Texas Tech University

Myotis occultus Hollister 1909

Order Chiroptera : Family Vespertilionidae

DESCRIPTION. A small bat; hairs of back with long, glossy tips that produce a conspicuous sheen; pelage long and full, longest hairs about 10 mm; upperparts ranging from bronzy brown to olive brown; underparts grayish with rich, buffy suffusion; interfemoral membrane sparsely haired above, about to line joining knees; foot relatively large, a little more than half the length of tibia; ratio of tail to head and body <0.80; dorsal profile of skull gradually rising from relatively short rostrum. Dental formula: I 2/3, C 1/1, Pm 3/3, M 3/3 × 2 = 38. Averages for external measurements: total length, 85 mm; tail, 35 mm; foot, 9 mm; ear, 13 mm; forearm, 39.5 mm. Weight, 7–9 g.

Myotis occultus

DISTRIBUTION. Only a single specimen of this bat has been collected in Texas. This specimen, consisting of a skin and skull (US National Museum, 21083/36121), was collected in June 1893 near Fort Hancock in Hudspeth County in the Trans-Pecos. It was probably a migrant individual, and it is doubtful that a resident population of this bat occurs in Texas. Myotis occultus is relatively uncommon in the southwestern United States and northern Mexico. In New Mexico, it is known from low-elevation riparian areas in the Rio Grande Valley and montane highlands, and its occurrence near Fort Hancock would be consistent with this interpretation.

Distribution of Myotis occultus

SUBSPECIES. Monotypic species.

HABITS. This bat spends the daytime in crevices in canyon walls, caves, attics, or other places of concealment and emerges shortly before dark. Its flight is erratic and relatively slow. In New Mexico, vegetation zone seems to be unimportant in determining their distribution, but in Arizona they are usually found in ponderosa pine or pine–oak woodlands.

In the western part of their range, they are thought to hibernate within their summer range. As cold weather approaches the bats move to suitable caves, mine tunnels, or other quarters where they hibernate and sleep through the winter. During the period of preparation for winter, males and females are found together and breeding takes place, but the ova are not fertilized at that time. This habit of breeding in the fall has led some students to estimate the gestation period of this species to be 300 days; however, the period of gestation is actually 50–60 days. The sperm from the fall mating are retained in the reproductive tract of the female, and fertilization of the ova does not take place until the following spring, shortly before the bats leave their winter quarters. This ability is known as delayed fertilization. A short breeding period may also occur in the spring. The single young is born in June or July. Food consists of insects captured in flight. Remains of small, night-flying beetles, true bugs, and flies have been identified in their stomachs. These bats forage primarily over or near water.

POPULATION STATUS. Extralimital. The southwestern little brown myotis is relatively uncommon throughout its range, and only a single specimen has been collected in Texas. This specimen was collected near Fort Hancock, Hudspeth County, in June 1893. It was probably a migrant individual, and it is doubtful that a resident population of this bat occurs in Texas.

CONSERVATION STATUS. The IUCN status of the southwestern little brown myotis is least concern, and it does not appear on the federal or state lists of concerned species. Populations have declined sharply in much of its historical range in California as a result of pesticides, control measures in nursery colonies, and disturbance at hibernation sites. It needs to be looked for elsewhere in the Trans-Pecos of Texas before its status can be definitively determined.

REMARKS. There has been a lengthy debate among taxonomists about whether M. occultus is a distinct species or a subspecies of the more wide-ranging M. lucifugus. Indeed, previous editions of The Mammals of Texas treated it as the latter, a subspecies of lucifugus. But recent molecular genetic studies published in the Journal of Mammalogy (volume 83, 2002) by Mike Bogan and associates suggest that occultus is separated from lucifugus by sufficient genetic distance to be considered a separate species. Those authors regard M. occultus as an uncommon but relatively widespread southwestern endemic bat that could be in jeopardy because of major population declines.

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From The Mammals of Texas, Seventh Edition by David J. Schmidly and Robert D. Bradley, copyright © 1994, 2004, 2016.  Courtesy of the University of Texas Press.

Natural Science Research Laboratory