The least shrew, Cryptotis parva, has expanded its distribution westward during the past 20–30 years in response to increased irrigation and a simultaneous trend toward more mesic climate. The species is recolonizing regions that it invaded coincident with increased summer precipitation during the late Wisconsinan (13,000 years ago) before being forced to the south and east during the establishment of modern desert-scrub vegetation 4,000 years ago. Allozymic patterns among three recently discovered peripheral populations of Cryptotis parva in eastern New Mexico compared to a population from within the continuous range of the species in Texas indicate that two populations probably resulted from recent dispersal from the east, while a third may represent a relict of late Wisconsinan and alti-thermal intervals. This population, at Bitter Lake along the Pecos River Valley, is isolated from eastern populations by an elevated, arid plateau and from southern populations along the Pecos River by 600 km; it has a unique allozymic complement and it resembles the southern subspecies (C. p. berlandieri) in cranial morphology. Range maps of species of mammals must be viewed with caution, particularly at their distributional limits, in light of recent documentation of substantial range shifts (up to 450 km) over periods of 2–3 decades.

Author notes

Present address of CJS: 14801 Eden Lane, Middleburg Heights, OH 44130
Associate Editor was Patricia W. Freeman.