Circadian activity rhythms in squirrel monkeys: entrainment by temperature cycles

Eight squirrel monkeys (six male, two female), kept singly in wire-mesh cages, were housed in groups of three in a climatic chamber. Locomotor activity was recorded by means of photocell systems or mobile climbing trees. For time spans of several weeks, the animals were alternately exposed to each of three different conditions: (1) constant conditions at various levels of light intensity and ambient temperature; (2) a 24-hr light-dark (LD) cycle at constant temperature; (3) a 24-hr cycle of low (about 17 degrees C) and high (32 degrees C) temperature in constant illumination (LL). In constant conditions, the free-running activity rhythms always had periods longer than 24 hr, without systematic dependencies on ambient temperature, but with a tendency to lengthen the period when the intensity of illumination was increased. All animals were entrained by a weak LD zeitgeber (10:1 lux). Exposure to cycling ambient temperature resulted in entrainment in about 45% of all tests. Usually, the entrained animals were active during the cold fraction of the cycle, but some records suggested entrainment in a warm-active mode. In addition, temperature changes exerted strong masking effects, mainly with increases of activity during the warm fraction of the cycle.