Gray Bat

Myotis grisescens

Description

The tiny gray bat, Myotis grisescens, weighs about 0.35 oz (9.9 g) at maturity and its forearm measures only about 1.6 in (2.9 cm) long. The gray bat is uniform dark gray for most of the year. For a short period after molt in July or August, gray bats are dark gray; but their fur usually bleaches to russet between molts. This difference in fur color is especially apparent in females during their reproductive season in May or June. The wing membrane connects to the foot at the ankle rather than at the base of the first toe, as in other species of Myotis.

Behavior

Bats are the only mammals capable of true flight, having forelimbs with the same general configuration as other mammals but with the bones of the fingers greatly elongated to support thin folds of skin that form wings and with hind limbs modified to allow them to alight and hang upside down by their toes.

Bats are nocturnal and navigate by uttering a continuous series of high-pitched cries that return as echoes when the sounds bounce off solid objects. This form of sonar detection is called echolocation. Gray bats roost in the daytime in caves, mines, or deep rock crevices. They feed almost exclusively on night-flying aquatic insects, including mosquitoes. A single colony can consume literally tons of these insects nightly.

One-way migrating distance between winter and summer caves may vary from as little as 10 mi (16.1 km) to well over 200 mi (321.9 km). Banding studies indicate the bats occupy a rather definite summer range with relation to the roosting site and nearby foraging areas over large streams and reservoirs. Summer colonies show a preference for caves not over 1.2 mi (1.9 km) from the feeding area. Colonies migrate between established maternity and hibernation caves. Gray bats mate on arrival at the hibernation cave in September and early October, and females immediately go into hibernation. Males feed for several additional weeks, replenishing fat supplies used during breeding. While hibernating, bats lower their body temperature to reduce metabolism and conserve energy. Adult females store sperm through the winter and become pregnant soon after emerging in the spring, giving birth to a single young in late May or early June. At that time, mature females and young congregate in maternity caves, while males and immature females congregate in "bachelor" caves within the range.

Because fat reserves are depleted and available food supplies are low, adult mortality is especially high immediately before and after emergence. During the period of peak nursing demand, when young are between 20 and 30 days old, nursing females feed continuously throughout the night to meet their energy requirements.

By August, all of the juveniles are flying and there is a general mixing and dispersal of the colony over the summer range. Fall migration begins around the first of September and is generally complete by early November.

Habitat

With rare exceptions, gray bats roost in caves carved out of limestone formations. In the winter they seek deep, vertical caves with narrow entrances that deflect the cold winds. Summer roosts and maternity caves are nearly always located near rivers or reservoirs where insects are abundant. Adult gray bats feed over water, along rivers or reservoir edges.

Distribution

Gray bats were abundant in Alabama, northern Arkansas, Kentucky, Missouri, and Tennessee. Fewer numbers were found at more restricted sites in northwestern Florida, western Georgia, southeastern Kansas, southern Indiana, southern Illinois, northeastern Oklahoma, northeastern Mississippi, western Virginia, and possibly in western North Carolina. Many millions of gray bats once inhabited the historic range.

During the Civil War, bat guano was used to produce saltpeter for the manufacture of gunpowder. Nearly every known gray bat cave was exploited for this purpose, and significant declines in the gray bat population can be dated from this era.

Populations are found mainly in Alabama, northern Arkansas, Kentucky, Missouri, and Tennessee, but a few occur in northwestern Florida, western Georgia, southwestern Kansas, south Indiana, south and southwestern Illinois, northeastern Oklahoma, northeastern Mississippi, western Virginia, and possibly western North Carolina. Distribution within range was always patchy, but fragmentation and isolation of populations have been a problem over the past four decades.

The number of gray bats is difficult to estimate and existing figures for some caves might be off by as much as 50%. The 1982 Recovery Plan estimated the total population of gray bats to be less than 1.6 million with the majority found in only nine major hibernation caves. But protective measures taken at high priority colony sites in the late 1970s and throughout the 1980s arrested the population declines at some major sites and those populations are now stable or in some cases are increasing.

Threats

Because the bulk of the gray bat population is restricted to so few hibernation caves, gray bats are particularly vulnerable to human disturbance. Bats aroused from hibernation often starve because they use up fat reserves that cannot be replenished until spring. In the 1950s cave exploration first became popular, and many caves were disturbed so often that bat colonies died out. Formerly isolated areas in the vicinity of caves are now used for recreation, bringing humans and bats into closer and more frequent contact. Some of the largest gray bat colonies ever known were lost as a result of cave commercialization.

Human disturbance and vandalism may have been primarily responsible for the decline. Disturbance of a maternity colony may cause thousands of young to be dropped to the cave floor where they perish; excessive disturbance may cause a colony to completely abandon a cave.

Deforestation and brush clearing near cave entrances favor predators such as the screech owl, which are able to capture bats more successfully in open habitat. Since female gray bats produce only one young per year, even slight increases in predation have a significant impact on population size. Bats are most susceptible to predation during migration when they typically fly along a forested corridor from summer to winter caves. Deforestation has deprived bats of safe migration routes. Another probable factor in bat decline is the routine use of agricultural insecticides and pesticides, which reduces the overall food supply and introduces poisons into the food chain. None of these factors, however, has been as devastating as human intrusion into bat roosts.

Conservation and Recovery

The most immediate strategy to protect the gray bat is to limit winter access to the hibernation caves and to protect important summer roosts. Most cave entrances on public lands have been fenced or gated to discourage intruders. The U.S. Fish and Wildlife Service (FWS) has sought the cooperation of landowners to allow gating and the posting of warning signs at cave entrances on private land.

After the gray bat was listed as Endangered in 1976, the Tennessee Valley Authority sponsored a research and recovery project that resulted in protection of two vitally important maternity sites, the Hambrick and Nickajack caves in Tennessee. Subsequently, the FWS acquired and protected Blowing Wind (the most important summer cave), Cave Springs, Sauta, and New Fern caves in Alabama. New Fern Cave shelters the world's largest known hibernating gray bat population.

In 1985, The Nature Conservancy, in cooperation with Bat Conservation International, acquired Judges Cave, the most important surviving nursery colony in Florida, and Hubbards Cave in Tennessee. Hubbards Cave is one of the three most important hibernation caves. A census that same year suggested that conservation efforts were already yielding important results. Before protection, four caves—Hambrick, Nickajack, Cave Springs, and Blowing Wind—were inhabited by a total of 128,000 bats. None of the caves supported a nursery colony. Each of these caves now supports an expanding nursery colony, and the total population has climbed above 700,000.

While other important hibernation sites remain unprotected, the momentum for protecting the gray bat is building. Recently, organized caving groups, such as the Tennessee Cave Survey, have identified bat habitats and placed them off-limits to their memberships during sensitive times.

Contacts

Regional Office of Endangered Species
U.S. Fish and Wildlife Service
P. O. Box 1306
Albuquerque, New Mexico 87103
http://southwest.fws.gov/

Regional Office of Endangered Species
U.S. Fish and Wildlife Service
Federal Building
Ft. Snelling
Twin Cities, Minnesota 55111
http://midwest.fws.gov/

Regional Office of Endangered Species
U.S. Fish and Wildlife Service
1875 Century Blvd., Suite 200
Atlanta, Georgia 30345
http://southeast.fws.gov/

Regional Office of Endangered Species
U.S. Fish and Wildlife Service
300 Westgate Center Dr.
Hadley, Massachusetts 01035
http://northeast.fws.gov/

Regional Office of Endangered Species
U.S. Fish and Wildlife Service
P. O. Box 25486
Denver Federal Center
Denver, Colorado 80225
http://www.r6.fws.gov/

References

Stevenson, D. E. 1981. "Survivorship of the Endangered Gray Bat (Myotis grisescens )." Journal of Mammalogy 65:244-257.

Tuttle, M. D. 1979. "Status, Causes of Decline, and Management of Endangered Gray Bats." Journal of Wildlife Management 43:1-17.

U.S. Fish and Wildlife Service. 1982. "Gray Bat Recovery Plan." U.S. Fish and Wildlife Service, Atlanta.