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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

Yuma Myotis

Myotis yumanensis

Category 2 candidate for listing by the U.S. Fish and Wildlife Service as Threatened or Endangered


Life History

The Yuma myotis is described as having short rounded ears and a pointed tragus without a keeled calcar. The body is light buff to dark brown with lighter underparts. The fur is darker at the base and is dull looking. The braincase rises sharply from the rostrum giving it a steep sloped appearance. Body measurements are: total length, 73 to 91 mm; foot, 9 to 11 mm (Ingles 1965); forearm, 32 to 38 mm; ears length, 11 to 14.5 mm; greatest length of the skull, 13 to 14.2 mm.

The Yuma myotis bat occurs along the western quarter of North America from Canada, south to Mexico, and eastward to Idaho and Texas, including parts of Montana, Utah and Colorado. Excluding most of Nevada and areas north eastward, The species comprises six subspecies with four subspecies (M.y. saturatus, M.y. oxalis, M.y. sociabilis, M.y. yumanensis) occurring in California (Hall 1981).

This bat is common in California and found throughout the state except in the Mojave and Colorado deserts of south eastern California. Occupies a variety of habitats below 11,000 feet (3300 meters); rare above 8,000 feet (2560 meters). Found in open forests and woodlands usually feeding over water. Emerges soon after sunset and feeds on a variety of flying insects low to the ground. Roosts in buildings, mines, caves, or crevices (Zeiner 1990).

Hibernates in areas and may make short seasonal migrations from higher elevations to preferred hibernacula. Forms large maternity colonies of several thousand in buildings, caves and bridge structures. Mates in the fall and bears one young between late May to mid-June. The Yuma bat has been found roosting with other bats including pallid, and mexican free-tailed bats. Animals have lived up to 8.8 years (Zeiner 1990).

Myotis yumanensis can be very difficult to distinguish from the little brown myotis (Myotis lucifigus) which is not listed. The two species may hybridize and ranges overlap in the mid to north western, north eastern and eastern parts of California. M. lucifigus usually has shiny tipped fur compared to dull tipped fur of M. yumanensis. Size is generally larger in M. lucifigus, but overlap does occur in measurements and slope of forehead. Some subspecies of M.yumanensis may have shiny fur.

Findings and Conclusions

Reasons of decline for this species include loss of suitable roosting sites habitat, including destruction and disturbance, and pesticides.

The Stanislaus River flows through the range of the Yuma myotis. Our surveys resulted in the capture of one animal at Caswell State Park on October 25, 1994. More animals are expected to occur along the length of the river.

The project should have minimal impacts on the Yuma myotis as long as foraging areas and roosting sites are not destroyed.