By utilizing a hybrid strain of rats, Rattus norvegicus, a paradigm of human culture is being explored. This paradigm is based on the following theoretical considerations: The evolution of species of mammals, including human beings, has resulted in optimum group sizes of 12 adults. Since each group occupies a fixed territory, eventual occupation of all available space by such groups culminates in a stable adult population of the species. However, if conditions permit, an elaboration of social roles (particularly collaborative ones) and development of a more complex communication network, then more individuals can survive in a given area without a stressful increase in rate of interaction among members of the population that would otherwise trigger limitation of population size. In our experimental study 24 rats were introduced into each of two bilaterally symmetrical, compartmentalized habitats. After initial and equal distribution of animals throughout the habitat by number and sex, restructured social organizations became evident in both experimental and control populations, with the alpha male and half the females in both occupying one side of the habitat, and all remaining animals inhabiting the other side.