ENTRAINMENT OF ACTIVITY RHYTHMS TO TEMPERATURE CYCLES IN DIURNAL PALMSQUIRRELS

Ambient temperature cycles entrain circadian rhythms of homeotherms. T he phase that entrainment occurs at varies, particularly in diurnal sp ecies. We investigated whether ambient temperature cycles of 12 h warm (34 to 40 degrees C) and 12 h cool (24 to 28 degrees C) entrained loc omotor activity rhythms of diurnal Indian palm squirrels (Funambulus p ennanti) that were free-running in constant dim light (3.2 to 7.6 lux) . Seven female squirrels were exposed to the temperature cycle for 21 days, after which a 5-h phase delay of the cycle was imposed. The cycl e then continued for a further 50 days. Three of the seven squirrels s howed entrainment to the temperature cycle. Of the three which entrain ed, one was warm-active and two were cool-active. Of the remaining squ irrels, two showed entrainment or relative coordination of one compone nt of the rhythm, and two did not show any entrainment. Positive and n egative masking of activity by the warm and cool fractions were observ ed regardless of whether or not squirrels entrained. These results sug gest that ambient temperature is an effective zeitgeber for F. pennant i. As has been reported for other diurnal species, interindividual dif ferences exist in the phase of entrainment to temperature cycles. (C) 1998 Elsevier Science Inc.