How bats survive the winter

Iowa has nine species of bats (you can read more about them here) and some of them hibernate while others migrate, and some migrate to hibernate.

Iowa's two most common bats, and those that you would find in Dickinson County, are the little brown bat (Myotis lucifugus) and the big brown bat (Eptesicus fuscus). Both of these bats hibernate through the winter.

Hibernation means that a bat's body slows down, including its metabolism, heart rate and breathing rate, so that it can survive a long time without food. A bat's heartrate will drop from 200-300 beats per minute to about 10 beats per minute, and it can go several minutes without breathing. They can stay in this state for up to a month in the winter, mixing that with periodic cycles of normal activity.

Places where bats hibernate is called a hibernacula.

Little brown bat

Little brown bats hibernate in colonies of hundreds or thousands in abandoned buildings and caves, typically. They may migrate hundreds of miles to their winter hibernacula.

little brown bat

During hibernation, little brown bats can withstand a temperature change of up to 120 degrees Fahrenheit without suffering any damage, and they can spend up to six months in hibernation while waiting for insects to arrive once again.

Big brown bat

Big brown bats may migrate to hibernacula or may not. They put on substantial weight in the fall in preparation for winter and find hibernacula such as caves, mines or abandoned buildings. They sometimes form clusters but also hibernate individually.


big brown bat

Big brown bat, USFWS/Ann Froschauer

Bat threats

Six Iowa bats, including the little brown bat, are listed as species of greatest conservation need in Iowa. The Indiana and northern long-eared bat are threatened or endangered under the federal Endangered Species Act.

One of the threats against bats today has to do with hibernation --- white-nose syndrome. WNS is caused by a fungus that affect hibernating bat colonies, causing bats to wake up and be more active in the winter, which in turn uses up their fat stores in a season without many insects to eat.

bat with white-nose syndrome

A hibernating bat with white-nose syndrome

WNS can be passed by humans if they come into contact with the fungus and spread it on clothing and footwear. Be sure to clean and disinfect clothing and fear before entering any caves or known bat habitats.

Migrating bats have also been found to be fatally impacted by wind turbines through direct contact or by pressure caused by spinning blades that causes a bat's lungs to fill with fluid as it flies near the blades. Research is ongoing in this area.

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