The Biological significance of Circannual Rhythms in Animals and Humans

Abstract

This presentation will first show examples of circannual rhythms within the animal kingdom and also give evidence for circannual rhythms in humans. A clear distinction must be made between „true“ circannual rhythms and „simple“ annual or seasonal rhythms. Although the biological significance probably is similar in both cases, the possible underlying mechanisms are very likely quite different. The biological importance lies in the fact that both circannual rhythms and seasonal rhythms allow physiological changes to be complete well in advance of seasonal environmental changes. Those environmental variables that, in the course of evolution, have exerted selection pressure to restrict an activity to a particular time of year are called „ultimate factors“. Physiological and/or morphological adaptations that require some time to develop must be initiated early enough to be completed on time, i.e. when the environmental challenges (the ultimate factors) meet with the adequate adaptations. For example, to guarantee that the young are born at the optimal time of year, the preceding process of gonadal development and gestation must be initiated long in advance and often at a time of suboptimal living conditions. For that, the animals need a forewarning environmental cue that reliably gives information about the season to come well in advance. Such cues that actually trigger the annual cycles by affecting physiological processes are called „proximate factors“, of which the photoperiod and its seasonal change is the most important one. The processes that are controlled by the photoperiod (and hence by the pineal gland and melatonin) comprise essentially all types of activities known to show seasonal variations. In contrast, circannual rhythms are generated endogenously (without an extermal trigger) and can persist for many years even under constant environmental conditions. Accordingly, true circannual rhythms can only be found in organisms with a life span of at least several years. In circannual rhythms the proximate factors such as photoperiod do not trigger and thereby initiate the seasonal rhythms, but they are used as zeitgeber to synchronize the endogenous rhythm with the outside world. It has long been questioned whether circannual rhythms exist, or whether they represent only a „sequence of stages“. Today, there are many good examples for endogenous circannual rhythms in many different species and even in humans there is good evidence for at least residual circannual rhythms. However, the underlying mechanism(s) and the anatomical location of a circannual pacemaker within the central nervous system are still under debate. Our present knowledge will be presented and discussed.