  We propose a new model in order to study behaviors of self-organized system
such as a group of animals. We assume that the individuals have two degrees of
freedom corresponding one to their internal state and the other to their
external state. The external state is characterized by its moving orientation.
The rule of the interaction between the individuals is determined by the
internal state which can be either in the non-excited state or in the excited
state. The system is put under a source of external perturbation called
"noise". To study the behavior of the model with varying noise, we use the
Monte-Carlo simulation technique. The result clearly shows two first-order
transitions separating the system into three phases: with increasing noise, the
system undergoes a phase transition from a dilute disordered phase to an
ordered compact phase and then to the disordered dispersed phase. These phases
correspond to behaviors of animals: uncollected state at low noise, flocking at
medium noise and runaway at high noise, respectively.
