Rhythms, Reproduction, and Photoreception1

Annual rhythms, in Which sexual activity, birth, rearing of the young, or other events in the reproductive cycle occur in a more or less precise temporal relationship with season, are widespread among plants and animals. Seasonality also occurs in processes such as dormancy, diapause, hibernation, and migration which, while not directly involved in the reproductive cycle, have profound effects on it. The primary adaptive advantage to be gained from seasonal reproductive activity is, of course, the temporal coincidence of the arrival of offspring with the maximum availability of nutrients to carry them through the rapid growth of the early stages of their lives. To insure this coincidence, the timing of reproductive events must be controlled by factors in the environment which are temporally related to the availability of nutrients. Other things being equal, the most useful controlling factor will be the one with the least variance in its temporal relationship with the food supply. However, because considerable time usually elapses between the initiation of reproductive activity and the appearance of the young, an ideal controlling factor should initiate reproductive activity at an appropriate time in advance of the best time for rearing the young. Environmental factors which satisfy these criteria are not numerous. On a priori