In wireless networks utilizing half-duplex transceivers, a mechanism is required to inform a sender of the outcome of its transmission. Traditionally, such feedback comes from the receiver. This approach cannot distinguish between a failure due to a collision at the receiver and a failure due to the receiver moving out of range. To address this problem we propose cooperative signalling, where nodes that overhear the transmission provide feedback to the sender in addition to the feedback from the receiver. Cooperating nodes also provide information to the receiver, allowing it to distinguish between failures which, in turn, enables the sender to respond to each outcome differently. We apply cooperative signalling in a carrier sense multiple access (CSMA) protocol using transmission power control. In simulation, CSMA/PC achieves substantial improvements in throughput per unit energy compared to IEEE 802.11 in both static and mobile ad hoc networks.