Merriam's Pocket Mouse
Order 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 man’s fist. Burrows were barely large enough to admit a man’s 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 other.

Photo credit: R. D. Porter.