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QUESTIONS AND ANSWERS: Foxes II

Content Updated: 8th October 2014

QUESTIONS:

What are Samson foxes?
Are American and European foxes different species?
Are British and European foxes different species?
Are urban foxes getting bigger?
What parasites and diseases do foxes carry?
Can foxes be rehabilitated and released back into the wild? [In Preparation]
What triggers dispersal in foxes? [In Preparation]
Why are foxes so smelly? [In Preparation]
Do foxes kill for fun? [In Preparation]

Q: What are Samson foxes?

Short Answer: Samson is a recessive genetic mutation of foxes (typically the Red fox, although it is known in other species) that prevents the long guard hairs that give the coat its lustre from growing and causes the underfur to be tightly curled. The result is that the fox has a ‘woolly’ appearance that may cover the whole body, or only tail and hind quarters, depending on whether the animal is a ‘complete’ or ‘partial’ Samson. Samson foxes tend to be larger than normal foxes and have a higher metabolism that provides them with a voracious appetite; they also have larger deposits of fat just under their skin and associated with their intestines that presumably helps compensate for the lost insulation that the fur would ordinarily provide. These animals apparently prefer to live around human settlements where they feed heavily on garbage. No recent work has been conducted on the genetic basis for the Samson trait and its origin is unknown; some suggest that it may have arisen in fur farmed animals and made it into the wild via escapees, although this seems unlikely. Occasionally, completely hairless animals are found (particularly in the USA), but these are alopecic foxes, rather than Samsons.

Samson foxThe Details: Red foxes come in an impressive range of colours and patterns, from jet black to pure white, with most hues of red, orange and yellow in between. The coat is composed of two main hair types: relatively short (ca. 35mm or 1.5 in.) fine grey underfur, sometimes called ‘vellus hair’, that covers the back and sides and provides insulation; and the longer, thicker guard hairs (or ‘terminal hair’) that provide waterproofing and give the coat the shiny ‘lustre’ so highly prized by hunters and fur farmers alike. Occasionally, however, a fox with a different coat appears – from a distance it looks pretty normal but, on closer inspection, it has a ‘woolly’ appearance resulting from a lack of guard hairs. These foxes are called Samson foxes, presumably deriving their name from the biblical character that lost the power with which God infused him when the Sorek temptress Delilah ordered a servant to cut off his hair. The Samson character can be expressed partially or completely: partial Samsons lack guard hairs around their rear end and on their tail (the remainder of the fur being normal), while complete Samsons lack guard hairs across the whole of their body.

A lack of guard hairs is not the only feature by which Samsons are distinguished from normal foxes. In a 1951 paper to the journal Papers on Game Research, Finnish Game and Fisheries Research Institute biologist Teppo Lampio described how Samson foxes are typically larger than normal animals, with a thicker layer of subcutaneous fat (that held just under the skin) and considerable amounts of fat associated with the intestines, which contributed to the animals being heavier than regular foxes; he suggested that this fat may provide insulation, possibly helping to compensate for the lack of guard hairs. Other traits noted by Lampio include ‘clearer’ footprints (there was only woolly fur on the underside of the paws, and even this was often worn away) that were narrower than those of normal foxes; claw marks were rare because the animals apparently wear down their claws quickly digging for food. In captivity, where food is provided, the claws apparently grow long and sharp. (Photo: A Red fox displaying the Samson character in North Carolina, USA. Note how the lack of guard hairs give the animal a 'woolly' appearance)

Lampio goes on to describe differences in the behaviour of Samson animals compared to normal Red foxes. It seems that Samsons tend to live around human settlements and are thus very accustomed to human activity; in fact, Lampio wrote that they were: “so bold as to be met in the middle of the day near the yard of a house”. Compared to modern day urban foxes such observations may be of little surprise, but remember that these accounts are of foxes in a hunting community during the 30s and 40s. The implication of Lampio’s summary is that Samson foxes are less well adapted to survive and find food in the wild than their fully-furred counterparts and that this explained their preference for living in such close proximity to humans; he described how a Samson will feed largely on refuse and “does not even shun the excrements of domestic animals”, which appears to refer to cattle dung. It seems that these Samsons were well-adapted to feeding on waste and, in an earlier (1948) study, Lampio found that they were less susceptible to the lung and intestinal parasites that regular foxes suffered with (although other explanations fit – see below). Finally, Lampio noted that experience of Samsons in captivity showed that their cubs were less viable (i.e. fewer survived) than normal fox cubs, initially growing more slowly (although catching up later on), being “obviously ill-tempered” and having an “abnormally large appetite”. It seems probable that these animals were at a competitive disadvantage to their normally-furred conspecifics.

Interestingly, it appears that Samson foxes were healthier during the early part of the Finnish population explosion than later on, and this is reflected in both the average weights of the animals caught and the number found with lung and intestinal worms. Presumably, when the population first became established and began to grow there were relatively few animals, meaning competition for resources was low; when the population became larger competition increased and conditions deteriorated for each fox. The graphs below show the data that Lampio collected on Samsons shot by hunters in Finland between 1945 and 1951 and they imply a change in the fortunes of the population around 1948. Caution should be used when drawing any firm conclusions from these data because they are based on relatively few Samson animals.

Samson Weight GraphSamson Lungworm Graph
Samson Intestinal Worm Graph
These charts graphically represent the findings of Teppo Lampio from his studies on Samson foxes shot by hunters in Finland. Lampio found that Samson foxes generally had fewer intestinal and lung worms than 'normal' foxes, although around 1947, the lungworm burden appears to increase among the Samsons. Lampio did not speculate as to the cause of this shift, nor to why Samsons may have fewer intestinal parasites. Caution should be used when interpreting these data because the sample size is small.

Despite the changes, Samson foxes were still able to spread throughout much of southern Finland by the end of the 1940s.  Indeed, by the 1946-47 hunting season, these animals accounted for 7.5% of the total catch, with substantial local variation (some populations had 80% Samsons). The distribution of Samsons remained relatively stable during the 1950s, although by 1960 there were only isolated populations based predominantly in the south of the country.

Samson fox spread in FinlandThe origin of Samson
In the Bible story, the Philistines approached Delilah and offered her cash to find out the source of Samson’s great strength. Much trial and error on Delilah’s part, and seemingly much teasing on Samson’s, led to the discovery that his hair was the crucial factor. In a similar manner, many theories have been proposed to explain the origin of the Samson character in foxes, not least because it is an economic disaster for fur farmers and many hunters; the pelt is virtually worthless.  I have been unable to establish when the first Samson fox was documented but such foxes have been recorded on Finnish fur farms since at least the 1920s. Northern Europe appears to be something of a ‘hub’ for these animals and most of the early (1930s and 1940s) records of wild Samsons come from Finland, Sweden, Norway and Denmark.  Indeed, the association between fur farming and capture of wild Samsons led some to theorise that the character originated in captivity and made it into the wild when animals escaped or were deliberately released from fur farms. Deliberate releases from farms happened from time-to-time and Lampio pointed out that, on the initiative of the Ministry of Agriculture, about 225 silver foxes (a black colour morph of the Red fox) were released into different parts of Finland during 1937/38, in the hope that they would cross-breed with wild stock and make it more valuable. (Image: A compound map showing the approximate spread of Samson foxes in Finland from between 1937 and 1948. The map is composite so the distribution in 1937 is shown in black, the 1943 distribution is black plus yellow, and the 1948 distribution is black, yellow and red. Compiled from data presented by Lampio in 1951.)

The problem with the ‘fur farm origin’ theory is that, as even Lampio noted, the earliest Samson foxes pre-date known releases; indeed, some maintain that they pre-date the establishment of fur farming in Finland.  It has also been argued that, if the Samson character originated on fur farms and entered wild populations through escapees or releases, we should expect more silver and ‘smoky red’ foxes in the wild populations (as these are the colour morphs likely to have been released) than were documented – wild black foxes were very rare in the country, but common on farms. Overall, it seems more likely that the character appeared spontaneously in the wild population and, because the fox population was very low (close to being exterminated, in fact) at the time, the mutation was able to gain a foot hold in conditions where competition with normal foxes was low. This theory also goes some way to explain the decline in Samsons during the late 1940s and 1950, as the population of normal foxes recovered.

Some early fur farmers considered that the Samson character was actually a disease; a response to a parasite infection or to the poor nutritional condition of many farmed foxes, owing to their heavily vegetable-based diet. Not everyone agreed with this theory and some considered there were genes at play. Between 1947 and 1950, University of Helsinki geneticist Tarvo Oksala conducted breeding experiments with Samson foxes; the preliminary results were published in a paper to Paper on Game Research during 1954.  Oksala had problems getting some of the foxes to breed in captivity, but he did have some limited success and made some interesting observations over the three years of his study. When he bred two wild-caught complete Samson animals, Oksala ended up with four complete Samson cubs, thereby providing a pure Samson line that -- given Samson and normal animals were kept in the same cages and given the same food -- implied diet is not a significant factor. Oksala wrote:

This does not imply that [disease or external circumstances] would not influence the Samson character, strengthening or weakening its expression. On the contrary its great instability suggests this very possibility. But the ultimate cause must be an inherited disposition.

Samson foxWhen Oksala talks of the “great instability” of the Samson character, he is referring to his observation that foxes can apparently grow in or out of it. During his study, Oksala found that some of his Samson animals moulted into normal foxes and vice versa. One male caught as a ‘normal’ cub in the south of the country during 1946 moulted into a Samson in its first autumn and then guard hairs grew back in the autumn of 1948. Also in 1946, this time in west Finland, a young male fox was caught with a normal pelt; in the autumn of 1948, however, he “changed into a fairly typical Samson fox”.  At the same time, the three other Samson animals (a male and two females) maintained the character for the duration of the study.  These findings caused Oksala to ponder the potential mechanisms of transference from parent to cub.

Oksala noted that, because the Samson character is of no commercial value, fur farmers do not breed from affected animals and yet the condition cropped up frequently on farms; this observation, and the results of his first breeding experiment, led Oksala to conclude that the trait must be recessive (i.e. only appears when two copies of the particular allele are present). He goes on to say:

It is tempting to suppose that selection in natural populations has created a modifier system which, when complete, is able to neutralize the effect of the gene or genes which give rise to the Samson character.”

In other words, another gene or gene complex is able to override or ‘switch off’ the Samson gene(s) and allow the normal growth of guard hairs. Genes can be switched on and off by the cell’s machinery during one of two genetic processes: transcription and translation. The turning on and off of a gene coding for the Samson character could explain how animals can moult between the two conditions. Unfortunately, to the best of my knowledge, no detailed genetic or dermatological studies have been conducted on Samson foxes and so the specific gene(s) responsible, as well as its origin, remains unknown. We have since discovered, however, that this is not a condition unique to Europe or to Red foxes. In 1974, Fish and Game Biologist Stephen Allen estimated that about 5% of North Dakota’s Red foxes had the Samson character; he tagged six normal cubs from the same litter in 1970 and found that two shot two years later had developed the Samson condition, while two killed the year before were normal. Samson Arctic foxes (Vulpes lagopus) have been recorded and, in a short paper to the Transactions of the Wisconsin Academy of Sciences, Arts and Letters, University of Wisconsin biologists David Root and Neil Payne described a male complete Samson Gray fox (Urocyon cinereoargenteus) that was shot in Richland County during late November 1979. The Wisconsin Grey Samson was in good condition and estimated to have been less than a year old.  It seems that other members of the Canidae (dog family) are susceptible to the condition and Samson Raccoon dogs (Nyctereutes procyonoides) are occasionally recorded in Finland. Outside of the canids, the ‘rex’ condition of rabbits and ‘crinkled’ character of mice is similar, although not identical, to the Samson character of foxes.

Samson fox skinHair today, gone tomorrow
Occasionally, completely hairless foxes have been recovered and several were photographed recently in the USA. In May 2004 a gentleman in Asheboro, North Carolina, photographed what he initially thought was an escaped dingo feeding on corn in his backyard, and a similar animal was seen several times in the Piedmont, on the east coast during the same year. Subsequently, in 2006, another hairless animal was photographed in the Piedmont, and one hit by a car and killed in Charleston, South Carolina – it is suggested that both were Samson Grey foxes. The Charleston animal was examined by Jaap Hillenius, a biologist at the College of Charleston, who found that it lacked hair follicles in its skin and so was apparently incapable of growing hair. A similar-looking animal was photographed foraging in a field in Raleigh, North Carolina during 2006 by Jerri Durazo. Ms Durazo’s photo was sent to North Carolina Wildlife Resources Commission biologist Perry Summer, who suggested it was a Samson fox (misspelt “Sampson” in the National Geographic article on the subject). Since these cases, there have been several reports of hairless foxes, including one bald Red fox that made CNN news in September 2009 when it was filmed stealing golf balls from a garden in Steamboat Springs, Colorado. More recently, during December 2011, a Samson fox turned up in the backyard of North Carolina resident Staci Wood. The animal was caught on her trailcam (a remote camera set to fire when something crosses its sensor beam) and she has kindly let me publish her pictures here.  Her observations are interesting: this individual appears to be a ‘loner’, which would tie in with the theory that they are competitively displaced by ‘normal’ animals. (Photo: Three variants of the Red fox are represented by these skins, from left to right: Silver fox; Cross fox; and Samson fox. Photo reproduced, with permission, from Steve Allen's article 1974 on Samson foxes in North Dakota Outdoors.)

Interestingly, it seems that, in some of the above cases, the definition of a Samson fox is being used more broadly than either Lopio or Oksala considered it. As we have seen, according to these authors (and most subsequent ones) Samson foxes aren’t bald; they simply lack guard hairs. Given that several of the recent examples lacked fur altogether, it seems there is a separate character being expressed here – especially if it is preventing the formation of hair follicles in the skin, which would make it impossible for the animal to grow fur at a later stage. On one Internet message board, a hunter claimed to have regularly trapped and shot Samsons (this board represents the first instance where I have seen Samsons referred to as ‘Cotton foxes’) in South Carolina, and noted that you can’t normally tell it’s a Samson until you get close (although all his were a striking yellow colour); this ties in with accounts from Oksala and others.  Indeed, Lopio noted that some foxes presented with very little underfur, or lacked it completely, but considered this to be a separate condition.  Consequently, to avoid potential confusion, I think it is more prudent to consider hairless animals to be alopecic foxes, rather than Samsons.

It is also worth mentioning that there are various disorders than can cause temporary hair loss, including malnutrition and severe cases of mange – in both cases, fur growth returns to normal once the animal has recovered.

Finally, a study of Red foxes on the Japanese island of Hokkaido during the late 1980s found that an inflammation of the dermis (top layer of the skin) can lead to the death of hair follicles and cause the hair to fall out; this is a condition referred to as hypotrichosis.  In a fascinating paper to the journal Polar Biology during 2007, University of Iceland biologist Pall Hersteinsson and his colleagues presented the results from their detailed study of hypertrichosis in Icelandic Arctic foxes. Hersteinsson and his co-workers found that hypotrichotic foxes presented with scarce or absent guard hairs and very thin underfur; in severe cases, the animals were almost hairless. These foxes shared other features in common with Samson animals, including a voracious appetite in winter (presumably a reflection of the loss of insulation and associated high metabolism), slower growth of cubs, lower survival rates of cubs and adults, and ill temperament in captivity. It seems that this condition has been known in Icelandic foxes since at least the 1930s; the earliest written record, from 1932, described cubs were born without fur (or lost their fur soon after birth) and suffering from swollen joints.  Hersteinsson and his team found that the hypotrichotic cubs they caught in the wild and reared in captivity grew much more slowly than, and never attained the same adult weight as, normal cubs.  Interestingly, Hersteinsson and his colleagues also observed that the hypotrichotic vixens were more fecund (i.e. reproductively productive) than normal vixens (females), while hypotrichotic dogs (males) were less likely to breed than normal ones. Both sexes were more affected by winter air temperature (doing better in milder winters) than normal animals, which is expected given their lack of fur.  Hersteinsson and his team found that the skin of hypotrichotic animals was inflamed and the hair follicles had started to die and, although they were unable to find the cause of the condition (i.e. a bacteria, fungi etc.), they suggested that it is the work of an infectious agent transmitted from mother to cub.

So, in conclusion, Samson foxes are those animals suffering from a heritable recessive genetic condition that can potentially develop and regress as the animal moults. Samson animals differ from normal animals by the lack of guard hairs and from alopecic animals by the presence of underfur. Given that the loss of fur appears to impose physiological costs (higher metabolic rate to compensate for heat loss, for example), it seems probable that such animals could increase in abundance as winters get milder in accordance with climate change. (Back to Menu)

Q: Is the North American Red fox a different species to the Eurasian Red fox?

North American Red FoxShort Answer: Most foxes in lowland North America are the same species as those in Eurasia, thanks in part to imports by early settlers, but there do appear to be some indigenous animals still surviving, although it is unclear whether these can be considered distinct species. Some early authors considered the common North American Red fox a different species to that found throughout Europe and Asia (Vulpes fulva and Vulpes vulpes, respectively). A comparison of the pelts and skulls of foxes collected from across their range (New and Old World) found a gradual change (cline) between foxes in Europe and those in America, concluding that there was no justification for separating the two as distinct species. Genetic surveys have shown two main groups in North America: foxes in Alaska, western Canada and most of lowland USA fit nicely with those in Europe and Asia, while those of central and eastern Canada and the western mountains of the USA form a group unique to the continent. Ultimately, high elevations (alpine meadows and parklands) have native fox populations, while low elevation habitats (agricultural lands, urban areas, rangeland etc.) have non-native populations. These populations are typically considered different subspecies of the same Red fox species (Vulpes vulpes).

The Details: Early naturalists described several species of Red fox from North America based on differences in size, colour, skull dimensions and geographical range. One of these species was described by Anselm Desmarest in 1820, based on a specimen from Virginia; he named this animal Canis fulvus.  With the rearrangement of the Canidae and grouping of foxes together in the genus Vulpes (see Natural History of the Red Fox), this species was re-named as Vulpes fulvus (photo, right). At some point, this binomen was changed to Vulpes fulva, presumably because Canis is a masculine noun, while Vulpes is feminine (fulvus is masculine and fulva feminine, so fulva belongs with Vulpes). These semantics matter to scholars, but to most of us they are of no consequence and the point is made here solely because both Vulpes fulvus and Vulpes fulva are widely found in the literature and both are referenced in this article.  The word fulvus pertains to the animal’s colouration; meaning ‘yellow’ or ‘tawny’ in Latin.

By the end of the eighteenth century, however, most North American foxes had been synonymised with (considered to be ‘types’ of) the widespread Vulpes fulva. Despite this, some authors continued to publish descriptions of new fox species from the continent and, in a short paper to the Proceedings of the Biological Society of Washington during 1897, Outram Bangs described a specimen from Nova Scotia that he named Vulpes pennsylvanica, and compared it to the European Red fox, listing seven features with which “these wholly distinct animals can always be distinguished”. These differences included differences in colouration and skull characteristics (namely the European foxes had larger, heavier skulls with a deep constriction between the eyes and a wide palate, compared with the Nova Scotia animals). The topic resurfaced three years later when, in March 1900, C. Hart Merriam published his revision of North American Red foxes, in which he described 10 species and two subspecies, including V. fulvus. Overall, based on their studies of various hunter-donated and museum-held specimens, Bangs and Hart Merriam argued that the North American fox was distinct from its Eurasian counterpart; being smaller, with a shorter and smaller tail, paler in colour with a rusty face, with more profuse blackening of the feet, longer fur and several skull and dental differences.

Not all naturalists saw things in the same light as Bangs or Hart Merriam and several subsequent authors have argued that the North American fox is conspecific with (i.e. the same species as) the Eurasian Red fox and the two should not be separated. The case for separation is not helped by Desmarest’s original description of fulvus; it transpires that he appears to actually have been comparing the skull of a European Red fox to that of a North American Grey fox (Urocyon cinereoargenteus), which are not closely related.

In a paper to the Journal of Mammalogy during 1959, Canada-based taxonomist Charles Churcher compared the skulls and skins of foxes from North America to those from other populations in Europe and Asia (including Austria, England, France, Ireland, Scotland, Spain, Assam, India, Siberia, Iran, and China). In total, Churcher compared 323 skulls and 89 skins and, although he did record some noticeable differences (including many of those alluded to by Bangs and Merriam), none were sufficient for designating them a species in their own right; he concluded that:

“… the North American, Asian and European red foxes be considered to be one species, all presently described forms being subspecies of this single species. This form must be known as Vulpes vulpes Linn. and Vulpes fulva (Desmarest) must be considered to be a synonym.”

British Red foxChurcher’s findings laid the matter to rest for many biologists, who took to referring to North American foxes as the subspecies Vulpes vulpes fulva (or not differentiating them from European animals at all) and this idea reigned for many years, even though morphological studies continued to register differences between the populations.  In a 1995 paper to Annales Zoologici Fennici, Università di Siena biologist Paolo Cavallini used various measurements to compare foxes from the Northern Hemisphere. Cavallini found that the animals clustered into three groups: North American foxes; British foxes; and central European foxes. British foxes being more similar to (but still distinct from) European animals than they were to those in North America. Cavallini explained:

“… foxes from North America are comparatively light, rather long for their mass and with a high sexual dimorphism.”

Some early authors had argued that the difference in appearance between Eurasian and North American foxes was probably a result of their adaptation to a new habitat in the 200 years since their introduction from Britain and Europe by early settlers. Cavallini, however, found that the animals from Australia, which were introduced from England and Wales about 160 years ago and now occupy very different habitats to their descendents, were still clearly grouped with their British ancestors. Consequently, Cavallini suggested that the differences in morphology that separate these groups reflects their phylogeny (i.e. how closely related the groups are), rather than the ecological niche they fill. Cavallini’s dataset was relatively small (comprising only 20 populations), but it certainly implies that a division of British and European animals may be warranted and it would be very interesting to see whether a larger study would yielded the same results.

More recently the debate was reignited as some biologists questioned Churcher’s study materials, pointing out that at least three-quarters of his North American specimens were collected from regions where non-native foxes had been established for at least a decade.  The problem is that humans have introduced foxes on to the continent on several occasions. Indeed, some early authors quote the Native American people as saying that the Red fox was not in North America before the arrival of European settlers, suggesting that the species was scare or absent from much of the continent after the last Ice Age. Churcher suggested that, based on historical accounts, the fox was native to North America north of about 45oN latitude and very scarce, or absent, from the unbroken mixed hardwood forests to the south.

Fossil evidence tells us that Red foxes originated in the New World, spreading out to colonise the Old World; the oldest Red fox remains from the continent were uncovered in central Alaska and date from the Pliestocene (160,000 to 130,000 years ago). It does seem that the foxes of North America declined as the species spread across Eurasia. Recent genetic work by Keith Aubry and his colleagues at the Pacific Northwest Research Station in Washington, however, has revealed new information on the spread of the Red fox in North America. It seems that foxes colonised Alaska from Alaska, presumably by crossing the Bering Land Bridge at some point before it closed about 130,000 years ago, and from here they expanded southwards across Canada and into the USA. The population was then split in half by the formation of a large continental ice sheet during the Wisconsin Glaciation (from 100,000 until about 10,000 years ago); this created two isolated populations (or refugia). In a fascinating paper to the journal Molecular Ecology during 2009, Aubrey and his co-workers described how the foxes in Alaska and western Canada can be grouped together with those from Eurasia to form a Holarctic clade, while those in southern and eastern states of the USA form a second, genetically-distinct, Nearctic clade containing foxes unique to North America.   So, the ice sheet led to the creation of a Holarctic clade to the north of the ice sheet (in the unglaciated parts of Alaska and the Yukon) and the Nearctic clade across much of the present-day USA. Furthermore, at some point while the ice sheet was in place a second wave of colonisation by foxes from Eurasia was happening in Alaska and the Nearctic clade split into three groups (subclades), with eastern and western foxes separated by a cold, sandy, treeless desert.

When the ice retreated about 10,000 years ago, Aubry and his colleagues suggest, the Holarctic clade spread south and east, while the Nearctic clade spread north, the two meeting in central Canada.  The eastern subclade appears to have followed the boreal forests north and colonised eastern and central Canada, while those of the mountain subclade colonised the alpine meadows and subalpine parklands of high mountains. The result is that the foxes in present-day North America are divided into three subclades; two were relatively recent divisions restricted to the southwestern mountains (mountain subclade) and eastern states (eastern subclade), while the third (widespread subclade) was older and more widespread in North America, probably representing the original nearctic population. The mountain clade, it seems, is actually made up of four geographically-restricted subspecies. Three subspecies are restricted to high-elevation montane and alpine habitats of the Cascades (V.v. canadensis), the Sierra Nevadas (V.v. necator), and the central Rocky Mountains (V.v. macroura). The fourth was recently identified by Benjamin Sacks at the University of California Davis and colleagues; the Sacramento Valley fox (V.v. patwin) is closely allied to the mountain clade, but restricted to the lowland grasslands of California.

Red fox distribution in USA
The approximate distribution of the Holarctic (red), Eastern (blue) and Mountain (yellow) subclades of the Red fox in North America. The delineations aren't quite as clear-cut as they appear here because, as you might imagine, foxes don't recognise state or federal boundaries and there is mixing. The foxes allied to the widespread clade are found throughout most of the continent, mixed in with animals from other clades; they make up about one-quarter of animals studied in the south west and around one-third of those in the north east. The proportion of foxes taxonomically linked with each subclade does, nonetheless, increase as you move away from the centre of the continent. In other words, as you move east from Ontario to Qubec, the number of eastern clade animals tends to increase such that there are very few holarctic or widespread clade members in along the eastern seaboard. Map drawn based on data presented by Keith Aubry and colleagues in their 2009 Molecular Ecology paper. Map produced by World Atlas / Graphic Maps and reproduced here with permission.

It seems that the mountain foxes are better adapted to the cold, harsh conditions at high altitude and do not mix with foxes from low elevations. Among the ‘mountain’ foxes, there appears to be a critical altitude -- about 2,100m (6,900ft) -- that separates them from their lowland counterparts. In Yellowstone, genetic data collected by biologist Bob Fuhrmann have revealed two genetically distinct populations (there’s almost no gene flow between populations, meaning highlanders don’t breed with lowlanders) separated by a line that can be drawn at 2,100m. Fuhrmann and his team found some morphological differences between the populations: high altitude foxes had larger bodies, smaller ears, and larger hind feet, with smaller toe pads that were more completely furred than foxes in low altitude populations. The lowland foxes are the same as those widely distributed across lowland North America and are probably hybrids.

European settlers started releasing foxes on the eastern seaboard (testifying to the apparent lack of native animals) for hunting purposes around 1750, and these spread out and appear to have interbred with indigenous populations (although, judging by the data presented by Sacks and his colleagues, perhaps less than originally believed).  Introduced animals were brought over from Britain and France (often considered V.v. crucigera) and Scandinavia (V.v. vulpes).  Consequently, many authors consider that the common North American Red fox (originally Vulpes fulva) is actually a hybrid of crucigera, vulpes and any indigenous subspecies that weren’t displaced by the non-natives.  The reason why these introduced lowland foxes didn’t (and still don’t) interbreed with high-elevation isn’t fully understood, but it seems that the may be poorly adapted to the conditions at high altitude.  In a recent interview to the Pacific Northwest Research Station’s journal, Science Findings, Keith Aubry notes that non-native lowland foxes living on the Olympic Peninsula haven’t colonised the high Olympics, where native foxes are found. If low-elevation foxes were capable of inhabiting high elevation habitats, Aubry considers they’d have done so by now, pointing out that:

Stanislaus foxThere are no obvious barriers to colonization – in fact, there’s a paved road right up to the alpine zone on the north side of the peninsula…

So, in the Olympic Mountain Range, it appears that some physical or physiological adaptation has led to highland animals being well adapted to the harsh conditions found at high altitudes that makes the habitat unsuitable for lowland animals. Elsewhere, however, there may be physical barriers of unsuitable habitat separating high- and low-elevation populations. In the Cascades, for example, earlier work by Aubry revealed that the thick west-side forests and sparse east-side sagebrush deserts seemed to act as barriers to up- or down-hill migration, preventing the high and low altitude populations from mixing. (Photo: An elusive mountain fox in the Humboldt-Toiyabe National Forest, Nevada in 2010. Photo courtsey of Sherri Lisius, US Forestry Service.)

Where does all of this leave us? In the end, the answer to the question of whether the North American Red fox is a different species is no. The point to remember is that the argument is not whether foxes from North America look different to those in Europe (most people who see the two side-by-side would be struck by the longer legs, longer fur and lighter colouration of the former), but whether these differences are sufficient to warrant them being considered separate species. In biological terms this really boils down to genetic isolation – i.e. can North American foxes interbreed successfully with European ones? Granted, this is not the whole story, because defining exactly what a species is can be a substantial challenge in itself (see How We Classify Organisms, for more info), but it represents a common starting point. Genetic data tell us that there appear to be some native animals -- which seem better adapted to harsh mountainous environments than the non-native animals (this is invariably part of what’s kept them around) -- remaining within restricted ranges, but these are subspecies at best.  The work by Sachs and his co-workers in California, Aubry and his team in Washington, and Fuhrmann in Yellowstone suggests that there is very little (if any) hybridization between the native and non-native populations, so there is the potential that these could eventually evolve into separate species (genetic isolation is a well known method by which species diverge). Nonetheless, considering that the common Red fox population of the lowland US was swelled by releases and fur farm escapees for at least 250 years following European settlement, it seems reasonable to consider North American foxes Vulpes vulpes, the same as those in Europe, Asia and Australia, with an acknowledgement that there are native and non-native subspecies present. (Back to Menu)

Q: Is the British Red fox a different subspecies to that found in northern and western Europe and Asia and how many ‘breeds’ of fox are there in the UK?

Short Answer: The Red fox found in central and southern Europe (from Ireland east to Greece) was described as Vulpes vulpes crucigera in the late 16th Century based on a specimen from Germany. The animal was smaller, with different teeth and a yellower coat than that described three decades earlier from Sweden by Linnaeus. The validity of this subspecies, indeed most subspecies, has been challenged and the current thinking is that the continuity of the Red fox’s range across the Northern Hemisphere makes it unlikely that it’s possible to distinguish individual subspecies. Indeed, importation of foxes from Europe and movements of foxes within the UK in recent history have probably served to mix and dilute any traits unique to British foxes. Most biologists consequently do not differentiate British foxes from those on the Continent. There are, nonetheless, genetic data from Europe suggesting that some grouping of foxes is warranted but it is unclear how widely this can be applied.

Turning to breeds, early authors suggested that as many as four different ‘types’ of foxes inhabited Britain; from the large, long-legged animals living in the Scottish highlands, to the small ‘cowardly’ foxes that ‘lurk’ around urban settlements. From these accounts it certainly seems that a separation of highland and lowland foxes was apparent, although it is unknown whether any such split is still applicable. Some have suggested that the highland (or Greyhound) fox is descended from Scandinavian foxes, which were imported for sport, and skeleton analysis certainly suggests it is certainly closer in size to those than to central European specimens.  That said, it has been argued that other factors (namely differences in habitat and diet) could equally explain the differences between highland and lowland animals. There is certainly a basic body plan that tends to appear more commonly (although not exclusively) in upland areas, but there are too few data to establish whether these can be considered a breed unto themselves, distinct from the foxes of the lowlands.

Cross foxThe Details: Within a species, some populations get separated from their neighbours by landscape obstacles (e.g. rivers, mountain ranges etc.) that prevent the two mixing, and mean any changes that evolve within the one population and would normally be inherited -- bigger skulls, larger teeth, longer fur etc. -- become restricted within the population, rather than spreading out through neighbouring groups.  The result is that these genetically isolated populations start to look, and sometimes behave, differently to their neighbours.  Carrying this on to its conclusion, they become completely reproductively isolated, which means that even given the opportunity they couldn’t breed successfully with their neighbours because the two groups are now too different; they’re now considered separate species under what we call the Biological Species Concept. Before things get that far (and there’s no guarantee they will), there is a period when the two groups could interbreed to produce viable young, if the chance came (i.e. if the barrier was removed); these populations are considered subspecies. (Photo: A Cross fox, showing dark stripe down the back and across the shoulders.)

In essence, therefore, a subspecies is a population of animals that can, based on their appearance and/or behaviour, be separated from the ‘type’ specimen (i.e. the first one ever described) and are separated from neighbouring populations only by a geographical barrier that, if removed, would lead to the mixing of the two populations.  Some authors have argued that a subspecies is designated purely on the basis of geographical separation, and that it should have nothing to do with how different the two groups look to the human observer.  The problem with this stance is that the number of subspecies spirals rapidly out of hand; foxes on the Isle of Wight (or any other island off our coast) would, for example, qualify as a subspecies distinct from those in mainland Britain. Nowadays we have genetic tests that can give us an indication of how closely related two groups of a species are and this has helped elaborate subspecies classification.  The problem for many early naturalists was that they were working solely on anatomy and physiology and this causes controversy over what constitutes a suitable taxonomic feature and, hence, when you drill-down to any grouping below species the debate is almost eternal.  For more information, see: How We Classify Organisms. So, what does this have to do with Britain’s foxes?

In 1789, German naturalist Johann Matthäus Bechstein described a Red fox caught in the central German state of Thüringen. This specimen was smaller than those typically found in Sweden, with smaller teeth, more widely space premolars that are seldom, if ever, in contact, and a coat bright yellowish or reddish in colour. This specimen also appears to have had the characteristic dark line running along the length of the back and across the shoulders that makes a ‘cross fox’ (see: Colour section of Main Red fox article), because Bechstein named it Vulpes vulpes crucigera (crucigera is Latin for "cross-bearer").  In his 1912 Catalogue of the Mammals of Western Europe, Gerrit Miller recognised this as the subspecies found in central and southern Europe from Ireland east to Greece. Overall, Miller considered this ‘central subspecies’ to be one of three in Europe; the other two being the ‘northern subspecies’ vulpes (i.e. the one first described by Linnaeus) of the Scandinavian Peninsula and the ‘southern subspecies’ silacea from the Iberian peninsula.

Not everybody shared Bechstein and Miller’s view that British and central European foxes could be separated from those elsewhere on the continent and, in 1931, in the second volume of his Mammals of Eastern Europe and Northern Asia, Russian zoologist Sergi Ognev argued that tooth size and premolar spacing are both very variable characteristics and concluded that Miller’s data don’t support the assertion that the vulpes subspecies is larger than crucigera. Indeed, in his 1980 opus Red Fox, former Ministry of Agriculture biologist Huw Gwyn Lloyd noted that the terrific variation among European Red foxes makes decisions to lump any together dubious. More recently, Stephen Harris at Bristol University has pointed out that the high levels of dental impaction (teeth packed close together in the skull) among British foxes make it difficult to positively identify them as crucigera.

In a 1995 paper to Annales Zoologici Fennici, Università di Siena biologist Paolo Cavallini used various measurements to compare foxes from various parts of their range. Cavallini found that the animals clustered into three groups: North American foxes; British foxes; and central European foxes. British foxes were more similar to (but still distinct from) European animals than they were to those in North America. Some authors have argued that the difference between Eurasian and North American foxes was a result of their adaptation to the habitat in the 200 years since their introduction from Britain and Europe by early settlers.  Cavallini, however, found that the animals from Australia, which were introduced from England and Wales about 160 years ago and now occupy very different habitats to their descendents, were still clearly grouped with their British ancestors by their morphology.  Consequently, Cavallini suggests that the differences in morphology that separate these groups reflects their phylogeny (i.e. how closely related the groups are), rather than the ecological niche they fill.  Unfortunately, Cavallini’s dataset was relatively small (comprising only 20 populations), but it certainly implies that a division of British and European animals may be warranted and it would be very interesting to see whether a larger study would yield the same results.  Although very common, studying morphology is not the only tool biologists have when it comes to deciding whether populations should be considered subspecies.

Vulpes vulpes schrenckiRecent genetic work outside of Britain has suggested that some grouping of foxes at the subspecies level is warranted. In a 1998 paper to the Journal of Zoology, a team of biologists led by Sandro Lovari at the University of Siena in Italy reported on the genetic structure of foxes in the Mediterranean Basin. The biologists analysed special enzymes and DNA from foxes collected from Spain, Italy, Austria, Bulgaria and Israel and found four main groupings: Spanish foxes grouped together; Austrian foxes grouped with those in mainland Italy; Bulgarian foxes were closely allied with those on Sardinia and Sicily (the latter two having been introduced by humans); and Israeli foxes formed a group of their own.  Overall, the study found remarkably little inter-mixing of the groups, suggesting that they were “genetically fairly isolated from one another”. Each group would potentially qualify as a subspecies, although the authors don’t propose it. A similar study on foxes from across Denmark between 1997 and 1999 found that animals from Copenhagen differed significantly, in terms of their enzymes and craniometry (skull measurements), to those in all other regions, suggesting that these urban foxes formed an isolated population, with little gene flow (inter-breeding) with other Danish populations. In this case, the researchers suggest that differences in behaviour between urban and rural foxes may account for this isolation, because geographic separation didn’t appear to be the cause. Finally, a study published in Zoological Science during 2007 reported that foxes from the Japanese islands of Honshu, Shikoku and Kyishu grouped together with those from Europe and East Asia, while those from Japan’s northern island, Hokkaido, were genetically distinct and had their own genetic background; Hokkaido’s foxes are considered to be the subspecies schrencki (Photo, right).

None of the foregoing studies helps us untangle the ancestry and ‘mixing potential’ of Britain’s foxes, but they do illustrate how genetic data can be invaluable in demonstrating where grouping may be warranted and how it’s not unreasonable to expect that foxes in the UK might be genetically isolated from those in much of Eurasia.  Nonetheless, despite the subspecies crucigera commonly cropping up in the literature to denote foxes from Britain and central Europe, without any genetic studies from Britain, the debate goes on.

Humans, as a species, have an inherent desire to produce order from chaos and this invariably explains our tendency to want to group things, be they underwear in your bedroom or foxes that look similar across different continents. The variation that we see in nature, however, can be on a far more localised scale and, even to the exclusion of any differences between fox populations in Britain and Europe or North America, many early naturalists recognised differences within the British population; they spotted that, much like the situation in North America, in many areas our highland foxes were different to our lowland ones.

Greyhound foxO ye'll tak' the high fox and I'll tak' the low fox
In his 1816 A History of the Earth and Animated Nature, poet-turned-naturalist Oliver Goldsmith wrote of how there were three ‘types’ of fox in Britain: the largest, tallest and boldest Greyhound fox (pictured left in an engraving in Colonel Talbot's 1906 Foxes at Home) that would “attack a full grown sheep”; the small, but strongly build Mastiff fox; and the smaller still and least common Cur fox, which “lurks about hedges and out houses”. Goldsmith’s view was popular among later writers, although not all agreed on the names.  John Sherer in his book Rural Life, published in 1860, for example, reckoned Britain was home to the Greyhound, Common and Little Red foxes.  In 1950, Oliver Pike went one further and described four races of fox from Britain: Lowland foxes; the smaller Welsh mountain foxes; small Terriers in northern England and southern Scotland; and Mountain foxes in Scotland. Pike alluded to these types having evolved in response to the harshness of the environment in which they lived and, in his Wild Animals of Britain, he wrote:

If a fox from an English county was transferred to the wild mountains of Scotland, I doubt very much if he would survive. The Scottish mountain fox is a larger and more powerful animal, and able to attack prey as large and even larger than himself. The long and arduous distances he must travel to find food provides him with powerful and well-formed muscles.”

Many naturalists were slightly more reserved, considering there to be only two types: the Highland (or Greyhound) and Lowland (Common or Terrier) fox. The nineteenth century Scottish naturalist and ornithologist William MacGillivray described the differences between the two types of fox in 1838:

The largest kind, or that which occurs in the Highland districts, has the fur of a stronger texture and of a greyer tint, there being a greater proportion of whitish hairs on the back and hind-quarters, while two or more inches of the end of the tail are white. The fox of the lower districts is considerably smaller, more slender, of a lighter red, with the tail also white at the end. … The skull of the Highland fox appears remarkably large and strong beside that of the ordinary kind, and the breadth is much greater in proportion.”

In his 1904 Mammals of Great Britain and Ireland, John Guille Millais agreed with the separation of lowland and highland foxes, although he placed a greater emphasis on colouration, writing:

It may, however, be taken as a broad rule that the large dark and grey forms are found inhabiting the mountains, whilst the smaller red and pale types frequent the valleys and plains.”

 In 1941, the late Bristol Museum zoologist H. Tetley published a short paper to the Proceedings of the Zoological Society of London in which he looked at museum collections of Red foxes and compared those from Scotland with those from central Europe, based on skull dimensions, skeleton size, teeth arrangement, coat colour etc. Tetley concluded that Scottish foxes were considerably larger than those from central Europe; their measurements more closely matched animals from Scandinavia. Tetley explained:

On the whole, therefore, I cannot see that there is any distinction between the Scandinavian and the Scottish Fox as represented by specimens from the Scottish Highlands, and I consider that both are Vulpes v. vulpes.”

L. Harrison Matthews, in his 1952 British Mammals, agreed with Tetley in separating the highland and lowland foxes, and a study comparing 87 English and Scottish foxes that was published in 1956 by Ivan Hattingh also supported Tetley’s findings, concluding that the Scottish fox was a distinct race, although he failed to find any significant differences between highland (Westmorland) and lowland foxes in England.  In his 1968 book Wild Fox, Roger Burrows also agreed, arguing that -- based on body and skull measurements -- the Scottish fox is conspecific with (the same species as) the Scandinavian fox. Thus, these authors considered the Scottish fox to be Vulpes vulpes vulpes, while the English fox was Vulpes vulpes crucigera. These studies haven’t, unfortunately, laid the matter to rest for many naturalists, because there are many early hunting reports from highland areas of England (e.g. the Cumbrian Fells) that clearly describe ‘long legged’ Greyhound foxes leading hounds on exceptionally long runs, seeming to have almost endless stamina compared with their smaller, shorter legged lowland cousins. Of course, many examples of very long chases (three days being the longest I’ve come across!) probably involve several foxes; a weary fox running into undergrowth and ‘putting up’ one resting there. In his Town Fox, Country Fox, Brian Vezey-Fitzgerald points out that:

Hounds will always follow the fresher line. And you can always tell when they change to a fresher line by the change in their cry.

Red fox sittingVezey-Fitzgerald suggests that the hunted fox may ‘know’ where neighbouring foxes tend to lie up and may head for that location in order to shake its pursuers. There is, to my knowledge, no proof of this, but it doesn’t seem entirely unreasonable and (deliberate or not) it would certainly explain some of the exceptional runs described by hunters. This doesn’t, however, explain the morphological differences the hunters describe. My friend and native ‘Lakelander’ Ron Black has researched (indeed, continues to research) highland fell foxes in some detail and has suggested to me that, if one unites Scottish and English highland foxes, the Greyhound fox may have been the ‘original British fox’, until animals from the continent were imported during the mid-19th Century for sport. Foxes were certainly imported to Britain from Europe (and translocated within Britain) as numbers declined following heavy persecution and severe outbreaks of sarcoptic mange. Indeed, Vezey-Fitzgerald notes that foxes were imported from continental Europe during the mid-1800s and released into the countryside at a rate of more than one thousand per year and animals were translocated from other parts of Britain to Somerset and Devon as late as the 1920s following a serious mange epidemic. Similarly, in his excellent book Running with the Fox, Oxford University zoologist David Macdonald wrote:

Where [fox] numbers ran short foxes were bought and released (such ‘bagged’ foxes sold for 10 shillings at the Leadenhall Market [in London] in 1845) and included a brisk trade in imports from the Continent.”

Based on the descriptions of early Cumbrian hunters it seems almost without doubt that the foxes they hunted were larger and faster than those we see across most of Britain today. Unfortunately, there is little evidence to suggest that these animals still survive in the Fells. In my experience (personal, photographic and video recordings), I have seen foxes that were large, slim, greyish in colour and with proportionally long legs and pelts in several lowland destinations; these seem to meet the basic criteria for ‘highlanders’. It is not, however, difficult to see how such a separation could occur. Nonetheless, I remain to be convinced that there is now any real distinction between highland and lowland foxes and there are certainly no empirical data supporting any groupings other than Scottish Highland animals as a distinct race. 

To my mind, it seems possible that the foxes which originally recolonised Britain following the retreat of the ice sheets at the end of the last Ice Age probably spread widely throughout the country.  Those inhabiting highland regions could subsequently have evolved to suit the terrain, with a larger body size and longer legs offering advantages (reduced heat loss, lower metabolic rate, ability to take larger prey etc.) in cold, wet and snowy environments. Those in lowland regions, however, probably remained close in appearance to their European ancestors. Any unique traits evolved by British lowland foxes could easily be diluted by interbreeding with imported animals. Adaptation of highland and lowland forms could have reached the point where the lowlanders were simply unable to compete effectively with the highlanders for high altitude habitats, while the highlanders were easily out-competed at lower altitudes by the lowlanders – the result being a separation, ultimately genetic isolation, of the populations. This is, of course, speculative and, while it works in theory, it remains to be supported by any genetic evidence. (For a detailed summary of the hill hunting literature pertaining to Greyhound foxes, the reader is directed to Ron Black’s article The Lost Foxes of Lakeland – and other places besides).

I suspect that the argument will never be fully resolved, and most recent authors have adopted the view of Gordon Corbet who, in his 1978 book The Mammals of the Palaearctic Region, concluded that the continuity of the Red fox’s range is so great that it’s doubtful any discrete subspecies can be recognised. There are some genetic data from foxes in the Mediterranean that suggest distinct groups can be made and that there may be a case for assigning some subspecies, but the data simply don’t exist for a sufficiently large geographical area to be certain whether similar groupings can be applied to other Eurasian populations. Consequently, the majority of biologists now consider that Vulpes vulpes is just a highly variable species that ranges throughout Europe and Asia and do not attempt to categorize it further. (Back to Menu)

Red foxQ: Are urban foxes getting bigger?

Short Answer: Possibly, although direct evidence is lacking. A large fox killed in Kent during 2010 sparked fears that urban foxes, fed on a mountain of scraps and bin waste, were getting bigger and ‘more vicious’. Many factors interact to control the size to which an animal grows; although they’re predominantly under genetic control, they can be significantly influenced by external factors, such as food availability and climate. It is certainly a plausible notion that, with a plentiful, high quality food supply, foxes living in urban areas could increase their body size, but there is no empirical evidence that they have done so.

The Details: The capture, in December 2010, of a large fox in a suburb of Kent prompted comments in the media about how urban foxes are growing larger; one headline read: “Foxes are getting bigger... and more deadly”. This article, written for the Daily Mail by a London-based pest controller and published in January 2011 made disturbing reading; statements were made about how foxes the size of the Kent specimen are becoming far more common since fox numbers apparently exploded, “as a direct result” of the rounding up of stray dogs by local councils. The Kent specimen was a dog fox weighing 12 kg (26.5 lbs) and measuring 123 cm (4 ft) from nose-to-tail.  The author of the Daily Mail article noted that, three months earlier, he had shot a 2 stone 3 lb (14 kg) animal in South London and, during his 36 year career, the largest he had seen was 2 st 7 lb (just under 16 kg). Unfortunately, the article made many claims that are either entirely inaccurate or for which there is simply no evidence.  It was, nonetheless, part of a larger ‘ground swell’ of public concern -- since the disturbing case of the Koupparis twins in June 2010 -- that urban foxes now pose a significant risk to people and their pets.  The debate was reignited in March 2012 when an even larger fox (weighing just over 17kg/38 lbs and measuring 145 cm/4ft 9in) was shot on a farm in Aberdeenshire. So, are such fears grounded and what is it that controls how big a fox grows?

Little or large?
Exactly what controls body size in mammals is a complicated question that biologists are only now starting to understand. Data from several species (including mice and humans) have shown that proteins and short chains of amino acids called peptides control the behaviour of cells growing in developing tissues – these molecules are collectively called Growth Factors. Many growth factors are hormones and, in essence, the pituitary gland (which sits at the base of the brain) is instructed to release these growth factors by ‘releaser factors’ secreted by the hypothalamus. Hormones are essentially chemical messengers that tell the tissues to put all their resources into making a certain protein; proteins being the building blocks of a body. One hormone in particular, Somatotropin, known more colloquially as Growth Hormone, plays a major part. Somatotropin is produced, stored and secreted by special cells, called somatotropes, in an area of the brain called the adenohypophysis (the front part of the pituitary gland). Somatotropin stimulates protein production on a ‘body-wide’ scale and, when released into the bloodstream, it takes one of two main pathways: it either acts directly on the body (increasing muscle mass, strengthening bones, promoting organ growth etc.), or it acts on the liver, causing the production of hormones called insulin-like growth factors (IGFs), which promote bone growth. Increases or shortages of Somatotropin can have profound effects on body size (extreme cases being gigantism and dwarfism, respectively), although much depends on when during the growth phase the hormones are altered; new-born mammals are largely insensitive, while puberty appears particularly important.

SomatotropinSo, Somatotropin (right) is an important hormone (one of several) when it comes to building a body and how much of it is flowing around an animal’s system is influenced by the environment, genetic make-up and nutrition. Building tissue is an energy-hungry process, so it makes sense that the amount of food available can ultimately influence final size and we see this in many species; malnourished individuals end up smaller than their well-fed counterparts. We also know that aberrations in the pituitary that impact hormone secretion (cancers, for example) can lead to developmental changes in the animal. The question, however, is why we don’t see much larger variations in size if nutrition is so important? The answer lies in the animal’s genetic blueprint, behaviour and physiology.

In 1945, experiments on salamanders found that two different genetic types of these amphibians (differing in the number of paired chromosome sets) grew to the same size by changing their cell number. Basically, the size of a given object is dependent on two factors: the size of the building blocks and the number of building blocks.  In this example, the cells of ‘polyploid’ salamanders are twice the size of those of ‘diploid’ salamanders and yet the two animals grow to the same size; the reason is that polyploid salamanders had half the number of cells of the diploid ones. Observations such as these led scientists to speculate that developing tissues ‘know’ how big they should grow. Several subsequent experiments have yielded similar results, with transplanted body organs growing to their normal size, even though they did so ‘out of context’.

The other aspect of growing larger is maintaining proportions, so that you don’t end up with a big pair of legs, but small head, for example. In October 1980, two German geneticists published a paper in the journal Nature in which they described 15 genes that were of fundamental importance in regulating the structure of a developing fruit-fly. These genes, which we now call Hox genes, set out the basic structure and segmentation of an organism, controlling what tissues develop into the head, the chest, the midriff etc. The Hox genes build ‘hox proteins’ that activate or deactivate other genes responsible for building the body. If you fiddle with the genes, you can change the structure of an organism; you can cause a fly’s antennae to develop into a leg, for example.

Since 1980, more genes that regulate growth have been discovered. A gene called c-Myc, for example, was recently found to regulate body size in mice by controlling the number of cells in a developing tissue; when the geneticists reduced the amount of c-Myc protein produced, the mice were much smaller than normal. The gene IGF-1 was also recently identified as a significant factor responsible for the great variation we see in dog breeds – a mutation of this one gene is largely responsible for the vast difference in size between the Chihuahua and the Great Dane. When you look at the tissues of the c-Myc mice and different dog breeds it becomes apparent that, in mammals, variation in body size is almost exclusively a result of variations in the number of cells.

So, it seems that genes and hormones combine to affect the overall size of an animal. Indeed, in a very interesting paper to the journal BioEssays during 2008, Michael Crickmore at New York’s Rockefeller University and Richard Mann at Columbia University’s Medical Center described how starvation in (or general disruption of) the signalling of the hormone insulin results in the development of smaller animals with smaller, although still properly proportioned, organs.  Insulin and Somatotropin fluctuate in relation to the amount of sugar in the blood. Crickmore and Mann suggest that nutrient signals (i.e. blood glucose) provide information to tissues about the overall size of the animal in which it’s growing, while selector genes (such as Hox and c-Myc) provide organ-specific information. In other words, the available food controls the overall size of the animal, while these selector genes control the relative sizes of the organs within the animals. There are several examples of how these ‘nutritional signals’ play a key role in the body size of wild mammals.

The North Atlantic Oscillation and Sub-Polar Gyre (two large circulations in the Atlantic Ocean that affect continental weather) both affect the food available to the Arctic fox (Vulpes lagopus) and changes in these circulations over time have been linked to changes in Arctic fox body size. Similarly, when Red deer (Cervus elaphus) were introduced to New Zealand, the abundance of food led to an increase in body size over their British ancestors; they returned to their normal size when food conditions deteriorated. In a fascinating paper to the journal Biological Reviews in 2011, Israeli biologists Yoram Yom-Tov and Eli Geffen discussed some of the recent changes in body size that have been documented in land animals.  In their paper, the authors presented a flowchart containing 15 inter-related factors that affect the final body size of an animal; these included celestial factors (movements of the Earth and Sun affect plant growth and weather patterns), air pressure, temperature (big animals suffer in hot conditions, while small ones suffer in the cold), precipitation, human influences (management, garbage etc.) and the availability of food. Yom-Tov and Geffen note that the quantity and quality of nutrition during growth is a key predictor of body size and the effects of this nutrition on skeletal growth carry over into adulthood.

Fox cub eatingSo, if the overall body size is controlled by nutrition, why aren’t all well-fed foxes giants? Well, part of the reason, I believe, lies in the narrow window during which nutrition makes a difference. We have already seen that, in many mammals, puberty is a key point when the body is most sensitive to growth factors circulating around the body – mammals aren’t very sensitive when first born, nor once they’ve finished their growth phase. In the case of foxes, the cubs are usually full grown by the time they’re finding food for themselves and so, even in urban areas, they’re largely dependent on what their parents bring back to the earth. During the phase of fastest growth, they don’t stray far from the earth and their hunting skills are too poor to catch more than the occasional insect. Even if a cub had access to unrestricted food around the clock, it cannot eat constantly. A cub is limited in the amount of food it can take in by various factors: stomach size (foxes have proportionally small stomachs compared to other dogs), the time taken to digest a meal, and digestive efficiency (most young mammals have relatively inefficient digestive systems) all influence how much nutriment an animal can obtain. On top of this, cubs are losing energy in the form of heat (they have a larger surface area to volume ratio than their parents and thus lose heat more rapidly), burning energy during their rambunctious play and sleeping a lot, which further reduces the time they can spend eating. Competition with litter mates also impacts the food available to each cub.

In the end, genes affect hormones and they, in turn, influence growth rates and patterns. Hormone secretion can be affected by the animal’s nutrition, but there are physiological and behavioural factors that serve to limit the amount of food an animal can eat and thus affect growth. Access to above average food, in reduced competition environments (being an only-infant, for example) can lead to above average sizes. The question is: is this happening to urban foxes in Britain?

They might be giants
We’ve seen how multiple factors interact to determine body size, but food (especially early in development) is an important component.  One of the reasons foxes thrive in our towns and cities is a direct result of human activity, both conscious and subconscious.  Sadly, many people are wasteful and untidy – few people can fail to have noticed the discarded fast food that litters many streets in our major towns and cities on a Friday and Saturday night, and I have woken up to find the remains of a kebab stuck to the windscreen of my car before now. The idea of eating our rubbish is repulsive to many, but to much urban wildlife, our waste represents a valuable source of nutrition. Moreover, this waste is so abundant that relatively little effort is required to obtain it; a fox has a higher net gain of calories eating the remains of a fried chicken meal dumped on its front door step than it does from finding, sneaking up on, chasing and catching a rabbit. The result is that more resources can be devoted to growth and more fat gets laid down.

A study conducted in Sweden during the late 1970s by Erik Lindstrom found that fat accumulation was related to the amount of food available to the fox and the amount of exercise the animal got.  Lindstrom looked at the fat deposits in 463 Red fox vixens shot during the autumn and winter between 1975 and 1979; he found that the number of wild berries available determined how much fat was laid down in the autumn and the depth of snow determined how much fat was lost during the winter. Snow is difficult to walk through and doing so burns a lot of calories; thus in harsh winters (with heavy snowfalls) foxes lost more fat than when there was little or no snow on the ground.  Obviously there are other factors at play (such as food being more difficult to locate in heavy snow), but it illustrates something most of us are now all too familiar with: lots of food and little exercise leads to larger fat deposits.

Scavenge isn’t the only food that foxes have access to in our parks and gardens: in many areas people actively feed their local foxes.   Indeed, the spreads I have seen some people lay out (sausages, eggs, cheese, chicken, even chocolate gateaux and roast dinner carcasses) gave me the impression that their foxes ate better than I do!  This supplemental feeding has three main impacts: it can increase the number of foxes in a given area, which has consequences for disturbance (see Q/A); it can make the foxes less wary of humans (see: Q/A); and it can potentially lead to an increase in weight as foxes get their entire nightly energy budget within only a couple of gardens, thereby getting less exercise. Moreover, some pet foods are high in energy, which means that a small amount can provide the animal with the equivalent number of calories that it might get from eating half-a-dozen small mammals, birds or insects; the difference is that the former is less filling that the latter, which means the fox is likely to eat more.  While it is true that most people are sensible in the rations they put out, there is a cumulative effect. The mammalogists at Bristol University found that, when the fox population was at its highest (during the early 1990s), there was at least 150-times more food put out by residents in the north-west of the city than the foxes actually needed. The study, led by Professor Stephen Harris, found a positive feedback loop: as the number of people putting out food increased, more fox sightings were reported and more food was put out, leading to each given patch being able to support more and more foxes.

Throughout their range foxes exhibit considerable variation in body size and weight, males typically being bigger than females and both sexes tending to be larger in the north than in the south of their range (in accordance with Bergmann’s Rule). Studies in Scotland, during the late 1960s and early 1970s, by the eminent naturalist Hugh Kolb found that foxes were smaller in the south of the country, where population density was highest; more foxes equates to less food per head and correspondingly smaller animals. Similar data were also found by Paolo Cavallini for foxes in central Italy, which are the smallest among the European foxes – in Pisa (in the south of the study area), as in Scotland, foxes are smaller and their population larger than in the north of the country. One of the conclusions of Cavallini’s study was that the “body size of the red fox may be variable even within a small area”. Cavallini considered that population density and phylogenetic distance (how closely-related the populations were) were the driving factors behind this variation in body size, although some researchers have found a correlation between fox size and food availability.

Foxes at food dump
Many people enjoy feeding their local foxes and, when this happens more food it often provided than is required by the visiting animals. If food alone controlled the size our foxes grow to, we could expect an increasing number of larger than average individuals. The fact that such large animals are rare, suggests more than simply food availability is at play.

In her 2008 children’s book, Foxes, Sally Morgan wrote:

Scientists have discovered that the urban fox is developing a different jaw to that of the rural fox as it scavenges for food rather than kills it.”

I’ve not been able to find any research that comes to this direct conclusion (indeed, many scientists don’t make a distinction between urban and rural foxes because the two intermix), but it stands to reason that an urban fox with deformed jaw could potentially eke-out a living, while the same animal living in the country would starve. I am not, however, aware of any apparent trend in jaw structure towards one that is better adapted to scavenging than hunting; current fox jaws seem well adapted to both, because most hunt and scavenge.  Indeed, it should also be remembered that, even in urban areas where scavenge makes up a considerable proportion of a fox’s diet, they still actively hunt small mammals (especially rodents such as mice, rats and squirrels), birds, insects and amphibians such that around 30% of the diet consists of ‘wild-caught’ prey (see: Diet). There are, however, data from researchers based in Europe, suggesting that human activities may be having an influence on fox skull size.

In a 2003 paper to the journal Evolutionary Ecology Research, Yoram Yom-Tov and Shlomith Yom-Tov at Tel Aviv University and Hans Baagøe at the University of Copenhagen presented their analysis of 272 fox and 308 badger skulls collected from Denmark between 1862 and 2000.  Yom-Tov and his colleagues found that, over the last 140 years, there has been an increase in three of the four characteristics they measured (some by almost 10%); these skull characteristics (zygomatic breadth, length of fourth upper premolar and canine diameter) are all associated with diet and body size, suggesting that foxes are now larger than they were in the previous century and capable of handling larger prey.  The increase in zygomatic breadth, which allows for the attachment of a larger jaw-closing muscle, was much more noticeable in foxes from Zealand (where there are more large farms and estates where gamebirds and Roe deer are abundant) than those of the Jutland Peninsula, suggesting that a better quality of diet may have driven the change.  The authors also point to an increase in road-kill and an increase in living standards in Denmark (and the corresponding increase in garbage) as providing a readily available source of food for foxes. A similar trend was found by Yom-Tov in Spanish foxes; analysis of 267 skulls held at the Natural History Museum in Madrid, showed that foxes from agricultural areas of Spain were significantly larger than those from non-agricultural areas. The authors suggested that increased food (from animal husbandry) was the cause of the increased body size.

So, these data imply that, in some areas, foxes have evolved larger skulls in response to the presence of anthropogenic food; gamebirds and Roe deer kids are larger than mice, voles and rabbits and therefore require larger jaws to tackle. It should be recognised, however, that this prey is still essentially wild (or at least free-ranging) and foxes are actively hunting it. The question is: are we seeing something similar in Britain?

The short answer is: nobody knows. I conducted a ‘vox pop’ of some pest control companies, asking if they kept records of the foxes they were called out to remove; the answer was no. The suggestion that urban foxes are getting bigger is, it seems, more of a ‘general feeling’ within the industry, rather than a statement based on any particular set of figures. Some of the controllers work closely with the Food and Environmental Research Agency (a branch of DEFRA), who collect the carcasses and examine them for any signs of parasitic Trichinella roundworms. FERA weigh each fox they collect before testing for the parasite, but they haven’t conducted any analyses on size, either across the country or over time, and thus could not comment on any trends.

Red foxWith the pest controllers and FERA out of the picture for data, I contacted several rescue and research organisations to ask whether they’d noticed any trends in fox size. Nobody at Oxford University’s WildCRU unit is currently working on these animals, so couldn’t help, but Bristol University’s Mammal Research Unit told me that they hadn’t seen an increase in size during the 50 years of their on-going study. Similarly, the Fox Project -- a charitable organisation who rescue and rehabilitate foxes from across the country -- said “average fox weights are the same as they’ve ever been”; they’ve handled more than 7,000 animals during their 16 years in operation and the bulk were around 4.5kg (10 lbs), with only one animal being much heavier, at just under 10kg (22 lbs). The picture was the same at Vale Wildlife Rescue in Gloucestershire, who celebrated their 26 year anniversary last June (2010); in that time, they have dealt with an average of two foxes per week (some 2,700 animals in all) and haven’t noticed a significant change in the size of the urban foxes in that time, although they do tend to find urban ones are slightly heavier than their rural counterparts.

Foxes aren’t legally considered vermin, which means that local councils aren’t obliged to control them, but most councils are responsible for removing foxes found dead in their jurisdiction owing to the potential public health risk; some have designated patrols that look for carcasses, while others rely on members of the public to report them. I contacted 55 local councils in England, Wales, Scotland and Ireland asking what happened to the carcasses they recovered and whether any information about them (length, weight, sex etc.) was recorded. The majority of councils that responded -- 30 at the time of writing -- sent the carcasses for incineration, although a few (notably in Scotland) sent them to landfill and Dublin city council sent theirs for ‘deep burial’. In no case did councils collect any data on the animals collected other than (in a few cases) the date and location. Ultimately, if there is evidence that larger than average foxes are being seen in urban areas more frequently, it exists only with a select few pest controllers.

Fox on loungerRed Fox (Spells Danger?)
So, there are no data to support claims that urban foxes are getting bigger; how about more ‘deadly’? Statements have been made in the press that people are now frequently bitten by foxes, but that most incidents go un-reported. It is true that there is no organisation responsible for maintaining records of animal bites, but without the bites being reported, and the details provided, we cannot draw conclusions. If these people are bitten while trying to hand-feed or touch/handle a fox, or having cornered an animal, then one cannot say that foxes are more dangerous than they were a decade ago. Foxes are not pets: they’re wild predators and should be treated as such. Instances where people have turned around to find a fox sitting on the sofa next to them invariably occur because someone has previously encouraged the behaviour. This may sound improbable, but I have seen photos of foxes lying on people’s sofas, eating off their kitchen floors and lying in front of the fire with their pet dog.

Foxes aren’t stupid animals and I don’t believe that they mistake one house for another, but if entering a house was beneficial (got them food) in one instance, the fox is unlikely to pass up similar opportunities if they arise. This is not, however, to say that the foxes are then always on the alert for an open door or window.  I suspect there has to be a set of specific triggers (e.g. a specific time, location, smell of food etc.) that initiate such exploratory behaviour. I say this because foxes learn very quickly what to persevere with and what to ignore. One animal that I have watched for several nights over the past month has sat in roughly the same spot on the lawn of a block of flats to await the food that’s thrown out of one of the windows. These flats have two blocks with four areas of grass arranged around the outside. The fox trots casually across each lawn, stopping periodically to scratch and sniff, but it only sits on this lawn and under this one window – it ignores the other seven windows bordering this lawn from which food could potentially come. This implies that the fox doesn’t just see a lawn and a window and assume it’s a good place to sit and wait for food – there are specific aspects of this particular lawn, this particular window and this particular time that cause the fox to sit and wait.

Putting out food for your local foxes invariably makes many of them less wary, especially if you stand around near them while they’re eating. The fact, however, that these animals are bolder than one might expect doesn’t necessarily mean that they’re also more dangerous or more likely to bite. The only concomitant factor that I can see is that a bolder fox allows a person to get closer to them (or vice versa) and that person may then attempt to touch, stroke or hand-feed the animal, which may in-turn result in a bite. The fact that an animal stands and looks at you when you yell at it may be infuriating, but it does not mean that the animal is aggressive.

So, in conclusion, an increase in food amount and/or quality (especially early in development) can have an influence on the size to which an animal grows, although there are limits imposed by genes and the physics of large body sizes. To the exclusion of the FERA study, it appears that there are no data being routinely collected on fox size or weight and as such no evidence is available to verify the claim that urban foxes are growing larger, so we are left with only anecdotal evidence. Indeed, the data that are available from long-term studies, such as that conducted by the University of Bristol, and the experience of wildlife charities suggests that there is no apparent trend towards larger foxes. Bolder does not necessarily mean more aggressive, but in the end we must accept that foxes are wild dogs, not pets, and we should treat them as such. (Back to Menu)

Q: What diseases and parasites do Red foxes carry?

Red fox groomingShort Answer: Foxes are known to harbour a range of different parasites, both internally and externally, including various species of intestinal worms, flukes, lungworm, heartworm, ticks, mites, fleas, protozoans, bacteria and fungi. Some of those of greatest concern owing to them being zoonoses (i.e. can be transferred to humans) include Toxocara canis (dog roundworm), Echinococcus multilocularis (hyatid worm) and Trichinella spiralis (muscle worm).  Toxocara is present in the UK, although we currently don’t have evidence suggesting that foxes are a significant link in the ‘infection chain’.  Echinococcus is currently considered by DEFRA to be absent from Britain, while the last confirmed case of Trichinella in a fox from Britain was in 1957. Angiostrongylus vasorum (canine heartworm) and Sarcoptes scabiei (mange mite) are also important parasites of foxes that can be passed to domestic dogs. In Britain, the average prevalence of the heartworm is 7% (although locally prevalence may reach almost 30%) and it is widely considered that slugs and snails are a more significant source of infection than foxes. Mange can be a significant problem and cause large-scale declines in the fox population; the mites can be transferred to domestic dogs, but infection is easily treated.

Possibly the pathogen of greatest concern is rabies, for which the Red fox is the major sylvatic (wildlife) carrier in Europe.  The virus is transferred through a bite and can be fatal to both humans and other animals (including foxes); large scale vaccination of foxes has served to control the spread of rabies in recent years, eradicating it altogether from parts of western Europe. Foxes are also capable of carrying bovine tuberculosis (although apparently aren’t infectious) and Weil’s disease, although they aren’t considered significant vectors for either disease.

The Details: Contrary to popular misconception, Britain’s fox population seems generally healthy, although there are still pockets of severe mange infection in parts of the country. In the International Fund for Animal Welfare’s 2006 report After the Hunt – The Future of Foxes in Britain, Stephen Harris and Phil Baker concluded:

Like most wild mammals, foxes carry a range of diseases.  There is no current evidence that these pose a significant disease risk to humans and/or domestic animals, although more monitoring is needed to determine the prevalence of current disease levels and assess their potential economic impact.”

Broadly-speaking, parasites can be divided into two groups: those that live inside the host (endoparasites) and those that live on the skin/fur of the host (ectoparasites). The most common endoparasites of foxes are worms and, in a paper published in the journal Parasitology Research during 2003, Valdmir Shimalov and a colleague at Brest State University in Belarus (eastern Europe) presented their analysis of 94 fox carcasses and 1,213 faecal samples collected from Southern Belarus between 1981 and 2001. The scientists found a total of 32 helminth (intestinal worm) species -- including Alaria, Pearsonema, Taenia, Toxocara, Trichinella larvae and Ucinaria -- all of which are considered significant for medical and veterinary health. A similar study of foxes from metropolitan Copenhagen (Denmark), published in 1996, looked at 68 carcasses and found many of the same helminths, with 86% of animals carrying Uncinaria stenocephala (a hookworm), 81% with Toxocara canis (a type of large roundworm well known to infect dogs), and 28% with Angiostrongylus vasorum. A team of ten biologists from the UK and Germany carried out a study of 588 foxes from across Great Britain to look at disease-causing parasites. The results, which were published in Veterinary Parasitology during 2003, showed that the most common gut parasites were Uncinaria stenocephala and Toxocara canis, occurring in 41% and 62% of the foxes, respectively.

Among the parasitic worms, the species that are generally considered to be of most concern in the UK, because they’re zoonotic (i.e. can be transferred to humans), are: Toxocara canis; Echinococcus multilocularis; and Trichinella spiralis.

Toxocara canisToxocara canis (the dog roundworm - left) is an intestinal worm that can cause toxocariasis in humans; depending on to where in the body the parasite migrates affects the syndrome experienced, which include ocular (blindness), visceral (fever, coughing, pneumonia etc.), asymptomatic (carrier) or covert (mild) infections. Much concern has been raised in the media following stories of children having been blinded by the parasite, although most cases are visceral and not serious. Infection is via ingestion of the eggs, which are generally picked up through contact with old faeces or contaminated soil – the parasite develops in the faeces for several weeks before becoming infective, so fresh faeces aren’t a source of contamination, although the eggs are very hardy and may persist in the environment for years. Simple hygiene (such as washing your hands before you eat) is an essential preventative measure. Unfortunately, there is still much to discover about this species, which makes it difficult to assess how important any given host species is in its spread. What we do know is that most infections in the UK (and, on their website, the Health Protection Agency note that there are about 100 cases of toxocariasis in Britain each year) originate from domestic dogs, although this parasite has long been known in wild foxes.

During his 1966 Ph.D study at Liverpool University, Bertram Cook found Toxocara in half of the foxes examined, although burdens were low (only 13% had more than four worms). Subsequently, in a 1993 paper to the journal Parasitology, Royal Holloway biologists David Richards and John Lewis, along with Bristol University’s Stephen Harris reported on the presence of this parasite in 521 foxes collected in Bristol between January 1986 and July 1990. The biologists found that 58% of male and 44% of female foxes had the parasite and that young animals, cubs in particular, were more susceptible than adults; worm burdens were highest in cubs, lower in subadults and lower still in adults. The authors note that cubs appear to become infected either prenatally (i.e. while in the uterus) or in the days and weeks after birth; this happens via the colostrum while suckling, and by coming into contact with the faeces and vomit of littermates. This study also found seasonal differences in infection, with the parasite being more common during the spring and summer months, but found environmental contamination to be low. They recovered Toxocara eggs from 2% of the 100 soil samples they collected; a single egg from a fox earth, and a single egg from a daytime lying-up site.

The infection picture is still unclear for foxes: some studies have found higher infection rates among adults than cubs, some (including an extensive study of more than 1,000 foxes in Denmark) support the findings of Richards and his colleagues that cubs have significantly higher worm burdens than adults, while others failed to find any difference between adult and cub worm burdens. Indeed, in a 2001 paper to the Journal of Helminthology, Richards and Lewis found relatively high egg counts in the faeces of both cubs and adults -- with the largest number of eggs (2,145 per gram) recovered from an adult male -- suggesting that both adults and cubs can harbour considerable Toxocara worm burdens. Either way, the initial results presented by Richards and his team are similar to those obtained for dogs, which generally show that puppies are a much greater source of infection than older dogs.

Despite many gaps in our knowledge, in recent years our understanding of this parasite and its transmission has improved. From Richards and Lewis’ 2001 study we now know that there is no significant relationship between the number of worms an animal has and the number of eggs in the faeces, so one must be cautious about attempting drawing conclusions from eggs in scat. Additionally, it seems that eggs may be transferred via fur. In a 2008 paper, Gillian Roddie and colleagues at the University of Dublin report that, of the 87 foxes they studied, 24 (28%) had Toxocara eggs on the fur around their anus (61% had worms in their intestines).  The number of eggs on the hair wasn’t related to the number of worms in the intestines and neither factor was related to the animal’s sex or age (although it should be noted that there were no cubs in the sample).  Interestingly, the study also suggested that eggs on fox fur may suffer more environmental degradation than those on dog fur, with just over twice as many unviable (essentially ‘dead’) eggs on fox fur than dog fur. Despite these, and other, recent advances in our understanding of the disease, there is currently no evidence to suggest that foxes are a significant reservoir of Toxocara infection for either dogs or humans.

Echinococcus multicularisEchinococcus multilocularis (the ‘hyatid worm’ - right) is a tapeworm that causes the potentially serious condition of alveolar echinococcosis (AE) in humans. The parasite is widespread throughout the northern hemisphere, in both North America and Europe, and causes tumours in the liver, lungs, brain, and other organs and, if left untreated, is often fatal; treatment can be both difficult and expensive. It is currently unknown precisely how many cases of AE occur globally each year; figures range from 18,000 to 500,000.

The tapeworm is carried by several members of the Canidae (dog family), including wolves, jackals, coyotes, and domestic dogs. Several studies have, however, implicated the Red fox as the main carrier in Europe, being largely responsible for the infection of humans by shedding eggs into the environment via their faeces. Foxes harbour the parasite in their intestines, which allows the worm to shed eggs into their faeces and subsequently pass into their surroundings. Once in the environment, humans, livestock (especially swine) and dogs can become infected. Fox density has been correlated with Echinococcus multilocularis abundance in parts of Europe (notably Germany and Switzerland, where as many as 30% of the fox population may be infected), although the precise reason(s) for this association is unknown. Rodents act as intermediate hosts for the parasite (i.e. foxes get it from their prey) and some have suggested that the risk of infection may thus be associated with the distribution of the foxes’ food. A recent study in France found that fox scats were deposited most commonly in areas with a high density of small-eared voles (Microtus spp.) and water voles (Arvicola terrestris). The fact that large populations of Microtus voles live on urban wastelands in France led the authors to suggest that the presence of this fox prey may increase the potential for transmission of this tapeworm. Indeed, a recent study of foxes collected in Geneva, Switzerland between 1998 and 2002 found that this parasite was more prevalent in rural (52% of foxes infected) than in urban areas (30% infected), which the authors attribute to fewer rodents in the latter habitat.

Echinococcus multilocularis is currently believed to be absent from Britain. Between 1999 and 2000, DEFRA tested 604 fox carcasses for various tapeworm parasites and did not find this species; a further 384 carcasses were assessed between 2005 and 2010 and all were negative for this tapeworm. DEFRA cannot say for certain that the parasite does not exist in Britain (they’ve not tested every single fox), but they do currently estimate that, if it is present, its prevalence is less than 0.1% (i.e. one infection per 1,000 individuals). Baiting foxes with anti-helminthic drugs (those that kill intestinal worms) has been used to decrease the prevalence of the parasite locally in parts of Europe, but there is currently no wide-scale solution to the problem.

There is another species of worm that causes a different hyatid disease in humans: Echinococcus granulosis causes cystic echinococcosis. There is only one record of this worm having been found in a fox from the UK, and this is widely considered to have been a misidentification of Echinococcus multiocularis. In 2009 there were nine reported cases of this disease among humans and all were associated with the consumption of imported meat.

Trichinella_spiralisTrichinella spiralis (the ‘muscle worm’ - left) is a small parasitic roundworm, typically found in the muscle tissue of pigs, that is sometimes referred to as the ‘pork worm’ owing to many cases of human infection following the consumption of undercooked pork. In humans the worm can cause trichinosis, a disease manifesting with various symptoms depending on the number of larvae (trichinae) in the body; common symptoms include swelling around the eyes, muscle pain, swollen joints, fatigue and fever. If the worms enter the nervous system, they can cause paralysis and, occasionally, death. While pigs are the most commonly infected animals in Europe, other species --including bears, wild boar, wild game (deer, for example) and foxes -- can also carry the worms and consumption of their undercooked muscle can lead to infection. In the UK, the Food and Environmental Research Agency (part of DEFRA) routinely screen foxes from across the country (killed by various groups, most notably by gamekeepers and pest control officers) looking for this worm. Since 1999, the FERA team has screened more than 3,500 foxes; all have been negative for Trichinella and, according to DEFRA’s most recent (2009) UK Zoonoses Report, Britain is currently considered to be free of this disease. The most recent case of infection from UK-sourced meat was recorded in pigs from Northern Ireland during 1979. The first reported human case of trichinosis in the UK came from London during 1835, while the most recent case of a person contracting the disease from British meat was in Liverpool during 1953. There have been cases of trichinosis in recent years, but all have been traced to imported meat (eight cases in 2000) or travel to an infected country (one case in 2008). The last confirmed case of Trichinella in a fox from Britain was an animal from Truro in Cornwall caught during 1957.

Outside of Britain, Red foxes are considered the main wildlife reservoir for Trichinella; the species most commonly found is T. britovi, although T. spiralis is also known, depending on the habitat. Prevalence of this worm in fox populations varies according to country, from low in Denmark and Spain (0.1% and 3% respectively) to very high (80%) in parts of Finland. Closer to home, two foxes from Northern Ireland recently tested positive for this parasite. In a brief paper to the journal Veterinary Parasitology during 2009, a team of 11 vets led by FERA biologist Irene Zimmer note that, between 2003 and 2007, 483 foxes from across Northern Ireland were examined for Trichinella and one individual tested positive for larvae – it was shot near Eniskillen, County Fermanagh during 2007. According to the UK Zoonoses Report 2009 another animal, also from Northern Ireland, tested positive in 2009. In the Republic of Ireland, a survey of 454 foxes -- by a team of biologists fronted by Paul Rafter at the Central Meat Control Laboratory in Dublin -- caught from across the country during 2002 found four infected animals (a prevalence of 0.9%): one in Waterford County, another in Donegal County and two in Cork County. Prior to the 2002 study, three animals from a sample of 70 foxes collected from the counties of Cork, Waterford and Tipperary during the late 1960s tested positive – all infected animals were from West Waterford, indicating that reservoirs can be highly localised. A survey of 120 foxes from Limerick County in 1971 found two infected individuals. The most recent data from Ireland, a study of 510 foxes from across the country by the Department of Agriculture Fisheries and Food Ireland conducted during 2008, found Trichinella larvae in two animals – the national prevalence is thus considered to be 0.4% (or one in 255).

Some fox parasites can cause diseases in domestic animals and some of those of greatest concern include: Angiostrongylus vasorum and Sarcoptes scabiei.

Angiostrongylus vasorum (the canine heartworm) is a roundworm parasite, first discovered in France during 1853, that causes chronic, sometimes fatal, infection in domestic dogs – the worms grow and reproduce in the lungs and move into the heart causing, among other symptoms, coughing, breathing difficulties, anorexia, vomiting and weight loss. The larval worms are coughed up and swallowed, passing into the faeces; faeces are eaten by gastropods (slugs and snails) and these are then eaten by foxes or other dogs, passing on the infection. Wild foxes have been implicated in the spread of this worm, although the parasite does require this intermediate host (the gastropod mollusc) and doesn’t appear to be passed directly from dog to fox or vice versa, although it seems that it may pass back-and-forth between dog and fox reservoirs. This worm was first reported in Britain’s foxes in 1995, when a young male fox was found wandering aimlessly in the Cornish village of Mousehole – it was taken to a rescue centre but its condition deteriorated and it was euthanized. The post-mortem of this animal was carried out by Vic Simpson at the Veterinary Investigation Unit in Truro and he confirmed Angiostrongylus vasorum infection. In a paper to Veterinary Record during 1996, Simpson described a further three animals from Cornwall, examined between May and December the same year that were also found to be infected with this parasite. It is interesting to note that the first records of this parasite in domestic dogs in Britain came from Cornwall in the early 1980s, although there are insufficient data to draw conclusions.

A recent survey by Central Science Laboratory (CSL) biologists of 546 foxes culled in Britain between 2005 and 2006 found that 40 animals were infected with A. variosum, giving an overall prevalence in the UK population of just over 7%. The level of infection varied according to region, with no reported cases from Scotland and northern England compared with an average prevalence of 23% (range 16% to 32%) in the south-east of England, which closely mirrors the perceived infection among domestic dogs. Despite these findings, the relative contribution of foxes to the spread of this parasite remains unknown and, in a recent letter to Veterinary Record, biologists at the CSL in York noted:

Geographical spread of A. vasorum in the UK could be associated with the movement of foxes, the movement of molluscan intermediate hosts and/or the movement of dogs (including overseas travel).

Currently, A. vasorum is known from Britain, Ireland, northern Spain and Portugal, France, Belgium, The Netherlands, Germany, Denmark, southern Sweden and Finland, Switzerland, Italy, Croatia, western Hungary and north-eastern Greece. Outside of Europe, it is also found in North America, parts of Asia (esp. Turkey) and Uganda in Africa. In Britain, it is widely considered that molluscs pose a greater threat of infection than foxes, and dog owners should ensure that their pets are vaccinated.

Sarcoptes scabieiSarcoptes scabei is a parasitic mite (right) that causes sarcoptic mange in foxes; the condition can also be passed to domestic dogs and, occasionally, humans (although the mite tends not to survive long on a non-canid host). The newly-mated female mite burrows into the skin (several thousand may infest a single square-centimetre of skin in severe cases), feeding as she lays her eggs – the irritation leads to intense itching and the associated scratching causes fur loss and wounds to the skin which can subsequently become infected. Without treatment, the condition is generally fatal within about four months. Mange can have a significant impact on fox populations – an outbreak in Sweden during the late 1970s caused a 95% reduction of fox density and populations remained low for 15 to 20 years. There have been many outbreaks in Britain and it has been hypothesised that mange was part of the reason fox numbers dropped sufficiently low to warrant imports from the continent during the mid-19th Century. In Bristol, an epidemic that began in 1994 caused a massive decline in fox numbers, with at least 90% of the animals dying. The subject of mange in foxes is covered elsewhere on this site and the impact of such substantial reductions in population density are discussed in the Territory and Home Range and Behaviour and Sociality sections of the main Red fox article.

In Europe and parts of North America the fox is a significant vector (carrier) of the rabies virus, which can be spread to humans and domestic animals. The subject of rabies in Red foxes is discussed at length elsewhere on this site and won’t be reiterated here.  A viral disease that can cause symptoms similar to rabies is canine distemper (CD) and this is occasionally recorded in foxes from Europe; unlike the rabies virus, which can only be spread through direct fluid injection (normally a bite), distemper is airborne. The CD virus is a member of the Morbillivirus genus (part of the same family that causes mumps in humans) and, despite the development of vaccines against it, it is still a significant problem in central and eastern Europe – a survey of foxes shot in Germany between 1996 and 1998 found 5% of animals sampled tested positive for the virus, the majority in urban areas. As far as I have been able to establish, there haven’t been any confirmed cases of CD in British foxes, although epidemics of distemper have been known to occur elsewhere, causing considerable mortality locally in Europe and on fur farms. Fox encephalitis (a type of canine viral hepatitis) has been recorded from North America but not, as far as I can establish, from Britain.

So, looking at endoparasites more generally, the most common tapeworms found in foxes are Taenia spiralis and T. pisiformis, while the most common roundworms are Toxocara canis and Uncinaria stenocephala, which are found in the digestive tract. A 1976 study suggested that the intestinal tapeworm Mesocestoides was fairly common among foxes in Scotland and south east England. The Capillaria aerophila and Crenosoma vulpis worms have been found infecting the lungs of foxes and the Capillaria plica roundworm (bladder worm) is prevalent in parts of Europe – a recent study found 78% of the foxes examined in southern Germany to have the parasite. Recent work in Norway has shown that Red foxes can act as hosts for the protozoans Sarcocystis alces, S. hjorti, Eimeria, Isopora and Hammondia, some of which are important parasites of cervids (deer), especially moose; it’s thought foxes may pick-up the parasites while feeding on moose carcasses. A recent study found low levels of the protozoan Cryptosporidium parvum (that causes the intestinal disorder cryptosporidiosis) in foxes from Ireland. The authors of the study, published in Veterinary Research Communications during 2007, note:

"Overall, this preliminary study, which represents one of the first published surveillance reports of Cryptosporidium in foxes, demonstrated the presence of C. parvum in our sampling of the Irish red fox population to be at a relatively low rate (1.6%)."

Foxes have also been found with Neospora canium and Toxoplasma gondii, parasitic protozoans that cause neosporosis and toxoplasmosis, respectively, in humans. During 1994 and 1995 a study of 16 foxes shot on a farm in Cornwall found two to be inconclusive and one to be ‘weakly positive’ for Neospora antibodies (suggesting an old exposure to the parasite). So, despite living in an area where the disease was rife in livestock, the local fox population seemed largely free of the disease. The FERA study of 587 foxes from across Britain between 1999 and 2000 failed to find any infected with Toxoplasma, although a more recent study (published in 2009) found 20% of the foxes examined to have this parasite. Elsewhere in Europe, Toxoplasma prevalence ranges from 0% in Austria to 60% in Spain and Hungary. In Belgium one 1997 study found 98% of animals to be infected, while a more recent (2010) study found a prevalence of 19%.

Foxes are also susceptible to infection by various species of trematodes (flukes), including: Alaria alata, Heterophyes heterophyes and Echinostomum melis that live in the intestines; Opisthorchis conjunctus in the bile duct; O. felineus and Pseudamphistomum truncatum in the gall bladder; Metorchis albidus in the gall bladder and liver; and Microphallus similes, Maritrema linguilla, Cryptocotyl lungua, Echinochasmus perfoliatus and Paragonimus kellicotti in the lungs.

New Forest fox cubsFinally on the subject of endoparasites, bacterial and fungal infections sometimes occur and can be fatal to foxes. In 1964 paper to the Veterinary Record, former Massey University (New Zealand) veterinary pathologist David Blackmore reported that eight (13%) of the 60 natural fox deaths he studied in Britain were caused by streptococci bacteria. A subsequent paper to the Journal of Wildlife Diseases, published in 1985, reported that foxes appear particularly susceptible to group G and C streptococcal infection, although the route of infection was found to be important; when the bacteria were injected into the muscle, they were considerably less virulent than when ingested (killing 17% and 100%, respectively). This study also found that, once the foxes had been exposed to (and survived) infection with either G or C streptococci, they were far less likely to succumb to subsequent infection with group G bacteria. This suggests that the foxes were able to launch an effective immune response against the infection. Blackmore, in his 1964 paper to Veterinary Record, also noted that foxes were prone to these bacteria:

From the limited number of examinations carried out, it would appear that streptococcal infection of British wild foxes is relatively common and that the infection usually gains entry via some traumatic lesion.”

Foxes can also contract the bacteria Leptospira icterohaemorrhagiae, which causes Weil’s disease. Rats are the main intermediate (carrier) host for this bacterium and, presumably, foxes contract this disease by eating rats; it’s then passed to other foxes in their urine, which is an important territory marker. In a study of 91 wild foxes during the early 1960s, David Blackmore found a high prevalence (61%) of nephritis (kidney inflammation), which he linked to Leptospiral infection. In 2009, DEFRA recorded 24 indigenous cases of Weil’s disease (i.e. caught within the UK) and between 2000 and 2009 there were 528 cases. I am not aware of any specific figures for Leptospira prevalence among Britain’s foxes but, in their After the Hunt report for IFAW, the Bristol University team note that “Up to 70% of foxes” carry antibodies to this parasite (which implies exposure), although they don’t cite the original study.  To the best of my knowledge, there are no data on whether foxes are an important carrier of this disease. Other occasional bacterial pathogens include Proteus (P. vulgaris and P. mirabilis), Pasteurella, Listeria, Mycoplasma, Staphylococcus, Salmonella and Escherichia. Mycobacterium bovis (the bacteria that causes bovine tuberculosis) has been recorded in foxes from Britain, although numbers are low (less than 1%) and none were infectious. This bacterium was also recently isolated from a fox in the Doñana National Park, southwest Spain. Generally, foxes are not considered a significant carrier of this disease. Similarly, some farmers have suggested that foxes scavenging foetal membranes of livestock may spread the Brucella abortus bacterium that causes abortions in cattle. Antibodies for Brucella have occasionally been found from foxes in Britain and Northern Ireland (I’m only aware of one animal in which the bacteria itself was found) but, in their 2000 Report on Contract 5 to DEFRA, Bristol’s Pirian White and colleagues wrote:

It seems highly unlikely that foxes play even a minor role in the spread of brucellosis in the UK…

In terms of fungal infections, ringworm (fungi of the Microsporum genus) is commonly encountered by rescue centres in the UK, although Trichophyton mentagrophytes (a related fungus) has also been recorded in foxes. It seems that foxes can pick-up ringworm from other species and, in a 2009 paper to the Iranian Journal of Veterinary Research, a team at the University of Tehran in Iran document the first case of a Red fox contracting Microsporum canis infection from an apparently healthy cat.

The ectoparasites of foxes are typically fleas and ticks, of which there are many species. Fleas are probably the most commonly encountered and numerous ectoparasites and, in his 1957 study on the distribution and hosts of fleas in Britain, Frans Smit recorded seven species of flea on foxes: Pulex irritans (normally on man, badgers, pigs); Archaeopsylla erinacei (hedgehog flea); Ctenocephalides canis (dog flea); Spilopsyllus cuniculi (rabbit flea); Paraceras melis (badger flea); Malaraeus penicilliger mustelae (vole flea); and Monopsyllus sciurorum (Red squirrel flea). In a 1980 paper to the Journal of Zoology, Bristol biologists Alan Buckle and Stephen Harris reported that, in a sample of 252 foxes collected from suburban London between October 1971 and July 1973, 65 (26%) had fleas; of these, 40 carried a single species and 25 had mixed fleas of up to four species. All-in-all Buckle and Harris found eight species of flea from seven genera: Archaeopsylla erinacei; Paraceras melis; Orchopeas howardi (Grey squirrel flea); Spilopsyllus cuniculi; Ctenocephalides felis (cat flea); C. canis; Pulex irritans; and Nosopsyllus fasciatus (rat/mice flea).  Other flea species recorded from foxes include two genera of fleas typically associated with rodents (Malaraeus and Monopsyllus), and two species of bird flea (Dasypsyllus gallinulae and Ceratophyllus gallinae).

Despite the considerable list presented here, in The Red Fox, Lloyd noted that foxes commonly carry dog fleas, but the other species are ‘stragglers’, probably picked up from other animals; being predators, Lloyd suggested that foxes probably acquire fleas from many of their prey species. With this in mind, it is interesting that no bird fleas were found by Buckle and Harris; in their dietary analysis they found birds to be the most significant prey item, but hedgehog fleas were most common, despite hedgehogs rarely appearing in the diet. Most flea species have co-evolved with their host such that they tend to be fairly host-specific (i.e. only survive on one or two species) and, in 1957, the eminent entomologist (insect researcher) George Henry Evans Hopkins described the flea Chaetopsylla globiceps as being unique to foxes. Jiri Preisler, of the State Veterinary Institute in Liberec, Czech Republic noted something similar in a 1983 paper to Folia Parastiologica; he found that this flea was widespread in Europe (although absent from Britain) and specific to Red foxes, with only the occasional flea collected from another carnivore. Preisler’s collection at that time contained 4,744 of these fleas collected from 186 foxes, one polecat and one stone marten. Similarly, a recent study in Germany found that C. globiceps was occasionally found on species other than foxes (on three domestic dogs in their sample), but the prevalence was low (about 1%). Sometimes fleas may occur in considerable numbers and the largest ‘haul’ that Preisler had was 237 fleas from a single fox. It should be noted that, despite the conclusions of Hopkins and Preislet, many authors consider that foxes don’t have their own flea species.

Fox with rabbitFoxes have also been recorded to carry ticks. Ticks are small members of the Arachnida class (which also contains the spiders and scorpions) that feed on their host’s blood; they feed until gorged and then drop off in a process that generally takes eight to 16 days. In a sample of 331 foxes collected from suburban London between 1971 and 1973, Bristol biologists Stephen Harris and Gordon Thompson found only two species of tick (Ixodes hexagonous and I. canisuga, typically found on hedgehogs and dogs, respectively). In their literature review, published in the Journal of Zoology during 1978, Harris and Thompson note that hexagonous is more common in southern Britain and Ireland, while canisuga is common in the north. Indeed, in their sample, 108 (33%) foxes had hexagonous, while only 10 (3%) had canisuga, and one fox had both. The researchers didn’t find evidence of acquired immunity against the ticks (i.e. older foxes weren’t less likely than younger ones to have ticks) and one female was severely infected (she had 315 adult ticks and 65 nymphs that caused several open sores seeping blood), but seemed otherwise healthy and above average weight. In his 1980 opus, Lloyd notes that foxes have also been found with Ixodes ricinus (sheep), I. negicallis, I. icuminatus and Dermacentor retulatus (cow tick). Once a tick has dropped off and digested its meal, it will sit and wait on vegetation for another suitable host to go past. Hence, the presence of ticks commonly associated with livestock is a result of the fox sharing the same habitat. (Photo: Foxes may pick up parasites -- including tick, fleas and intestinal worms -- from their prey species.)

Foxes have been implicated in the spread of the zoonotic tick-borne encephalitis (TBE). In a 2008 paper to the Scandinavian Journal of Infectious Diseases, Paul Haemig and colleagues in Sweden found a positive correlation between the number of foxes and the number of cases of TBE in humans; as fox numbers increased, so did human TBE cases. Correlation doesn’t, of course, equate to cause, but these findings have spurred further study.

Other ectoparasites that have been recorded on foxes include lice (Trichodectes vulpis is the usual louse found on foxes, although it is found infrequently) and the relatively harmless mite Trombicula autumnalis, which has frequently been observed on the ears and eyelids of foxes during September and October.

Finally, some of the more miscellaneous diseases from which foxes suffer include pneumonia, which is apparently a significant source of cub mortality, jaundice, paradontal disease, choking and hydrocephalus (‘water on the brain’). In his 1980 book The Red Fox, Huw Gwyn Lloyd mentions a single case of a fox from Denmark with pseudorabies and foxes are occasionally found behaving in a manner suggestive of brain damage. Lloyd described how, in 1932, George Tickner reported three foxes near Oxford displaying ‘a kind of madness’, including a male running in circles in a field snapping at air and ignoring the shouting people trying to drive it away; this appears to have been a result of some unknown brain injury causing cerebral lesions, rather than distemper. Foxes also appear susceptible to bioaccumulation of heavy metals (which is of particular concern in areas where foxes are eaten by humans). A recent study in Poland found that, in coastal and island populations where foxes feed heavily on fish and fish-eating seabirds, they can be particularly susceptible to mercury bioaccumulation, especially in their kidneys, which can lead to central nervous system damage, including lethargy, various visual impairments, ataxia, limb paralysis, tremors, sensory and motor disorders.

So, in summary, foxes are hosts for an array of both internal and external parasites, some of which are important for human health.  There is, however, no evidence to suggest that they are a significant vector for zoonotic disease in Britain. (Back to Menu)

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