Properties of Free-Running Circannual Rhythms

Abstract

Circannual rhythms differ widely among species with regard to their tendency to persist or to dampen. The rhythms of hibernation in golden-mantled ground squirrels (Spermophilus lateralis), those of body weight and other parameters in an eastern chipmunk (Tamias striatus) and the cycles of molt in garden warblers (Sylvia borin) and blackcaps (S. atricapilla) have been found to continue for at least five, six, and nine cycles, respectively (Table 2.1). This means that the circannual clock can function essentially for the whole life of an organism (Pengelley and Asmundson 1969; Richter 1978; Berthold 1978). In other instances, circannual rhythms have shown more or less pronounced tendencies to dampen. This is true, for instance, of the hibernation rhythm in several species of ground squirrels and chipmunks (e.g., Pengelley and Kelly 1966; Heller and Poulson 1970), the body-weight rhythms of the dickcissel (Spiza americana: Zimmermann 1966) and the rhythms of molt and migratory disposition in the chiffchaff (Phylloscopus collybita: Gwinner 1971a) and whitethroat (Sylvia communis: Gwinner 1983). Particularly interesting from a physiological aspect are cases in which, under one particular constant condition, the annual rhythm of some functions persists in an organism whereas that of others fade away. Examples of this will be given in Chap. 5.