Seasonal immune function and sickness responses.

Winter is a particularly difficult time to breed and survive. Animals monitor day length (photoperiod) to engage seasonally appropriate adaptations in anticipation of harsh winter conditions. I propose that photoperiodic information, mediated by melatonin, might also influence immune responses. Individuals could improve survival if seasonally recurring stressors were anticipated and countered. Recent studies suggest that short day lengths reroute energy from reproduction and growth to bolster immune function during winter. Short days can either enhance or suppress components of immune function, as well as reduce fever and the expression of sickness behaviors. The net result of these photoperiod-mediated adjustments is enhanced immune function and increased survival. Melatonin appears to be part of an integrated system that coordinates reproductive, immunological and other processes to cope successfully with energetic stressors during winter and to balance trade-offs between reproductive success and survival.