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Myotis grisescens A.H. Howell, 1909Gray Bat
Federal Protection: Listed Endangered
State Protection: Endangered
Global Rank: G4
State Rank: S1
SWAP High Priority Species (SGCN): Yes
Element Occurrences (EOs) in Georgia: 30
Habitat Summary for element in Georgia: Caves with flowing water or with large creeks or bodies of water nearby, also storm sewers and artificial caves in other states. Unknown summer roosts-eastern GA. range.
Gray bats have woolly-looking fur that is uniformly gray from root of fur to tip. Their fur is short with an orange spot of fur under their chin / ears and their belly is white. Gray bats’ pelage varies from dark gray immediately following molt in July or August, to russet, which is especially evident in females during the spring. Their calcar is not keeled, and they have a long and pointed tragus. The rear edge of the wing membrane attaches at the ankle, and toes hairs do not extend beyond the tip of the claws. This is Georgia's largest myotid bat, with a body length (excluding tail) of 47 – 52 mm (1.8 - 2 in), tail (base to tip of tail) of 33 – 44 mm (1.3 – 1.7 in), hind foot (ankle to tip of claw) 9 – 11 mm (0.3 – 0.4 in), ear of 13 – 16 mm (0.5 – 0.63 in), mass of 5 – 14 g (0.17 – 0.49 oz), and forearm length (outer edge of elbow to wrist) of 40 - 46 mm (1.57 – 1.81 in).
The gray bat is very similar to several other species in the same genus: southeastern bat (Myotis austroriparius), little brown bat (Myotis lucifugus), northern long-eared bat (Myotis septentrionalis), and Indiana bat (Myotis sodalis). However, these other Myotis spp bats have banded dorsal fur (not uniformly colored from base to tip) and the wing membrane attaches to the base of the first toe rather than the ankle. The gray bat is typically larger than other Myotis spp.
Perhaps the most cave-dependent mammal in this country, the gray bat roosts and hibernates exclusively in suitable caves in the southeastern U.S. However, some of the few specimens collected in Georgia were found in a drainage tunnel under the University of Georgia football stadium. Less than 5% of available caves in the southeastern U.S. have the right properties of temperature, humidity, and structure to make them suitable for gray bat occupation. Wintering caves are deep and vertical and serve as cold air reservoirs. The bats need constant cold, but not sub-freezing, temperatures to remain in hibernation and conserve energy; temperatures at hibernation roosts range 6 - 11 °C (43 - 52 °F). Summer caves, on the other hand, must be much warmer, especially at maternity colonies where females are raising young; temperatures there range 14 - 25 °C (57 - 77 °F). Domed ceilings help trap heat, including body heat produced by the bats, at these sites. Summer caves are almost always located within 1 km (0.6 mile) of a river or reservoir that serves as a foraging site. Most foraging occurs within 5 m (16 feet) of the surface over open water near a forested shoreline. The bats will forage 20 km (12 miles) or more from the roost sites and seem to prefer traveling within forested areas, probably because they are less vulnerable to predation from owls there. Young bats just learning to fly need forest cover in the vicinity of the maternity cave in which to forage and take shelter.
The diet is composed entirely of night-flying insects such as moths, beetles, and flies; primarily from the orders Ephemeroptera, Coleoptera, Diptera, Trichoptera, and Lepidoptera.
Gray bats are highly colonial and gather by the hundreds of thousands into only a few known caves in the southeastern U.S. to hibernate during the winter; nine caves are believed to house about 95 percent of the entire population each winter, with one cave sheltering from 50 - 66% of this total. These bats are known to migrate from 17 - 525 km (11 - 326 miles) between suitable summer and winter caves. Upon arrival at the wintering caves in September or early October, the adults mate. The females begin hibernation immediately, storing the sperm until springtime when insemination actually occurs. However, the males remain active for a few more weeks, replenishing fat supplies depleted during mating activities. The males and juveniles join the females in hibernation by early November. Gray bats hang from the ceiling, clustered tightly together, and averaging 1,828 individuals per square meter (yard). (Population estimates are derived by measuring the surface area covered by roosting bats and multiplying by the known average density.) Historical numbers have been estimated by measuring ceiling stains where the bats roosted in the past, or by measuring the old piles of droppings, or guano, beneath the roost sites.
In late March or early April, the adult females emerge, begin their pregnancy, and disperse to maternity caves within the summer ranges to bear their young. The adult males and juveniles (gray bats take 2 years to mature) emerge in mid-April to mid-May and disperse to bachelor caves within the same summer ranges. The spring migration is especially hazardous because fat reserves and food supplies are low; mortality is high at this time. The summer caves are always near a reservoir or large river. Each adult female delivers a single offspring during late May or early June. In large maternity colonies, the pups are able to fly after about 20 - 25 days and are completely weaned by 2 months of age; development is enhanced by the warmer temperatures created by large numbers of bats. However, colonies that have been reduced in size typically experience lower temperatures and slower development of the young. If development is slowed too much, the young might die before learning to fly. Once on the wing, growth rate and survival of juveniles is inversely proportional to the distance they must travel to reach the nearest foraging area over water. During late summer, most gray bats leave their summer caves and disperse and mix throughout their summer range. A few weeks later, they again assemble at the winter caves for mating and hibernation.
Gray bats can usually be detected by mist netting over canopied streams during spring and summer in their range. New roost sites can be identified through knowledgeable cave exploration, but hibernating bats should not be disturbed. Mist nests and harp traps are sometimes used at roost entrances to confirm species identification and gather demographic data. Mist netting surveys in Georgia should follow guidelines laid out on our Bat Survey Guidance webpage (http://www.georgiawildlife.com/BatSurveyGuidance). Cave surveys (especially during winter months) should be done with care as to prevent the spread of the Pseudogymnoascus destructans, the causative fungus of white-nose syndrome (https://www.whitenosesyndrome.org/static-page/decontamination-information).
In Georgia, gray bats are known to occupy only three caves regularly during the summer in Chattooga, Walker, and Catoosa counties. However, it is likely that additional roost caves in the northwest part of the state have yet to be discovered. The most important caves to gray bats, those that house large populations, are found in Alabama, Missouri, Arkansas, Kentucky and Tennessee. The range of the species also includes parts of Florida, Kansas, Indiana, Illinois, Oklahoma, Mississippi, Virginia, and North Carolina. The summer range extends across the Upper Tennessee River Basin (Virginia, Tennessee, North Carolina, and Georgia).
Gray bats have been listed as federally endangered since 1967. Their status is primarily due to anthropogenic disturbance to their hibernacula (caves). Gray bats are intolerant of disturbance, which usually comes in the form of intentional vandalism or by careless, inexperienced cave exploration. Disturbance can alter physiological responses in hibernation, causing individuals to arouse from torpor, which depletes fat reserves. Fat reserves are not easily replaced during the winter and each arousal from hibernation consumes enough energy to carry the bat through 20 - 30 days of torpor and are therefore energetically expensive. During the summer, disturbed females sometimes drop or abandon their young, resulting in mortality. Alteration of important caves due to vandalism, closure, commercialization, and inundation as a result of dam construction can negatively impact large percentages of the gray bat population. Since so few available caves are suitable, it is likely that gray bats do not find alternate caves once their primary ones become unavailable.
The confirmed infection of gray bats by the invasive fungus Pseudogymnoascus destructans (causative agent of white-nose syndrome, WNS) is a possible threat to the species. This fungus was first detected in upstate New York in 2006 and has subsequently spread to many gray bat hibernacula across their range. Powers et al. 2016 reported no significant changes in gray bat body-mass index (BMI), suggesting that their body condition was not affected by the introduction of white-nosed syndrome. They also report no changes in juvenile recruitment between pre and post introduction of the fungus. However, significant population declines could occur in the future thus monitoring gray bat populations should continue.
Deforestation of flight corridors between caves and foraging areas leads to increased predation by raptors. Pesticide use, along with pollution and siltation of waterways has decreased and contaminated food supplies. At the turn of the century, gray bats were probably among the most abundant mammals in the eastern U.S. However, by 1970 the population had declined an estimated 47. During the next six years the population dropped another 54%.
In more recent years, the numbers appear to have been increasing at surveyed roost sites and the species could be delisted. This species is relatively stable in Georgia but continued monitoring of summer and winter hibernacula is crucial to understand population trends and stability.
Protection of occupied caves from commercialization and inundation, as well as suitable management of the surrounding forest and nearby aquatic foraging sites, will be necessary to maintain and conserve gray bat populations. Caves can be gated or fenced to prevent human entry but the gate must be properly designed such that bat movement and cave microclimate are not affected. Protection of vital caves by government agencies and conservation organizations will greatly benefit the species. A continuing educational effort must be aimed at the general public, but efforts should especially be focused towards cave owners and explorers.
Baker, W. W. 1965. A contribution to the knowledge of the distribution and movements of bats in North Georgia. M.S. Thesis, University of Georgia, Athens.
Barbour, R. W., and W. H. Davis. 1969. Bats of America. University Press of Kentucky, Lexington. 286pp.
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Jordan, J. R. 1986. Gray myotis, Myotis grisescens (Howell). Pages 106-107 in R. H. Mount (ed.) Vertebrate Animals of Alabama in Need of Special Attention. Alabama Agriculture Experiment Station, Auburn University, Auburn.
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Whitaker, J. O., Jr., and W. J. Hamilton, Jr. 1998. Mammals of the Eastern United States. Cornell Univ. Press, Ithaca, New York. 583 pp.
J. Ozier, Aug. 2008: original account
K. Owers, Sept. 2009: updated status and ranks, added picture
K. Torrey, Oct. 2018: updated status and ranks, survey recommendations, and threats
P. Sirajuddin, April. 2019: edited sentences that included physiological phrases, morphology, and edited overall
S. Krueger, Jan. 2020: edited sentences with comments, and proofread for final