to count only the hedgehogs seen on the carriageway they were using and, for safety reasons, not to attempt to scan the other half of the road at the same time (which anyway might be the other side of a wide central reservation). Thus, an unknown fraction of the hedgehogs killed on dual carriageways remained uncounted, perhaps as many as half. Since dual carriageways account for about 17 per cent of all major roads, this omission could represent a significant number of animals. The MoR surveys also instructed participants not to count in the rain because of reduced visibility and because wet, flattened hedgehogs are less conspicuous than dry ones. This ‘detection factor’ is another of the methodological problems with counting roadkills. It reflects the proportion of hedgehogs that escape notice, perhaps also because observers are driving too fast or they are distracted by more important things around them. Yet another issue is the ‘carcass persistence time’. This too is an unknown quantity. If the tough and durable remains of a dead hedgehog continue to be recognisable for a week, they are more likely to be noticed and counted than say a frog whose body might be obliterated by passing traffic within a day. Sometimes a squashed hedgehog can actually remain recognisable for up to a month, its skin baked hard on the road by hot summer sunshine. However, only a small amount of rain will soften up a flattened body and traffic will pulverise the remains sufficiently that it will soon become difficult to spot from a moving vehicle. In wet conditions, a body may be lost within a few days. There is also the possibility that local authority street cleaners or scavenging foxes will remove a proportion, again unknown, before they are seen and counted by MoR volunteers.

So there are several key factors that affect the arithmetic when estimating total roadkill numbers. All of them will have a significant effect if taken into account, but all of them are also unquantified and perhaps impossible to estimate accurately. However, if some of these issues are added into the calculations, the total hedgehog roadkill may be as many as 167,000–350,000! (Wembridge et al., 2016). This is an order of magnitude greater than suggested earlier, but the extent of the variation in that estimate is itself an indication of the uncertainties involved in its calculation. In the Netherlands, it is estimated that between 113,000 and 340,000 hedgehogs are killed on the roads annually; in Belgium, the annual mortality is said to be 230,000–350,000 (Holsbeek et al., 1999). These figures are higher than estimates for Britain, but suggest that road traffic could be killing enough hedgehogs to make a significant contribution to a decline in the size of the hedgehog population.

The question remains as to the extent of the threat to the British hedgehog population as a whole. Assuming that there are about 1.55 million hedgehogs in Britain (Harris et al., 1995), then 15,000–20,000 deaths per year represents a minor loss of around 1–2 per cent killed by traffic annually. If roads really did account for as many as 350,000 hedgehogs each year, that would be a massive 23 per cent loss every year. This seems both unsustainable and improbable, but it could be even worse, since the estimate for the total population size used here (1.55 million) is based on dodgy data from long ago (see Chapter 10) and there are now also considerably fewer hedgehogs about.

Another way of assessing the severity of roadkill at the population level would be to consider a more limited area than the whole of the country. For example, a survey of road casualties on Jersey in the Channel Islands recorded more than 600 hedgehogs killed in a year. This implies a major loss to what is a limited island population (Morris & Burdon, 2008), possibly in the region of 10 per cent.

One way of quantifying the significance of roadkill on the population as a whole would be to estimate population densities by capture-mark-recapture methods (see Chapter 10) in areas of the countryside with differing levels of traffic flow. But those methods are subject to many uncertainties and there would also probably be other factors affecting hedgehog numbers, including habitat type or the presence of badgers. However, in the Netherlands, an index of abundance (the frequency of finding hedgehog footprints in feeding tunnels) was used to compare areas where there were many roads with locations where there were few roads. On that basis, it is claimed that roadkill may reduce hedgehog population densities by up to 30 per cent (Huijser & Bergers, 2000).

Although they represent only small samples, we can also look at the numbers killed during studies of marked populations of hedgehogs. Of the 80 or so marked animals on a golf course in west London, very few were found dead on the road that runs alongside most of its length despite its fast and heavy traffic load, but they comprised 18 per cent of all known deaths (Reeve, 1994). Traffic accounted for 33 per cent (12 known deaths) of the 30 hedgehogs translocated in Oxfordshire (Doncaster, 1992), a mortality rate that may have been exacerbated by transferring the animals to an unfamiliar place, resulting in increased activity on their part. At a study site in New Zealand, only 4 per cent of marked hedgehogs ended up dead on adjacent roads, but traffic densities there are low. In the Netherlands, the comparable figure was 12 per cent within two years. A detailed four-year study of road casualties on a 16.5 km section of road in southern Sweden (Göransson et al., 1976) found 18–39 (17–22 per cent) of the local hedgehogs were killed on that road annually, although this appeared not to result in a dwindling population. Hans Kristiansson monitored the survival of 220 hedgehogs in a Swedish village for eight years, during which road traffic accounted for between 2 per cent and 22 per cent (average about 8–10 per cent) of the population each year. In a Finnish study, three quarters of known deaths were caused by human activity, 97 per cent of which were due to road traffic (Rautio et al., 2016), and in Poland, 24 per cent of a population of 78 hedgehog were run over in a single year (Orlowski & Nowak, 2004). In one of my own studies, 2 out of 12 animals (17 per cent) were killed on local roads in just a couple of months, but in special circumstances that were different from other studies and in a habitat that was not typical of the country as a whole.

The upshot of all this is that we simply do not know how serious road traffic is as a mortality factor for hedgehogs, but in terms of percentage loss the figure is not inconsiderable. Having evolved for millions of years into an animal that is largely unaffected by predators, the sudden onslaught of road traffic is an additional form of ‘predation’ to which the hedgehog’s population dynamics have not had time to adjust. The numbers killed this way certainly cannot be doing the species any good and may be a serious threat to small and isolated populations like those that occur in many urban areas. Some evidence in support of this was published in Bavaria, where the possible loss of entire village populations of hedgehogs was attributed to roadkill (Reichholf, 1983). Ironically, road verges can be important foraging sites and dispersal corridors for hedgehogs, despite the hazard they represent.

Bristling spines present an impenetrable panoply to any would-be predator and a hedgehog can remain in a spiky rolled-up condition almost indefinitely. If a paw (or finger) is trapped when it curls up, the combination of sharp spines and powerful rolling-up muscles can create a very painful experience for any attacker. Rolling up might also result in squeezing and emptying a full bladder, giving rise to the suggestion by Pliny (and copied down the ages by many other authors) that a hedgehog will do this deliberately as a form of defence, increasing its unattractiveness for a meal. Most predators leave hedgehogs alone.

Eagle owls seem to take them occasionally (Andrews, 1990); in Germany, the remains of 160 hedgehogs were found among 2,211 vertebrate prey taken by eagle owls and I once found the bones of a young hedgehog in a disintegrated pellet regurgitated by a large predatory bird in Turkey. But eagle owls are not a problem for British hedgehogs, nor would they be much troubled by buzzards or golden eagles, which are active during the day, although red kites probably take a few scavenged off the roads. There were no hedgehog remains among the thousands of prey items I have identified in barn owl pellets, but I have seen an eyewitness account of a tawny owl that attacked a hedgehog in Lancashire and probably carried it away, but this is likely to be a highly unusual occurrence. A short video clip was posted on YouTube in 2014 showing a tawny owl swooping down beside a moving hedgehog, which flinched and then scuttled away to hide, but the owl made no attempt to attack it. Magpies will often go for sickly hedgehogs that wander about in daylight and they may sometimes inflict serious wounds, perhaps even resulting in death. In New Zealand, hedgehog fur was found in 3 out of 358 cat droppings and once in the gut of 45 cats, but this could represent scavenging rather than direct predation. British cats are unlikely to kill many hedgehogs.

Dogs may be more of a problem, but hedgehogs are able to defend themselves and incidents involving dogs probably more often result in injuries (to both species) rather than death (Fig. 190). Dog bites are a common reason for hedgehogs to end up in rescue centres and dogs off the lead will endanger any defenceless young that are found in a hedgehog’s nest. The BHPS issues periodic warnings about the need to keep dogs under control and just their presence in gardens may deter hedgehogs from visiting.

FIG 190. Dogs, especially the larger breeds, will often interfere with hedgehogs, perhaps just lifting them in the mouth, but sometimes actively biting and causing injury. With so many dog owners often taking their dog for a walk after dark, dogs have become a significant threat to hedgehogs.

It is likely that the majority of predated hedgehogs are in fact juveniles, in which the spines are not only thin and relatively soft, but the skin musculature is also incompletely developed. These younger animals are less able to resist determined efforts to attack them by rolling up tightly. Their skin is also insufficiently voluminous to allow the animal to be fully enclosed by its spiny skin and a determined predator could exploit this weakness, but we have few wild species that could tackle a hedgehog successfully. Over the centuries, bears and wolves were heavily persecuted and their extinction removed them as a threat to hedgehogs.

FIG 191. Many of the old accounts of hedgehogs tell tales of foxes rolling them into water or urinating on them. Allegedly, this causes the creature to unroll, thereby becoming less well defended against being killed and eaten.

There are occasional old records of hedgehogs in the diet of polecats (see Macpherson, 1892, for example), and humans might also be added to the list of occasional predators (see Chapter 12). Foxes can and do eat hedgehogs, but these are probably mainly road casualties, the fox acting simply as an opportunistic scavenger. However, foxes have been seen to attack live hedgehogs (Harris, 1986) and they have been accused of depleting hedgehog numbers in urban areas, especially through their predation on nestlings. There are also persistent folk tales of foxes taking a hedgehog to water or urinating on it to provoke the animal into uncurling (Fig. 191). Foxes might kill the odd one or two occasionally, but generally it is not worth the effort, especially in urban areas with abundant pet food and bird tables. Some hedgehogs brought in to care centres show injuries to their hind legs which are attributed to foxes, but dogs and other hedgehogs will often snap at the protruding hind legs of a hedgehog as it runs away, causing at least superficial injury. In many cases, the damage is far more serious and must be due to a large animal. It would be difficult to attribute the wound to foxes with certainty, but there is abundant direct evidence that foxes (and dogs) do attack hedgehogs and both are now very abundant in many urban areas where hedgehogs ought to be safe from predation. Nevertheless, observers who have kept careful records of the wildlife in their gardens through the PTES Living with Mammals Project showed no significant increase in fox numbers at around 3,000 sites, so it is unlikely that foxes are exerting greater pressure on hedgehog populations than they did in the past and they are probably not responsible for a chronic decline in hedgehog numbers (Fig. 192). By contrast badgers are increasing and there is more and more evidence accruing that this species is having a serious impact on the hedgehog (Pettett et al., 2017a)

FIG 192. Regular surveys of householders by the PTES reveal hedgehogs and foxes often coexisting in gardens, suggesting that the latter are not a widespread or serious threat. (Angie Davidson/ Hedgehog Street)

The badger is undoubtedly the most important predator of hedgehogs. It is the one carnivore that has claws long enough to reach down to the hedgehog’s skin among a mass of its bristling spines. Its forelimbs are also powerfully developed for digging, enabling a hapless hedgehog to be easily torn apart. The badger is supremely well equipped to prey upon any unfortunate hedgehogs that it comes across and its attacks have been well chronicled in the literature. Topsel (1658) recorded it as a killer and most wildlife authors since have done so too, sometimes mentioning the victim’s ‘hideous cries’ while being caught and devoured. Usually, the hedgehog is ripped open and its body torn away from the dorsal part of the skin and eaten. Only the spiny skin remains, a fairly unequivocal indication of who the predator was (Fig. 193). Some keen supporters of the badger have sought to deny its sins, but I have seen these pathetic remains and also viewed several gruesome video sequences obtained using trail cameras. Despite this, and much of what I have written in this book, I like badgers and still clearly recall the thrill of seeing them for the first time, even though that is now more than half a century ago. Nevertheless, it has become increasingly clear that interactions between badgers and hedgehogs are an issue that cannot be ignored.

FIG 193. Typical remains left by a badger after killing and eating a hedgehog, leaving behind only the spiny skin.

Badgers are omnivores, but also predators. They feed on animals including worms, beetles, insect larvae and occasionally on mammals, birds and eggs. In fact they eat many of the things that sustain hedgehogs and that also form part of the diet for shrews, moles, fox and polecat. These are all species that do much the same thing: prey upon macroinvertebrates. In this way, badgers and hedgehogs belong to the same feeding ‘guild’ and compete with each other for some of the same resources. In addition, badgers will kill and eat hedgehogs. This is referred to as ‘intraguild predation’ and is much discussed as an ecological principle in the literature. Eating a hedgehog has the double advantage (to the badger) of not only getting a useful meal, but also eliminating a competitor for food. This may seem like a good idea from the point of view of the top predator, but if such predation becomes too successful it may disrupt a long-standing natural balance. If one layer of a food pyramid is removed in this way, then the top predators may need to expand their diet, perhaps to include unexpected and valuable alternative prey such as chickens or domestic stock. If the middle-ranking predator is severely reduced or eliminated, perhaps its erstwhile prey will become embarrassingly abundant. These are interesting issues, but perhaps not of pressing concern in relation to hedgehogs. What matters is that badgers kill hedgehogs and also compete with them for certain forms of macroinvertebrate food such as earthworms.

So long as badgers remained relatively scarce, they would have had only a minor impact on hedgehogs at the population level, but in recent times their numbers have hugely increased. In the late 1980s, it is thought there were roughly a quarter of a million badgers in Britain living in about 45,000 ‘social groups’. The badger population of England and Wales was put at 225,000 (Cresswell et al., 1990). Within in a decade, there had been a 24 per cent increase in the number of badger social groups in Britain, rising further to 81,000 by 2013. The actual total population of badgers that this represents depends upon the average number in a social group. If it is still 5.9, as in the 1980s, the total number of badgers may have doubled, in England at least. The most recent estimates, taking account of several factors that may vary across the country (including social group size), suggest the total population of badgers in England and Wales is approximately 485,000, at an average density of between 0.26–5.98 badgers per km2 (Judge et al., 2017). The reason for this big increase in numbers is unclear, but it may be a consequence of changes in habitat quality. Another possibility is better survival among cubs during warmer, wetter summers, leading to a larger number of breeding animals the following year, repeated for several years in succession. Some would argue that the increase is a welcome confirmation that legal protection given to the badger since the 1970s has had the beneficial effect of allowing the population to bounce back to its proper level, having been previously suppressed by excessive levels of persecution, especially by badger diggers. However, there is no indication as to what the population size might have been in the distant past. Records of bounty payments made by churchwardens prior to the 1830s (Lovegrove, 2007) show remarkably few badgers (called ‘greys’) being killed despite a generous bounty payment of one shilling each, more than many men earned in a whole day. Despite this incentive, few parishes where badgers were killed averaged even three per year. This suggests that badgers were uncommon animals for centuries, and that modern numbers are way above the historical norm (Fig. 194).

FIG 194. Badgers forage for earthworms and other food items that support hedgehogs. In this way, as competitors, they can suppress hedgehog populations. Preying on hedgehogs exacerbates the threat they pose.

Further evidence comes from studies of the relationship between the size and abundance of mammals. Across the world and covering many species, there is a direct correlation between an animal’s size and its population density. Small mammals are more abundant than big ones. In this respect, badgers are out of line, with far more of them present than would be predicted from their body size. The total biomass of badgers in this country is 36 per cent greater than that of all our other terrestrial carnivores put together (Harris & Yalden, 2008). Put another way, there is a far greater tonnage of badgers in the British countryside than would be expected based on both logic and biological principles. One could argue endlessly and fruitlessly about whether there are ‘too many badgers’ (as Prince Charles once told me very firmly!), but just being numerous is the problem. Badgers are large animals. Each one eats about the same amount as five hedgehogs. Every extra badger eats the food that would otherwise be there to support other wildlife, including thrushes and starlings, whose numbers are also declining. More badgers has to mean fewer of something else, even without actually preying on them. The hedgehog loses out twice over: badgers compete with it for the same foods and badgers also eat hedgehogs directly. A survey carried out by the Central Science Laboratory in 2006 found a negative correlation between numbers of hedgehogs and badgers – where there were more badgers, there were fewer hedgehogs, a pattern that has been confirmed by several other radio-tracking studies and independent surveys. Indeed, almost all the recent field studies of hedgehogs conclude that badgers are a significant threat and their presence affects both hedgehog behaviour and distribution (Fig. 195).

FIG 195. Badgers are sometimes recorded feeding in the same gardens as hedgehogs. Perhaps the additional food put out by people means there is sufficient for the badger not to bother the hedgehogs. It is also possible that predation on hedgehogs is limited to particular individual badgers, and not all of them are guilty. (Angie Davidson/Hedgehog Street)

As with road casualties, it is difficult to know the extent to which badgers threaten the hedgehog population as a whole, but again we can consider the fate of individuals that were followed in the course of particular studies. In one of mine, 3 out of 12 (25 per cent) were killed by badgers within two months (compared with only 2 run over on local roads). Beate Johansen experienced similar losses in Norway – 6 out of 17 eaten by badgers (35 per cent), but only 3 run over. Patrick Doncaster found badgers ate 3 of his 12 adult hedgehogs (25 per cent) within two months of starting a study in Oxfordshire. Similarly when he introduced 30 radio-tagged hedgehogs into Wytham Woods near Oxford, 7 (23 per cent) were eaten by badgers. There is also abundant evidence that the presence and number of badgers affect the nightly movements of hedgehogs, with avoidance behaviour perhaps keeping them away from good feeding areas. Even just the smell of badgers is sufficient to elicit avoidance behaviour on the part of hedgehogs (Ward et al., 1997). Clearly, badgers have become an important issue in modifying the ecology, behaviour and survival of British hedgehogs (Doncaster, 1993; Hof et al., 2012) and a similar situation is found in the Netherlands (van de Poel et al., 2015). Principal component analysis suggests that the combination of badger presence and food availability is the main determinant of hedgehog abundance, and local extinction was predicted to occur when the density of badgers exceeded about 2.27 setts per 10 km2 (Micol et al., 1994). That density of badgers had already been exceeded by the time of the first national badger survey in the 1980s (Table 24). Over England and Wales, the average density of badger setts is now almost double that (Judge et al., 2017), with a probable increase in social group size as well. This does not bode well for the hedgehog (Fig. 196).

FIG 196. Data from the first national badger survey showing numbers present in the late 1980s. Studies in Oxfordshire suggested that hedgehogs faced the likelihood of extinction once badger numbers exceeded about 13 per 10 km2, a figure already exceeded in some areas 30 years ago. Badger numbers have greatly increased since then.

It has been widely assumed that urban areas were a refuge from badgers and this is why hedgehogs are often more numerous in and around towns than in the countryside. Several studies of hedgehogs in rural areas have found them gravitating towards villages, farm buildings and gardens. Again this is often presumed to be evidence of avoidance behaviour and assumes that such places are safe from badgers. However, as our urban fringe relentlessly extends into the countryside, and as badger numbers continue to increase, we find them living commensally with humans increasingly often. The urban refuge is no longer safe and the probability of hedgehogs surviving in urban areas declines towards zero in places where there is a high density of badgers (Young et al., 2006). We might reasonably assume that the badger population will continue to exert a downward pressure on hedgehog numbers.

In the early 1970s, bovine tuberculosis (bTB) was found in wild badgers. This country had been almost free of the disease, but since then there has been a significant increase in the incidence of bTB in dairy herds. The cost to the taxpayer was estimated to be about £100 million per year in 2015, with around 30,000 cattle having to be slaughtered annually. Badgers are a reservoir for the causative bacteria, although their role in actually transmitting it to cattle has been endlessly disputed. Culling badgers might help reduce the problem, or might make it worse by causing the established territorial systems of badger clans to break down, leading to more dispersal movements and greater risk of spreading the disease. Years of heavily criticised badger culling strategies failed to resolve that issue or result in controlling bTB. Culling is very unpopular with the public, but politically necessary given the devastation that the disease has caused in the dairy industry. For several years, badgers were culled in a large and structured experiment to measure the effects of doing so in relation to controlling the disease. They were removed from a selection of specially chosen areas in the Randomised Badger Culling Trials of 1998–2008, often called the ‘Krebs trial’ after Sir John Krebs who designed the experimental protocol for the cull. Five years after badger culling began, it was found that in areas of amenity grassland, a preferred hedgehog habitat (at least in those days), their numbers had more than doubled, from 0.9 per ha to 2.4 per ha. Hedgehogs remained fewer (0.3 per ha) and unchanged where no badgers were killed (Trewby et al., 2014). The apparent increase in numbers seen after removal of badgers was perhaps an illusion resulting from the same number of hedgehogs being bold enough to range more widely once the threat posed by badgers had gone. However, radio-tracking showed no major changes in hedgehog behaviour after badgers had been removed, so the observed increase in their numbers was probably real.

TABLE 24. Regional densities of badgers. Data from the National Badger Survey of 1985–88 (Cresswell et al., 1990). Studies in Oxfordshire suggested that once the density of badger setts exceeds 2.27 per 10 km2, equivalent to a population of about 13 badgers, the local hedgehogs face extinction (Micol et al., 1994). This figure had already been exceeded in several parts of Britain by the 1990s (marked * in the table) and badger numbers have risen substantially since that time, perhaps even doubled.

The Krebs trials had indicated that culling could exacerbate the bTB problem by causing the badgers to move about more. Despite this, a political decision was taken to instigate further badger culls. About 10,000 were killed in the autumn of 2016, making a total of more than 14,800 in three years. For political and economic reasons, badger culls may be extended in the future, in spite of widespread protests. Ideally, we should have a ‘before and after’ survey of hedgehogs at more than one culling site to measure the effects, but the cull locations were kept secret beforehand because the protesters were becoming seriously threatening. The PTES and BHPS jointly rejected the proposal to cull badgers mainly on the grounds that it would not solve the disease problem and that culling might make it worse.

This is a difficult issue. On the one hand, killing small groups of badgers results in others moving into the vacated territory, potentially spreading disease further. On the other hand, if enough badgers are killed over a big enough area, it could at least remove them as a reservoir of bTB. This was the approach recommended by the Government’s Chief Scientist Sir David King and led to the recent culls. Farmers are desperate to see the bTB problem resolved and they can put pressure on their MPs, but culling alone will not solve the problem because there are other factors involved. There needs to be an integrated approach that includes much greater control of cattle movements and better cattle management, perhaps even a reappraisal of the farming industry, including the genetics of dairy cattle and more accurate methods of testing for bTB. Where badger culls have taken place, hedgehog numbers have increased, but few people want to kill badgers and nobody should advocate killing them just to help hedgehogs. Belatedly, extensive trials are in operation deploying vaccines to control the disease, and from 2018 the policy in Eire will change from culling badgers to a strategy based on their use. If vaccination proves to be successful, British badgers will not need to be killed to control or eradicate bovine TB. Their numbers will then remain high, an ongoing threat to rural populations of hedgehogs. If culling proves to be a more cost-effective solution to the bTB problem than vaccines, culling will probably resume on a larger scale. Local increases in the number of hedgehogs could be that cloud’s silver lining.

As a rather generalist feeder, the hedgehog is vulnerable to a wide range of poisons and contaminants that are released into the environment for different reasons. These will include heavy metals from a variety of sources, consumed via the hedgehog’s prey. Cadmium, for example, accumulates with increasing age in an animal’s liver (Rautio et al., 2016), although there is no evidence to suggest that this or other industrial contaminants pose a significant threat to hedgehogs. Agricultural chemicals appear to be more of a problem, especially where efforts to increase the yield from arable farming include elimination of invertebrates that eat or damage crops. In the past, crops were sprayed with organochlorine compounds (like DDT and Dieldrin), killing weevils, aphids, caterpillars and anything else that compromised plant viability. Seeds were coated with Aldrin to kill insects that invade seed stores or have larvae that damaged the seeds after they have been sown. These agricultural insecticides proved to be extremely harmful because they are bioaccumulators. Tiny amounts consumed from each contaminated insect built up in a predator’s tissues. Carnivores further along the food chain would accumulate small sub-lethal quantities from each of their prey until a toxic level had accumulated internally. Widespread deaths and reproductive failures among birds and bats in the 1960s highlighted the problem posed by these toxic organochlorine compounds and their physiological and ecological effects were intensively studied. Little attempt was made to establish the effects on hedgehogs, either as individuals or at the population level. However, the use of agricultural pesticides cannot have been benign, if only because their purpose was to reduce or eliminate the very prey that hedgehogs rely on for most of their food (Fig. 197). At the individual level, small non-lethal doses of organochlorine compounds would also have accumulated in the hedgehog’s fat, to be released as reserves were consumed over winter, especially in a burst at the end of hibernation as the last dregs of fat were used up. This could have led to an increased mortality rate among hedgehogs at that critical time of the year, especially the smaller ones with lower fat reserves. These harmful chemicals were mostly banned in the 1970s, so they have not therefore been responsible for reducing hedgehog numbers in the past 30 years, but they may well have contributed significantly to earlier losses.

FIG 197. Some crops receive repeated doses of chemicals whose purpose is to eliminate the very wildlife that hedgehogs need for food. Ploughing also reduces soil invertebrates.

Just as insecticides could easily be assumed to be harmless to hedgehogs, because they were aimed at insects, one might be forgiven for assuming that hedgehogs were safe from weedkillers. These kill grass and weeds, so what’s the problem? In fact one of the most commonly used herbicides, paraquat, was actually highly toxic to animals and a foraging hedgehog pushing through rank vegetation that had recently been sprayed would easily pick up such a substance and then ingest it when licking its fur during a session of grooming. The sale and use of paraquat has been banned, and now the future of glyphosate is under review. This has become the universally used herbicide, but turns out to have more widespread effects, including causing liver damage in mammals at very low levels. As one ecological villain is removed, another comes along, including endocrine disruptors. These are often polychlorinated biphenyls (PCBs), which act by blocking hormone receptors in an animal’s tissues, resulting in reduced fertility. These chemicals are components of a vast variety of modern substances ranging from plastics and flame retardants to printing inks. PCBs can be biologically active at much lower concentrations than the older organochlorines. Endocrine disruptors mostly seem to be an issue with aquatic species, which have to live in water contaminated by factory effluent or treated sewage outflows, but terrestrial species are not immune. Hedgehogs frequently forage around accumulations of waste and will lick or eat things that could easily be contaminated, especially as so much litter and other refuse is nowadays discarded in the places where they live. Endocrine disruptors and the bioaccumulators are not directly toxic, the way we normally think of ‘poisons’. Their effects are at the population level not just on the odd unlucky individual.

FIG 198. There is increasing evidence that hedgehogs can only thrive around the uncultivated margins of arable fields, leaving large areas of the countryside devoid of them.

The need to protect food and horticultural plants from insect pests is real and requires fresh substances to be deployed as soon as those with undesirable side effects are removed from the market. Since the 1990s, new synthetic pesticides have been available in the form of organophosphorus compounds called neonicotinoids. These prevent the nervous system from working properly (and not just in insects!) by blocking the connection between nerves and muscles, resulting in paralysis. They also have a cumulative toxic effect, so multiple exposures to the chemicals amplify the toxicity for anything that eats contaminated insects. In recent years, it has become common practice to use such chemicals to control leatherjackets (Tipulid fly larvae) and ‘chafer grubs’, which damage turf by eating away at the grass roots. This is necessary, not just to maintain an attractively uniform green sward without patches of dying grass, but also to retain a tight root mat in order to support the increasingly heavy demands placed on it by rising numbers of human users. Pesticides that are used to sustain ‘sports turf’ such as football pitches, golf courses and general recreation grounds may have resulted in eliminating most of the invertebrates that live there. The chemicals used to control the ‘pests’ were usually chloropyrifos and imidacloprid. These have trade names that imply a direct enhancement of the grass plant or suggest they have a specific target like the large larvae of the cockchafer beetle (‘Maybug’, Melolontha melolontha) (Fig. 199). But they kill other insect larvae too, including leatherjackets, a favourite food of the hedgehog. Serious concern about the unintended effects of neonicotinoids on bees, worms and ecosystems has resulted in at least a temporary ban on the use of these chemicals across the whole of the European Union, as from October 2016, but the ban is only partial (effectively to protect bees that visit flowering plants) and poisonous seed dressings appear to be still allowed. Chemical treatments for ‘grubs’ will probably also continue, at least for now. But ‘grubs’ are among the key items of a hedgehog’s diet.

FIG 199. An adult cockchafer beetle, whose larvae live in the soil for several years before emergence. The larvae eat grass roots and are deemed a pest as they damage lawns and sports pitches. They are eliminated by using poisons that kill them and many other soil invertebrates that are important food items for hedgehogs.

The problem with all pesticides is that the chemicals do not confine their effects to pest species. Neonicotinoids kill insects, and that includes bees. They have been blamed for the rapid and ecologically disastrous collapse in honey bee colonies and bumble bee populations. Bees are vital pollinators, of valuable agricultural crops, not just wild flowers and garden plants. But it turns out that these substances are also very harmful to earthworms, more so than other modern synthetic insecticides (Wang et al., 2012). Earthworms, like bees, are ecologically vital animals. They are key elements of the hedgehog’s diet too. Plant sprays and pesticides appear to be remote from hedgehogs, but when it rains, sprays get washed off into the soil. Neonicotinoids are absorbed by plants, which later die or may be eaten by caterpillars, slugs, herbivorous mammals; who knows where they might end up?

Pesticides that appear safe and irrelevant to hedgehogs are potentially implicated in declining hedgehog populations even if there are no direct effects on the individual animals as a result of eating contaminated prey. The purpose of pesticides is to reduce or eliminate many species of invertebrates upon which hedgehogs (and many other creatures) depend for food. Neonicotinoids and other pesticides are usually not species-specific; they tend to kill a wide range of creatures, including most of the prey upon which hedgehogs normally depend (see Chapter 4).

Since 1937, metaldehyde has been recognised as an effective poison for killing molluscs and mixed with bran it was widely marketed in the form of small pellets. Over 350 tons of slug pellets were deployed on almost a million hectares of land in 2000 (Bourne, 2017). The pellets are highly visible and very good at killing slugs and snails, giving rise to a widespread belief that they might also pose a threat to hedgehogs. The pellets are normally dyed blue, as this is thought to be an unattractive colour to birds and should minimise the risk of being eaten by them. But this is irrelevant to hedgehogs, as they lack colour vision and anyway colours are not distinguishable to a nocturnal animal in the dark. Toxicity to molluscs does not mean that it is safe for other things to eat slug pellets containing metaldehyde, only that they are particularly lethal to slugs and snails. These will be killed by a dose of between 5 and 20 micrograms per gram of body weight, whereas dogs, cats and guinea pigs require 40 to 50 times as much to kill them (200–1,000 micrograms per gram of body weight). Dose rates like this are obtained experimentally by seeing how much poison is needed to kill 50 per cent of a sample of luckless laboratory animals. The four- or fivefold difference in what is needed to kill them suggests that there is plenty of variation in the threat posed to individuals and some (maybe even half) might not be killed at all.

Moreover, hedgehogs may be more susceptible (or more resistant?) than the types of experimental animals used for testing. But the little evidence available does suggest that hedgehogs are neither more nor less vulnerable to metaldehyde poisoning and that this substance will cause sickness and perhaps death if enough slug pellets are eaten directly. That is probably an infrequent event because hedgehogs do not normally like to eat dry pellets, but these can become soft in the rain and might then be eaten more readily. In the last 30 years, there have been plenty of post-mortems carried out on hedgehogs confirming that they can and do ingest slug pellets, although that might not always have been the main cause of their death. In 1991, a report describing results from 74 hedgehog autopsies identified 3 that had eaten slug pellets. In one case, the animal had absorbed the poison, not just swallowed it, and in sufficient quantity for it to be hazardous to its health.

FIG 200. Slug pellets can be deployed safely by covering them with a heavy slab raised on sticks or small pebbles and perhaps baited with orange peel. The slugs seek out cool shady places and can get under the slab, but hedgehogs and pets cannot. The poisoned slugs die there, but if they crawl away first, their bodies contain only harmless residues of metaldehyde.

Metaldehyde, intended to kill slugs and snails, kills dogs too and can now be found in some of our drinking water, despite its passage through purification processes (Bourne, 2017). It appears not to harm plants or non-molluscan invertebrates, but there remains a fear that hedgehogs might still be at risk from eating poisoned slugs. Happily, that appears not to be a serious problem. If a poisoned slug retained in its body all of the dose needed to kill it, an animal the size of a hedgehog would probably need to eat hundreds of contaminated slugs, perhaps more than a thousand, to be in danger of dying. Metaldehyde is not a cumulative poison, unlike DDT and the organochlorines, so those slugs would have to be eaten all at once. What actually happens is that the metaldehyde in a dead slug rapidly decomposes to acetaldehyde (a natural substance anyway) and this swiftly breaks down into carbon dioxide and water, neither of which will harm the hedgehog. A visiting researcher agreed to analyses some poisoned slugs for me and found no toxic residues in them shortly after their death. Independently of that, two studies in Switzerland fed poisoned slugs to hedgehogs with no discernible ill effects, even though each consumed up to 200 of them. It seems safe to conclude that eating poisoned slugs is not dangerous and that slug pellets are less of a threat than is often imagined. Nevertheless, it is important to deploy slug pellets in such a way that they cannot easily be ingested by hedgehogs or by any other non-target species, including pets and children (see Chapter 13).

This all relates to using the type of slug pellets that are normally available in shops and garden centres. Pellets containing a different molluscicide, methiocarb, were widely deployed for agricultural use with apparently little interest in what effect they might have on wildlife. Hedgehogs were said to eat methiocarb pellets, which seemed to be three times more palatable than metaldehyde-based pellets (Gemmeke, 1995). The recommended rate of application on farmland was equivalent to about 30 pellets per square metre and a foraging hedgehog would encounter sufficient to have a 50 per cent chance of death in just two square metres if it ate all the pellets available. Such incidents would probably be infrequent as the main areas where these pellets were used, in extensive wheat fields for example, would be rarely used by hedgehogs. Our limited experiments suggested that methiocarb did not decompose within the poisoned slugs, so their consumption would be harmful. If hedgehogs ate pellets or contaminated prey it could lead to physical symptoms such as diarrhoea and weight loss, but also potentially affect crucial behaviour patterns. However, our hedgehogs appeared to avoid eating contaminated slugs, suggesting a reduced risk in the wild. Happily, methiocarb is no longer an issue since the European Commission withdrew permission for it to be used anywhere in the EU, giving farmers until mid-2015 to use up their stocks of it.

The risks associated with controlling slugs and snails will be further reduced if biological control methods become more widely adopted. For example, Phasmarhabditis hermaphrodita is a nematode parasite that is lethal to slugs and appears not to attack other things (not even snails). It works by transmitting a deadly bacterium to the slug host. This prevents the slug from feeding, leading to its demise. Meanwhile the nematode can continue to multiply, allowing a build-up of the parasites in the soil, reducing slug numbers at least as efficiently long-term as conventional slug pellets (Rae et al., 2006). The nematodes are harmless to hedgehogs and can be supplied in a form suitable for horticultural applications and for use in protecting high value crops. Organic slug pellets based on iron phosphate are also available along with various forms of slug repellent (see Chapter 13 on creating safer gardens).

Several studies in recent years have found traces of ‘rat poison’ in dead hedgehogs, often in a high proportion of those examined: two thirds of 120 sick animals brought in to the RSPCA for example. Clare Dowding found rodenticide residues in a majority of the hedgehogs she analysed, with 22.5 per cent of her animals carrying residues of more than one compound, suggesting multiple exposures to rodenticides (Dowding et al., 2010). Although the rodenticide may not be an immediate cause of death (if it is found in a road casualty for example) nobody knows what might be the effect of sub-lethal amounts of these poisons in hedgehogs. Rodenticides may well weaken an animal, slowing its movements or compromising reproduction or in some way reducing its potential lifespan. Small quantities might reduce blood clotting and cause injured animals to die that might otherwise have survived. Whatever else, the poison certainly would not be doing the hedgehog any good. Higher doses will be fatal, but the body is unlikely to be found and submitted for toxicological analysis, so we can never know how many hedgehogs die in this way and therefore how serious a threat rodenticides might be to the population as a whole. What we do know is that a high proportion of dead hedgehogs are found to contain rodenticide residues. The question is, where is this stuff coming from, and how do hedgehogs end up eating it?

FIG 201. Slow-acting rodenticides allow poisoned rats and mice to disperse and be eaten by scavengers and invertebrates. The poison in the dead animals can then kill a wide variety of unintended victims, including hedgehogs. Limiting sale and use of rodenticides to trained operatives makes little difference; the poisoned rodents are still a threat no matter who was responsible for poisoning them.

Normal rat poisons are dispensed in grain baits, wax blocks or some other material that is attractive to rodents but unlikely to be gnawed by hedgehogs and many other wild animals. So it is regarded as safe to use these poisons to kill rats and mice in places where they are not wanted. In the past, that has meant dispensing the poisons indoors, such as in houses, chicken sheds and food stores. Inside a building, most wildlife species will not be directly at risk. However, in recent times it has become commonplace to deploy powerful rodenticides out of doors, not just around farms, but in towns and gardens too. Metal shelters containing the poison can often be seen outside supermarkets and in their car parks, even around garages where the ‘rat problem’, if it ever existed, is likely to have been minimal. If undesirable rodents are attracted to rubbish bins and other debris, then the best way to solve the problem is to tidy up! Waste food and empty food containers can be disposed of in rodent-proof bins. Putting out poison is less like hard work, but should be a last resort. What actually happens is that poison dispensers are deployed ‘just in case’, even when there is not a rodent issue. The manufacturers and corporate users will say that the metal shelters protect dogs, cats and birds (and hedgehogs) from gaining access to the poison, so they are safe. However, the entrance hole is 60 mm in diameter, a bit of a squeeze for an adult rat, but easily accessible to harmless species of wild mice and voles that rarely, if ever, venture indoors (Fig. 202). One of the most frequent autumn visitors to buildings around my home is the yellow-necked mouse (Apodemus flaviciollis), a nationally rare species, maybe getting rarer still thanks to the six poison dispensers in the yard at my local supermarket. Who knows?

FIG 202. Poison bait stations are now commonly installed as permanent fixtures, albeit not always with poison inside. Although intended to control rodent numbers, they have implications for other wildlife, including hedgehogs.

The reason why this is important to hedgehogs is that the poisons used are anticoagulants. These are not acute poisons that suddenly kill a victim, often painfully. Instead, after being ingested, they prevent mammalian blood from clotting so that wounds or minor leakages continue to bleed indefinitely, especially from the blood vessels around the intestines. The animal dies slowly, becoming weaker over several days and is unaware of its fate. This helps to prevent the rats from learning to avoid poison baits because they do not know what is happening. A single dose is not intended to cause death immediately and the poisoning strategy relies on animals living long enough to eat some more of the poison. Meanwhile, the poisoned rodent continues to walk about for up to 12 days with one or more doses of anticoagulant in its body. Small doses, insufficient to cause death to a rat or mouse, accumulate in the animal’s liver.

The contaminated animal is now a hazard for any predator or scavenger that might catch and eat it. This is especially easy in its weakened state, and easier still for any mammal or bird to feed on its carcass after death or for a hedgehog that eats the invertebrates that have been assisting with the poisoned animal’s decomposition. Poisoned rats can be found up to a hundred metres away from a bait station and it is likely that more wide-ranging species such as wood mice could carry the poisons even further away. This is how larger animals can be at risk, even though the poison is not intended for them and they cannot get at the actual poisoned bait inside the metal or plastic dispenser. Predators and scavengers get the poison second-hand so to speak, a process known as ‘secondary poisoning’. This is often a threat to predatory birds that feed on rodents weakened by anticoagulants. Rodenticide residues have been found in 92 per cent of British red kites, up to 91 per cent of barn owls and 80 per cent of kestrels, for example. Polecats are another proven victim of secondary poisoning, as a result of feeding on rats and mice around farmyards (Shore et al., 1999). Dogs and cats are at risk too, although few of their owners appear to realise this. Modern rodenticides appear not to be metabolised within animals, so they remain as active molecules in faeces and dead bodies, easily picked up by worms, beetles, maggots and other decomposers, the very things that hedgehogs eat. Like many other modern chemicals released into the environment, it is hard to know where they will end up. In the USA, thousands of children have apparently been found contaminated with rodenticides after showing signs of poisoning. Secondary poisoning is a serious issue and not just for hedgehogs. Like the fabled genie, once poisons are released, they are difficult to get back into the bottle and you never know where they will turn up next.

It is illegal to leave poisons about without using devices to exclude larger animals, and although hedgehogs cannot get into rat poison dispensers, they are very vulnerable to secondary poisoning, an unintended consequence of making sure there isn’t a rat problem. Hedgehogs will readily consume dead mice, voles and rats along with any rodenticide residues they may contain. Moreover, slugs and other invertebrates that feed on poison baits or dead rodents can do so with impunity as they have a different blood chemistry, but crawling about with anticoagulant in their gut, they offer another way for a hungry hedgehog to become an unintended victim.

Poisons are supposed to be a last resort, but it is quite normal to see bait stations set out with no attempt made to clear away attractive food waste nearby in spite of good practice recommendations. The rat poisons used today are referred to as SGARs (‘second generation anticoagulant rodenticides’) because they largely replace the anticoagulants used in the past such as warfarin. The new ones are much more potent and the danger they pose is obvious, so there are regulations that limit their use. For example, brodifacoum is supposed to be restricted to indoor use only (although poisoned rats and mice could still wander away and be eaten outside). Only a couple of SGARs are permitted to be used out of doors, the usual one being bromadiolone. Officially, this must only be deployed in dispensers that prevent access by non-target species, but there is no way of controlling secondary poisoning as a result of rodents (including non-target species) wandering away carrying a dose of rodenticide within their body. That could be a particular danger where farmers use rodenticides around cowsheds or chicken pens, remembering that studies show farmland hedgehogs (particularly in arable areas) tend to gravitate towards farm buildings. Another obvious focus would be in woodland areas where gamekeepers rear pheasants and use rodenticides to reduce losses of feed meant for the birds. The intended targets may be rats or grey squirrels, but large numbers of wood mice and bank voles (Clethrionymus glareolus) will be killed along with any rats that might (or might not) be living there. Their contaminated bodies will be available for consumption by hedgehogs, foxes, badgers and dogs too. It is small wonder that rodenticide residues are now turning up in all manner of wild animals.

To reduce risks to wildlife (and humans), there are rules about who may buy and use rat poisons, and penalties for misuse. One of the rules is that bait stations should not be used continuously for more than 35 days. But ‘better to be on the safe side’ and they often seem to be permanently in place. Who checks whether they are baited or not? When I checked a few of them near my local shops, one still had the poison inside, evidently forgotten when the last poisoning episode had ended. When poison baits are being deployed, users are required to search regularly for dead rats so as to remove them and prevent a hazard to pets and wildlife. Despite the fact that people have been prosecuted for not doing this, a survey reported by The Barn Owl Trust showed that this regulation was ignored by 98 per cent of users, probably because poisoned animals could have hidden themselves away almost anywhere and looking for them was considered to be a waste of time.

The risks associated with outdoor use of SGARs represent a higher level of threat than would normally be regarded as acceptable. Recognising this and under pressure from the EU, new rules mean that after mid-2016 SGARs would only be available to professional pest controllers and others (such as farmers) who have been appropriately trained in proper procedures. That did not prevent me buying packets of poison pellets in early 2017, with a cheery assurance from the shopkeeper that all was well. This tightening up should prevent casual use by gardeners and householders and place more emphasis on controlling an actual rat problem, rather than using poisons ‘just in case’. However, a poisoned rodent is a danger to any hedgehog or other animal that eats it, regardless of whether or not the poison was put out by a properly certified and regulated operator. Better regulation might reduce the risks to wildlife, but will not eliminate them. In particular, it will be impossible to prevent secondary poisoning, of hedgehogs and anything else, so the danger will remain. Poisons kill things, that’s what they are for, but maybe our modern, highly efficient poisons such as SGARs and neonicotinoids are just too efficient in what they do and the collateral damage they cause to wildlife is too high a price to pay for what they achieve. Perhaps the more potent anticoagulant rodenticides will be banned, just like DDT and the other ecologically dangerous cumulative organochlorine poisons of the 1960s. Meanwhile, rodenticides seem to be a likely contributor to recent declines in hedgehog numbers, adding to the modern dangers that confront this ancient species.

The basic message here is that if we want to increase food production, as we must in view of the rising human population, then we have to consider the law of unintended consequences. Whenever a new chemical is added to the tools of farmers and gardeners, we don’t necessarily know where it will end up or what other unexpected effects it might have. Ivermectin is another example. It is used as a ‘de-wormer’, for the desirable purpose of ridding pets and other animals of intestinal parasites such as tapeworms and nematodes. It is routinely given to cattle (and hospitalised hedgehogs!), but cows then create contaminated cow pats that cannot support the usual crop of flies and insect larvae that are important foods for many birds, bats, small mammals and hedgehogs. Using poisons of any kind is like releasing a mad dog – you never know where they might go or what they might do.

For millions of years, the hedgehog has only needed to cope with natural hazards and its population dynamics have evolved to ensure continued survival of the species. Suddenly, within a few decades it faced an unprecedented increase in the numbers of its most significant predator, the badger. Numerous other novel threats have also emerged, including road traffic, and even more abruptly it has been confronted with highly effective pesticides that reduce or eliminate many of its main prey. The most efficient of these chemicals are so dangerous that their use has been banned throughout the European Union, a situation that hopefully will survive Britain’s departure from that political entity. But the disruption that they have wrought among invertebrate populations may persist for decades. Hedgehogs also face a threat posed by insidious poisons intended to kill rats and mice. These substances are actually a hazard to any other species that may consume the poisoned rodents. They are widely distributed and regardless of measures to control their use, they continue to spread into the environment, creating an almost universal threat of secondary poisoning for hedgehogs and many other predators and scavenging species. Curiously, most people appear unaware of this, despite the danger to wildlife, their pets and perhaps even their own children. There is a widespread assumption that weedkillers kill weeds, rat poisons kill rats and insecticides aren’t a worry either. Hedgehogs pay the price for this ignorance.

CHAPTER 9

Death on the Roads and Other Dangers