ABSTRACT
Most Djungarian hamsters exhibit multiple adjustments in response to a short-day (SD) photoperiod, such as a decrease in body weight, increased thermogenesis and cold resistance, occurrence of torpor, molt to the winter pelt and reproductive regression. In contrast, some individuals fail to exhibit short-day adjustments. These hamsters maintain body mass, retain a summer pelt, stay reproductively competent, and never enter torpor. In this chapter, the authors identify circadian differences between SD-insensitive and SD-sensitive hamsters. To define a phase response curve (PRC), 15 min light pulses (350 lux) were given at random times, and the magnitude and direction of each light-induced phase insensitive hamsters were exposed to various intensities of constant light to assess incidence of splitting. Splitting occurs when a circadian rhythm dissociates into two components which eventually establish a stable relationship of 12 h. The PRC differences demonstrate that SD-insensitive hamsters respond to light input differently from their SD-sensitive counterparts.
