Are urban foxes getting bigger?


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 plausible that foxes with a plentiful supply of high quality food could evolve larger body size, but there is no empirical evidence that they have done so yet.

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 have apparently exploded “as a direct result” of local councils rounding up stray dogs.

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 the largest he had seen during his 36 year career 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 founded 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 the pituitary gland that sits under the brain is instructed to release these growth factors 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 your body. One hormone in particular, Somatotropin (known more colloquially as Growth Hormone), plays a major part.

A graphic showing the structure of somatotropin, a growth hormone that plays a key role in protein production in mammals. - Credit: Роман Беккер / WikiMedia Commons

Somatotropin is produced, stored and secreted by special cells (somatotropes) in the front section of the pituitary gland (the adenohypophysis). 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 to increase muscle mass, strengthen bones, promote organ growth etc.; or it acts on the liver, causing the production of hormones called insulin-like growth factors (IGFs), which promote bone growth. Overproduction or shortages of somatotropin can have profound effects on body size with extreme cases being gigantism and dwarfism, respectively. Much depends, however, on when during the growth phase the hormones are altered; new-born mammals are largely insensitive, while puberty appears particularly important.

So, somatotropin is an important hormone (one of several) when it comes to building a body; 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 body 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 two different genetic ‘types’ of salamanders (differing in the number of paired chromosome sets) found they both grew to the same size, but had different overall numbers of cells. 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, for example, really big hands but a small head. 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. 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 at least, variation in body size is almost exclusively a result of variations in the number of cells and not the size of those cells.

A Red fox with a grey squirrel. Food is a key requirement for growth in mammals, but it's not the whole story. - Credit: Mary Lee Agnew

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 such as blood glucose provides 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) can affect continental weather and, as a result, the food available to animals living on those continents. Changes in these circulations over time have been linked to changes in Arctic fox (Vulpes lagopus) 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 published 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.

Foxes are deceptively small canids. They are about the same size as a large domestic cat, but often appear much bigger, especially when viewed from distance. - Credit: Lee Ashbourne

So, 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 fully 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, cubs 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 that, 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 child, for example) can lead to above average sizes. Is this happening to urban foxes in Britain?

They might be giants

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 – discarded fast food litters many town and city streets 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. To us, the idea of eating rubbish is generally repulsive; but, to a lot of urban wildlife our waste represents a valuable source of food. Moreover, this waste is so abundant that relatively little effort or risk involved in getting it. A fox has a higher net gain of calories eating the remains of a fried chicken meal dumped in its territory than it does from searching for, 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, just like in humans, 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.

In some urban areas, food put out by householders may surpass the nutritional needs of the territory holder, resulting in territorial changes. - Credit: Marc Baldwin

Scavenge isn’t the only food that foxes have access to in our parks and gardens: in many areas people actively feed their local wildlife. Indeed, the spreads I have seen some people lay out for foxes visiting their gardens (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 the area, which has consequences for disturbance; it can make the foxes less wary of humans; 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 very high in energy, which means a small amount goes a long way; but it’s also less filling that, say, a mouse or a bird, and so foxes may be inclined to eat more of it.

While it is true that most people are sensible in the rations they put out, there is a cumulative effect. The mammologists at Bristol University found that, when the city’s fox population was at its highest in 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, uncovered 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 70s 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 presented 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 forces behind this variation in body size.

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.”

Having spoken with Sally about this, it appears to be a misinterpretation of work by Yom-Tov and colleagues, which we’ll look at in a moment. 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. There is, however, no data to suggest any trend in the evolution of a jaw structure that‘s better adapted to scavenging than hunting; current fox jaws seem well adapted to both, because most hunt and scavenge (see the diet section in main Red fox article). There are, however, data 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. The data show that, over the last 140 years, there has been an increase in three of the four characteristics they measured, some now almost 10% larger; these skull characteristics (zygomatic breadth, length of fourth upper premolar and canine diameter) are all associated with diet and body size, suggesting that Danish foxes are now larger than they were in the previous century and capable of handling larger prey.

Some data suggest foxes living in agricultural areas have increased skull size in recent decades, possibly in response to an abundance of larger prey such as pheasants and hares. - Credit: Tony McLean

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 with abundant gamebirds and roe deer, 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, the latter resulting in an increase in garbage, as having provided easily available food for foxes. In a separate study, published in 2007, Yom-Tov and his colleagues observed a similar trend in Spanish foxes, with analysis of 267 skulls held at the Natural History Museum in Madrid showing that foxes from agricultural areas of Spain were significantly larger than those from non-agricultural areas. The authors suggested that increased food, largely as a result of more livestock in Spain, was the cause of the increased body size.

So, these data imply that foxes in some areas 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 larger jaws offer a distinct advantage. It should be recognised, however, that this prey is still essentially wild (or at least free-ranging) and foxes are actively hunting it. The data do not suggest that foxes evolve bigger skulls from feeding on human rubbish.

The picture in Britain

Unfortunately, there are no equivalent data on skull size over the decades for British foxes to compare with Danish and Spanish foxes. In 2010, I conducted a ‘vox pop’ of some pest control companies, asking if they kept records of the measurements 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.

With 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 they 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, told me “average fox weights are the same as they’ve ever been”.

A fox in a garden in London. - Credit: Paul Calder

The Project have handled more than 7,000 animals during their 20 years in operation and the bulk were around 4.5kg (10 lbs); only one animal was much heavier, at just under 10kg (22 lbs). The picture was the same at Vale Wildlife Rescue in Gloucestershire, who recently celebrated their 30 year anniversary; in that time, they have dealt with an average of two foxes per week, more than 3,000 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 councils have designated patrols that look for carcasses, while others rely on members of the public to report them. In May 2011, 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 the 30 councils that responded 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 they dealt with 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.

Red 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 people are bitten while trying to hand-feed or touch/handle a fox, or having cornered an animal, for example, it is difficult to conclude that foxes are more dangerous than they were a decade ago. In the end, 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 most likely 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 (e.g. got them food) in one instance, the fox is unlikely to pass up similar opportunities if they arise. This is not to say, however, that the foxes are then always on the alert for an open door or window. I suspect there must 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.

Foxes are a common site in most British towns and cities, where they seem to have learnt humans pose little threat. Some people confuse this complacency with the animal being dangerous. - Credit: Paul Cecil

One animal that I watched for several nights a few years back sat in roughly the same spot on the lawn of a block of flats to await the food 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 that these animals are bolder than one might expect doesn’t necessarily mean that they’re also more dangerous or more likely to bite, though. The only concomitant factor that I can see is that bolder foxes allow people to get closer to them 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 just stands and looks at you when you yell at it, rather than running away, may be infuriating; but it does not mean that the animal is aggressive.

So, in conclusion, an increase in the amount and/or quality of food can have an influence on the size to which an animal grows, particularly if this is provided during the crucial growth phase, 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 in Britain and as such no evidence is available to verify the claim that urban foxes are growing larger, leaving us with only anecdotal evidence or personal ‘feeling’. 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 not expect them to instinctively conform to our expectations.