_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.
Two distinctive maturational pelages are seen in our material--juvenal
and post-juvenal or subadult, which generally resembles adult pelage (of
season) in texture but is duller of color. Collins (1918) for _P.
maniculatus_, Hoffmeister (1951) for _P. truei_, and Brown (1963) for
_P. boylii_, among others, have described maturational pelages and
sequence of maturational molts similar to those observed in our
specimens.
Deer mice from Harding County clearly are referable to _P. m.
nebrascensis_ (rather than to the smaller and paler _P. m. luteus_,
which occurs to the east and southeast), even though adults average
somewhat paler than adults of typical populations of that subspecies. A
tick of the _Ixodes ochotonae-angustus_ complex was obtained from one
specimen.
~Onychomys leucogaster missouriensis~ (Audubon and Bachman, 1851)