Propensity for Lunar Periodicity in Hamsters and Its Significance For Biological Clock Theories. ∗

Abstract

Two alternative hypotheses exist for the timing system of the rhythmic daily activity patterns (circadian rhythms) persisting in animals shielded from all ordinary environmental variations. The endogenous hypothesis is that the basic timer is a very stable autonomous oscillation which in constant illumination and temperature runs freely revealing its individual inherited period. This postulated inherent rhythm is assumed to depend upon the daily light and temperature variations (Zeitgeber) for its synchronization to the day. The exogenous hypothesis is that organismic responsiveness to pervasive rhythmic geophysical variations constitutes the basic timer for the persisting circadian rhythms, and that the diverse frequencies observed even among individuals of the same species under the same conditions of unvarying light and temperature are derived through intraorganismic variable frequency transformation. Both rational biological explanations (1,2) and theoretical models(3) for such frequency transformation have been suggested. Thus, either timer hypothesis is now tenable, with personal preference and plausibility determining the choice between them for interpreting any given observed circadian characteristics.

The following two reports of observations of mice with accurate mean 24-hour periodicities illustrate two very different possible interpretations.

It has been noted(4) that in blinded mice held under controlled environmental temperature (24 ± 0.5°C) and on a 24-hour cyclic lighting regimen—12 hours of light alternating with 12 hours of darkness—several circadian rhythms that were desynchronized from the lighting schedule during the first few months after the operation returned to an average 24-hour period about 5 months after blinding. The interpretation favored was that the mice had become responsive to 24-hour variation in such subtle potentially effective factors as unidentified laboratory noises and perhaps odors which, for the same mice, were initially ineffective as synchronizers of postulated inherent rhythms.