||The Mammals of Texas -
Chiroptera : Family
Vespertilionidae : Pipistrellus hesperus (H.
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 X 2 = 34. External measurements
average: (males), total length, 66 mm; tail, 27 mm; foot,
5 mm; forearm, 28 mm; (females), 73-30-5-28. Weight, 3-6
Distribution in Texas. Western Texas east
to Uvalde, Knox, and Haskell counties.
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.
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. Pipistrelles 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 mid-day, during which time they water to
alleviate stress caused by the arid environment they
Western pipistrelles forage from 2 to
15 m 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 species 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 less than 1 g
at birth but grow rapidly. By August they can fly and are
difficult to distinguish from adults.
Photo credit: John L. Tveten.