Circadian locomotor activity rhythms of the diurnal Indian palm squirrel in constant light

The parametric or tonic effects of light were studied in a recently established diurnal circadian model-the Indian palm squirrel, Funambulus pennanti. Sixteen squirrels (7 female, 9 male) were housed individually under varying constant light conditions (0.1 lux to 46 lux) with gross locomotor activity continuously monitored. Free-running period (tau), amplitude, mesor and day-to-day stability of the activity rhythm were determined using modified periodogram and iterative harmonic analyses, while the ratio of activity to rest time was estimated by eye-fit. The main findings were as follows: 1) tau did not vary between sexes or between light conditions, although a trend for tau to lengthen when light intensity was increased was noted; 2) amplitude and mesor did not show sex differences, but both sexes showed a decrease in amplitude and mesor when light intensity was decreased; 3) the stability of the activity rhythm was greater in males than in females, and a trend was observed for rhythm stability to decrease when light intensity was reduced. These descriptive data contribute to the growing literature on this diurnal species.