Seasonally breeding mammals typically use the annual change in the photoperiod cycle (light-darkness ratio) to drive rhythmic daily melatonin signals from the pineal gland, providing a critical cue to time seasonal reproduction. The daily light cycle resets the animal's internal clock (circadian clock) roughly every 24 hours, keeping it synchronized with the environment. Production of melatonin and other hormones rises and falls with this daily cycle, regulated by the internal clock. At high latitudes, however, where there is continuous light or continuous darkness for months at a time, no photoperiod information is available for much of the year. Lu et al. (2010) studied two circadian clock genes in fibroblast cells of Arctic Reindeer and found they did not turn on and off on a daily cycle, as in most other animals that have been studied. However, melatonin production responded strongly to light and darkness (dropping in the light and rising in the dark), regardless of the activity of the circadian clock genes being monitored. Lu et al. suggest that in the Arctic environment, where a 24 hour cycle has little meaning, natural selection has reduced or eliminated the circadian clock in Reindeer (and perhaps other Arctic animals). Instead, they speculate, informative melatonin signals associated with the spring and fall equinoxes (when daily light cycles are available) may directly entrain a ‘‘circannual clock’’ [24, 29] that may not involve circadian mechanisms. (Lu et al. 2010) This phenomenon must be investigated in other Arctic animals, and using other genes in Reindeer, but these data suggest that, as might be expected, time-keeping in the far north may be quite different than at lower latitudes.
