||The Mammals of Texas -
Merriam's Pocket Mouse
Rodentia : Family Heteromyidae
: Perognathus merriami Allen
Description. A very small, silky-haired
pocket mouse, similar to but smaller than P. flavescens; upperparts ochraceous buff mixed with black;
sides brighter, less blackish; underparts clear white;
spot behind ear clear buff, the one below the ears,
white; eye ring light; tail slightly darker above than
below; winter pelage brighter than in summer; young
grayer, less ochraceous. External measurements average:
total length, 116 mm; tail, 57 mm; hind foot, 16 mm.
Weight, 7-9 g.
Distribution in Texas. Known from western
two-thirds of state, but absent from extreme northern
Panhandle and extreme western Trans-Pecos.
Habits. In southern Texas, these
tiny mice are most common on sandy soils where vegetation
is sparse or at least short. In Trans-Pecos Texas, they
are more common on stony and gravelly soils covered with
sparse vegetation. They seem to have difficulty in
traveling through heavy vegetation, and if forced into
grass several centimeters high, their progress is
materially impeded. Near Oiltown, Texas, they were
especially common in stands of low Bermuda grass on the
shoulders of the highway where they were gathering seeds.
With the aid of a lantern, it was easy to capture a dozen
or more of them alive at night by hand. Their movements
in the grass resembled those of large, wingless
grasshoppers, but their leaps and bounds were neither so
long nor so high as those of the insect. When captured
they made no attempt to bite, and usually they emitted no
sound, although they can produce a high, metallic squeak.
When first caught they would not tolerate the company of
their own kind in close quarters. Several of them placed
in a cloth bag fought a battle royal, and some of them
were killed; however, six were kept together in a cage
for nearly a year without evidence of animosity.
Their tiny burrows are usually dug at
the base of a shrub or a clump of cactus. Several were
also found in the nearly vertical banks left by road
graders at the sides of the highway right-of-way. One den
consisted of three tunnels, 30-45 cm in length, that
converged under a flat rock to a nest chamber about the
size of a mans fist. Burrows were barely large
enough to admit a mans index finger. These mice
also make use of abandoned burrows of pocket gophers.
Their food consists largely of seeds of
grasses and weeds. They also feed on juniper seeds. In
captivity they are fond of millet seeds. They refuse to
drink; in fact, they can live for months without water.
The breeding season appears to extend
from April to November, and possibly two or more litters
of three to six young are reared each season. Young in
"gray" juvenile pelage have been captured in
June, July, and late November. In the Big Bend region,
Richard Porter found that the annual population turnover
was 84%; in a study on the Black Gap Area and in the Big
Bend, Keith Dixon found the turnover to be 75%. Dixon
recorded a maximum life span of 33 and 22 months,
respectively, for two mice on the Black Gap.
Remarks. The taxonomic status of
P. merriami has had a confusing history. In 1973,
Don Wilson presented morphological evidence indicating
that P. merriami and P. flavus
represented one species, and combined both under the name
P. flavus. Subsequent study using genetic analyses
has shown, however, that two species are indeed
represented. Using karyology and starch gel
electrophoresis, Tom Lee and Mark Engstrom have shown
that, although the two taxa are highly similar
morphologically, they do not appear to interbreed in
areas of sympatry. Thus, in the central Trans-Pecos
region and perhaps in the extreme northern panhandle
region, these nearly identical species of pocket mice
occur together but are reproductively isolated from each
Photo credit: R. D. Porter.