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An earlier analysis demonstrated that the mass-independent energy expenditure of five species of the genus Peromyscus Gloger, 1841 decreased with increasing aridity along a mesic-xeric climatic gradient. Each species was represented by two populations that were located along the gradient. These data are reexamined with new analytical techniques. A mass analysis accounted for the basal rates of six populations within 10% of the measured rates. The analysis accounted for the rates of all eight populations within 10% when it included the gradient. An apparent limit to this response may restrict the geographic distributions of Peromyscus in desert environments, which may have distributional and survival consequences with climate change. One species, the California mouse (Peromyscus californicus (Gambel, 1848)), did not conform to the analysis of the other species, which may reflect the presence of unidentified biological or environmental factors present in this and the other species. A successful analysis of the energy expenditures of species must include their characteristics and those of the environment in which they live because these are the factors that define species and their performance. To determine the effectiveness of an analysis, the estimated expenditures are compared with the measured expenditures, acceptable estimates being within 10% of the measured expenditures.