California Department of Fish and Wildlife

Bay Delta Region

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Acting Regional Manager:
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

Fringed Myotis

Myotis thysanodes

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


Life History

This myotis is characterized by light to dark brown fur, a well developed sagittal crest, large ears that extend 3 to 5 mm beyond the muzzle when laid forward, and a conspicuous fringe of hair along the border of the interfemoral membrane. Body measurements are: total length, 80 to 95 mm; foot, 8 to 11 mm (Ingles 1965); ear, 16 to 19 mm; forearm, 39.8 to 46 mm; and greatest length of skull, 16.2 to 17.2 mm (Hall 1981).

The fringed myotis is distributed from British Columbia south to southern Mexico and in the United States from the west coast to south west Montana, down through Idaho, Utah, the south west quarter of Colorado, New Mexico and western Texas. Three subspecies are recognized with only one, M.t. thysanodes, found in California (Hall 1981).

This bat is found over the majority of California up to 9,350 feet (2850 meters), excluding the Central Valley and southern deserts. It appears to be common locally in main habitats including pinyon-juniper, valley foothill hardwood and hardwood conifer forest from 4,000 to 7,000 feet (1300 to 2200 meters) (Zeiner 1990).

M. thysanodes feeds on a variety of insects, primarily beetles, and may feed from the ground or glean insects from foliage. This bat is also capable of hovering and forages in open habitats of streams, lakes, and ponds. Activity begins soon after sunset, usually one to two hours afterward and sometimes four to five hours afterward. This species is fairly tolerant of cold and hibernation occurs from October to March. Short migratory movements to hibernating sites may occur.

This myotis may roost in caves, buildings and crevices with adults and sub adults forming separate groups. Mating occurs in the fall and large maternity colonies of up to 200 individuals form from late April to September. One young is born from May to July, mostly in late June, and lactating females can be found through August.

Findings and Conclusions

This bat is easily disturbed at roost sites and may be causing a decline in the species. The destruction of suitable roosting sites, pesticide use, eradication from buildings and destruction of foraging habitat could also play a critical role.

The fringed myotis may be found in the upper reaches of the Stanislaus River. Our surveys occurred during the colder part of the year and did not resulted in the capture of any animals.

The project should have minimal impacts on the fringed myotis as long as foraging areas and roosting sites are not destroyed. Further surveys for foraging and roosting areas should be carried out and all sites identified.