Duty cycling is an effective method for prolonging network lifetime in WSNs. In recent studies, dynamic duty cycle schemes are adopted with the use of asynchronous MAC protocols. The opportunistic routing employing forwarder set is applicable for duty-cycled WSNs. Moreover, remaining energy of nodes could be taken into account as a restriction when adjusting duty cycles. In this work, in order to prolong the lifetime of WSNs, a dynamic duty cycle scheme which is based on an opportunistic routing technique is introduced. The technique also pays attention to the energy level of the nodes. The technique introduced is developed on the XMAC protocol. The performance of the technique introduced is compared with those of the node degree based heterogeneous duty cycling technique and the constant duty cycle technique. It is observed that the forwarder set based duty cycling technique introduced gives promising results in terms of network lifetime and delay.