1. A method of operating an ad hoc network, the network comprising a plurality of devices each of which includes communication means for communicating with other ones of the plurality of devices when they are in range, the method comprising: storing on each device one or more nodal policies which specify rules for determining how a device should behave in response to various prevailing circumstances; controlling each device to operate in accordance with one or more of the stored nodal policies; storing on each device a fitness parameter; each device adjusting the value of the fitness parameter stored thereon in dependence upon the level of activity of the respective device consistent with its stored policies; each device monitoring the value of its stored fitness parameter and the activity of its communication means; and each device transmitting, to other ones of the devices which are in range, one or more of its stored nodal policies in the event that its fitness parameter exceeds a threshold value and its communication means is not required for other purposes.