Photoperiod-Nonresponsive Morphs: A Possible Variable in Microtine Population-Density Fluctuations

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Abstract
Microtine rodents tend to breed in the spring and summer. Winter breeding occurs in many species and often precedes large increases in population density. Short photoperiods (less than 12 hours of light per day) arrest reproduction in a majority of animals. Laboratory studies have demonstrated that some individuals within a population are inherently unresponsive to day-length cues and continue to breed when maintained in winter photoperiods. Other studies have indicated that nutritional and other extrinsic factors can override reproductive inhibition in animals that normally respond to short day lengths. It is proposed that responsiveness to photoperiod is the common physiological pathway whereby both intrinsic and extrinsic factors can interactively influence breeding. In order for animals to breed during the short days of winter, photoperiodic information must be ignored. This masking of reproductive inhibition can occur if the proportion of non-photoperiodic morphs within a population increases, if abundant food or other favorable conditions override responsiveness to short days, or if some complex multifactor interaction occurs between intrinsic and extrinsic factors that determine responsiveness to day length. Factorial experiments that tease apart the influences of various biotic and abiotic factors on photoperiodic responsiveness are necessay in order to understand the factors that permit winter breeding and subsequent increases in population density.