Posts Tagged ‘swift fox’

arctic fox, swift fox kit fox

Canid taxonomy and evolution have long-stand ining debates, but one I don’t think has been discussed much is the evolution of the arctic fox-kit fox-swift fox clade.

For most of my life, there was always a long standing debate as to whether kit foxes and swift foxes are distinct species. The current thinking is the they are distinct but closely related species with a very narrow hybrid zone in parts of Texas and New Mexico.

And for most of my life, arctic foxes weren’t even considered part of the group. Indeed, they were considered so different from other foxes that they were placed in their own genus (Alopex).  We’ve since discovered that arctic foxes have mitochondrial DNA sequences that are very similar to swift and kit foxes, and in terms of their mtDNA, they are as distinct from kit and swift foxes as they are from each other.

Since then, arctic foxes have been classified within genus of “true foxes” (Vulpes), with the arctic fox being V. lagopus, the kit fox as V. velox, and the kit fox as V. macrotis.

Broader genomic analysis has revealed that the arctic fox is actually quite closely related to the kit fox, and analysis of mitochondrial DNA suggests that arctic and swift foxes diverged only 250,000 years ago. If similar results are confirmed in nuclear DNA comparisons, it means that these foxes diverged from each other after Old World and New World red foxes split.  A recent genome-wide study revealed that red foxes from North America diverged from those of the Old World 400,000 years ago, and in recent decades, it’s generally been accepted that red foxes are the same species. (There is currently a move to split them into two species).

Now, it’s pretty obvious that a similar study needs to be performed on swift, kit, and arctic foxes, and there is actually an obvious question that one of these studies could answer:

Are arctic foxes polar-adapted swift or kit foxes or are swift and kit foxes arctic foxes that have adapted to the Great Plains, Rockies, and arid regions of the West?

Arctic foxes are found in the arctic of North America and the arctic of Eurasia.  They come in two basic phases: white, which sheds out to brown and white in summer, and blue, which sheds out to blackish gray in summer and is gray-tinged in winter.

Arctic foxes could be descendants of swift or kit foxes that wound up adapting to a polar environment, which then allowed them to access the Old World. However, there is a paleontology study that says arctic foxes evolved in the Himalayas.  I am a bit skeptical of this study, because it seems to contradict the genetic data that connects arctic foxes with swift and kit foxes of the Americas.

However, if swift and kit foxes are temperate-adapted arctic foxes, then it could be possible that arctic foxes did evolve in the Himalayas. They came across the Bering Land Bridge, and then some of them became isolated in environments that became more moderate in climate, and they lost their adaptations for changing their coat colors. After all, least weasels, long-tailed weasels, and stoats (ermines or short-tailed weasels) have some populations where the animals turn white in winter, and populations where they don’t.

It could be that this is the real difference between arctic and swift/kit foxes is they just represent divergent populations where some populations turn white and some don’t. We only think of them as separate species because they are quite geographically different from each other.

It also could be that these animals are actually more genetically distinct from each other than we’re currently thinking, but preliminary genomic analysis suggests a very close relationship between kit and arctic foxes.

And those two are much more geographically isolated from each other.

So we do know that these three fox species do form a clade within the Vulpes genus, but how they exactly fit together is a good question. Maybe they actually are more closely related to each other than Old and New World red foxes. Maybe they aren’t.

And we don’t know which type came first.

But we can answer these questions, and we can answer them the same way we figured out that red foxes in North America aren’t derived from English imports from the colonial period.

This is a study that I’m pretty sure will be done but just hasn’t yet.



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This arctic fox looks a lot like a swift fox.

Classifying the various dog species has become a bit contentious in recent years.

Assays of various types of DNA has called into question the validity of many assumed and proposed species.

For example, a study that involved a large sample of nuclear DNA from  coyotes and wolves from many different populations found that the s0-called red wolf (supposed Canis rufus) is actually almost entirely coyote in its make-up.  It does have some wolf ancestry, but this wolf ancestry comes from the Holarctic wolf (Canis lupus), not any supposed endemic North American wolf species.

Now, wolves are fairly charismatic animals. They have quite a following in the popular culture–there is even a “wolfaboo” subculture on the internet– and in many ways, they have come to symbolize the modern conservation movement. People know that dogs are derived from wolves, and most people are willing to accept that dogs are part of the wolf species, even if there is still some institutional and cultural resistance to that notion.

Science knows a lot about wolves, and the popular culture knows a lot about them, too.

However, wolves are not the only wild dogs in which genetic analyses may reveal that species status is a bit more blurred than one might expect.

Let’s take three foxes in the genus Vulpes.

Vulpes is the big fox genus that currently includes almost everything called a fox in the Old World– the exception is the bat-eared fox (Otocyon). The red fox (Vulpes vulpes) is the most widespread of the genus, but there are several lesser known species, like the Tibetan and Blandford’s fox.

Among these less known species are the kit fox (Vulpes macrotis) and the swift fox (Vulpes velox). Both of these foxes are found in Western North America, and there is a huge debate about whether these foxes represent a single species or two distinct species.

This swift fox looks very much like the arctic fox at the top of this post.

Now, at the crux of classifying these species is figuring out how closely related they are to their closest relative, which is, surprisingly, the arctic fox (Vulpes lagopus).  Traditionally, the arctic fox has been placed in its own genus (Alopex), but the great preponderance of the genetic evidence shows that it belongs in the genus Vulpes. 

These foxes all have 54 chromosomes, and the swift and kit foxes regularly interbreed in West Texas and New Mexico, where the two “species” have overlapping ranges. Dragoo and Wayne (2003) examined kit and swift fox morphology and nuclear and mitochondrial DNA, and they found that the bulk of the evidence suggests that kit and swift foxes represent a single species. The authors contend that most of the greater genetic diversity within arid land foxes is actually result of them not dispersing great distances from their natal territories. Over time, populations in certain localities wind up with very distinct genetic characters, which might resemble those of a unique species.  Animals that travel a great distance from their natal territories, like lions and wolves, tend not to have such distinctiveness within a local population. There was always a gene flow across wolf and lion populations that may not exist at the same extent with smaller, less mobile species.

Animals identified as kit and swift foxes regularly interbreed in parts of New Mexico and West Texas. For whatever reason, hybrids have not been confirmed in southern Colorado and Western Kansas, where their ranges also overlap.

Now, the notion that swift and kit foxes are part of the same species is not universally accepted, and this is where the arctic fox comes into play.

Mercure (1993) examined the mitochondrial DNA of kit, swift, and arctic foxes and found that kit and swift foxes different from each other as much as they differ from arctic foxes. At the time, arctic foxes were still listed as belonging to their own genus, and this study was used as evidence to suggest that swift and kit foxes really were unique species. (Wayne was also an author of this study.)

However, now that we know that arctic foxes are actually in the genus Vulpes, we have another issue that needs to be considered.

Yes. I’m aware that mtDNA studies can be quite flawed. They examine only maternal inheritance, and very often, these studies have unusual biases. For example, initial mtDNA studies underestimated the date when forest and savanna elephants, which are now classified as separate species, split from each other.

But it doesn’t mean that these studies are inherently useless. When combined with studies that look at more of the genome than mtDNA, they can provide an interesting picture about the evolution of a species.

And here, I think these two studies are suggesting something quite radical:

Not only are swift and kit foxes the same species, the arctic fox is part of this same species.

Now, going with this classification is very contrary to what is generally accepted.

Arctic foxes are believed to have evolved in Europe 200,000 years ago. That is when they first appear in the fossil record, and the ancestral swift fossils have been dated to 500,000 years ago.  The Mercure study strongly suggests that arctic foxes evolved in North America, even though the earliest fossil remains were found in Europe.

However, it is also possible that the ancestral swift fox fossils are not in the same lineage as modern swift and kit foxes and that they actually evolved from the arctic fox. Traditional accounts of arctic fox evolution trace them to a common ancestor it share with the red fox, Vulpes alopecoides.  

It is possible that arctic, swift, and kit foxes all radiated from that ancestor, or arctic foxes are actually the ancestors of the swift and kit foxes. Perhaps, these foxes of arid and semi-arid lands are nothing more than arctic foxes that have adapted to a very different climate.

Further, there are several relatives of the swift/kit fox and arctic fox in Asia– the corsac and Tibetan fox. They are relatives, but they aren’t as closely related as swift/kit foxes are to arctic foxes. The fact that these three have such a close relationship with each other is really quite interesting.

The truth is we simply don’t know how these three foxes evolved, but it is clear that they are very similar to each other.

In fact, they differ less from each other in terms of their morphology than different breeds of dog or even different subspecies of wolf do. It is very likely that fertile hybrids can be produced with arctic foxes and either swift and kit foxes.  Arctic foxes live well to the north of where both swift and kit foxes are found, so no wild hybrids have been documented.

Thus, it might be more useful to think of them as representing a single species in which some populations have specialized adaptations. Kit foxes living in desert environments have big ears and pale coats, while arctic foxes have pelts that change colors depending upon the season.

More research must be performed on these three foxes, and the possibility that the represent a single species with three specialized subspecies has to be considered.

We know far less about the evolutionary relationships of these three foxes than we do about wolves, but they appear to have the same problem that wolves do.   Wolf taxonomy is always in flux these days, mainly because the studies don’t reflect what has always been assumed.

But it may be that wolves are not the only part of the dog family that might have these sorts of surprises in their DNA.

To me it is obvious that this foxes are different species from wolves, and because arctic foxes produce only infertile hybrids with red foxes, it pretty obvious that they aren’t the same species either. It’s when you start to find a great deal of interfertility between two wild  “species” that are also as distantly related to each other as they both are to a third species that one really should start to reconsider the taxonomy.

Perhaps the most parsimonious action would be to declare a single species for kit, swift, and arctic foxes.

I know that this is an extreme minority positions, but I think this reflects what has been found thus far.

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Video is here.

Swift foxes are prairie animals. They went extinct in Saskatchewan, Manitoba, and Alberta because of the practice of lacing carcasses with strychnine and other poisons to kill wolves. These poisons would kill any animal that scavenged the carcass, including swift foxes.  There are several other reasons listed here. The decision of Canada to tame its prairies for agriculture made it an inhospitable place for the swift fox.

The Smeeton fox breeding mentioned in this video is now run as a private organization. Their daughter, Clio, is probably the world’s greatest authority in swift foxes. The use of tribal lands in this part of Canada has been key to restoring them to the wild. Also, they have bred the foxes in open air pens, where they are occasionally preyed upon by hawks. This predation teaches the kits to avoid birds of prey, a very useful skill in the wild. However, I don’t know how any fox can learn to avoid coyotes from a captive situation.

These foxes are really interesting. Its scientific name is Vulpes velox, placing it in the genus of the “true” foxes, including the red fox.  It has a very close relative called the kit fox (Vulpes macrotis–“big eared fox”). It ranges into the deserts of the United States– Southwest and Great Basin– and Mexico. Its range is a bit west of the swift fox.

At one time, they were considered conspecific. Indeed, they have interbred and produced fertile offspring. Some people refer to the swift fox as a “kit fox,” but the current thinking of taxonomists is to place them as separate species.

Interestingly, their closest relative is the Arctic fox, with whom they both share the same number of chromosomes and a common ancestor within the last 250,000 years. However, the Arctic fox is usually classified in its own genus called Alopex.  This genus exists because the early taxonmists relied upon morphology alone to determine relationships between animals. The Arctic fox had a skull that was broader than a typical fox in the genus Vulpes but narrower than a dog’s skull. So it had to be a separate genus.

Now, we know that Arctic foxes are really closely related to the kit and swift foxes. In fact, if you look at white form of Artic fox in the summer and compare to the swift fox above, you can see a very strong similarity. (White Arctic foxes are brown with cream-colored patches on their necks, chests, bellies, and tails. They turn white in the winter. Blue Arctic foxes are silvery black in the summer months or silvery black with white markings. They turn a grayish blue color in the winter.)


White Arctic fox in summer. Compare with the swift fox above.

No one would have ever thought that the desert dwelling kit fox and the Arctic fox were such close cousins. In fact, it was only when fur farmers in Scandinavia crossed “blue foxes” with “silver foxes” that science began to realize the taxonomy wasn’t quite right. Blue foxes are the blue phase of Artic foxes, which are highly prized for their fur. Silver foxes are melanistic red foxes that have interspersed gray hairs on their coats, giving them a silver appearence. These hybrids were infertile, but it raised the question about whether the Arctic fox was truly a separate genus from the other foxes.

As far as I know, no one has tried to cross an Arctic fox with a Swift or kit fox, but it is probably possible. It is also possible that they could produce fertile offspring from the cross. However, kit and swift foxes aren’t fur farmed animals. There are no “domestic” strains of these foxes, unlike the various domestic forms red fox (including the truly domesticated foxes of Russia) and the farmed variety of blue Arctic fox.

Of course, I’m sure the first people to lay eyes on a swift or kit fox thought that it was a gray fox. However, the animal called a gray fox in North America isn’t even closely related to the true foxes. It is gray, just as the swift and kit foxes are, but it is an entirely different animal. It is the oldest canid in the Americas, and it may be the oldest canid in the entire world.

Its closest relative can be found on the Channel Islands. This species of fox is called the island fox, and it resembles a very small gray fox. How it got to the islands remains a mystery, but one theory is that it arrived there as a pet belonging to the native peoples of Southern California. Because it was living on the islands, where food was somewhat scarce, the fox became smaller and smaller to make better use of these limited resources. Today, it is a highly endangered animal.  These two foxes are not closely related to the others, and they aren’t related to any of the South American “wolf-foxes” either. They are an evolutionary relict of what the first wild dogs may have looked like.

The swift, the kit, and Arctic foxes should probably be placed in the same genus, which the share with the other Vulpine foxes, including the well-known red fox. The fact that we are only just now figuring out where these animals fit into their proper taxonomy and evolutionary history shows the limits of using morphology alone to determine these relationships.

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