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Myotis lucifugus - Little Brown Bat (Little Brown Myotis)

Physical Description:

The little brown bat weighs about 6 to 12 grams and is evenly brown over the entire dorsal side of its body.  The ventral portion of the bat tends to be a lighter brown.  Myotis lucifugus also has external targus that is usually blunt and about half the size of the bat’s pinna.  The keel of the calcar of this particular species either poorly developed or absent.  The bat also has characteristic hairs that extend past the claws of the hind feet.  The little brown bat skull shows an incomplete premaxilla and a complete zygomatic arch (Kurta 1995).


The little brown bats range spans north to Alaska, south to central Mexico, west to California, and east as far as Maine and Costal Canada.  The little brown bat is distributed state wide in Wisconsin however it is less common in urban areas.  Myotis lucifugus has been known to roost in attics of peoples’ houses and take up residence in barns and sheds.  Female little brown bats will establish a maternity colony commonly in attics where it is warm.  The bat roosts in dark secluded areas during the summer days, and hibernates in caves or abandoned mines during the winter (Kurta 1995, Fenton et al 1980).  It is not listed as endangered or threatened by either the Federal Government or the State of Wisconsin.

Ontogeny and Reproduction:

Little brown bats display delayed fertilization.  Mating occurs in the fall and the males’ sperm is stored in the females’ reproductive tract until after hibernation.  After arousal, the female ovulates and the egg is fertilized.  Gestation takes about 60 days and weather plays a role in the length of the gestation period (Fenton et al 1980).  If the weather is too cold, the bat will enter a state of torpor and gestation slows down.  Females will form nursery colonies and litter size is normally one.  In Wisconsin, females will form these colonies in caves, and warm attics of people’s homes.  The young are born hairless but are capable of flight within 3 weeks.  It is also around this time that the young are weaned.  Juvenile little brown bats begin foraging around 3 weeks of age when they begin to fly.  They are less maneuverable than the adults and will forage primarily in open areas (Adams 1997).

Ecology and Behavior:

Like most other bats, the little brown is active primarily at night.  The bats will congregate together before going out to feed at places called night roosts.  These roosting areas are different from the daytime roosting sites where the bats spend the day.  The bats will also take breaks while foraging at night to digest food and to regain energy reserves for additional foraging.  While taking these breaks, the bats will temporarily roost under tree bark, from braches, eaves of houses or under rocks.  Diets of the little brown bat include moths and aquatic insect larvae.  The preferred foraging area for the bat is open areas free of clutter like tree branches and tall ground cover.  Foraging little brown bats can be most frequently seen along the edges of forests, and over streams and lakes (Kurta 1995).  The adult bats will begin foraging in more cluttered areas when the juveniles begin to find their own food to decrease intraspecific competition with the young (Adams 1997).

In Wisconsin the little brown bat will roost during the day in buildings, under the bark of some trees, under rocks, and sometimes in caves and abandoned mines.  During the winter, the bats will find a different place than their summer roosting sites to use as their hibernacula.  Again, in Wisconsin, these sites tend to be caves or abandoned mines.  The bats will choose a winter roost based on humidity (greater than 90%) and choose places where the temperatures above freezing (Kurta 1995).

During hibernation, the little brown bat may lose up to 25% of its body mass from the energy requirements.  Duration of hibernation is about 13 days, bats will arouse during hibernation to drink moisture from the hibernacula wall and to defecate.  They will then go back into hibernation if needed.  Little brown bats will allow their body temperature to fall during hibernation to the ambient air temperature in the hibernacula, thus warming the body during arousal uses most of the bat’s stored energy (Thomas et al 1990).

Before the young are born, female little brown bats have an average home range size of about 30 hectares, but this are decreases to about 17 hectares after the young are born.  This is likely due to lactating females returning to the maternity roost a few times during their normal feeding time to nurse the young.  The females shorten the distance they fly from the roost at night to forage so that during their rest times they can return to the roost (Henry 2002).

Natural predators to the little brown bat include snakes, house cats, raccoons, and other small predators.  By far humans kill more little brown bats in Wisconsin than natural predators by exterminating colonies that may take up residence in attics of homes (Kurta 1995).


There have been reports of the little brown bat living up to be 30 years in age (Kurta 1995).

Literature Cited:

Adams, A. A.  1997.  Onset of Volancy and Foraging Patterns of Juvenile Little Brown Bats, Myotis Lucifugus.  Journal of Mammology 78:239-246.

Fenton, M.B., R.M.R. Barclay.  1980.  Myotis lucifugus.  Mammalian Species 142:1-8.

Henry, M., D.W. Thomas, R. Vaudry, and M. Carrier.  2002.  Foraging Distance and Home Range of Pregnant and Lactating Little Brown Bats (Myotis Lucifugus).  Journal of Mammology  83:767-774.

Kurta, A.  Mammals of the Great Lake Region.  United States: The University of Michigan Press, 1995.

Ransome, R.  Hibernating Bats.  Cornwall, Great Britain:  Hartnolls Ltd, 1990.

Thomas, D. W., M. Dorais, and J. Bergeron  1990.  Winter Energy Budgets and Cost of Arousals for Hibernating Little Brown Bats, Myotis Lucifugus.  Journal of Mammology 71:475-479.

Reference written by Steven Fullington, Biology 378 student.  Edited by Christopher Yahnke.
Page last updated 4-28-04.

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