Recent work on the adaptive functions of circadian and seasonal rhythms in animals

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Abstract
This paper reviews research published since the Cold Spring Harbor Symposium of 1960 on “Biological clocks”. It is concluded that, despite annual increases in the quantity of research which have appeared since then, knowledge of the fundamental physiology of rhythms has progressed relatively little. Adaptive functions have fared even worse. It is probable that no single explanation of the diurnal vertical migration of plankton will ever be found. There has, however, been further evidence for the rôle of circadian rhythms in relation to physical factors of the terrestrial environment. These include water conservation among invertebrates and thermoregulation in poikilotherms. The interaction of such factors is discussed in relation to photoperiodism. Biotic factors in which rhythmic activities are of importance include reproduction, feeding, escape from enemies and, presumably, competition, although there appears to be little factual evidence for this. Finally there is a discussion of the adaptive significance of seasonal rhythms, especially in the tropics where the advantages they confer are not immediately obvious as in temperate climates. The rôle of “clock” mechanisms in celestial navigation is not considered in this paper.