In WSNs, energy conservation is the primary goal, while throughput and delay are less important. This results in a tradeoff between performance (e.g., throughput and delay) and energy consumption. In this paper, the problem of energy-efficient MAC protocols in WSNs is modeled as a game-theoretic constraint optimization problem. After introducing incompletely cooperative game theory, based on the estimated game state (e.g., the number of competing nodes), each node independently implements the optimal equilibrium strategy under the given constraints (e.g., the used energy). Moreover, a simplified game-theoretic constraint optimization scheme (G-ConOpt) is presented in this paper, which is easy to be implemented in current WSNs. Simulation results show that G-ConOpt can increase system performance while still maintaining reasonable energy consumption.