Canis lupus hominis

Alan Hepworth killed this 38-pound-fox was killed in Scotland. The size of a coyote, this fox was more than capable of killing lambs.

Remember when I suggested that one reason why British red foxes were getting larger is because they were no longer subjected to hunting pressures that select for foxes that are smaller in size?

Well, it turns out that I was wrong.

In fact, even my assumption that the foxes were evolving to be larger was erroneous.

It turns out that these giant foxes have very little to do with evolution in size and everything to do with increased nutrition.

The BBC reports that although there are some selective advantages for foxes to become larger it hasn’t really affected their evolution much. Different fox populations have varied in average size over the years, but the best prediction of how big foxes will grow is the amount of rainfall during the summer.

What?

Well, there is a little known fact about red foxes: they eat a lot of earthworms. David MacDonald, who extensively studied red fox foraging behavior near Oxford, found that one fox could eat as many as 146 worms in a single night.

This behavior has only been documented in European foxes. North American red foxes haven’t been seen going around eating lot of worms at night.

However, they don’t dig for worms.

Instead, they wait for a good spring or summer rain to bring them to the surfaces. When it rains hard, the worms come to the surface to mate, and we’ve all seen what happens when worms get caught on wet sidewalks. They usually flounder around for hours and then they die.

But in Europe, they are very likely to wind up in the belly of a badger or fox.

So when there is a very wet year, there are lots of worms coming to the surface, and the fox kits (which the Brits call “cubs” for some odd reason) are able to grow quite a bit larger than they normally would. Discussing this finding in that same BBC article, the authors point to a study that found rainfall was the important variable in determining how large a fox might grow:

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

“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.

There is also no evidence to suggest that foxes are evolving to get bigger, though there are selective advantages about gaining large size:

A 2008 study in the Journal of Mammalogy,… 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.

So large sized foxes might be distributing genes for larger size in the fox population.

But it assumes that these foxes are larger because of some genetic variation.

In the end, the bulk of the evidence suggests that red foxes that reach larger size because increased nutrition from farm fields and from worms allows them to reach the larger size that many of them could reach if they were given similarly rich diets.

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.

Now, there is evidence that coyotes in the Eastern parts of North America are getting larger, but that’s so they can become more effective predators of deer. These larger coyotes also have some wolf ancestry, and as these coyotes have evolved to hunt deer, the larger size and more powerful jaws they got from their wolf ancestors has had a major selective advantage.

Something similar could happen in England.

Currently, one of the most common deer species in England is an introduced species– Reeves’s muntjac. Like all muntjac, it’s not a particularly large deer, weighing only 20 to 40 pounds. Some of these very large foxes exceed 30 pounds in weight, and they might be able to prey upon these muntjac, though I have not found any evidence that they regularly hunt either muntjac adults or fawns.

If these foxes do start to prey upon muntjac, there will be an advantage to having larger size, just as having a larger size in Eastern coyotes allows them to more effectively hunt white-tailed deer.

And maybe then we could see an actual evolution of larger sized foxes in England.

And that would be really interesting.

So it’s a rich diet of worms, not evolution, that is making some British foxes larger.

But it could lead to larger size if these foxes start to target larger prey on a regular basis.

In that scenario, the foxes could grow quite large.

One could almost imagine foxes reaching the size of wolves.

But I need to keep my imagination at bay.

At least for now.