Pallid bat (Antrozous pallidus)

Physical description

The pallid bat is the second largest bat in Washington. Its pelage is pale yellow with a tinge of brown on the back and creamy white on the underparts. Individual hairs are short, pale at the base, and darker brown or gray at the tips. Wing membranes are pale brown. Ears are large, pale, and not connected at the base. The tragus is long and narrow with a finely serrated outer edge. The snout has prominent glandular swellings on both sides and scroll-shaped nostrils. Eyes are relatively large in comparison to other bats in Washington. Pallid bats are the only bat species in the state with two pairs of lower incisors. The foot is large, about half of the tibia length, and the calcar is not keeled. In Washington, pallid bats can only be confused with Townsend’s big-eared bat, which is smaller and darker, has its ears joined at the base, and two prominent bumps on the nose. Pallid bats near their roosts commonly give a distinctive multi-syllable call that is audible to people. 

Food Habits and Foraging

Targeted prey are usually medium-sized to large ground-dwelling or slow-flying arthropods, such as crickets, grasshoppers, moths, beetles, and scorpions, but small lizards and small mammals are also rarely caught. Crickets represented 60% of the diet followed by moths (20%) in eastern Oregon. In the Okanagan Valley of British Columbia, scarab beetles are mainly eaten (primarily ten-lined June beetles and May beetles), with Jerusalem crickets, moths, and lacewings being minor prey items. Combined diet samples from Hanford and Winthrop, Washington, were comprised mainly of Jerusalem crickets (36%), beetles (18%), and short-horned grasshoppers (13%). Pallid bats also consume flower nectar in some locations.

Emergence from day roosts is often relatively late and may not occur until an hour after sunset. Prey are often caught on the ground, although some are also taken in flight or gleaned from vegetation. While foraging, pallid bats usually fly slowly with rhythmic dips and rises within a few meters of the ground. Terrestrial prey is detected by their rustling sounds rather than by echolocation . The bats then typically drop to the ground, landing on feet and wrists, beside or on top of the prey. Captured prey are carried off and either eaten in flight or taken to night roosts. Foraging occurs primarily in uncluttered, sparsely vegetated habitats. In British Columbia, most foraging occurs in large (>0.5 km in length) areas of exposed sandy soil with sparse shrubs and grasses.

Reproduction

Breeding occurs from October to December, and possibly into February. Sperm is stored in the female’s uterus over winter, with ovulation and fertilization occurring the following spring. Timing of births is dependent on local climate, possibly because increased use of torpor in cooler years may slow fetal development. Thus, gestation length is variable, ranging from 53 to 71 days (average = 63 days). Young are born from late April to July, with birth dates in the northern part of the species’ range occurring later in this period. Yearling females usually give birth to one young, whereas older females usually have twins annually.  

Geographic range

Pallid bats occur across much of western North America from the Okanagan Valley of south-central British Columbia to central Mexico. The species is present in at least 12 counties in eastern Washington.

Pallid bats primarily inhabit drier environments, such as deserts, canyon lands, shrub steppe, and dry coniferous forest, but also occur in mixed conifer forests, riparian forest, and oak woodland. Within these habitats, the bats are commonly associated with rock outcrops, cliffs, and water sources. Some use of vineyards also occurs. Elevations up to 2,440 m are occupied. In Washington, the species has been detected at lower elevations in buildings, shrub-steppe, sparsely vegetated dunes, riparian areas, bunchgrass, basalt cliffs and mounds, and planted hardwood trees.  Ponderosa pine forests near cliff faces are also used.

Roosting

Pallid bats roost both solitarily and gregariously in groups ranging from several to more than 200 individuals. Rock crevices, holes in rock overhangs, and large snags and decadent trees are often preferred as day roosts, but caves, mines, bridges, and other open human-made structures are also used At the Hanford Site, Washington, maternity colonies have been found occupying the inside of a former reactor building (>100 females present) and associated bat houses. Roosts may be shared with other bat species, especially species of myotis. When approaching or departing their roosts, pallid bats commonly give a loud contact call to communicate with roostmates.

Maternity colonies form in late March and early April and disperse between August and October. Nursery roosts in rock crevices often have southern or southeastern exposures that offer early warming by the morning sun and protection from the more intense afternoon sun. During cooler seasons, vertical crevices with temperatures widely fluctuating between cool morning and warm evening extremes are commonly selected. Roost trees and snags occupied by adult females are typically large in size with a high percentage of remaining bark, receive considerable solar exposure, and are generally located on south-facing slopes in patches of mature trees.

Males apparently enter torpor only in their day roosts. Pallid bats enter night roosts to consume prey, enter torpor, and probably socialize. Caves, mines, cliff overhangs, rock crevices, tree cavities, bridges, porches, and garages are all used as night roosts. The species makes frequent use of bridges as night roosts in eastern Washington. Activity at night roosts can begin soon after evening emergence and peaks 1-4 hours after sunset. More time is spent at night roosts during cooler weather in spring and fall than in summer. Under cooler conditions, pallid bats at night roosts may form clusters and enter torpor for up to 5 hours.