Davis and Dunford (1987) have made a case for the yellow-nosed cotton rat (Sigmodon ochrognathus) colonizing isolated highland patches of oak-pinonjuniper (Quercus-Pinus-Juniperus) woodland in Arizona and extreme southwestern New Mexico within the 50-year period preceding their study. To colonize these uplands, gaps in grasslands between suitable habitat must have been crossed. In a later paper, Davis et al. (1988) turned their attention to northern-affinity, forest-dwelling mammals in New Mexico that likely originally colonized highlands south of the southern-most Rocky Mountains during the Pleistocene. The modern presence-absence occurrences in 12 such mountains and montane complexes were consistent with the current distributions being a combination of local extirpation, vicariance, and post-Pleistocene colonization rather than as a result of vicariance and local extinctions alone.