Myotis Grisescens: The Grey Bat

Taxonomy

The genus Myotis falls under the order Chiroptera and family Vespertilionidae among the microchiropteran bats (Nowak 1994).

Geographical Distribution

Myotis grisescens is an endangered species of bat that inhabits only four states in the United State. These states include Missouri, Kentucky, Tennessee, and Alabama (Brack 1984). Colonies of these bats are occasionally found in nearby states as well. Myotis grisescens can only be found in the Southeastern United States, while members of its genus Myotis can also be found all over the world.

Physical Characteristics

All species in the genus Myotis possess the unique characteristic of six pairs of cheekteeth in their lower and upper jaws (Taylor 2000). Myotis grisescens weighs between seven and nine grams.

Habitat and Roosting Behavior

Myotis grisescens roosts primarily in caves. It is of a temperate origin. The grey bat has recorded roost temperatures of as low as thirteen degrees Celsius. This is the coldest recorded roosting temperature of any bat in North America (Altringham 1996). Part of the reason Myotis grisescens has cool roost temperatures is to delay pregnancy and facilitate torpor. Hotter caves facilitate growth in the young, however. Seasonal migrations are thus required between summer and winter caves (Tuttle 1976).

Diet and Foraging Behavior

The diet of the bats in the genus Myotis consists of a multitude of insects. These types of insects include Coleoptera, Diptera, Ephemeroptera, Lepidoptera, Neuroptera, and Trichoptera (Best et al. 569). Myotis grisescens in particular find most of their prey over streams and reservoirs. Their prey is made up of many different insects that fly at night.

Barclay and Brigham (1994) found that bats feeding on insects are not as selective under their natural conditions as they are in the laboratory (Best et al. 1997). The bats merely consumed prey based on its availability.

In contrast, Best et al. (1997) found that Myotis grisescens are selective in their choice of prey. Their study examined 1476 fecal pellets of Mytois grisescens and did not find any significant correlation among the availability of potential prey and the contents of these fecal pellets (Best et al. 1997). They found that the three main insects in the diet of Myotis griscens were Lepidoptera, Coleoptera, and Diptera.

Lacki et al. (1995) examined the food habits of Myotis grisescens. They found that the animal had eaten insects from eleven families and nine orders. Lackie et al. (1995) found that the three predominant orders of insects eaten by the Myotis grisescens were Coleoptera, Diptera, and Lepidoptera as well. This study found that grey bats took part in opportunistic feeding at times, mainly in the months of May and July when their choice of prey could be proportionally related to the most readily available prey.

Hibernation

Myotis grisescens hibernates during the winter amidst large colonies in select caves. It makes over 430 km fall treks from subtropical climates in Florida to the colder north for hibernation (Tuttle 1975). Myotis grisescens selects the coldest hibernating caves of any species in its genus. When spring arrives, they depart these caves and remain transient, staying in caves for only a few days at a time.

Mating Behavior

Bats typically mate in the fall, become pregnant in the spring, and give birth in the summer. Since the summer is short, births are usually synchronous and can occur in as little as two weeks in some species at certain locations. The Myotis grisescens is an example of a highly synchronous birth cycle (Altringham 1996).

Reproduction

Myotis grisescens can multiply quickly. A colony found in Pittsburg, Kansas, of 200 grey bats had 2000 by 1967 and 8000 by 1971 (Elder 1976). This site had a high humidity, which is the preference of the grey bat. There has only been one known maternity colony found located in a dry site. This site was in an abandoned Missouri barn which has since been destroyed (Elder 1976).

Myotis grisescens select caves with high humidity in which to form maternity colonies and bear their young (Elder 1976). In these maternity colonies the newborns remain with the females. Not many adult males occur in these colonies. They are usually located around the perimeter to defend the colony if present. This leaves tens of thousands of adult males unaccounted for in the summer. Only three summer colonies with numerous adult males were found among grey bat colonies studied in fifteen years of research (Elder 1976).

Homing Ability

Kennedy et al. (1976) took two-hundred grey bats from a cave in Tennessee and released fifty of them at points in each direction (North, South, East, and West) 161 km away. A total of seventeen bats returned during the hibernation periods. Returns were made from all four directions. There is room for experimental error because there were some inaccessible crevices in the cave. Some bats could have also returned to the cave but not remained there throughout the hibernation period. This study seems to prove the homing ability of the grey bat because the number of returns made was greater than chance would have predicted (Kennedy et al. 1976).

Preflight Growth

Tuttle (1975) found grey bats to have an average preflight growth rate of .20 to .39 grams per day. This figure may not remain the same from year to year. Tuttle (1975) also found that the size of a colony is a better predictor of the growth rate of the colonyπs young than the temperature of the cave. This is partly due to the fact that a colony with a greater size will generally have a higher ambient temperature because ambient temperature is affected by size. Myotis grisescens have recorded first flight times of twenty-four and thirty-three days (Altringham 1996).

Mortality Factors

Three kinds of bat mortality were observed by Elder and Gunier (1979) in Missouri. The first was direct influence of man. This included vandalism, mass removal, and eradication attempts. The second was indirect influence of man. This included bulldozing in caves, accidentally striking electrical wires, and pesticides. The third was natural predation. This occurred when predator tracks (minks, raccoons, and opossums) were found in caves and presumably would feed on their young (Elder 1979).

Only a few grey bats live to their potential lifespan. Tuttle et al. (1977) analyzed the effect of the Myotis grisescens migration patterns as a mortality factor. They found that migration was a major cause of stress. Bats found during migration would be more vulnerable and prone to attack by predators. The migration in the spring was found to be especially stressful because the bats have exhausted most of their energy reserves during hibernation. Food sources and weather are also less reliable at this time (Tuttle et al. 1977).

Works Cited

Altringham, John D. Bats: Biology and Behavior. New York: Oxford University Press Inc, 1996.

Best, Troy L., et al. (1997) Variation in Diet of the Grey Bat (Myotis Grisescens). Journal of Mammalogy 78.2: 569-583.

Brack, Virgil, Jr., et al. (1984) The Gray Bat (Myotis Grisescens) in Indiana. American Midland Naturalist 111.1: 205.

Elder, William H., and Gunier, Wilbur J. (1979) Dynamics of a Gray Bat Population Myotis Grisescens in Missouri. American Midland Naturalist 105.1: 193.

Elder, William H., and Gunier, Wilbur J. (1976) Sex Ratios and Seasonal Movements of Gray Bats (Myotis grisescens) in Southwestern Missouri and Adjacent States. American Midland Naturalist 99.2: 463.

Kennedy, Michael L., et al. (1976) Homing of Gray Bats, Myotis grisescens, to a Hibernaculumm. American Midland Naturalist 96.2: 497.

Lacki, Michael J., et al. (1995) Food habits of grey bats in Kentucky. Journal of Mammaology 76.4: 1256-1259.

Nowak, Ronald M. Walkerπs Bats of the World. Baltimore: Johns Hopkins University Press, 1994.

Taylor, Peter John. Bats of Southern Africa. Scottsville, South Africa: University of Natal Press, 2000.

Tuttle, Merlin D. (1975) Population Ecology of the Gray Bat (Myotis Grisescens): Factors Influencing Early Growth and Development. Occasional Papers of the Museum of Natural History 36:1-24.

Tuttle, Merlin D. (1976) Population Ecology of the Gray Bat. Occasional Papers of the Museum of Natural History 54: 1-38.

Tuttle, Merlin D., et al. (1977) An Analysis of Migration as a Mortality Factor in the Gray Bat Based on Public Recoveries of Banded Bats. American Midland Naturalist 97.1: 235.

John OπRourke

Dr. Jerry Wilkinson

HONR 278C

23 February, 2004