Mammals of Northwestern South Dakota

by Kenneth W. Andersen; J. Knox Jones

Seven adults of this woodland inhabitant were trapped along shrub-covered banks of the spring-fed stream and small impoundment in Deer Draw of the Slim Buttes (10 mi. S and 5 mi. W Reva). Deciduous trees grew in the bottom of the draw, but the slopes above supported ponderosa pine and juniper. No white-footed mice were found along the generally treeless tributaries of the Moreau and Grand rivers to the east of Slim Buttes nor were these mice found along the Little Missouri River or in likely-looking habitat in the North Cave Hills. The P. leucopus of Deer Draw likely represent, therefore, an isolated segment of a formerly much more broadly distributed population of white-footed mice on the Northern Great Plains in post-Wisconsin times. Other such populations may exist in Slim Buttes and perhaps elsewhere in the county. Zapus hudsonius and Microtus pennsylvanicus were trapped in Deer Draw in association with white-footed mice.

Females collected on June 15 and August 7 were lactating and one taken on June 20 contained six embryos that measured 15. Two males taken on June 2 had testes that measured 12 and 15. These two males and a lactating female taken on June 15 still were in winter pelage, whereas a non-breeding female obtained on June 15 and a male and female (pregnant) trapped on June 20 were in summer pelage or an advanced stage of molt to that pelage. A lactating female taken on August 7 was in summer pelage excepting that what definitely appeared to be new winter pelage was present on the head, cheeks, and below the ears, and molt was evident adjacent to these areas.

Selected average (and extreme) measurements of the seven adults from Deer Draw are: total length, 184.4 (175-199); length of tail, 77.4 (70-88); length of hind foot, 21.1 (20-22); length of ear, 16.7 (16-18); greatest length of skull, 27.8 (27.0-28.4); zygomatic breadth, 14.6 (14.0-14.9); least interorbital width, 4.1 (4.0-4.3); length of maxillary toothrow, 4.2 (4.0-4.4). Three males and two non-pregnant females weighed 34.9, 34.6, 30.5, 32.2, and 31.4 grams, respectively.

Peromyscus maniculatus nebrascensis (Coues, 1877)

Deer Mouse

Specimens examined (214).—19 mi. N, 1 mi. E Camp Crook, 5; 18 mi. N Camp Crook, 2; 2 mi. N, 5 mi. W Ludlow, 58; 4 mi. S, 7 mi. W Ladner, 14; 9 mi. N, 3 mi. W Camp Crook, 3400 ft., 3; 7 mi. N, 2-1/2 mi. W Camp Crook, 3300 ft., 2; NW 1/4 sec. 32, R. 1 E, T. 20 N, 4; 1/2 mi. W Reva, 2; SW 1/4 sec. 30, R. 7 E, T. 18 N, 5; 9 mi. S, 7 mi. W Reva, 3; 10 mi. S, 5 mi. W Reva, 64; 14-15 mi. S, 4 mi. W Reva, 33; 2 mi. S, 5 mi. E Harding, 16; 7 mi. S, 4-1/2 mi. E Harding, 3.

The deer mouse is the most abundant and widespread small mammal in northwestern South Dakota. We took specimens in all terrestrial habitats, although the species was commonest in upland situations such as grassy fencerows, rocky areas, and hillsides supporting shrubs, juniper, or pine.

Adult mice in reproductive condition were taken in each month from May through August, although most of our information is for the months of May and June. In the last half of May, seven pregnant females carried an average of 5.0 (4-6) embryos that ranged in crown-rump length from 2 to 10; three others taken in the same period had six, six, and three recent placental scars, and another was lactating. Twenty-three males collected late in May had testes that measured 5 to 15 (average 10.2). In the last half of June, 19 females contained an average of 4.9 (2-7) embryos that ranged in size from 3 to 30 in crown-rump length, and two more were lactating; seven males obtained in the period June 15-25 had testes that averaged 8.7 (8-10).

Additionally, we took lactating females on July 6, July 7, and August 7, and two individuals with recent placental scars on August 5. Twenty adult males collected in the period July 6 to 18 had testes that averaged 9.3 (6-11.5), whereas those of two taken on August 4 and 5 measured 10 and 12, respectively. Young animals in juvenal pelage were captured in each month, May through August, the earliest being taken on May 20. The first female young of the year that was found carrying embryos was trapped on June 16.

Molt from winter to summer pelage is evident on some specimens taken as early as the latter part of May, but most individuals from that period and from the first part of June still retained winter pelage. By the last half of June, some mice had completed (or nearly so) the seasonal molt, but many retained at least some worn pelage of winter into the first week of July.