Distribution and Ecology of the Hispid and Least Cotton Rats in New Mexico

Parts of three years were spent locating populations of cotton rats, Sigmodon hispidus and S. minimus , in New Mexico, with the objective of determining distributional limits and ecological preferences of the animals. One hundred twenty-one populations were located and 730 specimens were taken. Neither species seems to occur regularly in areas which have a mean annual temperature of less than 55°F or a growing season of less than 180 days. Both species have similar ecological preferences but, where the two kinds occur in the same area, S. minimus extends to somewhat greater altitudes than does S. hispidus . There is some evidence that both species have extended their ranges northward in recent years.