In wireless ad hoc networks, signal fading and interference have adverse effects on delivering signals. To alleviate this problem, MAC protocols for supporting cooperative communication have been introduced. One of the cooperative MAC protocols is using relay-based cooperative strategy. The performance of these MAC protocols depends on how to select relay nodes. We propose a novel cooperative MAC protocol with reactive relay selection that reduces collisions and overheads in relay selection. In our proposed protocol, using the overheard transmissions, each relay candidate node determines which group it belongs to, and estimates the number of nodes in the group. The source node transmits through the relay candidate node which belongs to "best" group in order to minimize the total transmission time. The reduction of the total transmission time leads to the increase in total network throughput.