Seasonal patterns of stress, disease, and sickness responses

The combined challenge of low food availability and low temperatures can make winter difficult for survival, and nearly impossible for breeding. Traditionally, studies of seasonality have focused on reproductive adaptations and largely ignored adaptations associated with survival. We propose shifting the focus from reproduction to immune function, a proxy for survival, and hypothesize that evolved physiological and behavioral mechanisms enable individuals to anticipate recurrent seasonal stressors and enhance immune function in advance of their occurrence. These seasonal adaptations, which have an important influence oil seasonal patterns of survival, are reviewed here. We then discuss studies suggesting that photoperiod (day length) and photoperiod-dependent melatonin secretion influence immune function. Our working hypothesis is that short day lengths reroute energy from reproduction and growth to bolster immune function during winter. The net effect of these photoperiod-mediated adjustments is enhanced immune function and increased survival.