Physiology & Behavior, Vol. 28, pp. 205-211. Pergamon Press and Brain Research Publ., 1982. Printed in the U.S.A. Entrainment and Masking of Circadian Drinking Rhythms in Primates: Influence of Light Intensity H. ELLIOTT ALBERS, 1 RALPH LYDIC 2 AND MARTIN C. MOORE-EDE Department of Physiology, Harvard Medical School, Boston, MA 02115 Received 1 August 1981 ALBERS, H. E., R. LYDIC AND M. C. MOORE-EDE. Entrainment and masking of circadian drinking rhythms in primates: Influence of light intensity. PHYSIOL. BEHAV. 28(2) 205-211, 1982.--The entrained drinking rhythms of squirrel monkeys were studied during exposure to 24 hr illumination cycles of three different intensities (60:0; 66:6; 76:16 lux). Increasing the intensity of ambient illumination significantly delayed the offset of drinking but had no effect on either the onset or the total amount of daily drinking behavior. Comparison of drinking behavior under an alternating schedule of LD 66:6 lux and constant light of 6 lux indicated that the twice daily light transitions consistently altered the temporal distribution of drinking behavior. The daily timing of squirrel monkey drinking behavior thus, depends not only on the mechanisms of circadian entrainment to the LD cycle, but also on the ability of the LD cycle to directly influence, or "mask" behavior. Circadian rhythms Light intensity Drinking Locomotor activity Squirrel monkey UNDER natural conditions the 24 hour light-dark (LD) cycle serves to synchronize or entrain the endogenous circadian (approximately 24 hours) rhythmicity of mammalian physi- ology and behavior. Specific physical and temporal charac- teristics of the natural illumination cycle, other than its 24 hour period, are also biologically relevant. For example, the daily ratio of light to dark and the intensity of illumination during day and night, are important factors in determining the precise temporal patterning of many behavioral and physiological events within the 24 hour day [3,4]. While a systematic relationship between photoperiod length and the timing of the onset of daily behavior patterns such as loco- motor activity has been extensively documented [8], the ef- fects of illumination intensity on temporal organization have been predominantly studied only under conditions of con- stant illumination. The intensity of continuous illumination influences the periodicity of many behaviors and their daily duration. These effects of light which have been extensively studied in nocturnal species, and to a lesser degree in diurnal animals [12], are formally expressed in Aschoff's rules [2]. The free- running period of circadian rhythms of nocturnal species generally becomes longer when light intensity is increased, while in diurnal animals increased light intensity shortens the period. An exception to this rule has been consistently ob- served in the behavior of diurnal primates where increasing intensities of constant light have been found to lengthen the period of several circadian rhythms [20,23]. Recent studies have indicated that light intensity can also influence the timing and duration of behavioral rhythms entrained to LD cycles in a manner which is predictable from the effects of light intensity on the free-running circadian period. For example, in avian species such as the finch where increases in light intensity shorten the free-running period, increases in the intensity of LD cycles advance the phase of daily locomotor behavior [17]. In the present study we examined the effects of the inten- sity of the 24 hour illumination cycle on the entrained drink- ing rhythms of diurnal squirrel monkeys. Since increases in the intensity of constant illumination lengthen the free- running period of circadian rhythms in the squirrel monkey, it was predicted that the entrained drinking rhythm would be phase delayed by increased levels of illumination provided in a LD cycle. The results of this study support this prediction, but also indicate that the LD cycle can have a masking effect on drinking behavior. Aschoff [3] used the term masking to de- scribe the influence of the LD cycle on the expression of behavior independently of its ability to entrain the circadian system. The present results demonstrate that these masking effects may facilitate (positively mask) or inhibit (negatively mask) specific temporal components of squirrel monkey be- havior. METHOD Adult male squirrel monkeys were housed individually in isolation chambers with free access to food (Teklad Diet TD 1Present address: Worcester Foundation for Experimental Biology, 222 Maple Avenue, Shrewsbury, MA 01545. zPresent address: Laboratory of Neurophysiology, Harvard Medical School, 74 Fenwood, Boston, MA 02115. Copyright © 1982 Brain Research Publications Inc.--0031-9384/82/020205-07503.00/0