Does Hibernation Make You Dumb?

Studies of hibernating ground squirrels reveal that long periods at extremely reduced body temperature can alter the biochemistry of the brain and lead to temporary memory loss upon arousal from hibernation (von der Ohe et al, 2007). Obviously, the ability to remember the location of food patches or recognize neighbors can be critically important for survival.

Long-term memory is likely to be even more important to long-lived mammals that forage in complex environments.
Vespertilionid bats, for example, can live more than 30 years in the wild and must forage for invertebrates in complex environments (Figure 1). Furthermore, many individuals make long distance foraging trips each night and even longer seasonal migrations to hibernacula where they spend several months in hibernation each winter. Thus, vespertilionid bats present an ideal model to test the effects of hibernation on memory retention.

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Figure 1. A greater mouse-eared bat (Myotis myotis). (From Manuel Werner, Germany, Wikipedia-Kontakt)

Ruczynski and Siemers (2011) conducted just such a study on greater mouse-eared bats (
Myotis myotis) in Europe. These bats hibernate for up to 5 months each year, during which their body temperature plummets to less than 8oC. To determine if hibernation affects memory retention, the researchers trained the bats to preform memory tasks for food. The bats were trained to locate food in one of 3 arms of a maze (Figure 2). After all the bats successfully chose the correct location 10 times in a row (i.e. after 5 weeks) the bats were randomly assigned to either control or hibernation groups. Those in the control groups were maintained in flight cages with food and water at 18-20oC, which prevented them from entering hibernation. The hibernation group was prepared for hibernation by slowly reducing ambient temperature over several days. They were housed at 7oC in a climate-controlled chamber for 10 weeks. Hibernating bats were monitored with infrared video cameras; the bats periodically aroused and returned to hibernation over the course of the study. The longest periods of uninterrupted hibernation lasted 25 days.

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Figure 2. An illustration of the memory test maze used in the study. (From Ruczynski and Siemers 2011)

After the bats aroused from hibernation for the final time, they were re-tested. The proportion of bats that correctly remembered the maze was the same for both non-hibernating and hibernating bats (Figure 3). Thus it appears that prolonged hibernation at low temperatures does not affect memory retention in bats. The authors suggest that the combination of long lifespan, complex foraging environments, and migration patterns in bats selects for memory retention even after prolonged hibernation.

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Figure 3. (a) Average proportions of correct decisions and (b) average time to complete a trial for hibernated and control bats before (grey bars) and after (white bars) the hibernation period. (From Ruczynski and Siemers 2011)

References

Ruczynski, I., and B. M. Siemers. (2011) Hibernation does not affect memory retention in bats. Biology Letters, 7:153-155.

von der Ohe, C.G., Garner, C.C., Darian-Smith, C. & H.C. Heller. (2007) Synaptic protein dynamics in hibernation. Journal of Neuroscience. 27, 84–92. (
doi:10.1523/JNEUROSCI.4385-06.2007)