Western Pipistrelle (Perimyotis hesperus)
- Protection Status Notes
- P. hesperus is not listed as Threatened or Endangered. It is one of the most common bats of the desert southwest, especially in the mountain ranges and rocky canyon country in the Trans Pecos.
- Pipistrellus hesperus is the smallest of all North American bats, and can be distinguished from the small Myotis species (californicus and ciliolabrum) by the club‑shaped tragus compared to the pointed tragus of Myotis. All three of these small bats have a keeled calcar. In Texas, there is a slight overlap in range with the eastern pipistrelle (P. subflavus) which is larger, has an unkeeled calcar and tricolored fur.
- Life History
- Western pipistrelles are often the first bats captured in an evening in mist nets set over isolated desert water holes or across mine entrances as they enter to night roost. During cooler winter months, pipistrelles hibernate in rock crevices (sometimes in mines), although on warm winter days, they may emerge to forage during the day. It is reported that females give birth to twins in late May through June, and mothers with their young may roost alone or in groups of fewer than 20 individuals. The young are volant within a month.
Western pipistrelles are also commonly known as canyon bats due to their association with rocky canyons and outcrops (usually at elevations below 6,560 feet), where they roost in small crevices. Occupied crevices may also be in mines and caves. They have been observed at dusk flying over creosote bush scrub several miles from rocky areas, and it is postulated that they may roost under rocks or in rodent burrows.
P. hesperus emerge early in the evening, often before sunset, and may be active after sunrise. Near rocky canyons, their small fluttery forms can fill the sky in the fading desert light. Stomach content analysis suggest they feed on small swarming insects such as flying ants, mosquitoes, fruit flies, leafhoppers and ants.
- P. hesperus occurs from the desert lowlands of the southwestern United States, into southern Washington. In Mexico, it ranges throughout Baja California and on the mainland to Michoacan and Hildago. While most commonly associated with arid, desert landscapes, it also occurs in association with significant rock features in lower elevation mixed conifer forest in mountain ranges in California and up to fir‑spruce forest in Arizona.
- Threats and Reasons for Decline
- Destruction of rocky areas due to renewed mining or other development activities (road construction, housing developments, water impoundments) can kill roosting bats and remove roosting habitat.
- Ongoing Recovery
- Establishing or maintaining open water sources may be a meaningful conservation tool for this species in Texas.
- Since this bat is too tiny to carry a radio transmitter, no data exist on individual foraging areas or range. Although P. hesperus is an ubiquitous bat throughout the arid southwest, limited information is available on social structure, microhabitat roost requirements, roost fidelity or longevity. Without more knowledge of natural history, it is difficult to assess potential threats to this species.
- Additional details can be found in the online version of The Mammals of Texas for the Western Pipistrelle