Groups of rats confronted to an increasing difficulty to reach food organize themselves. The emergent organizational structure is characterized by a distribution of two different behavioral profiles among the rats. This specialization is stable, robust and presents adaptive properties. Hamelin, the simulation system we propose is based on the coupling of two existing models: adaptive response thresholds and dominance relationships. It manages to reproduce the organization observed by biologists and presents some interesting adaptive properties. The originality of this model is that local internal needs (hunger) are distributed among the collectivity without an explicit global representation. The collectivity profits from individual adaptation abilities.