Sunday, 5 July 2015

The Dog Family: Foxes of the American West

Swift fox (in summer coat)
There are six living species of fox native to the North American continent. One of those is the red fox, which is also found through much of Asia and Europe. Three others, including the grey fox of the southern US, are not traditionally considered to belong to the genus Vulpes, and I will be returning to these at a later date. Which leaves two close relatives of the red fox that are unique to the continent. Both, as it happens, live out in the wilds of the American West.

Heading out into the west, the first of these two species we come to is the swift fox (Vulpes velox). The native range of this animal happens to line pretty up well with a series of interstate borders, and it's found from northwestern Texas/eastern New Mexico in the south to western South Dakota/eastern Wyoming in the north.

As one might imagine, given this area, they like flat, wide open short grass prairie, and apparently avoid everything else, although they don't seem to have a great problem with cropland. They are small foxes, standing only about 30 cm (one foot) high, similar in height to a small terrier or a King Charles spaniel, although, due to a slim build, they are also considerably lighter than those dog breeds. During the winter, they have a dark grizzled grey back with tan markings on the flanks and legs, but, aside from the black tip to the tail and the pale belly, the whole animal takes on a browner hue in the summer, when it sheds its thick pelt.

Swift foxes are mostly nocturnal, as well they might be, given the number of predators that they face - they seem to be quite a popular food for coyotes, never mind eagles, bobcats and so on. For themselves, they prey largely on small mammals, but, like most foxes, aren't terribly choosy about which ones. Thus, in South Dakota, their most common food is prairie dogs, in Kansas it's mice and rats, in Texas,  cottontail rabbits, and so on... whatever happens to be most available. Of course, they also eat birds, insects, and the like, and a small amount of vegetable matter, too.

As their names indicates, swift foxes can run fast - a useful trait out on the prairie, where cover can be minimal. Individuals can travel long distances over the course of a night, with about 18 km (11 miles) having been recorded as typical. They are highly territorial, living in mated pairs that, judging from field observations, drive away all rivals, although a single close relative might sometimes be tolerated. However, once we start to do any sort of paternity testing, it turns out they aren't exactly what you'd call sexually faithful - just over half of the pups born to a mother fox are apparently unrelated to her partner. This may partly be because it seems to be the female that calls the shots, taking a more active role in defending the pair's shared territory than the male does.

The core of each pair's territory is centred around their den, which typically has multiple entrances, to allow rapid escape if something goes wrong, and is located wherever food supplies are close at hand and coyotes aren't - low hilltops seem to be particularly popular. Indeed, dens seem to be more important to swift foxes than to most other fox species, helping them to avoid predators and to stay comfortable in extreme weather. Of course, this is also where they raise their young for the first month or so of their lives.

The mother comes into heat once a year, some time between December and March, depending on how far north she happens to be, and gives birth to a litter of three to six pups 51 days later. They leave home after about four to five months, sometimes travelling great distances to find a suitable den site of their own. Like all foxes, they are vocal animals, communicating with a wide range of different noises - one study identified nineteen distinct calls.

Today, swift foxes are relatively common animals, with, overall, a stable population. But that hasn't always been the case, since a great decline in their numbers began in the late nineteenth century, when they fell victim to collateral damage from efforts to keep down wolf and coyote populations. In fact, while today, their native range extends no further north than South Dakota, they were once perfectly happy even in the northern reaches of the Great Plains... until the last reliably recorded sighting of one in Canada occurred in 1938.

Or, at least, that was the case for several decades. From about the 1940s, swift fox numbers in the US began to recover again, the campaign against wolves having largely succeeded across much of their range. They never came back to North Dakota, but, starting in 1972, the Canadian government launched a program to bring them back to at least some of their former range. The initial success rate wasn't great, most of the foxes having been raised in captivity, and the most adventurous individuals faring least well when they had to face the rigours of the wild. Over the last twenty years or so, however, Canadian swift foxes seem to have established stable populations in southern Alberta and Saskatchewan - far less than their original range, which also extended into Manitoba, but nonetheless significant.

Kit fox
At one point, it was considered that the more northerly swift foxes belonged to a different subspecies than their southerly kin. This is not generally thought to be the case today, but controversy does continue over the status of the kit fox. Most agree that it is a separate species (Vulpes macrotis), but there are some who consider it no more than a collection of westerly subspecies of the swift fox.

Indeed, the two animals do look rather similar. The kit fox is slightly smaller on average , and it has a shorter coat than the swift fox, but, otherwise the two are easy to confuse. Perhaps the clearest point of difference is that kit foxes have larger ears than swift foxes. These large ears help to radiate away heat without losing water by panting, and are a common adaptation in desert-dwelling foxes, such as Blanford's fox in the Middle East.

Kit foxes are found through much of the American west, from western Texas to southern California, and from central Mexico in the south and southern Oregon in the north. In between, they are common in New Mexico, Arizona, Utah and Nevada. Like swift foxes, they avoid truly rugged terrain, but as their homeland indicates, they are more at home in arid chaparral and desert scrub than in the more fertile grasslands of their close relatives. Perhaps because this is more marginal terrain, they may travel even further each night than swift foxes do, with up to 32 km of nightly wandering having been recorded.

Their diet is similar to that of swift foxes, though... which is to say, they eat whatever they can find, and what they can find is generally prairie dogs and rabbits. Plants are, however, apparently an even smaller part of their diet than for swift foxes. They are also less territorial, being more willing to allow others of their kind to share the outer reaches of their home range, even if they try to keep them out of the core area around the den.

They, too, live in mated pairs, although, while these seem more stable than those of swift foxes,they, too, are hardly faithful to their partners, and even pups in the same litter may have differing fathers. Their dens are used year round, and they often have several, although many of them will have been pinched from other animals, saving them the trouble of doing too much digging. Living, for the most part, further south than swift foxes, they are able to breed earlier in the year, so that the young foxes are about eight months old when the winter approaches and they have to leave home for the first time. Even so, by one estimate, almost two thirds of them fail to survive that departure, dying within ten days of leaving home, and getting no chance to establish their own dens and find a suitable partner.

One reason that kit and swift foxes are sometimes considered to be two different examples of the same species is that the two are capable of interbreeding, and likely do so where their ranges abut, in New Mexico and Texas. However, this seems to be quite rare, even where the opportunity exists, and it is this segregation that has allowed them to maintain their distinct genetic profiles and, according to most researchers, their status as distinct species.

In part, this may be because kit foxes prefer more arid terrain. They are, of course, by no means the only foxes to do so. We've already seen Blanford's fox, but to see some even more truly desert-dwelling species we need to head to the last continent that is home to the genus Vulpes: Africa...

[Photos by Tony Ifland of the US Fish and Wildlife Service Mountain-Prairie Region, licensed under Creative Commons Attribution 2.0, and "Utahcamera", from Wikimedia Commons]

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