The present distribution of the Mexican vole (Microtus mexicanus) is not entirely the product of post-Pleistocene forest fragmentation and extinction; recent dispersal also is indicated. Literature records further suggest that this phenomenon may reflect a general pattern of northward range expansion in many southwestern mammal species.
Davis, Russell and Callahan, J. R.
"Post-pleistocene dispersal in the Mexican vole (Microtus mexicanus): an example of an apparent trend in the distribution of southwestern mammals,"
Great Basin Naturalist: Vol. 52
, Article 8.
Available at: http://scholarsarchive.byu.edu/gbn/vol52/iss3/8