Dark switch in the entrainment of circadian activity rhythms in night monkeys. Aotus trivirgatus humboldt.

1. The locomotor activity of the night monkey (Aotus trivirgatus) has been shown to be related to light intensity by an optimum function; here entrainment by LD cycles is examined to see whether the mechanism of synchronization of circadian periodicity in Aotus is based on this function. 2. Eleven night monkeys of various ages, previously in either a free-running phase or in LD 12:12 (10(2):10(-1) lux), were recorded in LD 12:12 with the optimal intensity (10(-1) lux) in the light part of the cycle and a suboptimal intensity (10(-3) lux) in the dark part. 3. In all cases the monkeys synchronized in such a way that their activity phase fell in the dark part of the LD cycle. 4. The implication is that Aotus is a true dark-active species, that the illumination-dependent activity maximum at 10(-1) lux does not affect the synchronization mechanism, and that the differential (direction of change) rather than proportional (absolute level) actions of light provide the decisive cue for synchronization of the circadian activity rhythm.