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Myotis grisescensgray myotis

By Vanessa Harriman

Geographic Range

Myotis grisescens is widely distributed in the southeastern United States of America. The distribution of gray bats within their range has always been patchy. Gray bats inhabit the cave regions of northern Arkansas, Missouri, Kentucky, Tennessee, and Alabama. There are also occasional colonies in northwestern Florida, western Georgia, southwestern Kansas, southern Indiana, southern and southwestern Illinois, northeastern Oklahoma, northeastern Mississippi, western Virginia, and possibly western North Carolina. (U.S. Fish and Wildlife Service Division of Endangered Species, 1991)

Habitat

Gray bats are restricted entirely to areas with caves or cave-like habitats. These caves are in limestone karst areas of the southeastern United States. Gray bats do not inhabit barns or other similar structures. This leads to extremely restricted nesting opportunities. Due to their requirement of unique cave types, Gray bats can only use 0.1% of available caves in the winter and 2.4% in the summer. (Tuttle, 1986; U.S. Fish and Wildlife Service, 1997)

Ninety-five percent of the total Gray bat population hibernates in only eight or nine caves. Two are located in Tennessee, three in Missouri, one in Kentucky, one in Alabama, and one in Arkansas. The Arkansas hibernation cave houses about 250,000 Gray bats. The winter caves utilized by Gray bats have deep, vertical passages with large rooms that function as cold air traps. The temperature of these caves ranges between 6 and 11 degrees Celsius (42 and 52 degrees Fahrenheit). (Kentucky Bat Working Group, 1999; U.S. Fish and Wildlfe Service, 1992; U.S. Fish and Wildlife Service Division of Endangered Species, 1991)

As they are for the winter sites, gray bats are highly selective for caves providing specific temperature and roost conditions in the summer. These caves are warm, ranging between 14 and 25 degrees Celsius (57 and 77 degrees Fahrenheit). As an alternative to finding a cave within this temperature range, they can roost in caves with small rooms or dorms that trap the body heat of the roosting bats. Summer colonies of gray bats occupy a home range that often contains several roosting caves scattered along as much as 81 kilometers of river or lake shore. Banding studies have indicated that gray bats prefer summer caves that have a feeding area (river or other reservoir of water) not over 2 kilometers away. Despite this, they have been known to fly as far as 19 kilometers from the colony to feed. (Kentucky Bat Working Group, 1999; U.S. Fish and Wildlfe Service, 1992; U.S. Fish and Wildlife Service Division of Endangered Species, 1991)

Physical Description

M. grisecens is the largest member of its genus in the eastern United States. They weigh between 7 and 16 g and are 75 to 101 mm in length. Forearm length ranges between 40 and 46 mm. Gray bats can be distinguished from all other eastern bats by their uni-colored dorsal fur (all others have bi- or tri-colored dorsal fur). They are also the only species of Myotis in which the wing membrane connects to the foot at the ankle as opposed to connecting at the base of the first toe.

Gray bats are dark gray in color directly after they molt in July or August. Between molts, they bleach to a russet color. This difference in fur color is most apparent in females during the reproductive season (May or June). (U.S. Fish and Wildlife Service Division of Endangered Species, 1991)

  • Sexual Dimorphism
  • sexes alike
  • Range mass
    7 to 16 g
    0.25 to 0.56 oz
  • Range length
    75 to 101 mm
    2.95 to 3.98 in

Development

After entering the winter cave, female Gray Bats are inseminated by sexually active male bats. The females exhibit delayed fertilization. After copulating, the females hold the sperm through hibernation. Fertilization between the sperm and ova occurs when the female emerges from hibernation. Females do not reach sexual maturity until they are two years old. For their size, bats are among the world’s slowest reproducing mammal.

One offspring per sexually mature female is born in June when the colonies have migrated to their summer ranges. The period between birth and weaning is two months. During these two months there is segregation between members of the colony. The adult females and their newborns roost in maternity caves. The adult males and yearlings of both sexes roost in bachelor caves. By August, all the juveniles are flying (most are capable of flight 20-25 days after birth) and general mixing and dispersal of the colony occurs over the summer range. The growth rates of young vary with the temperature at the maternity roosts. It has been discovered that young in warmer roost situations grow more rapidly. (U.S. Fish and Wildlife Service, 1992; U.S. Fish and Wildlife Service Division of Endangered Species, 1991)

Reproduction

Information on the mating system of these animals is sparse. They are reportedly polygynous.

Breeding in this species occurs shortly after the bats enter their hiberation caves. After entering the winter cave, the female gray bat is inseminated. Females exhibit delayed fertilization. After copulating, a female holds the sperm through hibernation. Fertilization of ova occurs when the female emerges from hibernation.

Females do not reach sexual maturity until they are two years old. For their size, bats are among the world’s slowest reproducing mammal.

Females give birth to a single offspring in June, after migration to the summer caves has taken place. The period between birth and weaning is two months. During these two months there is segregation between members of the colony. The adult females and their newborns roost in maternity caves. The adult males and yearlings of both sexes roost in bachelor caves.

By August, all the juveniles are flying (most are capable of flight 20-25 days after birth) and general mixing and dispersal of the colony occurs over the summer range.

The growth rates of young vary with the temperature at the maternity roosts. It has been discovered that young in warmer roost situations grow more rapidly. (U.S. Fish and Wildlfe Service, 1992; U.S. Fish and Wildlife Service Division of Endangered Species, 1991)

  • Breeding interval
    These animals breed once per year.
  • Breeding season
    Breeding occurs in the fall, when the bats enter their winter caves.
  • Range number of offspring
    1 to 1
  • Average number of offspring
    1
    AnAge
  • Average weaning age
    60 days
  • Average age at sexual or reproductive maturity (female)
    2 years
  • Average age at sexual or reproductive maturity (female)
    Sex: female
    456 days
    AnAge
  • Average age at sexual or reproductive maturity (male)
    2 years
  • Average age at sexual or reproductive maturity (male)
    Sex: male
    456 days
    AnAge

As in all mammals, the mother provides milk to her growing young. Neonate gray bats are altricial. The mother attends to her young in a nursery cave. This is especially interesting, because the mother can locate her own offspring among the hundreds of baby bats which may be in the cave. (Kentucky Bat Working Group, 1999; U.S. Fish and Wildlfe Service, 1992; U.S. Fish and Wildlife Service Division of Endangered Species, 1991)

  • Parental Investment
  • altricial
  • pre-hatching/birth
    • provisioning
      • female
    • protecting
      • female
  • pre-weaning/fledging
    • provisioning
      • female
    • protecting
      • female

Lifespan/Longevity

The maximum lifespan for gray bats is 14-15 years. (U.S. Fish and Wildlfe Service, 1992)

  • Typical lifespan
    Status: wild
    14 to 15 years

Behavior

Because of the limitations placed on them by using only a small fraction of the caves available, these bats are colonial.

Gray bats spend their daylight hours in caves. These bats are highly selective about the caves they will occupy, as discussed in the habitat section. During the night hours, gray bats forage on insects, usually flying above a river, stream, or reservoir.

Gray bats hibernate during the winter in special caves. Fall migration to the wintering caves begins around the first of September and is completed by early November. The one-way distance between the winter and summer caves may vary from as little as 16 kilometers to well over 322 kilometers. Transit or stop-over caves are used along the way.

Hibernating gray bats form clusters of several thousand bats. Density can reach 170 bats per square foot.

Beginning in late March, gray bats begin to come out of hibernation. Adult females emerge from hibernation first, followed by the juveniles, then the adult males. (Kentucky Bat Working Group, 1999; U.S. Fish and Wildlfe Service, 1992; U.S. Fish and Wildlife Service Division of Endangered Species, 1991)

Home Range

Home range varies in this species. Bats can fly a long way to a water area to forage at night, but do no use the entire area over which they fly to obtain food.

Communication and Perception

As in all mammals, there are a variety of means of communication. Bats use vocalizations to communicate with each other while they are in their roosts. Mothers and infants use tactile and vocal communication. There are probably some scent cues which help mothers to recognize their young.

Echolocation is used primarily to locate food. However, communication also occurs between predator and prey through echolocation. Some insects (particularly moths) can receive the sonar pulses from the bats and fly erratically to avoid being eaten. (Feldhamer, et al., 1999)

Food Habits

M. grisescens forages over streams and reservoirs where they consume night-flying aquatic insects. Like all microchiropterans, gray bats rely on echolocation to locate their food.

Most foraging occurs within 5 meters of the water surface over which they are feeding.

Until most recently, studies focusing on the diet of gray bats were not preformed. Data on this subject are therefore preliminary. Whether gray bats are opportunistic or selective feeders is still in debate. Recent studies suggest that this species feeds selectively, but more information is needed.

Originally it was thought that gray bats fed primarily on mayflies. M. grisescens has been seen feeding in large swarms of mayflies, but this insect has not been turning up in fecal analysis in the proportion that might have been expected. It is possible that mayflies are wholly digested, thus not often seen in the fecal records.

Analysis of gray bat feces has shown that thes bats most often select moths, flies, and beetles as prey when these species are present.

Other prey includes spiders, bugs, leafhoppers, scorpionflies, lacewings, dragonflies, stoneflies, grasshoppers, thrips and wasps. Various insects occur sporadically in fecal pellet analysis. (Best, et al., 1997; Lacki, et al., 1995; U.S. Fish and Wildlfe Service, 1992)

  • Animal Foods
  • insects
  • terrestrial non-insect arthropods

Predation

Gray bats may fall prey to howks, owls, skunks, foxes, mice, snakes, and housecats. Anti-predator adaptations include avoiding crowded cave entrances and flying more rapidly when exiting and entering the cave. (Ludlow and Gore, 2000)

Ecosystem Roles

One bat can catch up to 3,000 insects in one night. Because of this, they play an important role in the checks and balances of nature as the primary controllers of night-flying aquatic insects. (Tuttle, 1986)

Economic Importance for Humans: Positive

Insectivorous bats remove millions of insects a night, aiding in the control of these populations. Also, because of their roosting habits, inhabiting a small number of specific caves for long periods of time, these bats produce huge piles of feces on the floors of caves. Historically, this guano was used to make gunpowder during the civil war. Also, native americans used to eat these bats in stews. (Tuttle, 1986)

  • Positive Impacts
  • food
  • body parts are source of valuable material
  • produces fertilizer
  • controls pest population

Economic Importance for Humans: Negative

They do not adversely affect humans. Gray Bats are great!

Conservation Status

M. grisescens is considered endangered by both the United States Fish and Wildlife service, according to the Endangered Species Act, and the IUCN Red List. Although CITES does not list this species of bat on Appendix I, II, or III, it is difficult to see why.

Gray bats began encountering problems in prehistorical times when tribes of Native Americans began camping and living in the entrances of caves. The smoke from their fires likely suffocated the bats. It is also thought that they were placed in stews by Native Americans.

Guano was extracted from nearly every substantial gray bat cave in the south during the Civil War. This guano was used for gunpowder, not for fertilizer, as is commonly thought. It is thought that gray bat colonies suffered some of their largest losses during the Civil War. Studies of guano deposits in formerly occupied caves shows that gray bats (a highly resilient species) were able to prosper once again in spite of these losses. (Tuttle, 1986)

Currently, the biggest threat to gray bat populations appears to be human disturbance at hibernation and maternity colonies. The bats in the maternity colonies do not tolerate disturbance, especially when flightless newborn young are present. Thousands of baby bats may be dropped to their deaths or abandoned by panicked parents. A colony will even completely abandon a cave in the presence of excessive disturbance. This is particularly bad because so few caves are habitable for gray bats. Starvation in the winter can also be a problem. When bats are aroused during hibernation, their important fat reserves are used up more quickly. If the disturbance is intense or frequent enough the bats may starve to death. (Kentucky Bat Working Group, 1999; Tuttle, 1986; U.S. Fish and Wildlife Service Division of Endangered Species, 1991)

Despite once being one of the most abundant mammals of the southeastern United States, M. grisescens has been listed as an endangered species since 1976. In 1970 the population was estimated at 2.25 million bats after a census of 120 caves. However, a census in 1976 of 22 of the 120 caves found that these colonies had declined by an average of 54 percent each.

Other factors that influence the decline of Gray Bat populations are: vandalism, cave commercialization, toxins (like organochlorine pesticides, PCB’s, and lead), natural causes like cave-ins and flooding (killing bats and destroying important habitat), loss of caves by inundation by man-made impoundments, and reduction of insect prey over streams that have been degragaded by excessive pollution and siltation. (Arkansas Game and Fish Commission, 199; Clawson and Clark, 1989) (Tuttle, 1986)

Improper gating at cave entrances also presents a problem. Gates must allow the airflow, temperature, humidity, and amount of light entering the cave to be the same as it was prior to the gate installment. Although steel bar gates do provide excellent protection from humans, these gates may be detrimental to bats by giving predators a place to perch and wait for bats to emerge. It has also been found that bats prefer to use un-gated entrances. The alternative, if possible, would be to put up a chain link fence topped with barbedwire around the cave. This would prevent humans from entering the bats' caves, and allow the bats to fly OVER the gate, rather than through it. This would also protect them from predators perched on the gates. (Ludlow and Gore, 2000)

Due to protective increases at high priority colony sites, declines in M. grisescens populations have been halted in some locations, and others exhibit an increase in population. Currently there are about 1.5 million gray bats in existence. Important conservation measures that have been taken to aid in the stabilization of the population, especially the acquisition of caves by the U.S. Fish and Wildlife Service. This organization is currently in control of Blowing Wind Cave in northern Alabama. This is the most important summer cave for gray bats known. Fern Cave is the largest hibernaculum for gray bats and is also under the protection of the U.S. Fish and Wildlife Service. (U.S. Fish and Wildlife Service Division of Endangered Species, 1991)

Additional conservation measures are needed to help M. grisescens. The purchase and protection through proper gating and restricted use of other gray bat caves is very important. Education of spelunkers and other cave visitors who may unintentionally disturb the bats is key, as well as the continuation of federal efforts to reduce pesticide use (or at least limit their lifetime in the environment). (U.S. Fish and Wildlife Service Division of Endangered Species, 1991)

Temperate North American bats are now threatened by a fungal disease called “white-nose syndrome.” This disease has devastated eastern North American bat populations at hibernation sites since 2007. The fungus, Geomyces destructans, grows best in cold, humid conditions that are typical of many bat hibernacula. The fungus grows on, and in some cases invades, the bodies of hibernating bats and seems to result in disturbance from hibernation, causing a debilitating loss of important metabolic resources and mass deaths. Mortality rates at some hibernation sites have been as high as 90%. While there are currently no reports of Myotis grisescens mortalities as a result of white-nose syndrome, the disease continues to expand its range in North America. (Cryan, 2010; National Park Service, Wildlife Health Center, 2010)

Contributors

Nancy Shefferly (), Animal Diversity Web.

Vanessa Harriman (author), Humboldt State University, Brian Arbogast (editor), Humboldt State University.

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

acoustic

uses sound to communicate

altricial

young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

colonial

used loosely to describe any group of organisms living together or in close proximity to each other - for example nesting shorebirds that live in large colonies. More specifically refers to a group of organisms in which members act as specialized subunits (a continuous, modular society) - as in clonal organisms.

colonial growth

animals that grow in groups of the same species, often refers to animals which are not mobile, such as corals.

delayed fertilization

a substantial delay (longer than the minimum time required for sperm to travel to the egg) takes place between copulation and fertilization, used to describe female sperm storage.

echolocation

The process by which an animal locates itself with respect to other animals and objects by emitting sound waves and sensing the pattern of the reflected sound waves.

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

fertilization

union of egg and spermatozoan

food

A substance that provides both nutrients and energy to a living thing.

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

hibernation

the state that some animals enter during winter in which normal physiological processes are significantly reduced, thus lowering the animal's energy requirements. The act or condition of passing winter in a torpid or resting state, typically involving the abandonment of homoiothermy in mammals.

insectivore

An animal that eats mainly insects or spiders.

internal fertilization

fertilization takes place within the female's body

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

migratory

makes seasonal movements between breeding and wintering grounds

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

nocturnal

active during the night

polygynous

having more than one female as a mate at one time

riparian

Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).

seasonal breeding

breeding is confined to a particular season

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

social

associates with others of its species; forms social groups.

sperm-storing

mature spermatozoa are stored by females following copulation. Male sperm storage also occurs, as sperm are retained in the male epididymes (in mammals) for a period that can, in some cases, extend over several weeks or more, but here we use the term to refer only to sperm storage by females.

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

terrestrial

Living on the ground.

visual

uses sight to communicate

viviparous

reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

References

Best, T., B. Milam, T. Haas, W. Cvilikas, L. Saidak. 1997. Variation in diet of the gray bat (*Myotis Grisescens*). Journal of Mammalogy, 78(2): 569-583.

Clawson, R., D. Clark, Jr.. 1989. Pesticide contamination of endangered gray bats and their food base in Boone County, Missouri, 1982. Bulletin of Environmental Contamination and Toxicology, 42: 431-437.

Cryan, P. 2010. "White-nose syndrome threatens the survival of hibernating bats in North America" (On-line). U.S. Geological Survey, Fort Collins Science Center. Accessed September 16, 2010 at http://www.fort.usgs.gov/WNS/.

Feldhamer, G., L. Drickamer, S. Vessey, J. Merritt. 1999. Mammalogy: Adaptation, Diversity, and Ecology. San Francisco: McGraw-Hill Higher Education.

Kentucky Bat Working Group, 1999. "Gray Bat" (On-line). Accessed November 2, 2001 at http://www.biology.eku.edu/bats/graybat.htm.

Lacki, M., L. Burford, J. Whitaker, Jr.. 1995. Food habits of gray bats in Kentucky. Journal of Mammalogy, 76(4): 1256-1259.

Ludlow, M., J. Gore. 2000. Effects of a cave gate on emergence patterns of colonial bats. Wildlife Society Bulletin, 28(1): 191-196.

National Park Service, Wildlife Health Center, 2010. "White-nose syndrome" (On-line). National Park Service, Wildlife Health. Accessed September 16, 2010 at http://www.nature.nps.gov/biology/wildlifehealth/White_Nose_Syndrome.cfm.

Private Forest Management Team, 2001. "Gray Bat" (On-line). Accessed November 2, 2001 at http://www.pfmt.org/wildlife/endangered/gray_bat.htm.

Tuttle, M. 1986. "Endangered Gray Bat Benefits From Protection" (On-line). Accessed November 2, 2001 at http://www.batcon.org/batsmag/v4n4-1.html.

U.S. Fish and Wildlfe Service, 1992. "Gray Bat in North Carolina" (On-line). Accessed November 2, 2001 at http://nc-es.fws.gov/mammal/graybat.html.

U.S. Fish and Wildlife Service Division of Endangered Species, 1991. "Gray Bat" (On-line). Accessed November 2, 2001 at http://endangered.fws.gov/i/a/saa4l.html.

U.S. Fish and Wildlife Service, 1997. "Gray Bat" (On-line). Accessed November 2, 2001 at http://midwest.fws.gov/endangered/mammals/grbat_fc.html.

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