The gray bat (Myotis grisescens) is a medium-sized insectivorous bat with an overall length of about 3.5 inches and a wingspan of 10 to 11 inches. As the name implies, gray bats have gray fur, but the hair often bleaches to reddish-brown by early summer. The gray bat occurs in limestone karst areas, meaning a landscape marked by caves, sinkholes, springs and other features, of the southeastern and midwestern United States. It is estimated that more than 95% of the species range-wide population hibernate in only 15 caves. The gray bat was added to the U.S. List of Endangered and Threatened Wildlife and Plants on April 28, 1976.
At the time of listing, the main historical threats to the gray bat were human disturbance to roosting bats, environmental contamination, impoundment of waterways and roost modification or destruction. Such roost modifications include cave entrance or mine sealing and other modifications of the internal environment and entrances. The species is also negatively impacted by cave commercialization, improper gating and natural calamities, like cave-ins and flood events. Emerging threats, like interactions with wind turbines andhave been added as potential threats, since gray bats were first federally listed.
Gray bats are particularly sensitive to human disturbance via cave entry and exploration. Unlike some Myotis species in the midwest and southeast, like the Indiana, northern long-eared and little brown bat species, that typically roost high up in dead-standing trees and out of reach of humans, gray bats roost on the ceilings of caves and rear young in places where humans can disturb them with their presence through physical touch, noise and artificial lighting. To help recover gray bat populations, the 1982 Gray Bat Recovery Plan primarily focused on developing a plan to permanently protect important summer and winter caves from human disturbance. As a result, many gray bat sites were permanently protected through long-term voluntary landowner agreements, like stewardship plans, conservation easements, habitat management plans or memorandum of agreements, that protect sites in perpetuity. Protections typically involve posting signage that asks people not to enter caves when bats could be present. These protection measures can also involve more extreme measures, like building permanent gate structures at the opening of the cave or installing fencing around a cave to prevent human entry. While limiting public access through the use of gating is sometimes unpopular with recreational caving communities and the wider public, it has led to a steady increase in recovery of many gray bat and other bat populations. Some populations that reside within unprotected sites continue to suffer from repeated human disturbance, but overall, the threat from human disturbance has been greatly reduced since the 1970s.
Surveys conducted since 2009 indicate that gray bats do not appear to be susceptible to white-nose syndrome to the same degree as other affected Myotis. No mass mortalities have been documented, although to our knowledge, no studies have attempted to determine if sub-lethal impacts occur in gray bats as a result of white-nose syndrome. Based on the very few observed and confirmed white-nose syndrome affected gray bats and stable population numbers, gray bats appear to be resistant to the disease despite sharing hibernacula with other highly vulnerable species.
Efforts to prevent extinction?
Due to the diligent and hard work of many federal and state agencies and partners, 32 of 46, or 70%, of biologically significant summering roost sites across the gray bat’s range are considered permanently protected. Additionally, of the 15 major hibernacula, 14 are considered permanently protected. Thus, a significant proportion of the gray bat range-wide population is now protected from disturbance in its winter and summer habitat.
It is not advisable to enter gated caves, mines or sites with a sign at the entrance which indicates it is used by endangered bats. These gates and signs are in place to protect bat colonies that are sensitive to human disturbance. Disturbance during the summer before the young can fly can result in thousands of flightless young becoming dislodged and falling to their deaths. Every arousal during hibernation is energetically expensive. Fat reserves required to sustain the bats are utilized to some extent during each winter arousal. Too many arousals during hibernation can exhaust a bat’s?limited energy reserves and cause them to starve. The best thing the public can do to help bats is to admire them from afar, when they can be seen flying across the sky at sunset.
Conservation efforts also include protecting woodlands surrounding caves and woodedcorridors along streams near caves where gray bats forage. Removal of woodlands adjacent to water bodies and along riparian corridors may degrade and adversely affect gray bat foraging habitat.
Gray bats occupy caves or cave-like structures year-round. While gray bats prefer caves, summer colonies have been documented using dams, mines, quarries, concrete box culverts and the undersides of bridges. Summer caves must be warm or have restricted rooms that can trap the body heat of clustered bats. Winter hibernation sites are often deep vertical caves that trap large volumes of cold air; these caves are naturally very rare. Given that approximately 98% of gray bats roost in as few as 15 major hibernacula, natural calamities at any one of the hibernacula could result in the loss of a significant amount of roosting habitat or bats. Males and females typically hibernate together; however, because of the limited number of suitable caves, gray bats may migrate as many as 500 miles between summer and winter caves. However, based on band recovery data and the distribution of hibernacula and summer colonies across the range, most gray bats are considered regional migrants with migrations shorter than 200 miles.
A natural chamber or series of chambers in the earth or in the side of a hill or cliff. An irregular limestone region with sinkholes, underground streams and caverns.
Gray bats are powerful flyers that hunt over large areas during the breeding season that feed on flying insects over rivers, streams and lakes and other bodies of water. Mayflies, caddisflies and stoneflies make up?a?major part of their diet, but beetles and moths are also consumed.?The Asiatic oak weevil is a favorite summertime food, when it is abundant in forested cliffs along rivers. Most insects are eaten on the wing. Although gray bats prefer to roost in caves near large water bodies, there are records of gray bats commuting up to 20 miles to the nearest large water body from its roosting location each night.
The dorsal fur is gray, but often bleaches to reddish-brown by early summer. The most subjective way to identify gray bats is the presence of notched claws and forearm length, which is typically more than 40 millimeters. In places where the gray bat range does not overlap with the southeastern myotis, gray bats can be identified by the attachment of the membrane to the ankle rather than the foot, as documented by D. Sasse and others in 2019. However, the ankle attachment identification method should be avoided in a majority of the southeast, where the two species co-occur, because southeastern myotis may also share this physical trait.
Weights range between approximately 7 to 16 grams.
The gray bat is a medium-sized bat with an overall length of about 3.5 inches and a wingspan of 10 to 11 inches.
Gray bats require two years to reach sexual maturity. Gray bats mate in the fall when they begin to arrive at hibernacula. Female bats store the sperm and fertilization occurs in the spring when bats ovulate. During hibernation, males and females commingle and form large clusters with some aggregations numbering in the hundreds of thousands of individuals. Unlike Indiana bats, which stack tightly together, gray bats form loose clusters. Adult females begin to emerge from hibernation in late March through mid-April, followed by juveniles and adult males.
Based on band recovery data, the oldest known gray bat is at least 13 years and 6 months old. Most recaptured gray bats are much younger.
Females become pregnant in the spring, and form maternity colonies in caves of a few hundred to many thousands of individuals. Gestation takes somewhere between 60 and 70 days, with pups born in late May and early June. Gray bat summer colonies can use one or multiple caves located along a stream, river or reservoir, but are sometimes located further, yet within commuting distance of a water body. A single offspring is born in late May or early June. Newborns typically become volant within 21 to 33 days after birth.
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