Circadian activity rhythms of the pig-tailed macaque, Macaca nemestrina, under constant illumination.

Locomotor activity was recorded in singly caged pig-tailed macaques Macaca nemestrina in constant conditions with intensities of illumination varying between 0.003 lux and 100 lux. Each animal was kept in at least 5 different conditions for 2-3 weeks each. Three quantities of the circadian rhythm: the period, the duration of activity as opposed to rest, and the amount of activity per period, could be shown to be positively correlated with light intensity. As in 6 other diurnal primate species, these findings contradict the so-called circadian rule according to which opposite correlations should exist between the circadian parameters and light intensity in diurnal and in nocturnal species of animals.