Reproduction and Ecological Distribution of the Rockmouse, Peromyscus difficilis, in Northern Colorado

Abstract

Breeding activity of female Peromyscus diflicilis in northern Colorado in 1966 occurred from April to August with peak activity in May-June. The average number of corpora lutea per ovulation was 4.19 ? .92, the average number of embryos 4.25 ? 1.38, and the average number of placental scars 4.24 ? .84. Embryonic loss during gestation of P. diflicilis averaged 3.5-6.0% of ova ovulated. A marked annual testicular cycle occurred in adult male P. diflicilis demonstrated by the criteria of testicular weight, length, and histology, and length of seminal vesicle. Males were in breeding condition during the period March-August, 1966. New records of occurrence of P. diflicilis revealed that its geographical range extends northward of the previous marginal record to near the Colorado-Wyoming state line. A total of 151 trap localities sampled for P. difficilis were characterized in terms of 10 habitat categories. The rockmouse was most abundant in and showed preference for the mountain mahogany, pinyon-juniper, and yellow pine-juniper communities exhibiting numerous rock outcroppings. The species was nearly absent in these same communities where large surface rocks and boulders were lacking. Within the mountain mahogany zone, P. diflicilis occurred with greater frequency on granite and limestone outcroppings (90-100% of localities sampled) than on sandstone and shale outcroppings (50-54% of localities sampled). It did not occur to any extent in sagebrush, grassland, or Douglas fir-yellow pine communities within its northern geographical range. The altitudinal range of P. difficilis in northern Colorado extended approximately 1675 to 2300 m elevation along the eastern flank of the Front Range of the Rocky Mountains.