Hibernating Bats Don’t Forget

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This time of year, many animals are getting ready to hibernate or are already at it. Bats across Europe and North America, for example, started tucking themselves away a month or two ago to wait out the winter. While we often think of hibernation as a long winter’s nap, it’s very different from plain old sleep and studies indicate that it actually prevents animals from getting enough shut eye. During hibernation, animals go into and out of torpor, a state of lowered body temperature and slowed metabolism and heart rate. In this state, they get less rapid eye movement sleep and slow wave sleep. 

Sleep deprivation can wreak havoc on memory, impairing the brain’s ability to retain memories and shift them from short- to long-term memory. This, in turn, slows down learning accuracy and speed. Sleep-deprived humans are prone to memory errors, and missing sleep during hibernation and torpor can cause loss of spatial memory and object recognition in animals like squirrels and hamsters.

Lowered body temperature is one of the hallmarks of torpor, and Ireneusz Ruczyński from the Mammal Research Institute in Poland wanted to see if that’s what led to sleep deprivation and memory impairment. He used bats to explore the question because some species regularly go into daily torpor, even during the summer, and are also very long-lived (species in the genus Myotis live around 15 years on average, and the record for one of those, the greater mouse-eared bat, is 37 years), which makes good memories crucial. 

Ruczyński and German ecologists Theresa Clarin and Bjoern Siemers captured greater mouse-eared bats in Bulgaria and ran them through two experiments. In the first, the bats had to find a piece of food in a maze, and in the second, they had to work out an escape route in a flooded maze and find a dry perch. In between sessions in the mazes, the bats rested in two groups with slightly different accommodations. One group lived in a chamber that was kept at 71 degree F, while the other stayed in one that was 44 degrees. The researchers figured that in the chillier room, the bats’ body temperatures would dip as they do in torpor, and they would fare worse at learning and remembering their way through the mazes than their friends in the warmer room.

Over the course of a week, though, both groups of bats performed similarly in the two mazes. “Contrary to expectations, our study showed that daily exposure to a lower temperature has no effect on the bats’ learning abilities,” the scientists said. “This suggests that for bats living under natural conditions, daily torpor would have no significant cognitive consequence.”

Even longer and more frequent bouts of torpor don’t seem to affect bats’ memories. In a previous study, Ruczyński and Siemers kept bats in a cold chamber and let them hibernate for two months, after which they found food in a maze as easily as they did before hibernating and performed as well as a group that didn’t hibernate. 

Ruczyński thinks that because “many bat species live in complex environments and knowledge about this environment is probably important in their long life,” bats may have developed some behavioral or physiological trick that protects their memories during torpor and hibernation and has yet to be uncovered.