Movements, populations, and habitat preferences of three species of pocket mice (Perognathinae) in the Big Bend Region of Texas /

Abstract

Population characteristics and habitat preferences of Merriam's pocket mouse (Perognathus merriami), Nelson's pocket mouse (Chaetodipus nelsoni), and the Chihuahuan pocket mouse (C. eremicus) in the Big Bend region of Texas were studied intensively over 26 months. Steep slopes limited the distribution and abundance of Chihuahuan pocket mice but not the other two species. Merriam's pocket mice normally were not present on steep slopes because of the usual occurrence of tall, dense vegetation on these sites, but they did occur there when the understory vegetation was sparse and short and there were either large boulders or fine gravels. Nelson's pocket mice were most abundant on slopes >20%, P. merriami on slopes 3-10%, and C. eremicus on slopes <2%. C. eremicus was most abundant on deep, rock-free (<5% gravel) sands, loans, and sandy loams, and was rarely found on shallow, rocky, sandy, and sandy clay loam soils. P. merriami was common on deep sandy loans and sandy clay loans covered with erosion pavement (rocks usually <7.5 cm in diameter accumulated on the surface) and usually containing 40-60% gravel. Nelson's pocket mouse was most abundant on shallow sandy loam or sandy clay loam soils of the mountain slopes containing cobbles and boulders (70% rocks at least 7.5 cm in diameter and frequently much larger). It was rarely found on rock-free, deep loans or sandy loams. The three species segregated themselves in the habitat according to the number and size of the rocks and the density and height of the understory vegetation. The three species of pocket mice were not restricted to specific plant associations, although each species had preferences. Nine habitat types were identified in the Big Bend area, with C. nelsoni commonly found in rocky, often steep and densely vegetated habitats, and C. eremicus in flat, rock-free, sparsely vegetated habitats. Merriam's pocket mouse, which has a wide range of habitat tolerance throughout its geographic range, reached its peak of abundance in habitats intermediate to and not preferred by the two species of Chaetodipus. It is believed that despite its greater range of habitat tolerance, P. merriami is better adapted to these intermediate habitats, and that because of its smaller size, it further tends to be crowded into this habitat by population pressures from Chaetodipus. During colder years, males of P. merriami and C. eremicus emerged from hibernation earlier in the spring than females. These two species showed a much stronger tendency to hibernate than did C. nelsoni, which was generally active throughout the winter. Reproductive activities of C. nelsoni started earlier in 1959 than those of the other two species, probably due to a lesser tendency of C. nelsoni to hibernate. The principal period of reproduction for pocket mice was in spring with a smaller fall peak. Juvenile female P. merriami molted into adult pelage before pregnancy occurred; juvenile females of the other two species appeared more sexually precocious. The incidence of pregnant juveniles of C. eremicus was higher than that of the other two species. Based on sperm production, juvenile males of P. merriami were more precocious sexually than juvenile males of Chaetodipus. Although the general progression of molt among the three species was similar, details were different. In general, adults moved greater distances than juveniles (except in September). P. merriami moved significantly shorter distances between captures than the other species. Analysis of the shift in center of activity from one period of capture to the next revealed that these pocket mice are relatively sedentary animals. Adult females of all three species showed a stronger tendency toward territoriality than did adult males. Several species of arthropod ectoparasites were collected from pocket mice and associated rodents, including some species not previously described.