Desert Pocket Mouse
Order Rodentia : Family Heteromyidae : Chaetodipus penicillatus Woodhouse

Description. A medium-sized pocket mouse with long, heavily crested, and tufted tail; pelage coarse, but lacking spines on rump; sole of hind foot naked to heel; upperparts vinaceous buff finely sprinkled with black, imparting a grayish tone; sides like back; no lateral line; underparts and tail to tuft, white. External measurements average: total length, 205 mm; tail, 109 mm; hind foot, 25 mm. Weight, 15-23 g. Dental formula as in Perognathus flavescens.

Distribution in Texas. A southwestern pocket mouse that, in Texas, occurs mainly in the Trans-Pecos eastward to Val Verde and Crane counties.

Habits. This species in general occurs on sandy or soft alluvial soils along stream bottoms, desert washes, and valleys. In the Big Bend of Texas, large numbers of them have been trapped in loose sand along the Rio Grande where the dominant vegetation was Baccharis and mesquite and also in a brushy draw where the soil was hard-packed silt. We never found them on gravelly soils or among rocks, a habitat preferred by the externally similar Chaetodipus intermedius. Their burrows, from which the sand has been thrown well out to one side, are usually found near the bases of bushes and are closed in the daytime.

Their habits are not well-known. Like other pocket mice, they are strictly nocturnal. Their food consists of seeds; those of mesquite, creosote bush, and broomweed have been found in their cheek pouches.

Richard Porter found that in the Big Bend area the breeding season of this pocket mouse began in late February, the peak of pregnancies among females was in April, and the peak of juveniles in the population occurred in May. Lesser peaks of pregnancy occurred in June and August. The number of embryos per litter averaged 3.6 with extremes of two and six. Many of the young females reached sexual maturity early and became pregnant while still in their juvenile pelage.

The annual population turnover in this species is high — nearly 95%, according to Porter’s studies. Consequently, only 5% of the individuals present at the season’s peak survived 12 months in the wild. Only two juveniles of the 89 live-trapped animals he handled survived more than one year.

Remarks. Chaetodipus penicillatus is difficult to distinguish from C. intermedius and C. nelsoni. Table 3 gives some of the external features useful in identifying these species. In addition, the size and position of the interparietal bone in relation to the mastoid bullae is a most useful character in separating two of the species. Note in Figure 5 that in penicillatus the interparietal is separated from the bullae by straplike projections of the parietals and the supraoccipital, whereas in intermedius the interparietal is in contact with the bullae or nearly so. In addition, intermedius has a slightly narrower rostrum and the dorsal profile of the cranium is more highly arched.

TABLE 3. External features used in initial identification of Trans-Pecos Chaetodipus.

Photo credit: R.D. Porter.