Aggregation in quads but not pairs of rats exposed to cat odor or bright light

► The response of groups of two and four rats to cat odor and bright light is tested. ► Groups of four but not two rats huddle together in response to both stressors. ► Locomotor activity is inhibited by both stressors. ► Cat odor increases defecation and has a conditioned effect on locomotion. ► As with other species, huddling appears to be a defensive response in larger groups of rats.