Pipistrellus hesperus
(Western Pipistrelle)

Order: Chiroptera
Order Description: Bats
Family Description:

This very small bat, which flutters during flight, has dorsal Click word for definitionfur that is smoky to a yellowish gray brown color. The face and ears are blackish, the ears broadly rounded and short, not reaching beyond the nostrils when laid forward. The tragus Click word for definition, less than half of the length of the ear, is club-shaped. The calcar Click word for definitionis not keeled.

From southeastern Washington, south through western and southwestern U.S. to Michoacan and Hidalgo, Mexico. Known to winter in Nevada, California, Arizona, and Texas, but limits of winter range are not known. Distribution in Idaho is poorly documented.

Pipistrells occur in Eastern Washington and Oregon, Western Idaho through Utah and western Colorado to Texas, south into Central American to Columbia and west into Baja California and California. In Idaho, the western pipistrelle is found in Gem, Nez Perce, Twin Falls and likely other western counties.

Found (up to about 2100 m) in deserts and lowlands, desert mountain ranges, desert scrub flats, and rocky canyons. In Idaho, prefers cliffs and canyon walls close to water.

Pipistrells occur in arid rocky desert canyons near water, but have been collected in lower elevation mixed conifer forests. They roost in crevices, in mines, and buildings emerging in the early evening, especially in canyon areas, where they are often seen over slack water, feeding.

An important predator on small swarming insects, pipistrells feed on flying ants, mosquitoes, leafhoppers, and fruit flies, but often selects only one kind of insect that is abundant when feeding.

Pipistrells hibernate Click word for definitionsporadically, awakening during warm periods to feed. Because they are small, they would appear to be unable to accumulate sufficient fat reserves to migrate Click word for definitionto distant hibernal sites. As contrasted to other bat species in Idaho, they are active earlier in evenings and in the morning during early daylight but remain inactive in temporary roosts Click word for definitionin the middle of the night. The population ecology of this species is exceptionally difficult to study as a result of the small size of individuals and the type of habitat they occupy.

This species is known to swarm in large numbers, a condition that is likely important for reproduction. Mating occurs in the fall. Fertilization Click word for definition is delayed to spring and gestation Click word for definition lasts about 40 days. It has been suggested that two young may be produced but one appears to be the most common pattern, at least in California. Maternity colonies are small, comprised of fewer than a dozen individuals.

Status: Protected nongame species
Global Rank: G5
State Rank: S1

Important State References:
Keller, B.L. 1995. The status of bat populations in the Craig Mountain Area, Nez Perce County, Idaho. Cooperative Challenge Cost-share Project, Idaho Department of Fish and Game and Bureau of Land Management, Boise, Idaho. 21pp.

Information written by Barry Keller,© 2000
Map image provided by Stephen Burton,© 2000
Design by Ean Harker©1999, 2000.