Circadian rhythms have been found in virtually all organisms that have been studied [98] and the coupling of photoperiod and neuroendocrine control of reproductive physiology and behavior is often important among mammals. A mammalian species complex that spans wide latidunal swaths shows clinal variation in its responsiveness to photoperiod (from no response at all to a high responsiveness), suggesting rapid evolution [87]. In several non-human primates that live in the higher latitudes of the tropics and lower temperate zone, photoperiod has been associated with reproductive and behavioral changes [87].
