Gray bat
The gray bat 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. 95% of gray bats now hibernate in only 15 caves. M. grisescens has been listed as federally endangered by the U.S. Fish and Wildlife Service since 1976, and is protected under the Endangered Species Act. Gray bat populations were estimated at approximately 2 million bats around the time they were placed on the Endangered Species list. By the early 1980s populations of gray bats dropped to 1.6 million. With conservation efforts in place, in 2004, gray bat populations were estimated to have reached 3.4 million.
Description
M. grisescens are the largest members of their genus in the eastern United States. Of all U.S. mammals, gray bats 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. 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. Gray bats can live up to 17 years, but only about 50% of gray bats survive to maturity. Sexual maturity occurs at about age two. Although an adult gray bat's forearm measures only about 40–46 mm, Gray bats with forearm lengths of 39.5 mm or less cannot fly. The flight speed of the gray bat, M. grisescens, has been calculated at 20.3 km/h during migration. While foraging, gray bats have been clocked at a flying rate of anywhere between 17 km/h and 39 km/h.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. During grooming, gray bats also ingest ectoparasites such as chiggers that live in their fur. Gray bats are believed to groom extensively before beginning their nightly hunt. They then spend the nighttime hours hunting and digesting.
Gray bats in the wild may reach ages of at least 13–14 years, although this is considered exceptional.
Distribution
Gray bats live in limestone karst areas in the southeastern United States. Summer and winter ranges overlap but are not identical. Hibernation occurs in caves in northern Alabama, northern Arkansas, Kentucky, Missouri, and Tennessee. During summer, the species ranges further and can be found in eastern Oklahoma and Kansas, southern Illinois and Indiana, southwestern Virginia, western North Carolina, northwestern South Carolina, and northern Georgia. A very small number of records from West Virginia and Mississippi are considered to represent vagrants. Historically attested populations in Florida appear to be absent as of 2020. Gray bats are cave obligate bats, meaning that with very few exceptions gray bats only live in caves, not in abandoned barns or other structures as other species of bats are known to do. Less than 5% of all available caves are inhabited by gray bats. Thus, any disturbance to these cave habitats can be extremely detrimental to gray bat populations.Cave characteristics
Although the habitat range of the gray bat incorporates much of the southeastern United States, the largest summer colonies of gray bats are located within the Guntersville Reservoir. This reservoir, found in northeastern Alabama, contains the Sauta and Hambrick caves which can accommodate over 200,000 gray bats combined. Gray bats use caves differently at different times of the year. For example, populations of gray bats 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 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, gray bats prefer deep, cool caves with average temperatures ranging from 5 to 11 °C. Multiple entrances and good airflow comprise the other characteristics that gray bats find desirable. Winter hibernacula are already cold when gray bats begin arriving in September. Summer caves are usually located along rivers and have temperatures that range from 14 to 25 °C. Summer caves typically contain structural heat traps that capture the metabolic heat from the clustered gray bats, allowing the nursery populations to succeed. Preferred summer colony caves are within 1 km of a body of water and are rarely further than 4 km away from a lake or major river. The average roosting density of gray bats is 1828 bats/m2.
Fossil record
of the gray bat are known from the Three Forks Cave of Adair County, Oklahoma in the Ozarks; they utilised the cave to raise offspring.Biology and behavior
Foraging
Gray bats forage over water, including streams and reservoirs, where they consume night-flying insects most of which have aquatic larval stages, and in the riparian forests nearby these water sources. M. grisescens activity tends to be concentrated over slower-moving water or quiet pools than areas of fast-moving water. Foraging usually occurs below treetop height but above 2m. Gray bats tend to fly downstream more often than upstream, suggesting a potential preference for wider sections typical of downstream sections as opposed to upstream portions. M. grisescens tend to forage over extensive ranges, averaging 12.5 km but ranging from 2.5 km to 35.4 km.While gray bats have been shown to forage in small groups when prey is abundant, especially during the early hours of the night, when prey is scarce, gray bats can become territorial. Territories tend to be controlled by reproductive females. These females seem to claim the same territory year after year.
Diet
Gray bats consume a variety of insects including Coleoptera, Diptera, Ephemeroptera, Lepidoptera, Neuroptera, Trichoptera, and Plecoptera. Juveniles have a tendency to forage more in woodlands and eat more beetles than adults, perhaps they provide a greater energy reward per unit of capture effort. For example, beetles provide 1900–2800 calories/g wet weight versus 800–1400 calories/g wet weight for mayflies. M. grisescens juveniles also eat a less diverse diet than adults, possibly because juveniles are more dependent on high concentrations of prey or swarming prey. Gray bats are believed to be part opportunists, and part selective eaters. In their natural habitats, gray bats appear to attack any moving target that is of appropriate size, consistent with optimal foraging theory that predicts palatable insects of an appropriate size should be eaten when encountered. In captivity, under controlled laboratory conditions, however, insectivorous bats used echolocation to discriminate heavily among potential prey based on shape and texture of a target. This lack of discrimination may be because of the rapid flight of bats and the short range at which prey can be detected using echolocation, allowing bats only a fraction of a second after detection to capture prey. However, gray bats are believed to discriminate somewhat between insects when foraging in their natural habitat, consuming higher numbers of Lepidoptera, Coleoptera, Diptera, and in some populations Trichoptera, than their proportional prevalence would have otherwise indicated without selective foraging. Because of this tendency to select prey while being largely opportunistic, gray bats have been dubbed 'selective opportunists'. Scientists believe that food moves quickly through the digestive tract of M. grisescens, with feces being purged from the body within 1–2 hours after ingestion.Migration, hibernation, and reproduction
Fall migration occurs in approximately the same order as spring emergence, with females departing first and juveniles leaving last. Gray bats may migrate as far as from summer caves to reach hibernation caves, although based on band recovery data and the spatial distribution of summer and winter roosts, a migration range below appears to be the norm. The annual activity period of gray bats is April to October, though female gray bats enter hibernation in September.After arriving at winter caves, copulation occurs. Females 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 gray bats drops close to the ambient temperature, allowing the body to conserve fat. These fat reserves must last the approximately six months of hibernation and spring migration. Adult mortality is especially high during spring migration, as bats that do not have sufficient fat reserves have difficulties surviving the stress and energy-intensive migration period. After copulation, females store sperm in their uteri, ovulating only after they have emerged from hibernation. Gestation in gray bats lasts 60 to 70 days, with birth occurring in late May and early June. Gray bat females give birth to one offspring per clutch, thus giving birth to one offspring per year. Therefore, gray bats demonstrate an iteroparous life-history strategy. 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 four weeks of age.