Comparative aspects of circadian rhythms in homeotherms, re-entrainment after phase shifts of the zeitgeber.

Re-entrainment of circadian activity of 3 avian species (Fringillidae: Fringilla coelebs, Carduelis chloris, Pyrrhula pyrrhula) and 4 mammalian species (Rodentia: Eutamias sibiricus, Funambulus pennanti, Glis glis, Mesocricetus auratus), subjected to 12:12 h light-dark (LD) cycles as zeitgeber, was studied after shifting the LD cycle by different amounts (hours) and in different directions (advances and delays). The properties of the zeitgeber were changed by varying light intensity during dark-time (ID) and/or light-time (IL). After 6 and 9-h advance and delay shifts of the LD cycle, the 3 species of finches similarly re-entrained their activity rhythms in the direction of the shifted zeitgeber. After 10.5 and 12-h shifts, the majority of individuals re-entrained by delay phase shifts of the rhythms. After two successive 6-h advance or delay shifts (resulting in inversion of the LD cycle) most finches responded by advances. Generally, the time needed for re-entrainment of 2/3 of the full shift (2/3 tr) in finches dependend on: (a) the direction of the phase shift ('asymmetry effect'), and (b) the phase-angle difference between onset of activity and light-on (psio) before the shift. After 12-h shifts, species-specific differences in the direction of re-entrainment and in the slope of the relationship between 2/3 tr and psio were found. In two rodents, the nocturnal golden hamster and the diurnal Siberian chipmunk, the direction of re-entrainment following inversion of the LD cycle depended on psio. In both species, 2/3 tr after inversion of the zeitgeber was significantly longer than in the common dormouse. In chipmunks, changes in 2/3 tr were correlated with changes in psio due to systematic variation of the zeitgeber properties (range and/or mean level). After 6-h phase shifts, the 'asymmetry effect' was opposite in the two nocturnal rodents, the hamster and the dormouse. It is suggested that the time course (direction and rate) of re-entrainment was influenced by differences in the characteristics of the circadian systems and their responses ('sensitivity') to light changes in the various species of birds and mammals.