Waiting for the Sun: the circannual programme of reindeer is delayed by the recurrence of rhythmical melatonin secretion after the arctic night.

At temperate latitudes, the annual cycle of day length synchronizes circannual rhythms, and, in mammals, this is mediated via nocturnal production of the pineal hormone melatonin, proportional to the length of the night. Here, we studied circannual synchronization in an arctic species, the reindeer (Rangifer tarandus tarandus), which ceases to produce a rhythmic melatonin signal when it is exposed to extended periods of continuous midwinter darkness and continuous midsummer light. Using food intake, antler growth and moult as endpoints, we demonstrate that when animals living at 70°N are transferred from natural photoperiods in late autumn to either continuous light or continuous darkness, they undergo a conspicuous acceleration of the circannual programme. We conclude that rhythmical melatonin secretion, recommencing when the Sun reappears late in January, is required for proper timing of spring physiological responses, through a delaying effect on the circannual programme set in motion during the preceding autumn.