University Distinguished Lecture

Seasonal Rhythms in Sex and Death: A Time to be Born and a Time to Die

Randy J. Nelson
Distinguished Professor of Social and Behavioral Sciences
Director, Neuroscience Graduate Studies

Winter is often energetically challenging. At high latitudes, energetic requirements for thermoregulation increase while food availability generally decreases. Physiological and behavioral adaptations, including termination of breeding, altered metabolism, and altered neuroendocrine function, have evolved among nontropical animals to cope with this winter energetic bottleneck. Presumably, selection for mechanisms that permit physiological and behavioral anticipation of seasonal ambient changes have led to current seasonal breeding patterns for many populations. In addition to seasonal fluctuations in breeding, there are salient seasonal fluctuations in disease and death. Energetically challenging winter conditions can directly induce death via hypothermia, starvation, or shock; surviving these demanding conditions evokes significant stress responses to the extent that immune function is often compromised during winter. Thus, individuals would enjoy a survival advantage if seasonally recurring stressors could be anticipated and countered by shunting energy reserves to bolster immune function.

The primary environmental cue that permits physiological anticipation of season is daily photoperiod (day length), a cue that is mediated by melatonin. However, other environmental factors, including low food availability and ambient temperatures, may interact with photoperiod to affect immune function and disease processes. This talk will review laboratory studies that consistently report enhanced immune function and reduced sickness behaviors, in winter-like, short-day lengths. Prolonged melatonin treatment mimics short days, and also enhances immune function in rodents both in vitro and in vivo. Melatonin appears to be part of an integrative system that coordinates reproductive, behavioral, immunological, and other physiological processes to cope successfully with energetic stressors during winter. Because of finite energy resources, individuals appear to optimize energetically expensive activities so that compromised immune function often coincides with breeding activities or during thermoregulatory challenges. In order to understand the optimization of immune function, it is necessary to understand the interaction of factors, on both mechanistic and functional levels, that affect immunity and survival. This eco-immunological approach may prove valuable in understanding variation in seasonality induced by changing climate and by light pollution at night, as well as understanding the temporal influences on emerging diseases.