Activity cycles of the monkey(macaca mulatta)under different environmental conditions∗∗∗

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Abstract
Eight monkeys were subjected successively to four different schedules of daily feeding and exposure to the experimenter for the purpose of appraising the effects of these factors upon the form and amplitude of their daily cycle of activity. The subjects were confined in relatively unrestrained environments and committed to a 12:12 L:D ratio. Each of the four schedules produced a uniquely different curve of activity and the factor of predominant influence appeared to be exposure of the subjects to the experimenter. One finding, common to all of the experimental schedules, consisted of a high degree of photoperiodic dependency. To explore the limits of such dependency, four monkeys were subjected to three successive L:D schedules (12:12, 6:18, and 18:6). Essentially, the same degree of dependency manifested under the 12:12 schedule was demonstrated under the 18:6 schedule in that the onset and cessation of activity closely followed the onset and termination of the period of illumination. A considerably lesser degree of dependency occurred under the 6:18 schedule. Here, the onset of activity preceded the onset of illumination by 3–4 hours indicating a strong need for a minimum period of activity of about nine hours. Differences in habituation time were also observed. Stable daily curves were obtained within 2–3 days following the shift to the 6:18 schedule, 2–3 weeks following the shift to the 18:6 schedule, and 2–3 days following the shift back to the 12:12 schedule.

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