Photoperiodic Effects on Growth in Microtus pennsylvanicus

Growth and development in Microtus pennsylvanicus was significantly affected by different photoperiodic regimes in the laboratory. Young and subadults maintained under a long photoperiod (18L:6D) gained weight more rapidly than did those under a short photoperiod (6L:18D). Adults under natural conditions of light and temperature lost weight until winter solstice, while growth rates of subadults slowed through November and December. Both age categories gained weight after the winter solstice. Thyroid activity, as reflected by thyroxine levels and iodine-125 uptake by the thyroid, was significantly lower in animals maintained under a short photoperiod than in those maintained under a long photoperiod.