The Gray bat (Myotis grisescens) is a species of microbat endemic to North America. It once flourished in caves all over the southeastern United States, but due to human disturbance, gray bat populations declined severely during the early and mid portion of the 20th century.
Gray bats are the largest members of their genus in the eastern United States. Of all U.S. mammals, they are, perhaps, the most cave-dependent. Gray bats have uni-colored dark gray fur on their backs that may bleach to a russet or chestnut brown after the molting season (July or August). Unlike in other species of Myotis, where the wing membrane connects to the toe, in M. grisescens, the wing membrane connects to the ankle. Gray bats typically weigh between 7 and 16 grams.
Annual molting occurs between early June and early August, during which Gray bats eat larger amounts of hair than at other times during the activity season.
Gray bats live in limestone karst areas in the southeastern United States. Summer and winter (hibernation) ranges overlap but are not identical. Hibernation occurs in caves in northern Alabama, northern Arkansas, Kentucky, Missouri, and Tennessee. During summer, they range further and can be found in eastern Oklahoma and Kansas, southern Illinois and Indiana, southwestern Virginia, western North Carolina, and northwestern Georgia. Gray bats are cave-dependent bats, meaning that they only live in caves. Foraging takes place in riparian forests nearby water sources. These bats migrate in autumn, with females departing first (early September for fall migration) and juveniles leaving last (mid-October). They may migrate as far as 320 km (200 mi) to 500 km (310 mi) from summer caves to reach hibernation caves.
Gray bats are colonial and use caves differently at different times of the year. For example, they tend to cluster in caves known as hibernacula to prepare for winter hibernation. In contrast, their populations disperse during the spring to establish sexually segregated colonies. Females form maternity colonies (also known as summer maternity roosts) while males aggregate in non-maternity or bachelor colonies. These bachelor colonies also house yearlings of both sexes. Gray bats also utilize a third type of cave, the dispersal cave, which they inhabit only during migration. For their hibernacula, they prefer deep, cool caves with average temperatures ranging from 5 to 11 °C. Summer caves are usually located along rivers and typically contain structural heat traps (including domed ceilings, small chambers, and porous rock surfaces) that capture the metabolic heat from the clustered gray bats, allowing the nursery populations to succeed. Gray bats forage at night over water and over extensive ranges. When prey is abundant, Gray bats forage in small groups, especially during the early hours of the night, when prey is scarce, they can become territorial. Territories are usually controlled by reproductive females which seem to claim the same territory year after year.
Gray bats are carnivores (insectivores) and consume a variety of insects including beetles, flies, mayflies, moths, net-winged insects, caddis flies, and stoneflies. Juveniles eat more beetles than adults, perhaps they provide a greater energy reward per unit of capture effort.
The mating season of Gray bats starts right after arriving at winter caves. Females then immediately enter hibernation, while males may remain active for a few more weeks. Males use this extra time before entering hibernation to replenish fat reserves used during breeding. Males begin hibernation by early November. During hibernation, the body temperature of these bats drops close to the ambient temperature, allowing the body to conserve fat. These fat reserves must last approximately 6 months of hibernation and spring migration. After mating, females delay implantation until they have emerged from hibernation. Gestation typically lasts 60 to 70 days, with birth occurring in late May and early June. Gray bat females give birth to one pup per year. The young clings to the mother for about a week, after which they remain in the maternity colony until they are able to fly. Most young take flight by 4 weeks of age (late June to mid-July).
The main threats to this species include human disturbance, the creation of dams, which causes flooding in former bat caves, cave commercialization, natural flooding, pesticides, water pollution and siltation, and local deforestation. Direct human disturbance and vandalism is the major factor leading to the population decline in Gray bats. During the 1960s, bats were killed for entertainment purposes as they emerged from caves or were caught to be used for pranks. Many property owners attempted to exterminate entire colonies due to unsubstantiated fears that the bats may be carrying rabies. Bats that roost within 100 m (328 ft) inside the cave and only 2 m (6.6 ft) above the cave floor are especially prone to vandalism and high-intensity disturbance. Each human entry into a cave causes all bats within range of light or sound to at least partially arouse from hibernation. The arousal of Gray bats, while they are hibernating, can cause them to use up energy, lowering their energy reserves. Because these reserves must sustain the bats through hibernation and spring migration, if the bat runs out of reserves, it may leave the cave too soon, decreasing its chances of survival. When flightless young are present in June and July, females escaping a predator or other disturbance may drop their young in panic, leading to increased juvenile mortality.
According to Wikipedia resource, in 2004, Gray bat populations were estimated to have reached 3.4 million individuals. Currently, this species is classified as Vulnerable (VU) on the IUCN Red List but its numbers today are increasing.
