It is probably significant that the response to photoperiod is present to varying degrees in different strains of rats. The implication is that some of the wild ancestors of laboratory rats had the capacity to undergo both obligate and facultative inhibition of reproduction in SD, with longer reproductive delays in SD when food was scarce but faster maturation when food was sufficiently abundant to permit successful reproduction. The process of domestication of the wild Norway rat may have carried naturally occurring individual variation in seasonal reproductive responses into the laboratory. In the creation of relatively inbred laboratory strains (such as Sprague-Dawley and Wistar) and highly inbred lines (F344), different strains and lines were fixed for varying combinations of alleles that contribute to the regulation of seasonal reproduction. While this strain variation may not permit us to reconstruct or infer mechanisms of seasonal reproduction in wild rats, it may prove useful in elucidation of the contribution to seasonality of specific genes and the neuroendocrine phenotypes they control. buy ortho tri-cyclen
Our results should alter the interpretation of the significance of ‘‘unmasking’’ of photoresponsiveness in laboratory rats. Far from being a laboratory phenomenon of little relevance, our results suggest that these responses are important components of a seasonal response integrating multiple environmental inputs. In previous studies on other strains of rats, the effects of photoperiod have been modest enough to suggest that they might be of little or no biological importance in a wild rat (e.g., ). This study lends support to the hypothesis that seasonal reproductive regulation in temperate zone rodents, including Rattus norveg-icus, typically involves the integration of multiple cues. Ultimately, in order to fully understand seasonality, it will be necessary to understand the effects of these environmental cues both alone and in combination.