The Origins of the Island Fox by Courtney Hofman

(Thank you to our guest blogger, researcher Courtney Hofman) 

photo courtesy of Kevin Schafer

People have long wondered how the island fox first arrived on the Channel Islands. Did they swim? Were they swept out to sea on a piece of debris? Did Island Chumash and Gabrieliño people or their ancestors introduce them? Researchers have proposed a number of possible hypotheses of how the fox arrived but to test each hypotheses we must first examine the data on when foxes arrived on the islands.

For a long time scientists thought that island foxes had been on the islands for at least 16,000 years and some argued they had been there as early as 40,000 years ago (Aguilar et al. 2004). This is well before people arrived on the islands some 13,000 years ago. These early date estimates were based on island fox bones recovered from paleontological sites. However, direct radiocarbon dating of these same fox bones indicate that they are less than 7000 years old (Rick et al. 2009). Additional radiocarbon dates on island fox bones recovered from archaeological sites indicate that island foxes may have arrived on the islands approximately 7100 years ago, well after people.

When combined with radiocarbon dates, genetic data can also be used to test hypotheses about the origins of the island fox. Mitochondrial DNA is inherited from a fox’s mother and can tell us a lot about the history of the island fox. In my recent study, mitochondrial DNA was sequenced from 185 island and mainland gray foxes to explore how these different populations are related to each other (Hofman et al. 2015).

Median-Joining Network of Island and Mainland Mitochondrial DNA

By comparing these DNA sequences, we know that northern island (Santa Cruz, Santa Rosa, and San Miguel) foxes are closely related to each other while the southern island populations are more distinct (Santa Catalina, San Clemente and San Nicolas). Together with radiocarbon dates, mitochondrial DNA suggest that island foxes arrived on the northern islands between 9200 and 7100 years ago and were likely quickly moved by humans to the other islands. We cannot yet say how the foxes first arrived on the islands. More genomic and archaeological data are needed to distinguish between a human or natural introduction. 

Read the full paper at http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0118240

• Aguilar, A., Roemer, G., Debenham, S., Binns, M., Garcelon, D. and Wayne, R. K. (2004). High MHC diversity maintained by balancing selection in an otherwise genetically monomorphic mammal. Proc. Natl. Acad. Sci. U. S. A. 101: 3490–3494.
• Hofman, C. A., Rick, T. C., Hawkins, M. T. R., Funk, W. C., Ralls, K., Boser, C. L., Collins, P. W., Coonan, T., King, J. L., Morrison, S. A., Newsome, S. D., Sillett, T. S., Fleischer, R. C. and Maldonado, J. E. (2015). Mitochondrial Genomes Suggest Rapid Evolution of Dwarf CaliforniaChannel Islands Foxes (Urocyon littoralis). PLoS ONE 10:e0118240.
• Rick, T. C., Erlandson, J. M., Vellanoweth, R., Braje, T. J., Guthrie, D. A. and Stafford Jr., T. W. (2009). Origins and Antiquity of the Island Fox (Urocyon littoralis) on California’s Channel Islands. Quat. Res. 71: 93–98.

What Color Is An Island Fox?

photo courtesy of Kevin Schafer

What color is an island fox?

It might seem an easy question, but there is more to the color of a Channel Island fox than initially meets the eye.

At first glance an island fox (Urocyon littoralis) appears to be a mixture of white, reddish or rust, and gray markings, with a little black. The island fox's ancestor, the gray fox (Urocyon cinereoargenteus) has similar coloration, though typically less rust.

The primary color on an island fox is a grizzled gray along its back.

island fox back

This cryptic coloring provides very successful camouflage because it is not a solid color.

There is an underlying downy fur of a light tan, interspersed with long guard hairs. While some guard hairs are black, others along the back are variegated in color. A guard hair may be black, with a thin bit of brown, then white, and finally tipped in black. The combination of colors creates an overall appearance of gray.

island fox pup in the fog

However, the multiple layers of varying color create a break-up pattern; there is no solid color for the eye to discern. This allows the island fox to disappear into landscapes with low light levels–shade, twilight or fog.

island fox on mottled flooring at a medical facility

This characteristic also makes it challenging for auto-focus on a camera to successfully focus on island foxes. The fox in the photo to the left is actually standing, but it is hard to visually distinguish its back from the floor.

The variability in island fox fur also means that small hereditary changes in the fur can make large general changes in appearance. 

A little less black at the tip of the guard hairs and the overall appearance is much lighter. Such an island fox may appear more beige or brown and blend in better with the environment of the southern islands: San Nicolas or San Clemente. Lighter colored individuals may also be more successful hunting sand dune or beach habitats.

northern island foxes during captive breeding 2000-2006

A bit more black on the tip of the guard hairs and the overall coloring appears much darker and more gray. This coloring is more beneficial in habitats with denser vegetation, Santa Catalina, Santa Cruz and Santa Rosa Islands.

The plasticity of their camouflage coloring has helped island foxes remain successful hunters in varied Channel Island habitats.