  We study Nowak and May's spatial prisoners' dilemma game driven by mutations
(random choices of suboptimal strategies) on empirical social networks. The
time evolution of the cooperation level is highly complex containing spikes and
steps between quasi-stable levels. A statistical characterization of the
quasi-stable states and a study of the mechanisms behind the steps are given.
We argue that the crucial structural ingredients causing the observed behavior
is an inhomogeneous degree distribution and that the connections within
vertices of highest degree are rather sparse. Based on these observations we
construct model networks with a similarly complex time-evolution of the
cooperation level.
