Gray Bat

The gray bat, often called the gray myotis (Myotis grisescens), is an endangered species of bat that has a sporadic distribution throughout the southeastern United States, a region known for the limestone caves in which these bats live. In Alabama, gray bats are found throughout most of the state except in the southwestern coastal plains. The genus Myotis is derived from the Greek word for "mouse-eared," and the species name, grisescens, is Latin for "becoming gray."

The gray myotis is characterized by its dark uniformly gray fur, although the hair color may bleach to a russet or chestnut brown during molting season, which runs from June to August. Unlike other Myotis species, the gray bat's wing membrane connects to the ankle of the foot rather than to the base of the toes. The gray bat is one of the largest species of Myotis in North America and weigh between roughly 0.3 and 0.6 ounces (~ 7 to 16 grams). They are approximately 3.5 inches (~ 8.9 centimeters) in length with a wingspan between 11 to 13 inches (~ 27 to 33 centimeters). Gray bats mature at about two years of age and can live up to 15 years, but only about half live to maturity.

Although many bat species live in vegetation and even human-made structures, the gray bat lives almost exclusively in caves. With few exceptions, gray bats are found roosting in caves year-round and require specific conditions in order to establish a colony. When hibernating in winter, the bats prefer deep vertical caves with large open rooms that act as cold traps and keep the temperature between 42°F (~5.6°C) and 52°F (~11°C). In the summer, the caves must be warm with temperatures ranging between 57°F (~14°C) and 77°F (~25°C). Most colonies are found in caves that support these conditions and are typically located around rivers or lakes for feeding. About 95 percent of hibernating gray bats are found in only nine caves spread throughout the southeastern United States. They have also been known to migrate up to 300 miles between summer and winter hibernation caves. In Alabama, gray bats are found in the greatest numbers in north Alabama in caves around the Wheeler National Wildlife Refuge (NWR) near Decatur, Morgan County, around Lake Guntersville in Marshall County, and along the Tennessee River Valley. Fern Cave NWR in Jackson County near Gurley, Madison County, hosts a wintering cave and supports the largest colony of gray bats in the United States, with an estimated 1.5 million bats roosting there annually. The Sauta Cave NWR southwest of Scottsboro, Jackson County, and Hambrick Cave located just east of Guntersville Lake and Dam are used as summer caves and are home to more than 200,000 bats combined. Key Cave NWR near Florence, Lauderdale County, was established to protect the Alabama cavefish, but is also an important brooding site for thousands of gray bats.

During the summer months, gray bats forage over streams and large bodies of water close to where the colonies roost. They navigate and detect prey using echolocation. Gray bats primarily feed on night-flying insects such as moths, beetles, and various types of flies. Despite roosting near food-rich bodies of water, gray bats have been known to fly up to 12 miles from the colony to forage.

Gray bats breed in winter, after their migration to cooler hibernation caves. Female gray bats enter hibernation shortly after breeding and are able to delay fertilization of their eggs until they emerge from hibernation in the spring. Males use the few weeks after mating to restore fat reserves that were lost during the breeding process and thus go into hibernation later than females. During the winter, gray bats drop their body temperature to just above freezing, which allows them to conserve energy during their six-month hibernation. When the adults emerge in the spring, adult mortality is high in bats that did not accumulate sufficient fat reserves to fuel their journey to their summer caves.

Females give birth to a single offspring in June in the summer caves. Weaning takes two months, and during that period of time the adult females and their newborns roost in summer maternity caves. Gray bat males and yearlings form separate bachelor colonies in summer caves, or sometimes in the cooler areas of the maternity caves. All juveniles are able to fly by August, when the migration to the winter caves begins.

Prior to the Civil War, millions of gray bats were found throughout Alabama, however, regular disturbance for the extraction of the bat guano to be used for gunpowder contributed to a decline in their populations. The bats were placed on the Endangered Species List in 1976 and the U.S. Fish and Wildlife Service lists them as one with the highest conservation concern. Between 1960 and 1980, the gray bat population decreased by nearly 80 percent. They continue to face many threats, including disturbances to their caves that may cause them to completely abandon them. Disturbances in maternity caves may cause mothers to drop babies in places where they cannot be recovered or abandon them. Disturbances during winter can cause bats to waken and use up scarce energy reserves. Other threats include habitat loss to urban development and a reduction in the number of sites due to the gating of caves which can prevent access and alter temperature and humidity within the cave. Cave exploring, also known as spelunking, tourism, pesticides, and flooding caused by dam construction all threaten bat survival and the quality of their habitats.

Additional Resources

Clark, Donald R., Fred M. Bagley, and W. Waynon Johnson. "Northern Alabama Colonies of the Endangered Grey Bat Myotis grisescens: Organochlorine Contamination and Mortality." Biological Conservation 43 (1988): 213-25.

Decher, Jan, and Jerry R. Choate. "Myotis grisescens." Mammalian Species 510 (October 1995): 1-7.

Brack, Virgil, and Richard K. LaVal. "Diet of the Gray Myotis (Myotis grisescens): Variability and Consistency, Opportunism, and Selectivity." Journal of Mammalogy 87 (February 2006): 7-18.