Link asymmetry, arisen from varying coverage distance between two adjacent nodes, poses a real challenge for designing efficient receiver initiated MAC protocol for extremely low duty-cycled wireless sensor networks (WSN). One of the viable solution to overcome this limitation is to exploit the benefit of cooperative communication. In this paper, we propose a cooperative MAC protocol for WSN that effectively reduces asymmetry by taking advantage of alternative paths between a pair of nodes with link asymmetry. Simulation results show that our proposed scheme achieves significant improvement in performance compared with Asym-MAC, a state-of-the-art MAC protocol for WSN with asymmetric links. This performance improvement is effectively demonstrated in terms of energy consumption, average delay per packet and packet reception ratio.