California Department of Fish and Wildlife

Bay Delta Region

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  Napa, CA 94558
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  Stockton, CA 95206
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Scott Wilson

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Stanislaus River Report


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Stanislaus River Basin and Calaveras River Water Use Program
Threatened and Endangered Species Report - March 1995
Bay Delta and Special Water Projects Division, CA Dept of Fish and Game

Western Mastiff Bat

Eumops perotis californicus
Category 2 candidate for listing by the U.S. Fish and Wildlife Service as Threatened or Endangered and a California Department of Fish and Game "Species of Special Concern"

Life History

The western mastiff bat is a very large free tailed bat. Two of its distinguishing characteristics are long narrow wings and large rounded ears that are joined at the mid-line across the forehead and project forward, extending beyond the nose. An additional characteristic is the tail which extends far beyond the interfemoral membrane. The total length for this bat is approximately 157 to 184 mm (Ingles 1965) and the forearm is 73 to 80 mm (Hall 1981). The color of the body and membranes are dark to brownish gray while slightly paler below. The western mastiff bat is the largest bat in California.

This is an uncommon bat that inhabits arid and semiarid lowlands in the lower sonoran life zone of California. The distribution is not completely known and new sightings in northern California are expanding its previously recorded range. Currently in California, the western mastiff bat ranges from San Francisco across to the Sierra Nevada and south, encompassing the southern half of the state (Hall 1981).

The mastiff bat is apparently a permanent resident throughout its range in the United States (Barbour 1969). They primarily roost in crevices in vertical cliffs, usually granite or consolidated sandstone, and in broken terrain with exposed rock faces; they may also be found occasionally in high buildings, trees and tunnels. Roost sites may change from season to season. Due to its large size, this bat needs vertical faces to drop from in order to take flight. Nursery roosts are found in tight rock crevices with mating taking place in the spring resulting in one young born during the summer.

The mastiff bat is California's largest native bat. They are swift flyers with very poor maneuverability. They are active yearlong, limited only when temperatures drop below 41 degrees Fahrenheit (5 degrees Celsius). Foraging may involve flying in excess of 14.9 miles (24 kilometers) and last up to six or seven hours a night. Because of this they rarely use night roosts unlike other bats.

Findings and Conclusions

The mastiff bat is a high flying and foraging bat. Because of this, surveys with mist nets are unsuccessful unless positioned at bat watering holes.

The reasons for the observed decline of this species are currently unknown in California, but probably includes urbanization and human disturbance.

Surveys at the Two-mile Bar Reclamation Area resulted in hearing the feeding buzzes of at least two mastiff bats. A possible breeding colony was also noted at Table Mountain near New Melones/Tullock reservoir by Elizabeth D. Pierson, Ph.D.

It is highly likely that this bat is a year round resident along the upper reaches of the Stanislaus River roosting in the rock faces of the canyon reach and foraging over the river and surrounding grasslands. Other researchers (Barbour 1969) have heard mastiff bats foraging 15 miles from the nearest roost site; therefore foraging within the valley reach of this study area is likely.

Any project along the Stanislaus River affecting suitable foraging and roosting areas may have negative impacts on the mastiff bat.