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 (a) 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.
