Annual Rhythms: Perspective

Abstract

The habitats of most organisms are subject to pronounced seasonal fluctuations. Literally all physical environmental factors important to an organism—like temperature, day length, and rainfall—vary with season to such an extent that most plants and animals could not escape the necessity of developing adaptational strategies to cope with them. The most obvious expression of this necessity is that many biological activities are concentrated or restricted to the times of the year when they are most likely to be successful. This is true, for instance, of reproduction, which is timed in such a way that the growth of the offspring occurs when environmental conditions are most favorable, that is, late spring and summer. Other seasonally restricted activities include behavioral, physiological, and morphological processes by which organisms overcome or avoid severe winter or extreme summer conditions, that is, dormancy, diapause, hibernation, and migration. Developmental processes like molt of skins, furs, and plumages are often inserted between reproduction and the processes occurring in winter. In addition, numerous physiological functions like basal metabolism or growth rate change continuously with the time of year as a direct or indirect consequence of environmental seasonal variations (for reviews, see, e.g., Immelmann, 1963b, 1967, 1971; Aschoff, 1955; Lack, 1950; Murton and Westwood, 1977; Farner and Follett, 1979).