In this paper, we investigate a novel low-latency MAC protocol (ARQ-CRI) for low-power cooperative wireless sensor networks WSNs, while preserving (i.e. in high traffic mode) or even increasing (i.e. in low traffic mode) energy-efficiency. Based on the wakeup period of relay nodes, a new relay selection technique is proposed to decrease latency. Simulations show that ARQ-CRI can get latency gain and help to increase energy-efficiency in low traffic mode as compared to CL-MAC, a recently proposed cooperative MAC protocol for WSNs. Besides, regarding traffic variation, ARQ-CRI is claimed to be more stable than CL-MAC. Finally, the effect of the number of potential relays on the performance of ARQ-CRI is also considered.