  A generic property of biological, social and economical networks is their
ability to evolve in time, creating and suppressing interactions. We approach
this issue within the framework of an adaptive network of agents playing a
Prisoner's Dilemma game, where each agent plays with its local neighbors,
collects an aggregate payoff and imitates the strategy of its best neighbor. We
allow the agents to adapt their local neighborhood according to their
satisfaction level and the strategy played. We show that a steady state is
reached, where the strategy and network configurations remain stationary. While
the fraction of cooperative agents is high in these states, their average
payoff is lower than the one attained by the defectors. The system
self-organizes in such a way that the structure of links in the network is
quite inhomogeneous, revealing the occurrence of cooperator ``leaders'' with a
very high connectivity, which guarantee that global cooperation can be
sustained in the whole network. Perturbing the leaders produces drastic changes
of the network, leading to global dynamical cascades. These cascades induce a
transient oscillation in the population of agents between the nearly
all-defectors state and the all-cooperators outcome, before setting again in a
state of high global cooperation.
