Ad hoc networks are expected to be used in a number of very different situations. But a common characteristic is that the nodes have to cooperate with each other. This problem is particularly crucial, if each node is its own authority. Reckoning the relevance of this issue, several groups of researchers have proposed different incentive mechanisms, in order to foster cooperation between the nodes, notably for packet forwarding. However, the need for these incentives was not formally justified. In this paper, we address the problem of cooperation without incentive mechanisms and propose a simple model based on game theory. We then prove several theorems about the equilibrium conditions in a simple scenario. We investigate by simulation a more realistic scenario, which includes a real network topology as well as a mobility model. We show that the level of contribution of the nodes to reach cooperation is much higher than in the theoretical model, and we quantify the relationship between mobility and cooperation. We conclude that spontaneous cooperation is easier to reach when mobility is higher.