We study cooperative data transmissions in coalition-based wireless sensor networks, where neighboring nodes are organized into groups to form coalitions. We assume that data compression and cooperative communications can be carried out within one coalition, and that CSMA is used to reserve the channel from the coalitions to the sink. We examine three schemes for data transmissions from the coalition (with reservation) to the sink. In Scheme 1, one node in the coalition is selected randomly to transmit the data; in Scheme 2, the node with the best channel condition in the coalition transmits the data; and in Scheme 3, all the nodes in the coalition transmit in a cooperative manner. We investigate the corresponding delay performance and the energy consumption of the three schemes. Our numerical examples show that significant gains can be achieved by the coalition-aided transmission schemes, and as expected, Scheme 3 achieves the best performance.