Photoperiodic polyphenisms in rodents: Neuroendocrine mechanisms, costs, and functions

Annual changes in daylength figure prominently in the generation of seasona l rhythms in reproduction, and a wide variety of mammals use ambient photop eriod as a proximate cue to time critical reproductive events. Nevertheless , within many reproductively photoperiodic mammalian species, there exist i ndividuals-termed "photoperiod nonresponders'-that jail to adopt a seasonal breeding strategy and instead exhibit reproductive competence at a time of year when their conspecifics are reproductively quiescent. Photoperiod non responsiveness has been principally characterized by laboratory observation s-over half of the species known to be reproductively photoperiodic contain a proportion of nonresponsive individuals. The study of nonresponders has generated basic insights regarding photic regulation of reproduction in mam mals. The neuroendocrine mechanisms by which the short-day photoperiodic si gnal is degraded or lost in nonresponders varies between species: differenc es in features of the circadian pacemaker, which provides photoperiodic inp ut to the reproductive neuroendocrine system, have been identified in hamst ers; changes in the responsiveness of hypothalamic goradotrophs to melatoni n and as-yet-unspecified inhibitory signals have been implicated in voles a nd mice. Individuals that continue to breed when their con-specifics refrai n might enjoy higher fitness under certain circumstances. Statements regard ing the adaptive function of reproductive nonresponsiveness to photoperiod require additional information on the costs (metabolic and fitness) of sust aining reproductive function during the winter months and how these costs v ary as a function of environmental conditions. Reproductive nonresponders t hus continue to represent a challenge to theories that extol the adaptive f unction of seasonality. Several nonexclusive hypotheses are proposed to acc ount for the maintenance of nonresponsive individuals in wild rodent popula tions.