CORRELATION BETWEEN REPRODUCTIVE PHOTORESPONSIVENESS AND PHOTOREGULATED LOCOMOTOR-ACTIVITY IN MEADOW VOLES

Some rodent populations contain individuals that undergo complete gona dal regression under short day lengths, other individuals whose gonads are little affected by such treatment, and still others that are inte rmediate in their response. Meadow voles exhibiting this variation wer e used to explore the relationship between reproductive photoresponsiv eness and photoregulated locomotor activity. The activity patterns of the two extreme phenotypes-reproductively photoresponsive vs. nonrespo nsive-were compared, first under short day lengths and then under long day lengths. The primary component of the daily running wheel activit y pattern for both phenotypes under both conditions was ultradian. Wit hin that framework, reproductively photoresponsive voles were predomin antly nocturnal in their locomotor activity under both day lengths. In contrast, the nonresponsive individuals showed no significant circadi an variation in their activity under either day length. These results suggest that reproductively photoresponsive and nonresponsive individu als may have fundamentally different patterns of activity throughout t he year in the wild. Furthermore, the data suggest that the inability of some voles to respond reproductively to variations in photoperiod m ay be caused by a decoupling of the circadian system from the entraini ng effects of day length.