Ad hoc networks are expected to be used in many different situations. A common characteristic among these situations 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 investigate by simulation a 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.