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Most animals exhibit seasonal variation in their reproduction and survival
simply because most of them live in environments in which food availability
varies seasonally. This is true in the tropics as well as at the higher latitudes, but
at the higher latitudes the combined challenge of food shortage and low temperature
makes winter a particularly difficult time to reproduce and survive.
Traditionally, vertebrate physiologists interested in seasonal phenomena
have focused on reproduction and largely ignored survival. In part, this is
because of the discovery 70-odd years ago that some temperate zone animals
use variation in day length to synchronize their reproduction with seasonally
changing environmental conditions. This discovery meant that researchers
interested in seasonality could bring the power of the scientific method into
play simply by installing a light timer on the wall of an animal room. The
result of several decades of experimentation using this technique is a robust
body of knowledge about how photoperiod regulates reproduction. Indeed,
this is an epic, albeit an as yet unfinished epic, in biological research.
Randy Nelson and his colleagues have added a whole new dimension to
the study of seasonality by shifting the focus from reproduction to survival.
This book is a pioneering effort to define this field of study, and it will prove
to be a milestone in research on seasonality. The specific hypothesis under
consideration is that “individuals have evolved mechanisms to bolster immune
function in order to counteract seasonally recurrent stressors that may
otherwise compromise immune function.” Stated differently, the authors visualize
seasonal variation in survival as an interaction between two factors:
the suppression of immune response attributable to changing energetic conditions
and an endogenous rhythm of enhancement of immune response that
is dependent on photoperiod, clocks, and melatonin. |
