The black-eared mouse Peromyscus melanotis is distributed in conifer forests and highland prairies throughout the Trans-Mexican Volcanic Belt, Eastern Sierra Madre and Western Sierra Madre in Mexico, and isolated mountain ranges in Arizona, United States. We examined geographic variation in cranial measurements throughout the distribution of the species in Mexico. Because populations of these major ranges are isolated, we expected discontinuities in variation consistent with geographic gaps. Instead, we found no differences between the Sierras and significant differences between them and the Trans-Mexican Volcanic Belt. Moreover, a north-to-south trend of size increase was detected. By relating size to 6 environmental variables, we evaluated 4 hypotheses previously proposed to explain the observed pattern of variation: 1) seasonality, 2) body heat conservation or Bergmann's rule, 3) primary productivity, and 4) body heat dispersion. Significant positive correlations with elevation, productivity, and humidity, and negative correlations with annual mean temperature, temperature seasonality, and latitude led us to conclude that the observed pattern of variation in size is not related to body heat conservation or loss, but driven by primary productivity, a positive correlate of resource availability.