Hybrid rats, Rattus norwegicus, are used to investigate a model of nongenetic adaptation with rats living in a stable environment under crowded conditions. A control population received food and water under normal conditions with no demands for learning cooperative tasks placed upon them. Rats maintained at twice normal population density obtained food and water resources by learning cooperative tasks. Results indicate that these latter rats show: (1) reduced intensity of fighting, (2) increasing maintenance of body weight, and (3) development of modes of reproductive control, including killing of young. We conclude that acquisition of cooperative behavior reduces crowding-related stress.