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Keywords:

  • locomotor activity;
  • Circadian rhythm;
  • Soricidae;
  • free-run;
  • Crocidura cyanea

Abstract

The circadian rhythm of locomotor activity in a southern African shrew, the reddish-grey musk shrew Crocidura cyanea was investigated. Thirteen individuals were subjected to three successive light cycles, each cycle lasting approximately 2 weeks: an LD cycle (12 h light/12 h dark), a DD cycle (constant darkness) and a DL cycle (an inverse of the LD cycle). All of the animals exhibited entrainment of their activity to the LD and DL lighting regimes. Locomotor activity of C. cyanea occurred predominantly during the dark phases of the LD cycle and the DL cycle. Under LD, the mean active phase (α) of C. cyanea was 10.8±0.3 h and the total percentage of activity was 78.9% during the dark phase. When subjected to constant darkness, the mean active phase increased to 13.2±01.8 h and all animals expressed free-running rhythms of locomotor activity (mean±1 sd=23.0±0.55 h; range=22.4–23.7 h). On the reverse LD cycle, the mean active phase was lowest (09.3±0.16 h) and the total percentage of activity was 71.5% during the dark phase of the cycle. The reddish-grey musk shrew possesses a strong circadian rhythm of locomotor activity that predominantly occurs during the dark phase and consequently the musk shrew may essentially be considered a nocturnal mammal.