Circannual Systems

Abstract

The existence of circannual rhythms was first postulated for such organisms as exhibit pronounced seasonal cycles in their physiology and behavior despite their exposure to either relatively constant or unpredictable and highly complex environmental conditions. In the first category are a variety of tropical organisms that inhabit areas with little seasonal variability but still show conspicuous seasonality in various activities, like flowering in plants or reproduction in animals. It seemed reasonable to assume that these functions were primarily independent of external factors and rather were controlled by an endogenous annual clock (e.g., Moreau, 1931; Chapin, 1932; Baker, 1938; Aschoff, 1955; Marshall, 1960; Immelmann, 1973). The second category comprises, among other organisms, many migratory birds that experience peculiar seasonal patterns in photoperiod, temperature, and other factors as a consequence of their migrations across large ranges of latitude and longitude. Baron von Pernau speculated back in 1702 that such birds were ``driven by a hidden urge'' to commence migration at the appropriate time, and that environmental factors like food and temperature were of minor significance. Much later, the concept of an endogenous annual rhythmicity involved in the control of annual functions in such birds was proposed more explicitly by Rowan (1926), Chapin (1932), Marshall (1960), and others.