Geographical variation in populations of Pero-myscus maniculatus from north-central Colorado was studied during the years 1957-1958. A total of seven hundred twenty-eight mice was collected from seven sites at elevations from 5,000 feet to 11,500 feet. The sites represented several diverse habitat types. Variation among populations in lengths of the condyle-premaxillar distance, condyle-zygomatic distance, nasal, femur, maxillary tooth row, mandible, and width of the skull at the bullae was insignificant. The reflectance of the pelage, as measured by photoelectric reflectometer, was greater for plains specimens than for mountain specimens. It was observed that the color of the pencillation of the tail varied from brown to black. The intensity of the color in the pencillation was related to the general darkness of the pelage. Brown tails occurred more frequently in plains mice while black tails predominated in mice from high elevations. The darkness of pelages increased with elevations and was correlated with the darkness of the soil, although not strongly so. The differences in coat color among populations may have resulted through selection by predators. It was noted also that litters tended to be larger in female mice from the highest localities. The average litter at 5,000 feet elevation was 4.0 embryos; at 6,000 feet it was 4.6 and at 8,000 to 10,000 feet it was 5.0. The causative factors behind variation in the number of young produced per litter are not known. In the course of obtaining the specimens, observations were made of the patterns of distribution of Peromyscus maniculatus through the environment. At all sites, the mice were found most abundantly in a limited portion of the habitat. The distribution of mice, as a result, is dendritic or discontinuous, which may have had important effects in evolution of differences among populations.

Covers not scanned. Item deaccessioned after digitization.