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Cooperative communication techniques offer significant performance benefits over traditional methods that do not exploit the broadcast nature of wireless transmissions. Such techniques generally require advance coordination among the participating nodes to discover available neighbors and negotiate the cooperation strategy. However, the associated discovery and negotiation overheads may negate much of the cooperation benefit in mobile networks with highly dynamic or unstable topologies (e.g. vehicular networks). This paper discusses uncoordinated cooperation strategies, where each node overhearing a packet decides independently whether to retransmit it, without any coordination with the transmitter, intended receiver, or other neighbors in the vicinity. We formulate and solve the problem of finding the optimal uncoordinated retransmission probability at every location as a function of only a priori statistical information about the local environment, namely the node density and radio propagation model. We show that the solution consists of an optimal cooperation region which we provide a constructive method to compute explicitly. Our numerical evaluation demonstrates that uncoordinated cooperation offers a low-overhead viable alternative, especially in high-noise (or low-power) and high node density scenarios.