Getting an expert’s take on the mysteries of mammal diversity


While vacationing in New Mexico recently, I observed some chipmunks at the top of Sandia Peak in Albuquerque. I knew they couldn’t be eastern chipmunks, but I had no idea what species they might be. The tram staff at the top of the mountain was no help.

Chipmunk mystery

When I got home from the trip, I consulted my favorite mammal field guide, Mammals of North America, second edition by Roland Kays and Don Wilson (Princeton University Press), and quickly discovered that it included 26 species of western chipmunks. That compares to one here in the East, the eastern chipmunk, a familiar visitor to many backyards. Rather than just guess which species I had seen, I contacted Dr. Don Wilson, emeritus curator at the Smithsonian Institution.

I explained the chipmunk that puzzled me and asked which species he thought I might have seen. He replied promptly and said it was almost certainly a Colorado chipmunk. Having done his graduate work at the University of New Mexico in Albuquerque, he knows the area and its mammals well. With that mystery solved, I noticed the disparity in the number of species of chipmunks and ground squirrels between the East and West.

East vs. West

Here we have the eastern chipmunk and ground hog. According to Mammals of North America, 26 species of chipmunks, 23 species of ground squirrels (including four prairie dogs) and five species of marmots inhabit the West. (The eastern-most species of ground squirrels, the 13-lined ground squirrel, occurs in Ohio and Michigan.) Hereafter, for simplicity, I will refer to all these species as “ground squirrels.”

I could come up with a few explanations for the East vs. West diversity of ground squirrels, but I decided to ask the expert. I called Wilson again and asked for an explanation.

Habitats differ

“The basic explanation is that there is much greater diversity of habitats out West compared to the East, so we expect to see more species represented by any given group,” he said.

Wilson continued: “The longer answer relates to the geological history of the two areas. The four most recent glaciations affected East and West differently. The Western mountains are relatively young with many high mountain peaks and secluded valleys and basins that isolated and separated mammal populations. As the ice sheets retreated north, populations were left behind on isolated mountaintops and valleys, so speciation occurred rapidly, leading to the many species we see out West.”

Numbers may be equal

“Here in the East, the Appalachians are much older and more eroded, the topography is less severe and the habitat is more homogeneously forested. Consequently mammal populations did not experience the geographical isolation required for rapid speciation,” Wilson said.

“Despite the disparity of species diversity between the East and West, however, I’d bet that the total number of individuals (biomass) is probably similar, though the West has many more species, due to the geographical isolation caused by retreating glaciers,” he added.

Bats at risk

While I had this prominent mammalogist on the line whose area of expertise is bats, I asked about the long term effects of white-nose syndrome, the fungal disease that has killed millions of hibernating bats over the last 10 years.

“There is reason for hope,” Wilson explained, “because we finally understand exactly what the problem is, and we’ve actually seen immunity in some individuals. But it’s not yet realistic to believe we can treat individual caves for the white-nose fungus on a large scale. I’m hopeful for the future, and the more we learn the better our chances for success.”

Coyotes adapting

Finally, I asked Wilson about the proliferation of coyotes throughout the East and even in urban areas, places they’ve never been seen before.

“Coyotes are an extremely adaptable species; they do very well living close to people,” he said. “One reason for this is that we provide an abundant, easy-to-catch food supply. Chickens, lambs, goats, dogs and cats are easy prey for coyotes. And it seems the longer they live among us, the wilier they get.”

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