Effect of crowding on emotional reactivity in male rats.

The effect of two different population densities (3 and 9 animals/cage) on emotional reactivity, measured by their performance in an exploratory box, and pituitary-adrenal response to acute noise stress are compared. The crowded rats showed a higher defecation rate and lower exploratory activity than the control rats. Adrenocorticotropin (ACTH) response to noise was higher in the crowded than in the control rats. The results clearly indicate that crowding increased emotional reactivity in adult male rats.