Rats from litters of 4, 7, or 11 were placed 1, 2, 3, or 4/cage till 60 days old. Litters of four were less active in an activity wheel than litters of 7 and 11 (p < .05) and females were more active than males (p < .01). Neither cage density nor litter size affected time to begin eating. Although main effects were not demonstrated in a brightness discrimination, there was a cage × sex interaction. Males housed 1/cage were slower to learn than multiply caged Ss. Possible interpretations are facilitative effects of crowding, increased incentive value of food due to home-cage competition for food, or increased activity need generated by crowded living conditions.
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