Lunarperiodic variation of the phase-angle difference in nocturnal animals under natural Zeitgeber-conditions near the equator.

Several months' studies under natural illumination conditions in the vicinity of the equator show that night monkeys (Aotus trivirgatus), and leaf-nosed bats (Artibeus lituratus) exhibit species-specific lunarperiodic changes of the phase position of their activity rhythm to the Zeitgeber rhythm (Figures 1-3). These changes are closely to nightly illumination conditions. In Aotus the phase-angle differences (p.a.ds) of the onset and end of the activity phase, and in Artibeus the p.a.ds of the midpoint and end of the activity phase are significantly dependent upon the phases of the moon. In the African fruit bat Rousettus aegyptiacus, and in the leaf-nosed bat Phyllostomus hastatus no statistically significant lunarperiodic changes of the p.a.ds could be detected. These results cannot be explained by means of current models concerning the mechanism of synchronisation of biological oscillation. They appear to be, instead, expressions of a periodic masking of the real phase-position due to direct effects of light intensity on the level of activity which which itself is determined by the circadian rhythm. Aschoff's rule, which states that the earlier a species or an individual awakes the later it terminates its daily phase of activity, is partly confirmed for the nocturnal mammals examined in the present study.