Memory retention and cognition in hibernators

Hibernation is an extreme adaptation of some mammalian species to survive cold winter seasons or other unfavourable environmental conditions. In so-called torpor phases during hibernation the metabolism is extremely reduced, which causes the body temperature to drop to the level of the ambient temperature, reaching even zero degrees centigrade. Heartrate and breathing during torpor are minimized. Also, the activity and blood flow in the brain is strongly reduced. Phases of torpor last for several days or weeks and are interrupted by warming-up phases, so-called arousals, which last only several hours. We plan to investigate the effects of torpor and hibernation on the memory retention and cognition of animals. Do hibernators have to learn their surroundings every year anew? Do they recognise group members and relatives after hibernation? Earlier studies did not show a clear picture but pointed to some negative effects of torpor. Until today it is not entirely clear why torpor bouts are interrupted by arousals. Are arousals important to activate brain functions like memory retention and cognition? In several experiments we plan to investigate in detail whether factors such as frequency of arousals, minimum body temperature, and duration of hibernation, affect memory retention and cognition in hibernators. We have chosen the edible dormouse (Glis glis) as our study species. This species is for several reasons very interesting. With a maximum hibernation duration of 11 months, they hold the world record in naturally occurring hibernation durations. Thus, if hibernation has some negative effects, we would expect to observe them in edible dormice. On the other hand, dormice show an arboreal lifestyle, which is known to be challenging in terms of spatial orientation and cognition. For our studies of these relationships we will use innovative technics. We plan to implant miniature data-loggers into the animals, which can store activity and body temperature for two years. This allows us to closely follow seasonal changes in body temperatures and activity patterns. The dormice will be trained to find their way out of a maze. Further, they have to learn to jump on correct symbols to enter their home nest-box, the typical sleeping site for edible dormice. The success in these tests will be controlled after hibernation. Further, we will keep groups of dormice together in large enclosures to investigate via social network analysis, whether group formation is affected by hibernation.

This study investigated the effects of hibernation on the memory retenton of hibernators, focusing specifically on the edible dormouse. Edible dormice hibernate for an average of eight months, and in extreme cases up to eleven months, making them particularly well suited for this study. During deep torpor, the animals cool down to ambient temperature, and both heart rate and respiration are drastically reduced. This affects overall blood circulation; in the brain, circulation is also severely restricted during torpor phases. Torpor is interrupted by short arousal periods in which metabolism is reactivated and body temperature rises to normal levels. How does this extreme fluctuation, along with periods of reduced oxygen supply to the brain, affect memory? To investigate this, hibernators were hand-reared, trained in complex tests of spatial orientation (a vertical maze) and cognition (pattern recognition), and tested before and after hibernation. In addition, biologgers were implanted to precisely record body temperature. Preliminary analyses ensured that the implantation procedures and associated anesthesia did not bias the results. It was also verified that different housing conditions during winter, tested in a subgroup of animals, did not negatively affect cellular aging (specifically telomere length). The final analysis shows that the animals forget slightly over the course of hibernation. However, we found no evidence that animals kept artificially awake during winter (indoor housing with extended daylight periods) remembered better or worse than those that hibernated. The somewhat reduced performance in both tests after hibernation therefore appears to be more of a time effect rather than a direct consequence of hibernation. Animals whose environment was manipulated during hibernation showed that an increased frequency of arousal phases led to poorer performance in tests after hibernation. We conclude that milder winters with higher ambient temperatures could trigger these negative effects in the wild, as higher temperatures are likely to increase the frequency of arousal phases during hibernation. Our findings therefore point to a potential problem for hibernators in the context of climate change. The possible impacts on ecology and survival in the wild should be examined in future studies.