Keep cool: Memory is retained during hibernation in Alpine marmots

Abstract

Hibernators display severe changes in brain structure during deep torpor, including alterations in synaptic constitution. To address a possible effect on long-term memory, we examined learning behavior and memory of the hibernator Marmota marmota. In two operant conditioning tasks, the marmots learned to jump on two boxes or to walk through a tube. The animals were trained during their active season. Performance improved during the training phase and remained stable in a last test, four weeks before entrance into hibernation. When retested after six months of hibernation, skills were found to be unimpaired (box: before hibernation: 258.2+/-17.7 s, after hibernation: 275.0+/-19.8 s; tube: before hibernation: 158.4+/-9.0 s, after hibernation: 137.7+/-6.3 s). Contrary to these findings, memory seemed to be less fixed during the active season, since changes in test procedure resulted in impaired test performance. Besides the operant conditioning, we investigated the animals' habituation to a novel environment by repeated open field exposure. In the first run, animals showed exploratory behavior and thus a high locomotor activity was observed (63.6+/-10.7 crossed squares). Upon a second exposure, all animals immediately moved into one corner and locomotion ceased (7.2+/-1.9 crossed squares). This habituation was not altered even after hibernation (6.1+/-1.1 crossed squares). We thus conclude that long-term memory is unaffected by hibernation in Alpine marmots.