Circadian activity rhythms and sensitivity to noise in the Mongolian gerbil (Meriones unguiculatus)

Since consistent data on endogenous circadian rhythms of Mongolian gerbils are not available, the main aim of our study was to identify suitable condi tions to receive stable and reproducible free-running rhythms of activity u nder different light intensities. Another objective was to determine the ro le of social cues as an exogenous zeitgeber in the absence of a light-dark (LD) cycle. We performed two long-term sets of experiments with adult male gerbils kept in climatic chambers under Various photoperiods of at least 30 days each. In all cases, the time of lights on in the chambers differed fr om the daily starting hour of work in the animal house. Always, two animals per chamber were kept separately in cages with a running wheel while their activity was monitored continuously. During the first set, only three of e ight animals developed intra- and interindividual variable free-running rhy thms. The activity patterns seemed to be influenced by human activities out side, indicating high sensitivity to external factors. Subsequently, we dam ped the chambers and the room and restricted access to the room. In the fol lowing noise-reduced set, all gerbils developed comparable free-running rhy thms of activity. We determined the mean of the free-running period a, the activity rest relationship alpha/theta and the amount of running wheel acti vity per day: tau = 23.7h +/- 0.08h under low light (5 lux) and 25.5h +/- 0 .19h under high light intensities (450 lux); alpha/theta = 0.53 +/- 0.08 un der 5 lux and 0.34 +/- 0.04 under 450 lux. The amount of daily activity was 12 times as high under 5 lux as under 450 lux. There was no indication tha t the two animals in one chamber socially synchronized each other. In concl usion, the pronounced rhythm changes in accordance with Aschoff's theory su pport the view that gerbils are mainly nocturnal animals.