Pipistrellus hesperus
western pipistrelle

Western pipistrelles, Pipistrellus hesperus, occur from southern Washington to the southern portion of Mexico including the Baja peninsula. Stretching from the west coast of California, its range includes Arizona and New Mexico, spreading into western Texas and extreme western Oklahoma (Harris, 2001). These bats are also found as far north as Utah and southern Colorado in the eastern portions of their range (Klingel, 2000)

Western pipistrelles inhabit a variety of habitats, ranging from rocky canyons, cliffs, and outcroppings to creosote bush flats. They are the most abundant of North American bats that are found in deserts, but are found at higher elevations in arid brush lands, grasslands, and even some forests. Western pipistrelles spend their days roosting in rock crevices, beneath rocks, in burrows, mines, and buildings. It has been suggested that western pipistrelles use burrows made by kangaroo rats (Dipodomys spp.) and other rodents (Barbour and Davis, 1969).

Western pipistrelles spend their winter hibernating in mines, caves, and rock crevices.

Water is a very important resource determining the distribution of this bat. Because of the high proportion of protein in their diet, the arid environment that they inhabit, and the subsequent high levels of evaporative water loss, western pipistrelles generally roost close to a water source.

Availability of maternity roost sites is an extremely important factor in successful bat reproduction (Cockrum and Cross, 1964).

Western pipistrelles are small brown-gray bats with distinct black, leathery facial mask, ears, and patagium. Beneath, they are pale brown-gray. Western pipistrelles have a short, blunt, and slightly curved tragus. They have a dental formula of I 2/3, C 1/1, P 2/2, M 3/3 = 34.

The species is sexually dimorphic, with females being larger than males. Weight ranges from 3 to 6 grams. Males have a total length of 66 mm, tail length of 27 mm, foot length of 5 mm, forearm length of 28 mm, and a wingspan of 19 cm. Females have a total length of 73 mm, a tail length of 30 mm, a foot length of 5 mm, a forearm length of 28 mm, and a wingspan of 22 cm (Davis and Schmidly, 1994).

Longevity of these bats has not been reported.

Western pipistrelles begin their foraging flights very early in the evening hours, making them one of the most diurnal of North American bats. They may also be encountered later in the evening (4 hrs after sunset), or closer to the morning. Occasionally, individual bats have been observed on wing during mid-day, during which time they seek out water to alleviate stress caused by the arid environment they inhabit. Because these bats fly slowly, they are restricted to small foraging circuits. They have a very slow, fluttery flight that can often be observed along cliff faces, among pinyon trees, or other desert shrubs. They are often mistaken for large moths.

One of the smallest and weakest of all bats, a slight breeze can bring western pipistrelles to a standstill, and a stronger wind will cause them to seek shelter. In the winter, western pipistrelles will enter hibernation in the northern portions of their range, but also use hypothermia to conserve energy during periods of cold temperatures in the southern portion of their range (Cockrum and Cross, 1964; Cox, 1965). Males will exhibit more activity during these periods of reduced activity then females will.

Western pipistrelles are insectivorous bats that feed on a variety of insects depending on the time of year. Prey items are located using echolocation. Stomach content analysis has shown that these bats typically feed on one type of insect at any one time. This suggests that they will find a swarm of some type of insect and feed upon it exclusively. Western pipistrelles forage from 2- 15 m above ground on small, swarming insects and consume about 20 % of their weight in insects per night (Klingel, 2000).

Insects eaten include: Hemiptera, Hymenoptera, Lepidoptera, Diptera, Coleoptera, Homoptera, Tricoptera, Plecoptera, soft bodied insects are preferred.

Predation in this species has not been adequately described, but possible predators include owls and larger species of bats (Klingel 2001).

Due to their high-energy requirements, western pipstrelles have a substantial impact on insect populations, potentially affecting a variety of insect-related ecological processes such as herbivory, pollination, and disease transmission (Klingel, 2000).

Bats are known hosts of the causative virus for rabies and are known vectors of other zoonotic diseases.

Bat guano is an important source of fertilizer and houses unique organisms that are useful in the production of waste detoxifying agents (Snow et al., 1993). Because this bat preys heavily on insects, it also helps to control insect pests.

Although this species is common, and currently has no special conservation status, people should remain mindful that their activities could greatly reduce the western pipistrelle population, if steps are not taken to ensure that bat habitat is protected.

Like all bats, western pipstrelles are most vulnerable at their roost sites and therefore protection of these roost sites is the focus of most management. Since reopening abandoned or inactive mines can negatively affect bat colonies, it has been recommended that multi-seasonal surveys be conducted prior to allowing any renewed mining. These same issues should be considered when a mine is recommended for permanent closure. If bats are roosting in a mine scheduled for closure, installation of a bat gate is preferred method for preventing unauthorized human access while allowing the bats passage.

Human disturbance can be extremely detrimental to bat colonies, especially to non-volant young and hibernating adults, depending on the season and severity of the disturbance. Mines that are in close proximity to roads, towns, hiking trails or camp grounds are more susceptible to disturbance then those in remote areas with difficult access.

There is a need for greater assurance that roosts will remain undisturbed and that future (potential) roost sites will be left when managing for bats in pinyon-juniper habitat. Ideally, management should aim to sustain adequate food, water, and roost sites in close proximity to one another (Klingel, 2000).