Are red foxes getting bigger?

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A fox believed to be the biggest killed in Britain has been shot after attacking lambs on an Aberdeenshire farm. The fox led some commentators to suggest that food left by people is leading to more outsized red foxes.

But are red foxes getting larger, and if so, why, asks the BBC within its Question of Nature series?

The huge fox recorded in Scotland weighed an impressive 38lbs 1oz (17.2kg), sparking speculation about what caused the animal to become so big.

Generally, the availability of food does have an impact on the body size of red foxes - but the relationship is not as clear cut as might be expected.

Red foxes (Vulpes vulpes) are one of the most ubiquitous of all mammals, occurring throughout most of Europe, including the UK, and Asia and in parts of Africa and the Middle East.

This wild dog, or canid species has been introduced to Australia where it is now widespread, and in North America it is present throughout much of Canada and the United States.

Red foxes have also colonised a range of habitats including semi-arid deserts, tundra, farmland, boreal forests and metropolitan areas.

A fox's world

  • Members of the wild dog family are found on every continent except Antarctica, with red foxes being the most widespread of them.
  • The red fox is the largest and most successful of the true foxes, adapting quickly to new environments including deserts, mountains and tundra as well as urban areas.
  • There are currently 45 subspecies of red fox recognised.

Their adaptability is reflected in their diet; red foxes are omnivorous and feed on a variety of animals and plants, including insects, small mammals such as rodents and rabbits, birds, earthworms, eggs, berries, fruits and seeds. In some parts of their range they will take turtles, frogs, snakes, fish and even deer fawns and ringed seal pups.

In 1995, Paolo Cavallini of the University of Siena in Italy published a study called "Variation in the body size of the red fox", which has been regularly cited by scientists since. In that study, he examined the body size of red foxes worldwide.

Males tended to be 5-6% larger and more massive than females, but the size of both sexes varies considerably depending where in the world they lived, it found.

For example, that research found that male red foxes in Scotland averaged 16lbs (7.3kg), and Wales 14lbs (6.4kg), whereas those in England averaged somewhere between these two weights; 14lbs (6.4kg) and 16lbs 4oz (7.4kg) depending on the study area.

Red fox Rainfall influences how big baby foxes grow

Foxes in the state of Maryland, US however, were much smaller, averaging just 9lbs 6oz (4.3kg).

A more recent study published in 2008 by Carl Soulsbury and colleagues at the University of Bristol, UK, examined foxes living in the city up until 2004.

It found the average weight of the male fox to be closer to 13lbs 3 oz (6kg).

"We're not seeing urban foxes getting any bigger," says Professor Stephen Harris, a co-author of the report.

"I've been researching urban foxes for about 40 years and the biggest fox I've had has been about 23lbs in an urban area," he says.


Fox on a rubbish tip

"But we've had foxes under 10lbs and maybe 9lbs. You have quite a range of sizes."

He says the large fox caught in Aberdeenshire is simply a rare, but normal specimen. Media reports that the previous largest UK fox on record weighed 26lbs are not true, he says.

Similar-sized foxes have been recorded centuries ago, he says. "This fox was about 38lb and in the 18th Century hunting literature we've got foxes up to 36lb.

"This fox is unusual but it's an outlier. Once every so often we get a very big fox but I wouldn't expect anything else."

Eating waste

But in other countries, there is some evidence that red foxes are generally getting bigger.

In Israel, a study published this year by Yoram and Shlomith Yom-Tov from Tel Aviv University found that red foxes there have grown larger, with the trend starting in the 1930s and stabilising in the 1980s. During this period human populations grew 16-fold in the country, the standard of living increased 10-fold and the area of worked land increased by 50%.

Today garbage in Israel includes at least 40% organic waste, say the authors, providing a steady source of available food to birds and mammals, including foxes.

The researchers have found a similar trend among foxes living in Denmark, while studies in Spain have shown that red foxes living in good habitats are on average 14-19% heavier than those living nearby in poorer habitats.

However, no such trend has yet been found in the UK, though researchers have found that back gardens are the most important foraging habitat for urban foxes.

Urban fox (Image: BBC) Adapting to survive?

Outside of urban areas, other foxes are showing changes in body size.

Another study in Spain, also by Yoram and Shlomith Yom-Tov, collated measurements of red fox skulls over the 20th Century. It found that red foxes living in agricultural areas were significantly larger than those living elsewhere.

These larger foxes are living off food scraps, but leftovers used to feed livestock, rather than from people. They may also scavenge or kill the livestock itself.

But other, perhaps unexpected factors influence fox size as much as human leftovers and agriculture.

Dr Soulsbury's and Prof Harris's 2008 study also examined how diet and environmental conditions affect the body size of red foxes in the UK.

Surprisingly perhaps, variation in rainfall and the abundance of earthworms most influenced how large a fox will be when it grows up.

Generally, red foxes attain full grown mass during their first year of life, and the researchers found that Bristol foxes achieved their adult weight around October in the year they were born.

They then eliminated from their data any foxes suffering from sarcoptic mange, a potentially fatal disease that causes a loss of muscle and mass. They also accounted for any changes in population density that might affect body size.

After studying the Bristol foxes for a number of years, they found that larger cubs do, as perhaps might be expected, become larger adults.

But that effect is more than swamped by the effect of rainfall, according to the study published in the Proceedings of the Royal Society B journal.

Cubs growing up in areas that experience heavy rainfall in July tend to grow significantly larger than those that don't.

Moving on

  • According to the IUCN's Invasive Species Specialist Group red foxes are one of the world's worst invasive alien species.
  • Selective breeding experiments in Russia over the last 50 years have produced a domesticated form of the red fox. Known as the domesticated silver fox or Siberian fox they are tamer and more dog-like.

"The difference between a dry summer and a wet summer is about 18% in body size," says Prof Harris.

That is because heavy rainfall causes more earthworms to come to the surface. And earthworms are about the only prey that young foxes, unskilled at hunting, are able to catch and eat in the few weeks after they stop suckling from their mothers.

Conversely litter size and the size of the mother fox does not predict how big cubs will grow.

Is being bigger actually better for foxes?

A 2008 study in the Journal of Mammalogy, again by Carl Soulsbury and colleagues, found that heavier males hold larger territories and defend them better against their rivals.

This study, the first to show the affect of body mass in generally monogamous mammals, also revealed that heavier male foxes move over wider areas, and further from their territories. In doing so, they mate more with other females (known as extra-pair matings) and sire more offspring as a result.

So males at least do benefit from growing larger. Bigger females conversely do not appear to produce bigger litters.

Freaky or frequent?

But what will the future bring?

Despite these trends towards bigger sized foxes living in certain habitats, particularly those closer to human settlements and agriculture, foxes are not necessarily destined to become bigger.

Your tweets

Red fox photographed in South Gloucestershire
  • This #fox in South Glos wasn't huge but is a beauty (pictured)

Alix ‏ @thewoodstoday

  • I saw a quite big fox last week, haven't seen it since! But I'm currently filming foxes for a channel 4 documentary.

Yusuf Akhtar @WildYusuf

  • Last year our rough Collie was chased by a screaming vixen in heat - did she think him a suitable large mate?

Alvin Finch ‏ @alvinfinch

Even the reported trends towards larger size are likely to be local, and transitory.

A 2009 study published in Global Ecology and Biogeography showed that French red foxes are decreasing in size, while those in adjacent Benelux or in Alaska are not altering size at all.

That research examined the sizes of skulls held in museums of more than 4400 specimens from 22 carnivore species from 52 populations sampled around the world in the past few decades.

Overall it found that the majority of carnivores, including wolves, badgers and weasels among others, are not changing in size.

There is even less evidence that any size changes are truly adaptive; that the genes of carnivores such as foxes are evolving to produce bigger foxes.

More likely is that occasional changes happen on an individual level.

Where large foxes are found, they are not evolving to become bigger, but rather they have simply had the chance to eat more as they are growing, either earthworms or food left out by people, filling out their genetic potential.

Big foxes, it seems, happen to be normal foxes that have eaten a lot, not freaks or the product of a new evolutionary trend. And they have likely occurred throughout history.

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