Field Guide/Mammals/Little Brown Myotis

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Myotis lucifugus (Little Brown Bat)
Family: Vespertilionidae
Size: Little brown bats range in length between 2 to 4 inches (50.8 mm to 101.6 mm) and only weigh about 5 to 15 grams (0.18 oz to 0.53 oz). Their wingspan ranges from 9 to 10.5 inches (228.6 mm to 266.7 mm).[1] While both sexes appear similar, males are generally smaller than females.[2]
Description: Little brown bats have fur that is glossy and varies from dark brown, golden brown, reddish, to olive brown. The abdominal region is lighter in color.[1] Unlike the rest of their body, the wings of little brown bats are nearly hairless and dark brown or black.
Similar Species: The little brown bat lacks the brownish vascular membrane of the eyeball and folded retinas so they do not exhibit eye shine like other species.[1] They have a blunt tragus, or an extra piece of cartilage sticking out from the base of each ear, and fur is typically longer, darker, and more glossy than others. Other distinguishing factors include the length of the ears, pelage length and sheen, and ear color.[3]

Range: Little brown bats are found in abundance in southern Alaska, Canada, across the United States, and in high, forested elevations in regions of Mexico.[1] Little brown bats are absent from hot, arid lowlands such as areas in northern Canada, Florida, the Great Plains regions, southern California, and the coasts of Virginia and North and South Carolina. They are located in Iceland and Kamchatka, which may have been a result of overseas travel.[1]
Habitat: Little brown bats occupy three different types of roosts: day, night, and hibernation. Active bats occupy day and night roosts in buildings, trees, under rocks, and in piles of wood. Most roosts have very little to no light and provide good shelter and high humidity. Night roosts include confined spaces in which large clusters of about 300,000 bats can group together to increase the temperature.[1] Day and night roosts are different in order to prevent accumulation of feces and to avoid attracting predators.[4] In the winter, a hibernation roost is used. These hibernation roosts may be shared with the Myotis yumanensis and include areas such as abandoned mines and caves where temperatures are above freezing and humidity is high. Unlike day and night roosts, hibernacula are not found in or around buildings.[1]
Diet: Insects make up the primary diet of little brown bats. Like birds, little brown bats catch their prey by aerial hawking and gleaning. Typically, little brown bats feed on swarms of insects in order to save time and energy. If a feeding area is successful these bats tend to go back to the same site. Types of insects that are consumed by these bats are beetles, aquatic insects, caddisflies, moths, midges, mayflies, lacewings, and mosquitoes.[1]
Activity: Little brown bats do hibernate in the winter. Northern populations enter hibernation is early September whereas southern populations enter hibernation in November. Northern populations end their hibernation period in mid May whereas southern populations end in mid March.[4] Some populations may migrate for the winter but never too far from their original location. Little brown bats are primarily nocturnal and begin to hunt during the dusk hours and return before dawn.[1] In order to communicate, bats use ultrasonic calls that are beyond the range of human hearing. Calls usually last about 4 milliseconds. When hunting, these bats call about 20 times per second and increase their calls to 200 per second when approaching their prey. Calls allow bats to locate, track, and evaluate prey.[2]

Reproduction: There are two phases in the mating process. These include the active phase in which both partners are awake and alert and the passive phase in which males mate with inactive individuals. During the active stage, females often times mate with several males and during the passive stage males mate with multiple females.[1] Mating occurs during the late summer and fall and is called swarming.[1] Females delay ovulation and store sperm for up to seven months until fertilization takes place in the spring. Females have a 50 to 60 day gestation period and pups are born in June and July. Most bats hang upside down, but females giving birth hang with their heads facing upwards. Unlike other species, the little brown bat only gives birth to one pup each year. Pups are born with a full set of teeth and open their eyes and ears within hours of birth. It takes about three weeks before a pup can finally fly and feed on insects. After about four weeks, pups reach their adult weight and become self-supporting.[1]
Lifespan: Usually little brown bats live up to 6 to 7 years in the wild with the males leading longer lives than the females.[1]

Notes: Evidence suggests that the little brown bat may be entering an extinction stage because of the widespread disease called white-nose syndrome.[3] WNS is a disease that affects hibernating bats. When the disease is visible, the nose of the bat or other hairless parts, such as the wings, have a layer of white fungus. First detected in the winter of 2006-2007 in New York, this disease has continued to spread throughout the bat community into many regions of the United States. So far 5.7 to 6.7 million bats in eastern North America have been killed with a death rate of 90 to 100 percent in some hibernacula. Scientists are still researching the disease and trying to find a way to control it.[5] In addition to the day and night roosts, females occupy nursery roosts. These roosts are similar to day roosts but are warmer and are usually only occupied by females and their offspring.[4] The same nursery roost is used each year.[1] These types of roosts are usually located in and around buildings but may also be found in hollow trees and other natural crevices. Typically during feeding little brown bats will eat half their body weight, but nursing females can eat up to 110 percent of their body weight.[2]
Little Brown Bat

  1. a b c d e f g h i j k l m n Havens, A. (2006), "Myotis lucifugus", Animal Diversity Web,, retrieved September 24, 2012 
  2. a b c "North American Mammals:Myotislucifugus", Smithsonian Institution National Museum of Natural History NMNH, n.d.,, retrieved September 24, 2012 
  3. a b Kunz, T.; Reichard, J. (2009), "Status Review of the Little Brown Myotis (Myotis lucifugus) and Determination that Immediate Listing Under the Endangered Species Act is Scientifically and Legally Warranted", Myotis Status Review (1): 1–22,, retrieved September 24, 2012 
  4. a b c Barclay, R. (1980), "Myotis lucifugus", Mammalian Species (142): 1–8,, retrieved September 24, 2012 
  5. U.S. Fish and Wildlife Service (2012), "White-nose syndrome: the devastating disease of hibernating bats in North America", White-Nose 1–2,, retrieved October 8, 2012