The hibernation patterns of European hamsters were compared under different housing conditions (two different laboratory housings and a seminatural housing in an outdoor enclosure). In the laboratory, animals were kept either under a standardised light-dark cycle (LD 8:16) and constant ambient temperature (Ta:  8±1°C), or constant dim illumination (LL: l= 680 nm; light intensity < 1 lux) and the same constant Ta. In the outdoor enclosure, animals stayed all or almost all of the winter in their self made burrows under constant darkness (DD) and mediated Tas without any daily periodicity. One animal spent the winter in a laboratory cage above the soil and was therefore exposed to the natural LD and harsh Tas with a pronounced daily periodicity.
 
The objectives of this study were:
1. To define the natural hibernation pattern of the European hamster in comparison between the housing conditions and to address artefacts due to the animals housing.
2. To investigate the importance of the circadian system in the temporal organisation of hibernation and to rate the influence of Zeitgebers.
3. If an essential participation of the circadian system could be shown, it should be tried to decide, which of the proposed options for the circadian regulation of hibernation is correct for the European hamsters.
 
A comparison of all registered hibernation patterns in this study with results from free ranging ground squirrels , marmots and hedgehogs, indicates that the undisturbed pattern consists of many multi day hibernation bouts (HBs) spaced by only short intervals of normothermy. HBs of shorter duration than 24 h at the beginning (the so called „test drops“), in the middle or at the end of hibernation indicate disturbances of the animals. As they were much mor frequent in the laboratory studies it is reasonable to asign them as artefacts.

The different studies of this PhD thesis clearly show an important influence of the circadian system on the thermoregulation during hibernation. However, since a clear and doubtless rhythmicity of HBs only occurred under the     presence of an exogenous trigger or Zeitgeber, it is reasonable that both rhythmic, as well as homeostatic mechanisms affect the hibernation patterns. Both processes may be interdependent from each other, either enforcing  the influence of the other factor or masking it. Most propably the role of the circadian system during hibernation corresponds to the function, which it has in the regulation of sleep. It determines sensitive and nonsensitive phases for the beginning and end of homeostatic processes ("gating").

Due to the interdependence of metabolic and rhythmic factors no clear decision on the mode of the circadian regulation of hibernation could be done. In fact, evidence for all three hypotheses could be found. Although there is a clear correlation between body temperature (Tb) and the duration of HBs,  no evidence for a pure temperature dependent slowing down of the circadian clock could be found. In a few animals, the temporal organization of arousals from HBs fulfilled the conditions of a persistant temperature compensated mechanism. Finally, ultradian oscillations in Tb during the plateau phase of HBs in one out of 28 hamsters indicates that the duration of each HB is a multiple of a endogenous period.

An important, beforehand not expected result of this study presents evidence, that there are two completely different types of torpor in the hibernation of European hamsters:  short and shallow Hbs and typical multi day Hbs. The first type is known to occurre as daily torpor in non hibernating species.  As shown in the comparative part of this study, this torpor type represents rather an artefact in the hibernation patterns of European hamsters. In contrast to this, multi day HBs represent the typical torpor type of genuine hibernations. Entrance into this kind of torpor occurred at a distinctivly different time of day as compared to the shorter HBs. The variety of different torpor types in one and the same species shows an astonishing fexibility of hibernation.
 
 


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