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Parastrellus hesperus (H. Allen 1864)

Order Chiroptera : Family Vespertilionidae

DESCRIPTION. A small drab-gray or smoke-gray bat with distinct black, leathery facial mask and black membranes; tragus short, blunt, and slightly curved; underparts pale smoke gray. Dental formula: I 2/3, C 1/1, Pm 2/2, M 3/3 × 2 = 34. Averages for external measurements: of males, total length, 66 mm; tail, 27 mm; foot, 5 mm; forearm, 28 mm; of females, 73-30-5-28. Weight, 3–6 g.

Parastrellus hesperus

DISTRIBUTION. The American parastrelle is particularly abundant in the western part of Texas in the mountain ranges and rocky canyon country of the Trans-Pecos and adjacent parts of the Edwards Plateau, Rolling Plains, and High Plains vegetative regions. Its eastern distributional limits are approximately along the 100th meridian, with eastern records from Knox and Haskell counties in the north and Uvalde and Webb counties in the south. The southernmost record is from Laredo, Webb County.

Distribution of Parastrellus hesperus

SUBSPECIES. Parastrellus h. maximus.

HABITS. This bat is associated chiefly with rocky situations along watercourses. Its daytime retreat is in the cracks and crevices of canyon walls or cliffs, under loose rocks, or in caves. In winter, these bats are known to hibernate in mine tunnels and caves.

These are among the most diurnal of bats, beginning their foraging flights very early in the evening and often remaining active throughout the early morning hours. American parastrelles are slow bats and may be distinguished on the wing by their slow, fluttery flight, which is restricted to small foraging circuits. Occasionally, individual bats have been observed on the wing during midday, during which time they water to alleviate stress caused by the arid environment they inhabit.

American parastrelle forage 2–15 m (6.5–49 ft.) above ground on small, swarming insects and consume about 20% of their body weight in insects per feeding. Specific prey items include caddisflies, stoneflies, moths, small beetles, leaf and stilt bugs, leafhoppers, flies, mosquitoes, ants, and wasps. Stomach contents of individual bats often contain only a single species of insect, or, if more than one kind of insect is present, the remains are clumped together within the stomach, suggesting that they take advantage of swarming insects and feed intensively within such swarms.

The young, numbering one or two (usually two), are born in June and July after a gestation period of approximately 40 days. Maternity colonies may be established in buildings or rock crevices. The newborn bats weigh slightly <1 g at birth but grow rapidly. By August they can fly and are difficult to distinguish from adults. Small maternity colonies may be established in buildings or rock crevices.

POPULATION STATUS. Common, year-round resident. The American parastrelle is one of the most common bats of the desert southwest and populations appear to be stable in Texas.

CONSERVATION STATUS. The IUCN status of the American parastrelle is least concern because of its wide distribution, presumably stable population, occurrence in a number of protected areas, and tolerance of some degree of habitat modification. It does not appear on any federal or state lists of concerned species.

REMARKS. In all previous editions of The Mammals of Texas, Parastrellus hesperus and Perimyotis subflavus were arranged in the genus Pipistrellus and commonly called the western (P. hesperus) and eastern (P. subflavus) pipistrelles, respectively. However, based on recent molecular genetics evidence, it has been determined the two taxa are not one another's close relative nor are they close relatives of the Old World pipistrelles. This discovery led to changes in the taxonomy of this group and to the formal description of a new genus, Parastrellus. Thus, the bat formerly known as the western pipistrelle (Pipistrellus hesperus) has been placed in the new genus as Parastrellus hesperus. In addition, the genus Perimyotis was resurrected to apply to the eastern pipistrelle (Pipistrellus subflavus), now known as the American perimyotis (Perimyotis subflavus).

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From The Mammals of Texas, Seventh Edition by David J. Schmidly and Robert D. Bradley, copyright © 1994, 2004, 2016.  Courtesy of the University of Texas Press.