Is the island fox (Urocyon littoralis) a valid species?

"Shhh! Don't tell anyone, we might not be a unique species after all!"

Jess from DesertWindHounds and I had a discussion yesterday on this post on BorderWars, which discusses the real lessons we need to learn from the Isle Royale wolves.

As these discussions tend to go, there was mention of the island fox (Urocyon littoralis) of California’s Channel Islands.  It is a particularly inbred animal, and those on the island of San Nicolas are thought to be the most inbred wild mammal population ever documented.

This discussion led to me voicing skepticism that island foxes were actually a valid species.

Yes, I’m aware of a mitochondrial DNA study that found that all island foxes have a very different mtDNA sequence from mainland gray foxes, and I’m also aware of a morphological comparison study that shows a great deal of variation between island and mainland foxes.

Neither of these studies actually tell us that these foxes actually represent a unique species.

Morphological alone studies can be confounding when we’re dealing with members of the dog family.  All members of the dog family have an unusually high number of tandem repeats in their DNA. These tandem repeats are responsible for some morphology, and because of how these tandem repeats operate, morphological variation can rapidly evolve. Using dental morphology alone, it was assumed that the South American bush dog was most closely related to the dhole and African wild dog. Genetic analyses have shown that the bush dog is actually closely related to the maned wolf, another South American wild dog.

So be skeptical of morphological studies when they relate to members of the dog family.

Mitochondrial DNA analysis, as I’ve shown time and again on this blog, can often be quite faulty.

The studies that point to an East Asian origin for domestic dogs have looked almost exclusively at mtDNA– lots and lots of different samples of mtDNA.

And yet, I don’t think we can find these to be all that conclusive, especially when the genome-wide analyses show that dogs show a very strong affinity with Middle Eastern wolves.

The problem with mtDNA analysis is that it looks at only maternal inheritance. Matrilines have a tendency of dying out.

If we take the Urocyon foxes, we actually have a very good reason for why matrlines would die out and then be replaced by another one.

I have not seen an mtDNA study that compares the island fox population to a very wide sample of gray foxes. Gray foxes range from the US/Canadian border to Venezuela.  Not only are they widespread, the gray fox lineage is the oldest extant lineage in the dog family.

With such a widespread and ancient lineage, it is possible that there could be wide mtDNA variance across the mainland population of the gray fox.  No one has actually looked at gray foxes this closely.

The island fox could be nothing more than an insular subspecies of the gray fox, but no one has produce convincing evidence one way or the other.

My hypothesis goes something like this:

California gray foxes colonized the Channel Islands and became separated from the mainland population. Over time, the mtDNA sequences on the mainland became replaced, while those on the Channel Islands were not touched.  My guess is that the gray fox’s heightened susceptibility toward distemper destroyed whole populations and entire mtDNA lineages over time on the California mainland. New foxes with different mtDNA lineages were able to colonize California following these outbreaks, and this could explain why California gray foxes and Channel Islands foxes have very different mtDNA sequences.

(A major distemper outbreak greatly reduced the population of gray foxes in Southern California in the mid-90′s. Gray foxes are a major vector for distemper, and if one lives where they do, it is a very good idea to make sure dogs are vaccinated for the disease.)

However, there are some hints.

Jess pointed me to a new analysis that did some more sophisticated carbon-14 dating on the earliest fox fossils on the Channel Islands. These fossils were dated to a much later time than the 10,400 to 16,000 years when it was proposed that the ancestral gray fox came to the Channel Island.   The earliest proposed date from that study is 6,400 years ago, which is thousands of years after humans colonized the islands– and thousands of years after any of the islands were connected to the mainland.

If this later date is more accurate, then it means that the island fox was introduced to the Channel Islands– probably as a pet or semi-domesticated animal. Isotopic analysis revealed that the foxes ate almost nothing but marine life, which they would have obtained from the human companions.

It was not unusual for Native Americas to keep wild dogs as pets. The gray fox was a relatively common pet in some cultures, and on the exotic pet market, one sees gray foxes offered as being more tamable than red foxes.

Now, one should be a little bit cautious of studies that use carbon-dated fossils to tell us when animals arrived. It is possible that the 6,400-year-old fossil is just the oldest fox fossil that has been discovered. Genetic evidence shows that the dingo arrived in Australia earlier than its oldest fossil remains, so it is possible that the foxes came to the islands at an earlier date than the fossils are suggesting.

However, if this later date is correct, the island fox is a gray fox– and it is an old introduced species.  Something like the dingo of the Channel Islands.

The authors of this later study are curious about how quickly a fox could become dwarfed on islands, but as we have seen with so many dog species, morphological variation can evolve very rapidly. Not only do we have the tandem repeat issue, we have discovered that the genes that separate the many different breeds and types of domestic dog are controlled by just slight variance on just a few genes. Similar results have not been confirmed in modern village dog populations, but breed dogs vary from each other by only very tiny genetic variations.  Small size, for example, evolved very soon in the development of the domestic dog, and many modern small dogs have a variant of gene that causes small size that is also found in some Middle Eastern wolves.

Even the older dates associated with the split between island and mainland gray foxes are really recent.  10,400 to 16,000 years is actually much sooner than the date proposed for when dogs and wolves split from each other. The study that sequenced the dog genome revealed that the dog and wolf lineages began to split 27,000 years ago. The first mtDNA studies suggested that dogs and wolves split 135,000 years ago.

It is now nearly impossible to say that dogs and wolves are not the same species– and those who try cannot do so without twisting themselves into severe logical pretzels. But if it is now accepted that dogs and wolves represent the same species, why on earth would we assume that island foxes are a separate species when the split from their mainland ancestors such a relatively short time ago?

In order to resolve this issue, we need in depth analyses of nuclear DNA from gray foxes from a variety of locations and from island foxes.  As the authors of the recent carbon-14 study state, we also need to see if we can find ancient DNA in the old fox bones from the Channel Islands. Only when we get a really broadly-based analysis will we be able to see where island foxes fit.

My guess is the reason why such studies have not been performed is that it is just not a major priority among geneticists. Gray foxes themselves are not widely studied, and although they have been confirmed as canid lineage that dates back 10 million years, they just aren’t that interesting to most scientists.

Also, I don’t think the results of such studies would be received very well.

In all likelihood, the island fox will turn out to be a subspecies.

Subspecies can get special protection– see the Mexican wolf and Florida panther–but it’s easier to protect an endangered species under the Endangered Species Act when it is actually a species.

Now, it may be that if the island foxes are just a subspecies, they can experience some amount of genetic  rescue through the introduction of mainland gray foxes to the island.   The problem is that if that happens, the mainland gray fox genetics will spread through the population, just as the male wolf that colonized Isle Royale wound up swamping the inbred population with his genes.  The foxes on the islands would lose some of their unique morphology, although one should not assume that they would become larger. Gray foxes native to Central and northern South America are not much larger than the island foxes, and foxes from these populations would probably be the best choice for an outcross.

But if genetic rescue is attempted, one runs the risk of losing an inbred population of foxes that are able to produce pretty interesting data. The heavily inbred San Nicolas population has rather diverse MHC haplotypes– the result of balancing selection.

It’s much easier for science to operate under the assumption that Urocyon littoralis is a unique species.

It probably isn’t.

That’s my wager.

 

 

 

 

 

 

 

 

 

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