Astronomical and meteorological parameters and rest-activity rhythm in the spider monkey Ateles geoffroyi

All animals including primates show regular variations in their behavioural activities and physiological functions. In numerous species diel, ultradian, infradian, annual and/or lunar rhythms have been established; however, the use of the focal sampling method does not provide information on hour-to-hour nor day-to-day variations. In behavioural field studies dealing with primate activity rhythms and their modulation by environmental variables the possible dual, synchronizing and/or masking effect of these variables is often ignored. In the present study, we analyze the relation between astronomical and meteorological parameters and various parameters of the rest-activity rhythm in Ateles geoffroyi, a species scarcely investigated with respect to biological rhythms. These Ateles were kept under semi-natural conditions, exposed to natural light, temperature and humidity cycles, and sky state. Motor activity was recorded with actiwatch accelerometers during 180 days (5-min intervals), starting from the autumn equinox.

Ateles is active throughout the day, rests during the night and presents bimodal activity. Activity time, onset and end of activity as well as the times of their two activity peaks were significantly correlated with duration of the solar day and sunrise or sunset time. The behavioural parameters also correlated with ambient temperature. Climate factors and sky-state significantly influenced the duration and times of onset and end of activity. Our findings identified a clear modulation of the rest-activity pattern by astronomical variables serving as Zeitgebers, such as the natural light-dark cycle, and by masking effects of meteorological factors (temperature, weather and cloudiness), as well as of artificial variables introduced by the interaction with man.