Confronting groups of rats to a situation in which they have an increasing difficulty to reach food leads to the emergence of a social structure. Three profiles appear: supplier, non-carrier and autonomous rat. The regularity of this differentiation and the stability of the phenomenon let biologists assume that an underlying mechanism is responsible for the differentiation. The problem remains of a possible involvement of social cognition among the members of the group. We developed a multi-agent model without social cognition but with a self-organizing mechanism and ran computer simulations. The results match the biological observations. Furthermore such a model can be helpful for the assessment of the influences of both individual parameters and the environmental constraints on the group behavior.