|Circadian activity rhythms of dwarf hamsters (Phodopus spp.) under laboratory and semi-natural conditions|
Weinert D., Schöttner K., Surov A.V., Fritzsche P., Feoktistova N.Yu., Ushakova M.V., Ryurikov G.B.
The hamsters of the genus Phodopus (P. campbelli, P. sungorus, P. roborovskii) inhabit different ecosystems facing them with different environmental challenges. This should have behavioural and physiological consequences, manifested genetically. Therefore, the present paper compares the daily activity rhythm of the three Phodopus species under various conditions, from highly standardized laboratory to semi-natural ones. Motor activity was recorded by the mean of passive infrared sensors (PIR), running wheels (RW) and ring-shaped sensors (RSS) placed on the burrow entrances.
Under standardized laboratory conditions with artificial light-dark cycles (L:D=14:10h or 18:06h), all hamsters were active almost exclusively during the dark time. The amount of general activity (PIR method) per day was not different between males and females or between species. The onset of activity (RSS) was significantly later in P. campbelli than in the two other species, the activity offset was earliest in P. roborovskii. The latter had also the shortest duration of dark-time activity. Running wheels were used mainly during the dark time (on average 97%). P. sungorus did run significantly more than the other two species. On average they realized 9000 revolutions per day (ca. 3.5 km).
The rhythm stability was lowest in P. sungorus. In some hamsters of this species, the activity onset was delayed by several hours, and the activity period was strongly compressed. Also, a free-running rhythm despite the presences of a light-dark cycle or arrhythmicity was observed in those animals. Differences between the species were also found during the last weeks of life. Whereas P. campbelli and P. roborovskii revealed clear activity rhythms until the last decade, in P. sungorus the activity rhythm disappeared from the last 100 days of life.
First studies under semi-natural conditions were performed at the Biological Station of the A.N. Severtsov Institute of Ecology and Evolution in Tchernogolovka. These included experiments in a laboratory though with natural lighting and temperature conditions and in outdoor enclosures. In one of them (75 m2) artificial nest boxes were provided, in the other two (400 m2 each) the animals could dig their own burrows. The observed activity patterns are similar to those obtained under constant, standardized conditions. Moreover, the animals did clearly respond to the changing photoperiod.
The activity patterns of each species described in the present paper are rather similar under semi-natural and artificial environmental conditions. This can be taken as evidence that the obtained species-specific patterns have a genetic basis and are not caused by the different environmental conditions.
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