Effects of rearing condition on activity-induced weight loss.

This study evaluated the effects of differential rearing conditions on a rat protocol for various human syndromes. Subjects were 26 male Sprague-Dawley rats, 24 days old at the start of the experiment, matched according to weight, and randomly assigned to an isolation- or group-reared (4 rats/cage) condition. At 60 days of age (273 +/- 13 g), subjects were individually housed in cages allowing access to running wheels. Weight loss was produced through voluntary exercise and restricted food access. Animals in the isolation-reared condition lost weight at a faster rate and had heavier relative thymus weights than those in the group-reared condition. Animals in both conditions ran equivalent distances and ate equivalent amounts of food. The data show that postweaning rearing conditions impact the interpretation of behavioral and physiological outcomes of animal models. The results implicate a shift from maternal regulation of pup physiological and behavioral systems to the broader social niche.