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Cooperative communications is one of the promising techniques to enhance the performance of wireless networks. The extent of performance improvement needs to be carefully investigated, especially for wireless sensor networks, due to the overhead and energy costs involved with cooperation and the dependency on energy consumption models. In this paper, we propose a cooperative medium access control protocol, COMAC that enables cooperation in a realistic scenario using 802.11g based radios and leverages cooperative communications by making use of the overheard packets from neighboring nodes of a sender node. In an effort to determine the conditions under which cooperation is preferable, we evaluate COMAC's performance in comparison with standard 802.11 through detailed simulations, considering different physical layer data rates, varying transmission ranges, networks of different sizes and various energy consumption models. COMAC is shown to provide robustness to the wireless channel impairments, resulting in increased transmission range and improved packet success ratio in point-to- point scenarios. Throughput and energy efficiency performance is also quantified for multi-point-to-point scenarios with varying number of contending nodes. It is shown that cooperation with COMAC can provide significant enhancement in throughput, up to 23 times non-cooperative 802.11, together with energy savings of 50% even for high circuit energy consumption cases.