Gray Bat
Myotis grisescens
Summary 3
The gray bat (Myotis grisescens) 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. At one cave alone, the Georgetown Cave in northwestern Alabama, populations declined from 150,000 gray bats to 10,000 by 1969. In 1976, M. grisescens was placed on the U.S. Fish and Wildlife Service’s endangered species list and put under the protection...
Description 4
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. Gray bats can live up to 17 years, but only about 50% of gray bats survive to maturity. Sexual maturity occurs at about age 2. Although an adult gray bat's forearm measures only about 40–46 mm, Gray bats with forearm lengths of 39.5 mm (approx. 1.55 in) or less cannot fly. The flight speed of the gray bat, M. grisescens, has been calculated at 20.3 km/h (12.61 mph) during migration. While foraging, gray bats have been clocked at a flying rate of anywhere between 17 km/h and 39 km/h.
Diet 4
Gray bats consume a variety of insects including Coleoptera (beetles), Diptera (flies), Ephemeroptera (mayflies, of which Gray Bats consume at least six species), Lepidoptera (moths), Neuroptera (net-winged insects), Trichoptera (caddis flies), and Plecoptera (stoneflies). 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. (Outside of captivity, gray bats are limited by the sporadic emergences of potential prey. When prey emerges, there is only an abundance of a few taxa at any given time. The available taxa change based on the time of night, the month, and the time during the activity season.) 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 4
Fall migration occurs in approximately the same order as spring emergence, with females departing first (early September for fall migration) and juveniles leaving last (mid-October). Gray bats may migrate as far as 500 km (310 mi) from summer caves to reach hibernation caves. 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 (bout of reproduction), thus giving birth to one offspring per year. Therefore, gray bats demonstrate an iteroparouslife-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 (late June to mid-July).
Sources and Credits
- (c) U.S. Fish and Wildlife Service Headquarters, some rights reserved (CC BY), http://www.flickr.com/photos/50838842@N06/5687233983
- (c) U.S. Fish and Wildlife Service Headquarters, some rights reserved (CC BY), https://www.flickr.com/photos/usfwshq/5687233991/
- Adapted by floracliff from a work by (c) Wikipedia, some rights reserved (CC BY-SA), http://en.wikipedia.org/wiki/Myotis_grisescens
- (c) Wikipedia, some rights reserved (CC BY-SA), http://en.wikipedia.org/wiki/Gray_bat
