Sunday, 30 August 2015

The Dog Family: Arctic and Grey Foxes

Arctic fox
The same generalist habits and adaptability that have allowed foxes to colonise the deserts have also allowed them to evolve to suit one of the other great barren ecosystems: the Arctic tundra. Here, where even wolves are not particularly common, we find what else but the Arctic fox (Alopex lagopus)?

Arctic foxes are truly creatures of cold and desolate habitats. In Europe, they are found only in Norway, Iceland, and the coasts of the White and Barent's seas in northern Russia. Indeed, it is one of only two species of land-dwelling mammal native to Iceland - the other is the wood mouse, although there are some human-introduced animals there as well. Elsewhere Arctic foxes are found right round the coasts of the Arctic Ocean, in northern Siberia, in northern and western Alaska, northern Canada and its islands, and even in Greenland. Despite this wide, multi-continental, distribution, most belong to just one subspecies, although those living in Iceland, Greenland, and Svalbard form one or two distinct subspecies between them, and there are also distinct subspecies on the isolated Pribilof and Commander Islands in the Bering Sea.

We of course recognise the Arctic fox immediately by its pure white fluffy coat, and the great majority of pictures available on the internet will show it as looking exactly like that. But that's actually only the winter coat, and during the summer, most Arctic foxes shift to a much shorter, browner coat, dark enough that it's not easily mistaken for some other kind of fox, but not what we typically first think of. Not only that, but I say "most" because there is also the so-called "blue fox"...

Blue foxes are not one of the subspecies of Arctic fox, but rather, what's known as a colour morph. That is, some Arctic foxes are just born with a different coat colour to the one we'd expect, much as some humans are born blonde. Blue foxes have a much darker, greyer, coat in the summer, and they aren't pure white in winter, either, shifting to a sort of pale bluish grey. They aren't common; in most areas no more than 1% of Arctic foxes are born with this colour pattern. But in some places, including Iceland, and often also in coastal habitats, they may be much more prevalent, even forming the majority of the population in a few localities.

The Arctic fox lives primarily on the tundra, where the weather is so cold that the frozen subsoil never melts, preventing trees from growing. They do sometimes venture south into the pine forests, especially when food is in short supply, but for the most part they inhabit the treeless wastes of the north, especially close to the coasts. On the other hand, it has been argued that the southern boundary is marked, not so much by it being too warm, but just by the fact that red foxes become too common, and better able to compete for resources when the two are on an even footing. Be that as it may, since Arctic foxes are active year round, they need to be able to survive even the harshest of winters, and they have a number of adaptations to help them do this.

Arctic fox (summer coat)
Most obviously, there is the fur, which has been estimated to be the best, most insulating, fur of any animal. So effective is it that Arctic foxes never need to boost their metabolism to keep themselves warm - with a lower critical temperature of -40°c there is literally no temperature they could encounter in real life that's so cold they'll run into a serious problem. While all mammals can reduce blood flow to the extremities in cold weather to prevent heat loss (which is why we go pale and fingers and so on may even turn blue), Arctic foxes are particularly good at this, possessing such things as capillary heat-exchangers in the soles of their feet. They also have fur across the pads of their feet, a feature that, while it's not unique among foxes, was distinctive enough to be the source of the second part of their scientific name: lagopus means "hare-foot".

Besides the cold, the other problem facing Arctic foxes is finding enough food to eat. In most places, the bulk of their diet consists of moderately sized rodents, with lemmings being a common theme. However, that's largely because lemmings are the main thing that's around, and Arctic foxes exist in the first place at least partly because their ancestors could eat pretty much whatever they found. Even where lemmings are common, Arctic foxes also eat smaller rodents, such as mice and voles, a number of birds, and the carcasses of larger animals such as reindeer. In Iceland, for example, where there are no lemmings at all, the foxes mainly eat birds, and this is common on smaller islands, too.

Indeed, one of the advantages of living on the tundra is that, in the absence of trees, all the birds living there will be ground-nesting, which gives the chance for the foxes to raid their nests for eggs at the right time of year. Scavenging close to the seashore also gives other opportunities, and Arctic foxes have even been known to attack and kill seal pups. In general, though, the diet of Arctic foxes tends to shift throughout the year, depending on what is available as the seasons change. Unsurprisingly, they will also raid bins for human garbage in those few locations, such as air bases, where there is any sizeable human population so far north. And, being foxes, they will eat plant matter if that's all they can get. As is common with Arctic animals, their population tends to rise and fall significantly over the years as their food supply changes following particularly poor or good winters. To at least partially counteract this, they cache any excess food they find, hiding it for later use.

The social life of Arctic foxes is much the same as that of other foxes, being based around a mated pair that defends a clearly-defined territory, marking it out with urine, and signalling to their neighbours by barking, the sounds of which allow easy discrimination between one pair and another. Although some larger groups have been reported on small islands, presumably because they can't move far from one another, the pairs are typically independent, and the young leave home as soon as they can. Even members of a mated pair hunt alone and live apart when they aren't raising pups. Home ranges - the areas within which they regularly forage for food - can be large, especially in the more barren habitats, and they can be nomadic, roaming over areas of up to 1000 km2 (390 square miles) containing multiple separate feeding areas.

Mating occurs between February and May, with the pups being born 50 to 54 days later, followed by something of a mad rush to get them to functional adulthood before the winter closes in again. As might be expected, the size of a litter seems to depend heavily on how much food the adults can get beforehand, although the size and security of the birthing den is at least as important. Such dens can be huge, with some of them having over a hundred entrances, and being in use year after year. Some have been estimated to last for over three centuries, home to successive generations.

Arctic foxes evolved fairly recently, when, no more than half a million years ago, a close relative of the swift and kit foxes of the western US wandered north. As such, it clearly falls within the genus Vulpes, and, despite the traditional scientific name I've given above, should really be named accordingly. This has been slow to catch on, but is likely to become increasingly common as the years pass.

Grey fox
But, if Arctic foxes are closely related to red foxes and their kin, the same is certainly not true of the grey fox (Urocyon cinereoargenteus). In fact, quite where the grey fox places in the dog family tree is not at all clear. They are probably more closely related to red foxes than to wolves, but even that isn't certain. What we do know is that their ancestry can be traced back to the Pliocene, over three million years ago, and that they represent what is undeniably a very early branch in the evolution of modern dog species. With one exception (which I'll get to shortly), whatever they're closest to in evolutionary terms, they aren't very close to it.

Despite this, they do look quite clearly like foxes, being medium-sized for such animals, and with the typical narrow snout and bushy tail for which foxes are known. They are mostly dark grey in colour, with distinct brown markings on the flank and neck, and occasional paler patches about the face. They are found throughout much of the USA, save for the northwest and Maine, and across essentially the whole of Mexico and Central America. In the north, they just about cross the southernmost bits of the Canadian border, while in the south they reach northern Venezuela and Colombia just beyond the Isthmus of Panama.

Within this region, they are largely creatures of forest and scrubland, although they can't be very picky about the specific sort of woodland they inhabit, given how much that changes over their vast range. While they now expand into farmland as well, which likely explains their relatively recent arrival in Canada, this liking for woodland may be related to the fact that, with curving claws and  unusually flexible wrists and ankles, they are remarkably good at climbing trees. Especially for a dog. This ability stands them in good stead when attacked by predators and it's understandable that they'd prefer a quick escape route to hand.

Omnivorous even by the standards of foxes, they eat berries and nuts in the autumn, while switching to a more mixed diet for much of the rest of the year, with rabbits being a particular favourite prey animal. They also eat a fair amount of insects during the summer, along with pretty much whatever else they can catch, and the odd bit of carrion should they come across it.

Mostly nocturnal, like other foxes they live in mated pairs, although they don't always appear to be sexually faithful to one another. They may be less territorial than other foxes, but still put in some effort, scent marking in the usual manner for dog species, and barking and growling at intruders who get too close. They mate in the spring, giving birth to a litter of around four pups two months later.

This is the only time of the year that they really bother with dens, and they tend to be fairly simple ones even then. On the other hand, while it isn't common, they have been observed denning in tree hollows up to nine metres (30 feet) above the ground, which has to have some advantages when it comes to escaping from, say, coyotes. It is mostly the mother that looks after the young, but the male, and possibly some of the female's older children, do help out. Indeed, at least some males switch to looking after the pups on their own if their mate dies.

For all its unclear evolutionary affinities, the grey fox does have one close relative. That's because, no more than 16,000 years ago, a group of grey foxes somehow reached the Channel Islands, a small island chain off the coast of southern California, just west of Los Angeles. There, in isolation, their descendants evolved not only into a distinct species, but into no less than six subspecies - one on each island.

Island fox
These are the island foxes (Urocyon littoralis), and they look very similar to their mainland kin, save for being much smaller. At typically only half the weight of grey foxes, this is an excellent example of insular dwarfism, whereby animals evolving on small islands shrink over time to better cope with the smaller food supply. In most other respects, there isn't much difference between the two species, aside from island foxes having notably fewer bones in their tails (at least two fewer, sometimes more).

They don't stick to the woodlands, because with the islands being so small, they have to use all of the space that they can. And, of course, they eat mainly mice, seabirds, crickets, and cactuses because there isn't much else there that they could eat. Lack of resources probably also explains why their litters tend to be smaller. Other than that, their lives seem to be fairly similar to those of the grey foxes.

Fortunately for the foxes, five of the six islands form a nature preserve, and nobody can be bothered to hunt them. Their small population, which probably has no more than 4,000 adults, means that we can't entirely consider them safe should some natural disaster occur, but it's stable enough that we can't really consider them threatened, either. Back in the 1990s, four of the subspecies did suffer a major population crash, with one declining to as few as fifteen living individuals. Since then, however, they have recovered, largely due to active conservation efforts, and even that worst-suffering subspecies now numbers in the hundreds.

For the time being, at least, the island fox is secure.

All of the foxes I've looked at so far are (probably) "true" foxes. But, as that name implies, there are also not-true foxes. To find them we have to travel to the one inhabited continent that I've largely ignored so far: South America.

[Photos by "", Michael Haferkamp, and James Marvin Phelps from Wikimedia Commons, and the US National Park Service, in the public domain.]

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